Barking Mat
Cuddler of hedgehogs.
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Wow, and you landed it too.
- Thread starter Misterg
- Start date
Misterg
Member
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I went back to sheet metal for a bit - I fancied a go at making the tail cone. It's a cone that transitions to a parallel section via a radius.
I turned a mandrel to suit the ID by machining the conical part, then a series of steps at known positions / diameters through the radiussed section. These were roughly blended by turning a series of tapers (judged by eye), before finally blending with a file.
This is almost done - the blue rings are the remains of the layout fluid at the reference diameters
A few minutes with a file cleaned the mandrel up. Next to it is the rolled up cone that will form the blank (0.5mm 304 stainless).
The welded blank and the mandrel. The blank is over length, so I will be able to cut the end with the missing weld off.
It's actually the second one, as the first one cracked at the weld when I tried to spin it. I had used filler on the weld (to try and stop it burning back at the end) and needed to dress it flat, but I thinned the material out too much, I think.
The welded cone was spun over the mandrel using a brass bar in the toolpost of my lathe. I did anneal it a couple of times. It was held in place with pressure from the tailstock on the end of the blank to wedge the conical part tight against the mandrel.
The polishing is totally unnecessary, but I couldn't help myself!
The finished cone outside:
And inside
The seam is towards the camera here.
I was VERY pleased with that.
. I need to cut slits in it to take the support vanes, but I can't bring myself to do it just yet!
I also made the rear of the outer casing using a wooden former and a disc of 0.5mm stainless. Again, I made two, as I let the first one get a little too far out of shape which resulted in it cracking.
The former - the flat area is large enough the take the NGV flange, and the OD is correct for the ID of the casing, but between the two is just free-hand.
Let battle commence!
I made the blank a little bigger on the second try and when I got it to this stage, I trimmed the ripples off, rather than trying to flatten them (lathe is running backwards here, with the tool upside down).
It worked out OK, and I was able to drill the holes for the NGV and use these to secure the part to the former while the central hole was trepanned out. I also made the hole for the fuel feed as per the book (most designs run the fuel feed through to the front of the engine, but Kamps runs his straight through the rear casing).
This was the first try:
This was the second!
Again, very pleased with how it came out.
Thanks for reading
(I've omitted the links from the images to see if it makes any difference to them showing up - still none the wiser as to what's causing it
)
I turned a mandrel to suit the ID by machining the conical part, then a series of steps at known positions / diameters through the radiussed section. These were roughly blended by turning a series of tapers (judged by eye), before finally blending with a file.
This is almost done - the blue rings are the remains of the layout fluid at the reference diameters
A few minutes with a file cleaned the mandrel up. Next to it is the rolled up cone that will form the blank (0.5mm 304 stainless).
The welded blank and the mandrel. The blank is over length, so I will be able to cut the end with the missing weld off.
It's actually the second one, as the first one cracked at the weld when I tried to spin it. I had used filler on the weld (to try and stop it burning back at the end) and needed to dress it flat, but I thinned the material out too much, I think.
The welded cone was spun over the mandrel using a brass bar in the toolpost of my lathe. I did anneal it a couple of times. It was held in place with pressure from the tailstock on the end of the blank to wedge the conical part tight against the mandrel.
The polishing is totally unnecessary, but I couldn't help myself!
The finished cone outside:
And inside
The seam is towards the camera here.
I was VERY pleased with that.
. I need to cut slits in it to take the support vanes, but I can't bring myself to do it just yet!I also made the rear of the outer casing using a wooden former and a disc of 0.5mm stainless. Again, I made two, as I let the first one get a little too far out of shape which resulted in it cracking.
The former - the flat area is large enough the take the NGV flange, and the OD is correct for the ID of the casing, but between the two is just free-hand.
Let battle commence!
I made the blank a little bigger on the second try and when I got it to this stage, I trimmed the ripples off, rather than trying to flatten them (lathe is running backwards here, with the tool upside down).
It worked out OK, and I was able to drill the holes for the NGV and use these to secure the part to the former while the central hole was trepanned out. I also made the hole for the fuel feed as per the book (most designs run the fuel feed through to the front of the engine, but Kamps runs his straight through the rear casing).
This was the first try:
This was the second!
Again, very pleased with how it came out.
Thanks for reading
(I've omitted the links from the images to see if it makes any difference to them showing up - still none the wiser as to what's causing it
)
northwest
Searching for the Holy Grail.........
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In the Netherlands they can have Pulse Jets, very impressive. They get a special permission when they come over here to fly them at the NATS.*
*National model aircraft meeting
Misterg
Member
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I looked around for something to adapt into the outer casing, but didn't really find anything. I would have loved to use the original CV470 gas canister with the original markings intact, but it's not a big enough diameter for this engine. I believe some people use the housing from a Land Cruiser oil filter, but I didn't have one of them so I ended up rolling my own casing around a piece of 110mm diameter soil pipe.
The sheet was pulled tight with the hose clamp and tacked just off the end of the tube, then the whole thing was moved along a few mm and clamped again.
After tacking with it clamped tightly around the pipe it was round, but after I seam welded it, it went 'pear shaped'. I think because of the stresses locked in by springing the metal into position to tack it.
This was going to be my neatest ever weld - I set everything up so I was comfortable; I clamped a bar next to it as a rest for my wrist to slide on; back-purged the weld (hence the aluminium foil) but despite this I realised that I'm absolutely useless at trying to weld in straight lines! I did all the practice runs, etc. and all seemed well, but every time I did it for real, I ended up welding in a curve - a bit like someone walking in circles in the desert.
I'd made the casing over length, as I thought I'd end up with a bit of burn back at the end of the seam, but as sod's law dictates, I managed to make a reasonable job of it this time.
I plannished the weld against a length of 2" steel tube held in the vice and it came back round again - which was a relief. After cleaning it up with a dremmel:
It still needed trimming and truing up, so I made an arbour in the form of a wooden bobbin to keep the casing true.
At 110mm diameter, this is too big to swing over the cross slide of my lathe, so the bobbin had to be machined one end at a time.
The casing was a bit of a loose fit on the rear section, so after trimming the casing to length, I used a bearing tool to swage the rear of the casing down until it was a good fit. I actually went too far and had to open the casing up again a little bit. this had the completely unintended, but very useful consequence, of creating a lip that held the rear section square in the casing.
I silver soldered the fuel line boss before spot welding the casing to the rear section
The book seems to imply that the NGV is secured to the casing with nuts and bolts, but I didn't fancy the chances of something coming loose and getting drawn into the works. I made a clamping ring to fit inside the casing out of [what I'm pretty sure is] aluminium bronze instead - I've had several bad experiences of stainless-on-stainless galling which made me want to choose something else for the bolts to thread into.
Fitting the NGV and checking run-out. There was 0.1mm TIR runout measured on the inside of the NGV which I think is pretty good!
I decided to 'caulk' the joint with silver solder. I had envisaged a neat fillet all around the tail end, but as it happened, even though the solder flowed well inside the casing, it didn't make it all the way around the outside. I did try and clean it up and have another go, but all that got me was another clean-up job.
Can't be too unhappy though!
Thank you for reading
The sheet was pulled tight with the hose clamp and tacked just off the end of the tube, then the whole thing was moved along a few mm and clamped again.
After tacking with it clamped tightly around the pipe it was round, but after I seam welded it, it went 'pear shaped'. I think because of the stresses locked in by springing the metal into position to tack it.
This was going to be my neatest ever weld - I set everything up so I was comfortable; I clamped a bar next to it as a rest for my wrist to slide on; back-purged the weld (hence the aluminium foil) but despite this I realised that I'm absolutely useless at trying to weld in straight lines! I did all the practice runs, etc. and all seemed well, but every time I did it for real, I ended up welding in a curve - a bit like someone walking in circles in the desert.
I'd made the casing over length, as I thought I'd end up with a bit of burn back at the end of the seam, but as sod's law dictates, I managed to make a reasonable job of it this time.
I plannished the weld against a length of 2" steel tube held in the vice and it came back round again - which was a relief. After cleaning it up with a dremmel:
It still needed trimming and truing up, so I made an arbour in the form of a wooden bobbin to keep the casing true.
At 110mm diameter, this is too big to swing over the cross slide of my lathe, so the bobbin had to be machined one end at a time.
The casing was a bit of a loose fit on the rear section, so after trimming the casing to length, I used a bearing tool to swage the rear of the casing down until it was a good fit. I actually went too far and had to open the casing up again a little bit. this had the completely unintended, but very useful consequence, of creating a lip that held the rear section square in the casing.
I silver soldered the fuel line boss before spot welding the casing to the rear section
The book seems to imply that the NGV is secured to the casing with nuts and bolts, but I didn't fancy the chances of something coming loose and getting drawn into the works. I made a clamping ring to fit inside the casing out of [what I'm pretty sure is] aluminium bronze instead - I've had several bad experiences of stainless-on-stainless galling which made me want to choose something else for the bolts to thread into.
Fitting the NGV and checking run-out. There was 0.1mm TIR runout measured on the inside of the NGV which I think is pretty good!
I decided to 'caulk' the joint with silver solder. I had envisaged a neat fillet all around the tail end, but as it happened, even though the solder flowed well inside the casing, it didn't make it all the way around the outside. I did try and clean it up and have another go, but all that got me was another clean-up job.
Can't be too unhappy though!
Thank you for reading
Barking Mat
Cuddler of hedgehogs.
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- France, Brittany.
Wow, (again), superb work.
Morris
Member
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Seconded. Very impressive.
Misterg
Member
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Hello again Thank you very much for the kind words.
With the casing made, I could make and fit the lugs that centre the combustion chamber to suit the actual casing ID
...And once they were fitted, it meant I could silver solder the vaporiser tubes into place (apparently silver solder is OK here as the incoming air and liquid fuel keeps this end of the vaporiser tubes "cool".)
I also decided that I wanted to make a fairing for the intake end of the engine, just to make it look more 'jet-engine-ey' - I wanted to make the fairing next so that I could adapt the diffuser cover and intake to suit.
So it was back to a bit of metal spinning - aluminium is a *lot* easier than stainless, and 1.5mm aluminium is a lot easier than 2mm (made the first one out of 2mm aluminium, but cut the blank too small, so it ended up too short
- the next one was 1.5mm aluminium)
Trimming to size:
Trepanning out the bore
Next was the diffuser cover and intake.
The book suggests machining this from one piece - it would be a huge blank (about 120mm dia x 60mm long), most of which would end up as scrap. It also leaves the compressor wheel & front spacer trapped inside it when the engine is disassembled. For these reasons (and others) I decided to make the diffuser cover and inlet as separate pieces.
Because I'm stingy, I trepanned the centre out of the front cover with a hole saw. It made the lathe motor very hot!
It was bored out to leave a shoulder at the outer edge, where there should be a radius to match the outside of the diffuser vanes.
The radius was created by more 'turning by numbers'
The diffuser fitted nicely against the resulting curve, and actually stayed put on its own.
I could then gradually turn the OD down until the casing was a good fit over it.
There's an O-ring groove around the face but there's not enough cross slide travel to get the toolpost outside the part, so I had to use a bodged together boring bar.
There was *just* enough clearance to get my toolpost spindle in to spot the holes for the securing screws in both parts if I contorted the top slide to just the right angle and used the last thread of its leadscrew (there was about 3mm total travel available on the cross slide).
After drilling all the relevant holes, I reversed the part in the lathe and turned the front face, leaving a lip for my front fairing to sit on.
Thanks for reading
Thank you very much for the kind words. With the casing made, I could make and fit the lugs that centre the combustion chamber to suit the actual casing ID
...And once they were fitted, it meant I could silver solder the vaporiser tubes into place (apparently silver solder is OK here as the incoming air and liquid fuel keeps this end of the vaporiser tubes "cool".)
I also decided that I wanted to make a fairing for the intake end of the engine, just to make it look more 'jet-engine-ey' - I wanted to make the fairing next so that I could adapt the diffuser cover and intake to suit.
So it was back to a bit of metal spinning - aluminium is a *lot* easier than stainless, and 1.5mm aluminium is a lot easier than 2mm (made the first one out of 2mm aluminium, but cut the blank too small, so it ended up too short
- the next one was 1.5mm aluminium)Trimming to size:
Trepanning out the bore
Next was the diffuser cover and intake.
The book suggests machining this from one piece - it would be a huge blank (about 120mm dia x 60mm long), most of which would end up as scrap. It also leaves the compressor wheel & front spacer trapped inside it when the engine is disassembled. For these reasons (and others) I decided to make the diffuser cover and inlet as separate pieces.
Because I'm stingy, I trepanned the centre out of the front cover with a hole saw. It made the lathe motor very hot!
It was bored out to leave a shoulder at the outer edge, where there should be a radius to match the outside of the diffuser vanes.
The radius was created by more 'turning by numbers'
The diffuser fitted nicely against the resulting curve, and actually stayed put on its own.
I could then gradually turn the OD down until the casing was a good fit over it.
There's an O-ring groove around the face but there's not enough cross slide travel to get the toolpost outside the part, so I had to use a bodged together boring bar.
There was *just* enough clearance to get my toolpost spindle in to spot the holes for the securing screws in both parts if I contorted the top slide to just the right angle and used the last thread of its leadscrew (there was about 3mm total travel available on the cross slide).
After drilling all the relevant holes, I reversed the part in the lathe and turned the front face, leaving a lip for my front fairing to sit on.
Thanks for reading
Barking Mat
Cuddler of hedgehogs.
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- 13,535
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- France, Brittany.
Loving this build, keep it coming.
Misterg
Member
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- North Wales
I prepared the intake blank in a similar way and hogged out enough material that I could bolt the diffuser cover to it in the lathe so that they could both be turned together.
The intake needs to be machined to match the curved profile swept out by the blades on the compressor wheel. The depth of it needs to be just right so that the profile merges into the face of the diffuser cover. It's important to get a reasonably close and consistent clearance for the compressor wheel to work properly.
With the inlet faced off level with the diffuser cover and the through hole bored to the finished size of the narrowest section, it's ready for a mammoth session of turning by numbers...
I find it easiest to set the top slide parallel to the bed and use this for the Z feed, rather than the carriage. I set a stop on the lathe bed where the tool just touches the face of the part and zero the top slide dial. Cross slide dial is zeroed by touching off on the bore. I can then wind the carriage back, increment the cross slide and set the desired Z travel on the top slide without cutting any metal whilst making the adjustment.
I use an excel spreadsheet to calculate how far the tool needs to be fed in the 'Z' direction (towards the spindle) for increments in the cross-slide (X) direction, taking into account the desired finished radius and the radius of the tool being used. It converts these dimensions into the number of complete turns of the handwheel and the dial reading. (Vital for me, as my cross slide pitch is 1.25mm, so for consistent 0.2mm increments, I need to stop variously on the odd number graduations, the even number graduations or the '5s' between them!)
For an internal curve like the inlet, the first Z value will be high, and gradually reduce as the cross slide is wound out (decreasing X).
For each X value, I set the Z value on the top slide (off the work) then traverse the carriage up to the stop (using auto feed if needed) to cut to the Z depth specified, back the carriage off again, wind the X out 0.2mm and repeat. There's no need to reset the top slide to zero every time unless you lose track of where you are (but obviously the reading must be approached from a consistent direction to eliminate backlash - I make a sharpie mark on both slides at the zero position in case I get lost).
I've found machining this way to be quite straightforward as long as I work methodically (e.g. crossing out each setting after it was cut) and not too tedious. Trying to cut in the other direction (Constant Z increments and variable X) is much more nerve-wracking as the lathe will be cutting whilst the X values are being dialled in so a steady feed rate and no overshoot are important for critical curves. It is also necessary to wind the X slide in and out the full distance every time.
The book specified a 16.7mm radius curve for the inlet which I think was meant to give 0.3mm blade tip clearance on a wheel with a 17mm profile radius. I though I could do a little better than that, and to err on the side of safety, I went for 16.85mm radius which should have given ~0.15mm tip clearance (it could always be made bigger!).
This seemed to go well, but when I tried the compressor wheel in the hole, it wouldn't sit all the way down - there was a gap of about 0.3mm at the tips of the exducer.
Some sharpie pen and a wiggle showed that the problem was that the radius machined in the cover was tighter than the one on the the wheel:
I had tried to establish the profile radius of my wheel, but it's not easy to measure because the blades are swept back so far and I couldn't find any drawing specifically for this wheel. There is some ambiguity as the 2018 was seemingly superseded by the 2037 for which I have seen a drawing giving an 18mm radius. I had tried to verify which it was by eye, and had convinced myself it was 17mm. Looks like I was wrong! A bit of checking on CAD verified that the 0.3mm gap was also consistent with a 18mm radius profile against a 16.85mm radius cover.
Anyway, re-cutting to 17.85 mm radius was straightforward, and the wheel now fitted as expected (with the exducer tips able to go tight against the cover).
Before I disturbed anything, I wanted to make a check on the clearances in case I needed to cut the intake any deeper.
I stuck small pieces of mylar tape on the intake, in multiple layers, and offered up the compressor / diffuser assembly to see if it would still turn freely.
It turned out that the clearance was somewhere between 0.3mm (4 layers) and 0.375mm (5 layers) which was a bit more than I was hoping for. I knew that the diffuser vane height was accurate, and although it wasn't clamped down, the diffuser vanes seemed to be seating against the cover; the turning by numbers had ended up exactly where it should have done, with the last few cuts doing little more than scraping the layout fluid off the surface, so unless it pulls up a bit tighter when I can get the cover securing screws in, it is likely that the wheel is a little low in the diffuser.
In any case, I decided I didn't need to adjust the intake. It was finished it off by machining the short, 3° tapered, section between the edge of the wheel and the diffuser vanes.
I could have just left the outside of the intake as it was, but decided to turn it to an aesthetically pleasing curve...
It was then flipped around so that the inlet bell mouth could be machined in the same way. A 20mm radius was about right:
And a flange at the appropriate angle for the fairing to sit against...
Finished item - I tried not to polish it (because a slightly rough surface is supposed to flow better), but it ended up shiny anyway
(Fascinating fact: the finished intake only weighs 15% of the blank that it started from!)
The last job on the inlet was drilling and tapping some holes at an odd angle for the securing screws for the fairing:
Done.
That's all the major components machined!
Thanks for reading
The intake needs to be machined to match the curved profile swept out by the blades on the compressor wheel. The depth of it needs to be just right so that the profile merges into the face of the diffuser cover. It's important to get a reasonably close and consistent clearance for the compressor wheel to work properly.
With the inlet faced off level with the diffuser cover and the through hole bored to the finished size of the narrowest section, it's ready for a mammoth session of turning by numbers...
I find it easiest to set the top slide parallel to the bed and use this for the Z feed, rather than the carriage. I set a stop on the lathe bed where the tool just touches the face of the part and zero the top slide dial. Cross slide dial is zeroed by touching off on the bore. I can then wind the carriage back, increment the cross slide and set the desired Z travel on the top slide without cutting any metal whilst making the adjustment.
I use an excel spreadsheet to calculate how far the tool needs to be fed in the 'Z' direction (towards the spindle) for increments in the cross-slide (X) direction, taking into account the desired finished radius and the radius of the tool being used. It converts these dimensions into the number of complete turns of the handwheel and the dial reading. (Vital for me, as my cross slide pitch is 1.25mm, so for consistent 0.2mm increments, I need to stop variously on the odd number graduations, the even number graduations or the '5s' between them!)
For an internal curve like the inlet, the first Z value will be high, and gradually reduce as the cross slide is wound out (decreasing X).
For each X value, I set the Z value on the top slide (off the work) then traverse the carriage up to the stop (using auto feed if needed) to cut to the Z depth specified, back the carriage off again, wind the X out 0.2mm and repeat. There's no need to reset the top slide to zero every time unless you lose track of where you are (but obviously the reading must be approached from a consistent direction to eliminate backlash - I make a sharpie mark on both slides at the zero position in case I get lost).
I've found machining this way to be quite straightforward as long as I work methodically (e.g. crossing out each setting after it was cut) and not too tedious. Trying to cut in the other direction (Constant Z increments and variable X) is much more nerve-wracking as the lathe will be cutting whilst the X values are being dialled in so a steady feed rate and no overshoot are important for critical curves. It is also necessary to wind the X slide in and out the full distance every time.
The book specified a 16.7mm radius curve for the inlet which I think was meant to give 0.3mm blade tip clearance on a wheel with a 17mm profile radius. I though I could do a little better than that, and to err on the side of safety, I went for 16.85mm radius which should have given ~0.15mm tip clearance (it could always be made bigger!).
This seemed to go well, but when I tried the compressor wheel in the hole, it wouldn't sit all the way down - there was a gap of about 0.3mm at the tips of the exducer.
Some sharpie pen and a wiggle showed that the problem was that the radius machined in the cover was tighter than the one on the the wheel:
I had tried to establish the profile radius of my wheel, but it's not easy to measure because the blades are swept back so far and I couldn't find any drawing specifically for this wheel. There is some ambiguity as the 2018 was seemingly superseded by the 2037 for which I have seen a drawing giving an 18mm radius. I had tried to verify which it was by eye, and had convinced myself it was 17mm. Looks like I was wrong!
A bit of checking on CAD verified that the 0.3mm gap was also consistent with a 18mm radius profile against a 16.85mm radius cover.Anyway, re-cutting to 17.85 mm radius was straightforward, and the wheel now fitted as expected (with the exducer tips able to go tight against the cover).
Before I disturbed anything, I wanted to make a check on the clearances in case I needed to cut the intake any deeper.
I stuck small pieces of mylar tape on the intake, in multiple layers, and offered up the compressor / diffuser assembly to see if it would still turn freely.
It turned out that the clearance was somewhere between 0.3mm (4 layers) and 0.375mm (5 layers) which was a bit more than I was hoping for. I knew that the diffuser vane height was accurate, and although it wasn't clamped down, the diffuser vanes seemed to be seating against the cover; the turning by numbers had ended up exactly where it should have done, with the last few cuts doing little more than scraping the layout fluid off the surface, so unless it pulls up a bit tighter when I can get the cover securing screws in, it is likely that the wheel is a little low in the diffuser.
In any case, I decided I didn't need to adjust the intake. It was finished it off by machining the short, 3° tapered, section between the edge of the wheel and the diffuser vanes.
I could have just left the outside of the intake as it was, but decided to turn it to an aesthetically pleasing curve...
It was then flipped around so that the inlet bell mouth could be machined in the same way. A 20mm radius was about right:
And a flange at the appropriate angle for the fairing to sit against...
Finished item - I tried not to polish it (because a slightly rough surface is supposed to flow better), but it ended up shiny anyway
(Fascinating fact: the finished intake only weighs 15% of the blank that it started from!)
The last job on the inlet was drilling and tapping some holes at an odd angle for the securing screws for the fairing:
Done.
That's all the major components machined!
Thanks for reading
Maker
Most folk just call me; Orange Joe
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Good excuse to get an optical comparator.
Misterg
Member
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A few odd jobs:
I needed to increase the size of the cut-outs in the intake to clear the nuts on the diffuser cover securing studs - I couldn't get the intake off without also removing the nuts (best laid plans, etc...).
I just plunged through it with a 10mm end mill (I know, I know, but it was a plunge cut and the chuck was nice'n'tight).
While I had the vertical slide set up on the lathe, I could also drill and tap for the lubricating oil fitting and cut out the front fairing to clear it - I had wanted to bring the oil pipe out inside the fairing and hide the flow restrictor in there, but I couldn't figure a sensible way of doing this, so just brought the oil pipe through the very edge of the diffuser as per the book.
Scalloping out the fairing was a bit twitchy as the metal was very soft and gummy, having been annealed before spinning, and I was relying on the three small mounting screws to secure it. All went OK though.
The compressor intake clearance didn't close up any when the assembly was bolted together, so I also made up new front spacer that was 0.15mm thicker than the previous one.
Then it was time for a trial assembly!
(without the combustion chamber for now):
I was very pleased with the way it went together - everything seemed to fit, and the rotor even spins round!
Getting the clearances around the compressor and (especially) the turbine wheels is important to get the engine running well (or at all, even). The new front spacer has brought the compressor clearance to just less than 3 layers of tape (somewhere between 0.15mm and 0.23 mm - probably nearer the 0.23mm) which I'm reasonably happy with, and the compressor wheel looks level with the diffuser body (I'm blaming my change to a different set of bearings since I originally set the level of the wheel...)
The turbine disc isn't quite central in the NGV - it is spinning true, but the clearance is 0.13mm on one side and 0.23mm on the other (so something has gone awry by 0.05mm somewhere, but overall, I think it's gone OK. I fully expect the whole thing to move a bit when it gets hot, and I may need to skim the turbine wheel or NGV if it rubs when running.
(Feeler gauges photographed where I could see their thickness, not where they were to check the clearance!)
With the parts assembled, I could measure where the NGV sits on the shaft tunnel - the actual position depends on the tolerance stack-up from five separate parts
When I took the engine apart again, I could then use this measurement to double check the clearance between the front of the combustion chamber and the back edge of the diffuser. I wanted to do this before finally fitting the front of the CC in case I needed to shorten it. (I did shorten the 'tail pipe' on the CC by ~1.5mm - I think the various angles and tolerances in my work had stacked up to make it a bit longer than intended.)
There's about 6mm clearance, which I think is plenty (I think it needs a minimum of 3 or 4 mm).
Right, that brings us up to date, so things will slow down a bit now!
I've still got a few jobs to do:
- Fit the pre-heat gas injector and pressure tapping to the casing
- Attach the front of the combustion chamber
- Secure the fuel manifold
- Make and fit the oil pipe
- Thin down and fair in the front cover retaining screws where they pass through the diffuser vanes
- Finish the tail cone
- Make some sort of stand
- Figure out how to secure the fuel pipe through the rear of the casing. I thought I had figured it out, but the push connectors I got don't seem to grip on brass tube.
- I'd like to anodise some of the parts before final assembly
- Probably some other stuff that I've forgotten...
- And, erm... Balancing the rotor.
Don't start holding your collective breath just yet, but it *is* getting closer...
Thanks for reading!
I needed to increase the size of the cut-outs in the intake to clear the nuts on the diffuser cover securing studs - I couldn't get the intake off without also removing the nuts (best laid plans, etc...).
I just plunged through it with a 10mm end mill (I know, I know, but it was a plunge cut and the chuck was nice'n'tight).
While I had the vertical slide set up on the lathe, I could also drill and tap for the lubricating oil fitting and cut out the front fairing to clear it - I had wanted to bring the oil pipe out inside the fairing and hide the flow restrictor in there, but I couldn't figure a sensible way of doing this, so just brought the oil pipe through the very edge of the diffuser as per the book.
Scalloping out the fairing was a bit twitchy as the metal was very soft and gummy, having been annealed before spinning, and I was relying on the three small mounting screws to secure it. All went OK though.
The compressor intake clearance didn't close up any when the assembly was bolted together, so I also made up new front spacer that was 0.15mm thicker than the previous one.
Then it was time for a trial assembly!
(without the combustion chamber for now):I was very pleased with the way it went together - everything seemed to fit, and the rotor even spins round!
Getting the clearances around the compressor and (especially) the turbine wheels is important to get the engine running well (or at all, even). The new front spacer has brought the compressor clearance to just less than 3 layers of tape (somewhere between 0.15mm and 0.23 mm - probably nearer the 0.23mm) which I'm reasonably happy with, and the compressor wheel looks level with the diffuser body (I'm blaming my change to a different set of bearings since I originally set the level of the wheel...)
The turbine disc isn't quite central in the NGV - it is spinning true, but the clearance is 0.13mm on one side and 0.23mm on the other (so something has gone awry by 0.05mm somewhere, but overall, I think it's gone OK. I fully expect the whole thing to move a bit when it gets hot, and I may need to skim the turbine wheel or NGV if it rubs when running.
(Feeler gauges photographed where I could see their thickness, not where they were to check the clearance!)
With the parts assembled, I could measure where the NGV sits on the shaft tunnel - the actual position depends on the tolerance stack-up from five separate parts
When I took the engine apart again, I could then use this measurement to double check the clearance between the front of the combustion chamber and the back edge of the diffuser. I wanted to do this before finally fitting the front of the CC in case I needed to shorten it. (I did shorten the 'tail pipe' on the CC by ~1.5mm - I think the various angles and tolerances in my work had stacked up to make it a bit longer than intended.)
There's about 6mm clearance, which I think is plenty (I think it needs a minimum of 3 or 4 mm).
Right, that brings us up to date, so things will slow down a bit now!
I've still got a few jobs to do:
- Fit the pre-heat gas injector and pressure tapping to the casing
- Attach the front of the combustion chamber
- Secure the fuel manifold
- Make and fit the oil pipe
- Thin down and fair in the front cover retaining screws where they pass through the diffuser vanes
- Finish the tail cone
- Make some sort of stand
- Figure out how to secure the fuel pipe through the rear of the casing. I thought I had figured it out, but the push connectors I got don't seem to grip on brass tube.
- I'd like to anodise some of the parts before final assembly
- Probably some other stuff that I've forgotten...
- And, erm... Balancing the rotor.
Don't start holding your collective breath just yet, but it *is* getting closer...
Thanks for reading!
Hy Misterg, dont worry about balancing the rotor:The methode Kurt Schreckling described in his book works very well. I have balanced all my rotors in this way and
they all run absolut smoothly up to 115.000rpm (my personal limit) without any whistle (whistle is a sign of imbalance) since over 20 years. So keep your head up!
greets kappi
they all run absolut smoothly up to 115.000rpm (my personal limit) without any whistle (whistle is a sign of imbalance) since over 20 years. So keep your head up!
greets kappi
Misterg
Member
- Messages
- 1,548
- Location
- North Wales
Thanks very much for that - I know Kurt has described the 'rocking' method and the 'fingertip' method, but I'm not sure whether you mean these or something else - I've had a go with the 'rocking' method and made a lot of progress fairly quickly. I could call it done, but I've put it to one side for a little while and will come back and double-check it.
I've been making some progress on all the little jobs that need finishing;
I've silver-soldered a couple of threaded bosses onto the casing: One to take the pre-heat gas injection nozzle; and one as a tapping for the casing pressure gauge:
I've also fitted the two halves of the combustion chamber together - This is the view of the exit from the combustion chamber towards the NGV before the front / inner went on (the gasses leave through the annular gap with the vanes in - the central hole is blocked by the shaft tunnel):
I'll probably smooth out the chamfer on the front of the NGV and fettle some of the welds at the outside of the vanes before final assembly.
I decided to spot weld the two parts together on in the end (as a low risk option compared to tacking it with TIG). I had to make up a skinny lower arm for my spot welder to sneak through the gap between the inner and outer
The combustion chamber fitted into the casing:
It slots around the NGV and is held in place by the fuel pipe being silver soldered into a threaded fitting that fits into the hole in the back of the casing. This is a minor PITA since the fitting has to be in just the right place to tighten up as the same time as the combustion chamber bottoms on the casing.
So I fitted the fuel manifold, putting a little tweak in the end of each needle to try and ensure the fuel washes the wall of the vaporiser tube, before locking the fuel manifold to the combustion chamber:
Then everything could be fitted loose and the NGV removed to see what clearance there was between the end of the combustion chamber and the casing. The fuel feed pipe was gradually shortened until the gap was closed:
I'd made the fuel feed to come out of the manifold and bend around to go straight out of the back of the casing (as per the drawings in the book). This meant that the fitting needed to be almost on top of the bend, so a bit of fettling was needed. I've since noticed that one of the photos in the book shows the fuel feed pipe going forwards from the manifold and turning 180 degrees to come out of the back - I wish I'd noticed sooner as this would have given a lot more wiggle-room! (A lot of designs take the fuel feed out through the diffuser and front cover, but the thin, sheet metal, vanes of this diffuser mean that this would partially block the air flow.)
Back at the front of the engine, the book recommends turning the threads off the diffuser securing screws where they pass through the diffuser air passages and fairing them in with epoxy (so that they don't interfere with the airflow).
I reckoned I could get a stronger result, with less disturbance of the airflow by making some flatted studs instead.
I made up the studs and locked them into a plate in the tool holder on the lathe to mill a 5mm tall flat on opposite sides
Making the studs this way also meant I could leave a plain section where they pass through the front cover. To try and seal the hole properly I made some small stand-offs with an o-ring in the end which *should* seal against the stud and front cover.
The studs were screwed into the diffuser body and held in place by a locknut behind the diffuser. Once I was happy with the fit of things, they were removed and re-fitted permanently with epoxy.
I'm pretty pleased with the way that worked.
Also (mostly) at the front of the engine, I've anodised the aluminium components.
I gave the fairing a little colour, as the 1050 sheet it's made of comes out quite grey compared to the 6XXX of the machined parts.
(I got some weird streaking on the inside of the fairing, I think related to the grain in the raw sheet. I don't know what's caused this, but luckily it's much more feint on the outer surface which has been smoothed and sanded.)
Getting there!
Thanks for reading
AlexBrown_1
Member
- Messages
- 160
- Location
- Cheshire UK
Just wanted to add, like others, how impressed I am by this build. It's fantastic to see your work and a huge inspiration!
