Taylor Hobson Chasing Lathe

[RobCox]

I finished up the hob follower yesterday. This is the part that sits on the platform behind the hob and engages with it to move the headstock to the left. I had it in mind to make this out of a solid piece of bronze, until I found out how much that would cost x how many thread pitches do you want. I found an image of a hob follower on the web and figured I'd copy that design.

I started with a piece of PB1 bronze I had left over from another repair. This was the perfect size, 1 1/4" dia. I turned a 1/2" long, 1/2" diameter spigot on the end, parted it off, reversed it in a collet chuck and faced it flat, then over to the mill to cut the large end to a rectangle:

The finished size of the rectangular end was 0.75 x 1.00 inches, which fits perfectly in a 1.25 inch bar (think of a pythagorean 3-4-5 triangle). Add a couple of 45 degree bevels and this is the follower blank awaiting the threading setup:

This part now needed some female threads cut on it. I had the choice of making a fixture, holding it in the 4-jaw and offsetting it the right amount and conventionally single pointing an internal thread, but in the end decided to hold the blank on the compound and rotate the tool in a boring bar. To hold in on the compound, I got out my very first milling machine:

I originally bought this to do some milling on my minilathe in the days before I got a mill. It was so successful that I got a mill soon after. The general lack of rigidity in the whole machine made the whole exercise a waste of time, so this slide has sat on the bench for years unused. I almost sold it at one point but the buyer pulled out, by which point I'd already regretted trying to sell it and figured it'd come in useful one day.

Bit of modification required to mount it on the compound of the M300. The post that secures the QCTP is threaded M12, so I drilled the milling slide 12mm and machined a bolt to length:

Refitting the tool post is much more faff, as the compound slide has to come off to get at the locking grubscrew underneath.

I've got a nice little vice to go with this which I've abused (holding parts while I braze them) and repaired (new screw and nut after the original brass nut fractured). Here's the setup with a DTI used to check that its aligned to the lathe axis:

The compound is set at 30 degrees here, but I turned it parallel to the lathe axis - after I'd set it up the first time - as there wasn't enough cross slide travel. First machining op was to put the inside radius on the face. So first, measure where the face sits in relation to the spindle centre using an edge finder:

Move it out to the correct radius and use that distance to set the boring bar swing up, then machine away. The height of the milling slide was fine tuned during this machining op to get the workpiece centre bang on centre height:

I then ground a 60deg threading toolblt of the right length to fit in the boring bar, used an edge finder to reset the new position of the centre of the blank, then the threading is done as normal, except the tool is rotating and the workpiece feeds along the bed:

This is the threading tool, set to the root radius of the finished threads:

 

[RobCox]

And here's the finished follower:

It now needs a holder to mount it on the chasing fixture, but that will be done in steel as it's cheaper! Finally, the follower sat on the hob threads, where it fits really nicely

Rob

 

[Brad93]

Wish I’d bought that vice now it’s a work of art!

 

[Milkybars]

@RobCox quality engineering

 

[Pete.]

Great work Rob,

You often find big rectangular blocks (6" x 2.5" x 2") of bronze in demolition if you're in a building with the lifts being stripped out. I was pretty excited at the prospect in my current project because w had to remove four of the buggers. Imaging how disappinted I was to find that the lifts ran on rollers, not sliding wear-blocks

 

[Milkybars]

@Pete. Gutted is the word !

 

[RobCox]

Quick update on this weekend's progress. I made a carrier for the hob follower from a length of 30mm sq black bar. The material choice was driven by what I had.
Milled it down to 1 inch by 3/4 inch. Found the centre of the end using the dro on the mill and centre drilled it, then used that centre to true it up in the lathe so I could bore out a hole to fit the spigot on the follower:

After that it was back to the mill to drill and tap a hole for a 1/4-20 grubscrew:

Then cut a slot for the mounting clamp on the chasing lathe. The overall width was a tad tight when fitting it onto the follower rest on the chasing lathe, giving me the perfect excuse to pretty it up on the surface grinder making it an easy fit onto the lathe.

Finished assembly:

and mounted onto the lathe:

Seems to work ok.

BTW, after I cut the threads I went back to the image I used for inspiration for the design of the follower, read the comments properly, and found that they'd used buttress threads rather than the 60 deg form I used. When I get round to making a hob and follower for a different pitch I may well try that, although it presents the perfect opportunity to get the thread form backwards if you're not careful...

 

[Pete.]

Presumably a butress thread would resist partial engagement errors and it certainly will carry more load - in one direction.

 

[RobCox]

I think thats probably the idea - the sliding headstock only ever gets pushed to the left and when the headstock pivots to disengage the follower a spring pulls it back to the right. Turning the spindle by hand (clockwise, as the tool has to be on the side away from the operator) the 60 deg thread seems to work fine.

 

[RobCox]

Making the hob and follower turned out to be the right way forward as I've finally worked out how this machine manages to auto-reciprocate so all the operator has to do is to feed the tool in each time.

There are two motors (or there should be, I'm missing the one that drives the spindle). The spindle rotates constantly and there's a camshaft which raises the spindle slide so the hob engages with the follower to initiate the threading. What was confusing me was how was the rotation of the camshaft timed to the spindle so the camshaft motor didn't stall during the threading cut and how it released the spindle slide from the follower at just the right time. The answer is this pulley:

This drives the camshaft. The motor that drives it is geared right down with an integral gearbox (I'll get round to cleaning it up later). A round section belt drives the pulley. The flash of inspiration came to me when I remembered that there's no keyway and the camshaft spindle is hard and polished. Those "two washers" under the securing nut are a single spring washer. Like a muppet, I'd just whacked it up tight ... wrong! The nut needs tightening enough so that the washer provides friction to drive the shaft round, but when the camshaft eccentric (which is machined to have an abrupt stop) meets the moveable lever, the camshaft stops but the pulley can continue spinning.

This moveable lever is pushed aside when the sliding headstock meets the end stop, the camshaft rotates and the headstock retracts from the follower and the spring pulls it back to the start. A video would explain it a million times better, but lacking hands free videoing kit and needing one hand to rotate the spindle and one to rotate the camshaft pulley to demonstrate the action, you'd be disappointed by the results.

So I need the extra lever, which of course is completely absent along with it's pivot pin. I do have a picture of what it looks like... don't hold your breath! This is the best info I've got on this:

So some practical detective work was required. Guessing it was aluminium (most of the headstock parts are), I grabbed a piece of bar from the offcuts bin and milled, drilled and reamed it to suit. A length of drill rod was turned to make the pivot and a bit of bronze was turned into a spacing washer:

The brass rod engages in the groove on the pivot and is secured by a grubscrew to keep the assembly, uh, assembled. This are those parts on the lathe:

It worked but with limited travel. That cutout in the headstock casting behind the ally lever isn't decorative! So, lever Mk II:

Milled from solid, not bent (it's 8mm thick), from the gooiest aluminium alloy. Horrid stuff, a devil to get a decent finish, it needed my sharpest slab mill and plenty of paraffin. Where the end travel adjustment lever hits this I fitted a plug of steel as the prototype had marked with a few cycles by hand. The 10 degree bend clears the headstock and I milled some extra relief. Here's the finished assembly:

It still needs some finishing touches but it seems to work. Next step is to find a spindle motor and power the spindle, and clean and fit the camshaft motor. I'll post a video of it cycling when (if) I get it going.

 

[Shox Dr]

Brilliant detective work. Have you spoken the Taylor Hodson. You never know they might have some info in cupboard

 

[Munkul]

I still have no idea what this machine can do (apart from making threads in a strange fashion) but it's very cool

 

[Brad93]

Lovely work

 

[RobCox]

I still have no idea what this machine can do (apart from making threads in a strange fashion) but it's very cool

Basically it's a production machine from pre-CNC days. It's a second op lathe designed for turning / facing / threading relatively short parts. The handwheels have got pre-settable stops so it can be set up so a semi-skilled operator can churn out parts by turning the wheels to the stops - no measuring necessary. The threading mechanism uses a hob and follower to do the job that your leadscrew and half-nuts would do on a conventional engine lathe, except that it automates the disengaging half nuts, rewind carriage, engage half nuts process. Not as flexible as a lathe with a leadscrew and gearbox obviously.

I'll admit I've not real use for it when it's done, its just something thats fun to tinker with

 

[RobCox]

So, having worked out the basic mechanism, time to put some effort into getting it powered. I don't have a spindle motor (yet) and will have to buy new, prob 0.55kW, as I can't see this lathe needing to work really hard, but I do have the original camshaft motor. It was in need of a bit of TLC:

This thing was chock full of swarf. As its a 400V motor it needs connecting to some slightly hazardous voltages, so I took the whole thing completely to bits to clean it up. Once dismantled all of the bearings were well past their best before date so I took the opportunity to change them too. Economy jobs in the main as they'll be better than what came out and last long enough. The gearbox on the front was packed with grease, most of it had never got anywhere near anything that needed lubricating:

All the bits were then cleaned, at which point the real state of the bearings became evident:

The output shaft at the bottom presented a problem. The final gear had to come off the shaft to get the bearing off - a shoulder prevents it going the other way. The gear is tufnol with a steel hub, so I didn't want to risk breaking it. Trying to shift it with the arbor press didn't work, trying to drift it out without cracking the gear didn't work, so I went down the easiest route - mount the shaft on the mill, dial it in to cente it and drill the shaft out. A new shaft is easy enough to make, the gear less so (plus there would be the expense of an involute cutter):

It gave up easily enough:

And why was it tight? Because the key was wedge shaped and had been driven down the keyway. Trying to press the gear off only made it tighter:



I turned a new shaft, but instead of slotting a keyway through the threads I turned, hardened and ground a woodruff key cutter. This was getting close to model engineering sizes - a 5/16 dia, 80 thou thick key. Here's the cutter in action:

I'm not keen on stuff this small, its a bit tough to see! It fitted nicely though - 3 attempts at getting the key width right and having to widen the slot in the shaft as well:

 

[RobCox]

The stator windings needed cleaning, so I removed them from the housing and gave the whole lot a bath in white spirit:

I somehow didn't fancy my chances unpicking the star point and rewiring as delta - even though the solder blob on the left side is the star point, and as 240V in - 400V out VFDs cost more than I paid for the lathe, I intended to get a transformer and capacitor and run it in a Steinmetz arrangement.

The bearings arrived and it was reassembly time. The screws in the first pic were remade, 6BA. I bought a die as I thought I didn't have one. When I went to put the die in my tailstock die holder there was a 6BA die in there already . The sweet little woodruff key is also in the pic:

A judicious application of grease:

And so to the electrics. I got a 400VA transformer off ebay. Overrated for this application, but it might have to run more than one motor if the carriage feed motor needs 400V:

To check what voltage capacitor would be needed I used the circuit simulator at work (part of the day job normally). Turns out that an 8uF capacitor would only have about 270V across it, so no need for an expensive 600V rated cap. I had a couple of 400V ones kicking around (I've used a Steinmetz connection to get the coolant pumps on the lathe and mill to work - even if I don't run coolant - as they're a lot cheaper than VFDs!) so I wired these in parallel to get the value I needed and wired everything up to check the motor ran. It was rubbing somewhere inside slightly when I reassembled it - prob the rotor scratching the inside of the stator core - so I figured I'd run it for 10 minutes to let it bed in, which it did very nicely. No rubbing now. Here's the test arrangement, a lot safer than it looks thanks to the transformer isolating the HV from ground:

The DVM is showing the capacitor voltage (reading rms not peak to peak).

The motor now needs a coat of paint, black to match the original, and the tag re-fitting.

 

[RobCox]

Whilst I dither about spending money on a motor and some steel to knock up a frame to attach the motors to the lathe, I got on with sorting out a chuck for the spindle. This is the original collet chuck:

The knurled ring holds the collet body on the end of the spindle. The collets are bespoke and use a drawbar through the spindle to tighten. I've got lots of these rusty collets in a range of goodness knows what sizes, but I would prefer something more generic, like an ER series. ER25 would be the sensible choice as the collet body thread would fit through the knurled nut, but I don't have ER25 stuff and this lathe is costing enough for a flight of fancy so I went for ER32 which I use on the lathe and mill. This requires the chuck to be made in 2 parts, a backplate which fits the spindle and is captured by the knurled nut, and a chuck body which bolts to the backplate.

This unpromising piece of metal is the raw material:

It might look like junk, but this was a pivot pin in the rear bucket arm of a JCB (I did a job for the owner drilling retaining bolt cross holes in some spare pins and got to keep this worn out one). It's rusty as it spent a night in my heat treating oven (ok, the woodburner in the living room). These thing are hardened for use but once annealed the steel is really nice to turn. This is the backplate before being sawn off (my lathe doesn't like parting off, certainly not this stuff):

So I got to use my repaired bandsaw, which is making some "interesting" noises now under load which I've not had the inclination to investigate:

I needed this part reversed in the lathe as perfectly as possible, so drawing on my woodturning past I made a jam chuck out of some aluminium turned to an interference fit:

The back was cleaned up, over to the mill to put 3 backplate to chuck mounting holes in and a blind hole for the spindle drive dog and here it is mounted on the lathe:

The chuck body was made from half the remaining stock. Firstly, chuck up some 20mm stock, turn to 19mm so it's concentric, put a 19mm ER32 collet on it and adjust the compound for the exact taper angle:

Then re-chuck the stock, turn down a length for the M40x1.5 threads

and bore the taper:

Blue it up and check the fit on a collet:

There's a bit much blue in there, but I reduced the amount with a gloved finger (to avoid turning into a smurf).

 

[RobCox]

Here's the fit on the collet (with the 19mm stock inside keeping it parallel:

That'll do me. Then thread M40x1.5:

Bandsaw off the excess stock, mount some 20mm stock in the 4 jaw and turn down to 19mm (again, guarantees concentricity). Put a 19mm collet in the new chuck body and tighten the nut onto this 19mm stock. The runout on the outside of the body was 0.01mm or less... that'll do! I cleaned up the end and cut a register to fit over the backplate:

Dismount, drill the fixing holes in the mill using the DRO to get the positions right. Here's the backplate assembled to the body:

Notice the wonky drive dog hole. My measurements and calculations put the centre of this 0.572" from centre. Guess who dialled in 0.527", drilled a small pilot hole before thinking "that doesn't look right" . The groove between the two parts captures the knurled ring:

Fitted to the lathe and with a 12mm pin tightened in I measured the runout, admittedly close to the chuck (there's not enough carriage travel to put long stock in, lathe's not designed for it and there's no tailstock provision), <0.01mm. Very, very happy.

Thanks for reading. My posts are like buses, you wait for ages...

Rob

 

[Brad93]

Try nipping up the headstock bearings, might improve your parting ops.

 

[Milkybars]

@RobCox whats the material you require for motor mount ? and maybe worth putting up a wanted add for a suitable motor..most of us on here have loads of under the bench stuff