Harrison 140 thread cutting woes

[Royce Creasey]

Thanks for these replies, I appreciate the effort.

I've got to the bottom of this problem now. Basically, I had assumed that the 140 was similar to imperial lathes I've used in past decades. (last millenium actually), where if you went back to the start line it worked fine Now, thanks to a chance remark on 'Haxby Shed' I'ver found the little data plate on the right hand end of the carriage. It turns out that some pitches, including the first thread I cut for a customer, align correctly on any line on the indicator. Some pitches only align on the numbers. One pitch only aligns on two of the numbers. The rest need the lathe to be reversed with the half nut engaged.

One of these pitches is 1.75mm (M12), naturally the one I chose while improving the tool grind. Hence my bafflement. To add to the fun this lathe is driven by an inverter that has lost the reverse function. So no M12 threads then. The lesson here? 'Don't assume, Check'

I tried using the carriage reverse, keeping the half-nut engaged but it's not accurate enough for thread alignment. Disengaging the clutch and turning the lathe backwards by hand is a pretty heavy job, I'll save trying that until there's no alternative!

 

[Rrobson]

Just stop the lathe and turn the chuck backward by hand to the start of the thread if your really desperate, or run a die nut down

 

[Munkul]

To be totally sure, it's better just to stop spindle, wind out and reverse IMHO, but I don't cut many threads full stop.

The Imperial lead screw on metric threads has always flummoxed me, it depends on what leadscrew pitch you have as well as how many teeth are on your threading dial gear. I know it's just about divisible numbers, but I prefer simple.

However, something I recently learned from tinternet:
If you have a bed stop set precisely for the thread dial indicator to line up at one of the marks, and you engage the half nut each time at that position on the bed and that dial mark, then you can use whatever screw with whatever dial gear to cut whatever thread.
Essentially you're always picking up the exact part of the lead screw each time.

 

[addjunkie]

Fix your reverse,and and just do all threading with half nuts engaged, saves lots of cock ups with dial indicators etc

 

[Agroshield]

To add to the fun this lathe is driven by an inverter that has lost the reverse function.

It is a bit of a risky solution, but you could put a standard three phase reversing switch downstream of the VFD (i.e. one that swaps two of the phases). Just be sure not to touch it when the output from the VFD is active.

 

[E T]

However, something I recently learned from tinternet:
If you have a bed stop set precisely for the thread dial indicator to line up at one of the marks, and you engage the half nut each time at that position on the bed and that dial mark, then you can use whatever screw with whatever dial gear to cut whatever thread.
Essentially you're always picking up the exact part of the lead screw each time.

That doesn't work. The half nut will be at the same position on the leadscrew, but the leadscrew will not be in the same position in relation to the spindle most of the times.

 

[Pete.]

That doesn't work. The half nut will be at the same position on the leadscrew, but the leadscrew will not be in the same position in relation to the spindle most of the times.

Yep, this. It MIGHT work, purely by chance. It's not in any way reliable. The spindle will only by synchronised to the leadscrew once every so often.

 

[smashweld]

does adding whatever to the dial every time always work regardless of gears/changewheels...
the gears that came with my lathe don't match any of the combos listed on the charts for standard threads, but experimenting and found it's in line every 2.5 increments of the dial (dial goes from 1 to 4), so on the first cut if the halfnut was engaged on 1 I could disengage it at the end of the cut, keep an eye on the dial when winding back the carriage and put the halfnut back on at 3.5...then next cut 2, then 4.5, 3 etc

means having to constantly watch the dial and can't imagine you could go very fast without losing count but seemed to work!

 

[Munkul]

That doesn't work. The half nut will be at the same position on the leadscrew, but the leadscrew will not be in the same position in relation to the spindle most of the times.

This is why i stick to reversing less to think about! It took me ages to work out why it wouldn't work even with your description above. I'd forgotten about the relationship to the spindle

 

[addjunkie]

Fix your reverse,and and just do all threading with half nuts engaged, saves lots of cock ups with dial indicators etc

snap

 

[Agroshield]

the gears that came with my lathe don't match any of the combos listed on the charts for standard threads, but experimenting and found it's in line every 2.5 increments of the dial (dial goes from 1 to 4)

It is impossible to answer this as it is too vague.

First, to which gears are you referring? Do you mean the change gears that connect the spindle to the leadscrew? Do you mean the gear(s) on the thread dial indicator?

If you mean the change gears, the very first thing you need to do is calculate whether they do produce exactly the pitch you are intending to cut. If they do, use of the thread dial indicator is possible. If they do not, no matter how close an approximation they make, you cannot reliably use the TDI.

If you mean the gear(s) associated with the thread dial indicator, please tell us the pitch of your leadscrew and the number of teeth on each gear you have. It is then possible to say, for each conceivable pitch you might want to cut, which gear is suitable. Once we have this established, we can then worry about how the dial is graduated (it is possible to swap dials or attach an overlay dial).

 

[Royce Creasey]

Thanks for the replies, I apreciate the effort, I wrote about this immediately after the replies above but it didn't post for some reason.

I discover that the solution to this specific problem with thread cutting on the 140 can be found (Thanks to a chance remark on 'Haxby Shed' - Youtube) on the small data plate hiding on the right hand end of the carriage. Once the grime of ages has been wiped off it reveals that different pitches need different settings on the thread indicator. Most fine pitches align on any line of the indicator. Most other pitches align on the numbered lines on the indicator, one only aligns on two of the numbered lines, all other pitches need the lathes to be reversed with the half nut engaged. My bafflement was based on cutting a fine thread for a customer job that aligned on all lines and hence went normally, but I then chose 1.75mm as a conveniently large thread pitch to improve the tool grind. Naturally this is one of the pitches that needs lathe reversal... Adding to the fun is an inverter drive to this 3-phase lathe motor that's lost the reverse facility - and a carriage reverse that isn't accurate enough for thread cutting.

This problem probably relates to the imperial gearbox with added metric features, I don't remember it from previous lathe experience although that was a long time ago.

 

[Hopefuldave]

Sounds like time to sort out your VFD... Have you checked whether it's due to the VFD itself not goiing into reverse or the control terminal not gettinig the reverse command? Also, has its programming been changed, it's possible to set a parameter (possibly accidentally!) to disable reverse/force it to forward on most VFDs?

My own lathe is metric/imperial threading with an imperial leadscrew, and sadly lacks the single-tooth clutch on its bigger brothers that would make everything easy, so the handbook recommends reversing with half-nuts engaged except for very long imperial threads, when one should use the thread dial...

And the name rang a bell - are you the fella who built the Quasars? I remember reading about 'em in Motorcycle Sport a few decades ago!

Dave H. (the other one)

 

[smashweld]

It is impossible to answer this as it is too vague.

First, to which gears are you referring? Do you mean the change gears that connect the spindle to the leadscrew? Do you mean the gear(s) on the thread dial indicator?

If you mean the change gears, the very first thing you need to do is calculate whether they do produce exactly the pitch you are intending to cut. If they do, use of the thread dial indicator is possible. If they do not, no matter how close an approximation they make, you cannot reliably use the TDI.

If you mean the gear(s) associated with the thread dial indicator, please tell us the pitch of your leadscrew and the number of teeth on each gear you have. It is then possible to say, for each conceivable pitch you might want to cut, which gear is suitable. Once we have this established, we can then worry about how the dial is graduated (it is possible to swap dials or attach an overlay dial).

Yes the gears connecting leadscrew to spindle...I've thought about it more now and pretty sure you could have any gears/gearbox setting, the dial is just counting revolutions of the leadscrew....I suppose it would be possible for someone to make an electronic thread dial indicator that you could set for any thread imaginable( 'custom',not just standard imperial/metric etc), you would just need to know how many revolutions of the leadscrew until it's back in line, maybe!

 

[Hopefuldave]

Yes the gears connecting leadscrew to spindle...I've thought about it more now and pretty sure you could have any gears/gearbox setting, the dial is just counting revolutions of the leadscrew....I suppose it would be possible for someone to make an electronic thread dial indicator that you could set for any thread imaginable( 'custom',not just standard imperial/metric etc), you would just need to know how many revolutions of the leadscrew unti'l it's back in line, maybe!

If you're going to go with electronics, you need to know position of both spindle and leadscrew to sync and pick up threads correctly - by which point you may as well do the full ELS (Electronic Leadscrew) with an encoder on each and a stepper or servo to drive the leadscrew! There's a public-domain version or three out there, and they seem to be pretty well documented - all are comprehensive and will do metric, imperial, Mod and DP gears by just feeding a few numbers in!

Myself, I've considered doing a single-tooth clutch mod to mine, but it gets complicated... Particularly if you (like me) tend to look for problems to solve!

Dave H. (the other one)

 

[Agroshield]

...the dial is just counting revolutions of the leadscrew...

A slightly better way to phrase it is that the dial is counting leadscrew thread pitches. When the leadscrew is rotating AND the halfnuts are engaged, the dial is stationary, so leadscrew revolution is taking place but counting is not (unless we try weasel words to say that adding or subtracting zero is a form of counting).

A very good book is Martin Cleeve's 'Screwcutting in the lathe'. See: https://archive.org/details/03screwcuttinginthelathe/mode/2up The crucial bit to read is section 5, starting on page 71.

....you would just need to know how many revolutions of the leadscrew until it's back in line

If you look at the above book, he talks about the concept of 'minimum synchronisation distance' (MSD). What you say is absolutely correct, but there is a practical difficulty in that when cutting a metric thread on an imperial leadscrew, the number of leadscrew revolutions is very large, so it takes a long time to reach that number.

If you are using a 127 gear for an exact imperial-metric conversion, the MSD varies for each thread pitch and the lowest MSD you will ever see is 5" (127mm). On a 4tpi leadscrew, that is 20 leadscrew revolutions. In the 5" MSD case, you would need a gear on your thread dial indicator that registers multiples of 20 (most standard imperial TDIs are 16t so you would be trying to track 1 1/4 TDI revolutions using the standard gear). Let us say you are threading 1mm pitch at 300 rpm. After each pass it would take 127/300 of a minute or 25.4 seconds for things to resynchronise. This is generally slower than full reversal.

If you use an approximate conversion (e.g. the 63t gear on a Harrison 140), the MSD increases by at least an order of magnitude (see Cleeve's worked examples starting on p.73). The time taken to traverse this MSD is impractically long and the gear needed on any TDI would be impractically big.

 

[smashweld]

If you are using a 127 gear for an exact imperial-metric conversion, the MSD varies for each thread pitch and the lowest MSD you will ever see is 5" (127mm). On a 4tpi leadscrew, that is 20 leadscrew revolutions. In the 5" MSD case, you would need a gear on your thread dial indicator that registers multiples of 20 (most standard imperial TDIs are 16t so you would be trying to track 1 1/4 TDI revolutions using the standard gear). Let us say you are threading 1mm pitch at 300 rpm. After each pass it would take 127/300 of a minute or 25.4 seconds for things to resynchronise. This is generally slower than full reversal.

If you use an approximate conversion (e.g. the 63t gear on a Harrison 140), the MSD increases by at least an order of magnitude (see Cleeve's worked examples starting on p.73). The time taken to traverse this MSD is impractically long and the gear needed on any TDI would be impractically big.

thanks for the explanation...tbh I need to read up more to really understand the above (127t gear = 127mm is prob a good place to start..), but my question (although maybe poorly worded!) is answered, leadscrew revolutions/pitches will always be a whole number to get the MSD even if impractical

the thread indicator on my lathe is 16t btw...50t gear on the top/spindle, 60t on intermediate and 70t on the bottom shaft gives the 2 1/2 increments of the thread dial that worked...if that makes sense

 

[Royce Creasey]

OK... And a VFD is a ? If it's the Inverter it went through a bit of a head fit last year, losing variable speed control, reverse and jog. Now it's just a 'static' inverter that drives the lathe. I'd get it fixed or buy a new one if I could find anyone who can 'do' power inverters in Bristol (A post-industrial city). It's not really a problem, variable speed was unknown when I started out so I'm happy enough with old-school gearbox-wrangling. My wife is a software engineer and we've been through all the external switching and set up without result - It's lost part of it's mnd. (Invertek unit)

On the subject of change gears, I've just got the sets that come wth the 140 so I'm currently running the 25/80/100 'standard' set. I have absolutely no ambition to cut imperial/UNF threads with anything other than taps and dies! This is why I bought a metric lathe...

Answering HopefulDave, I'm not Malcolm Newell, the Quasar man, I worked with Malcolm, then did the 'Voyagers' that came after the Quasar, following on with a smaller, lighter and more practical vehicle that was equally unsuccesful commercially. I'm still driving one about. All this can be found on www.bikeweb.com with my own work at https://bikeweb.com/image/tid/10. I've retired from that effort now an am using the workshop it created for general light engineering. Building prototype vehicles made me a 'jack of all trades' and I'll never achieve 'master' status as a machinist, just an apprentice at present. Still learning how to use this forum too.