Why reverse a lathes direction?

[Ian S CParticipant @iansc]

I suppose it's posible to losen the jaws of a 3 jaw SC chuck by reversing with nothing held in the jaws.

ega you are right, but one ring could be bolted directly to the back plate on the chuck, you only need say 1/2" of spindle nose to clamp on.

Ian S C

[John Stevenson 1Participant @johnstevenson1]

Knee jerk.

In real life this happens once every 22 years. The rest is just regurgitated by armchair machinists on a regular basis.

[AjohnwParticipant @ajohnw51620]

I might be able to explain the screw cutting problem a bit clearer but some thought will be needed as it's best explained via a screw cutting indicator and what this does.

I did have some notes from a lecture that explained the problem via a formulae that initially used radians / second of the lead screw and the lathe spindle but lost them and can't remember anyway. Repeat distances are easier to grasp.

Take an imperial thread on an imperial machine, or metric on a metric machine. The pitch of the thread being cut will divide exactly into so many pitches of the lead screw. That is the repeat distance. The screw cutting indicator has a gear on it which is always engaged with the lead screw. The tooth count on the gear cause the marks on the indicator to always indicate repeat distances as the saddle is moved along the bed. So some thread is cut with the screw cutting indicator at some specific mark. The dial stops turning, at the end of the thread the nut is disengaged and the dial starts spinning again, the saddle is wound back by hand and the spin rate changes but the mark that was used is still in sync with the thread that has been cut so if engaged on that mark again the tool will travel down exactly the same path. I'm skipping the aspect that a number of marks can be used for certain pitches and that the leads screw pitch may be it's own repeat distance for some pitches so it doesn't matter where the nuts are engaged. The indicators are generally positioned so that the nut engage cleanly though so it's always best to use one to avoid the nut riding on top of the lead screw.

So take an odd imperial thread say 7 tpi. The pitch is 0.142857142 ……………. Say the pitch of the lead screw is 0.125. One divided by the other is 0.875. So if the indicator can show repeat distance of 0.875in it can be used. It can also be used if it can shown some exact multiple of 0.875in. I could just arrive at that by dividing the pitch of the thread by the pitch of the lead screw.

The imperial pitches that are used have been picked so that a screw cutting indicator can be used to cut all if not most of them but the indicators capabilities vary from machine to machine. The metric pitches that are used don't follow the same rules so more than one gear is needed for the indicator. Colchester did one with lots of gears but the catch is that at some extremely long repeat interval the marks take a long time to line up so may as well reverse the lathe. It's more normal for metric lathes to come with 1 or 2 indicator gears. 2 is usable on more pitches.

I don't know of a good web page on screw cutting indicator use but there can be other catches. Say a 4 tpi thread is being cut with an 8 tpi lead screw. If screw cutting isn't engaged at the right point it might cut a 2 start 8 tpi thread. Actually that sort of accident happens at times. All goes well and suddenly the lathe cuts the other start. It usually happens when the wanted thread is rather deep.

This all changes when conversion gears are added to cut a thread of the other standard, all down to the number 127 that is needed for exact conversion. There isn't a screw cutting indicator gear tooth count that can be used to obtain a correct repeat interval. I've never looked to see what would happen if one of them had 127 teeth. Take an example of say a 2mm pitch and a 8tpi lead screw the fraction is 0.078740147………/0.125. It's pretty obvious that the 0.0787…. can't be multiplied by any integer to provide another integer other than n*127. The teeth on the indicator has to be an integer number.

One way of screw cutting without an indicator is to take a cut, stop the lathe and then mark the gears where they mesh together. Then wind the saddle back to the start and run the lathe until the lines mark up exactly again and at that point engage screw cutting again. Waiting for the lines to line up ensures that the lead screw is synced to the thread being cut so it doesn't matter where along it the nut is engaged. The alignment has to be surprisingly accurate for this to work even accounting for the actual angles of the teeth on the gears. This is where using a tumbler reverse is likely to fall down. The gears in it would have to be rather precisely positioned so that they meshed perfectly when changed over. The gears would also need to be identical and engage in the same place on the spindle otherwise there could be an angular error. There are details of a gizmo that does this sort of thing around. It does it by having 2 gears rotating in opposite directions and locations to ensure things remain in sync when the lead screw rotation is reversed. I believe the person who copied that idea from other commercial machines used to have a large handle on the lead screw to wind it back quickly by hand. If the tumbler is used there is no guarantee that it will re engage with the spindle at the same angular relationship as it did before.

The real problem with reversing the lathe and leaving the nut engages is time – it takes a long time for the saddle to get back to the start again, it's also a bit of a problem screw cutting to a shoulder as the lathe needs to stop rather quickly. There is a method of using an indicator to help with that but the lathe is still stopped and reversed. A saddle can be wound back very quickly by hand and working to a shoulder isn't a problem at sensible speeds.

John

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Edited By John W1 on 24/08/2015 14:31:13

[KWILParticipant @kwil]

John, all you really need is a single dog clutch and a reversing gear set up. leaves the chuck spinning in same direction and reverses the leadscrew, all with the leadscrew nut still engaged.

Design earlier on this Forum works very well.

[Neil WyattModerator @neilwyatt]

Without wanting to seem too much of a clever ass…

If the OP wants to know why one would want to reverse a lathe, he probably doesn't need to worry about installing a reversing switch.

Neil

[AjohnwParticipant @ajohnw51620]

Posted by KWIL on 24/08/2015 14:49:48:

John, all you really need is a single dog clutch and a reversing gear set up. leaves the chuck spinning in same direction and reverses the leadscrew, all with the leadscrew nut still engaged.

Design earlier on this Forum works very well.

 

 

I did mention gizmo's. There is nothing new about them. The arrangement is shown on one rather old lathe whose name I can't remember on lathes co uk. It's also in one of the model engineering books and at least one of the main manufacturers has offered one.

My answer to metric on an imperial lathe is to not use them unless absolutely essential. If I were to copy a design for say a steam engine and it called for say 25x1mm I would simply use 25mm x 26tpi. As I would also be making the other part that would match. If I have to make something to fit a metric thread I cut metric, usually an approximation even though I have the exact gearing.

If I owned a metric lathe I would work the other way round from imperial designs and if it didn't have an indicator either buy or make one. There is a design for one for a 3mm pitch lead screw in my album. Some one makes and sells the same basic design on ebay. They claim they have copyright even though the idea has been around for a very long time. It looks like some of the small chinese hobby metric lathes come with indicators. Some don't.

I could program my inverter up to return the saddle at a very high rpm but past some point it wouldn't do the lead screw much good.

In short the best answer to the problem is not use the opposite standard unless really needed and do have a screw cutting indicator or even better an electronic lead screw but ideally that needs closed loop control of the lathe motor speed.

Actually sync distances can be worked out when an exact conversion gear set is added to an imperial machine but some of them are a bit long. They can be when cutting metric on a metric machine too.

That's the user name I usually use but finished up with the wrong one on here. The pitches in italics aren't in the extended iso specs. The rest appear to be.

John

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Edited By John W1 on 24/08/2015 16:10:10

[RainbowsParticipant @rainbows]

Posted by Neil Wyatt on 24/08/2015 14:58:07:

Without wanting to seem too much of a clever ass…

If the OP wants to know why one would want to reverse a lathe, he probably doesn't need to worry about installing a reversing switch.

Neil

My cheap ass just hasn't had a lathe with a thread cutting mechanism before. :<

[AjohnwParticipant @ajohnw51620]

If anyone tries to use the same idea along with their super accurate dro I would be interested to hear how it works out.

John

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[John Stevenson 1Participant @johnstevenson1]

yawn…………….

[AjohnwParticipant @ajohnw51620]

John

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[KWILParticipant @kwil]

And there was I thinking how easy and straight forward it is to cut metric and imperial threads on an imperial lathe

I am with you JS, yawn………

[AjohnwParticipant @ajohnw51620]

Posted by KWIL on 24/08/2015 19:39:55:

And there was I thinking how easy and straight forward it is to cut metric and imperial threads on an imperial lathe

I am with you JS, yawn………

To be honest I find running the saddle back via the lead screw even more boring than the comments and usually a total waste of time for pretty obvious reasons. Just as posting to answer some ones question is to people who don't want to know the answer. They don't have to read them and could possibly profit by doing something more useful.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Posted by John W1 on 24/08/2015 16:06:04:

My answer to metric on an imperial lathe is to not use them unless absolutely essential. If I were to copy a design for say a steam engine and it called for say 25x1mm I would simply use 25mm x 26tpi.

.

Here's today's fascinating fact, Chaps:

The 39mm screw thread on the original Leica lens mounts was actually cut 26tpi

[ offered as a little Leitz relief … to calm the bickering ]

MichaelG.

[Chris DentonParticipant @chrisdenton53037]

Screwcutting, taping threads, hydraulic profiling, rear mounted tool post, using a file to remove a sharp edge.

[Roger Provins 2Participant @rogerprovins2]

My cheap Asian Shaoxing 9×20 has a security bolt that the prevents the chuck being unscrewed until it is removed.

I know it works.

Edited By Roger Provins 2 on 01/09/2015 06:26:36

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 24/08/2015 20:28:03:

Here's today's fascinating fact, Chaps:

The 39mm screw thread on the original Leica lens mounts was actually cut 26tpi

[ offered as a little Leitz relief … to calm the bickering ]

Much googling for the thread on various telescope threads shows that (unlike microscope threads which are well defined) there appear to be no official standards and not even agreement as to whether some of them are imperial or metric…

The fact is that 99.999% of threads are fixings, not leadscrews, and need to be free enough to spin up, and worrying about tiny pitch errors is pointless.

Neil

[AjohnwParticipant @ajohnw51620]

LOL Some of the major microscope manufacturers have departed from the standards Neil. I think that even the objective threads may vary now from make to make.

Screw cutting ? We all know that lead screws are made from super hard material that never wears out and is temperature controlled on the lathe to 1 billionth of a degree C and that manufacturers get every lead screw checked by a national physics laboratory so microns must matter.

John

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[Peter G. ShawParticipant @peterg-shaw75338]

I wonder if I could just throw in a few thoughts.

I still remember when I first got involved in this hobby and how little I knew, eg "What's a milling machine?". Same with questions about screwcutting etc so in that respect I can sympathize with both Rainbows and Eugene. The big problem is that there are people on this forum who are professionals and have a great tendency to go off into the depths of technicals when people like Rainbows and Eugene need almost Yes/No answers. This doesn't only apply to this thread as I've noticed it on other threads as well. I'd therefore like to make a plea that people consider what they are saying in respect of the OP's request.

Having said that, I do like the humour that appears. Reversing the lathe to put back metal indeed! And John's bit about lead screws one posting above. Or maybe two if someone else beats me in submission.

You'll notice that I haven't attempted to explain the why's and wherefore's of the question. It's all there – if you can sort it out – I don't want to add to the confusion.

Regards, and please don't take it to heart what I've said; I do mean it for the best – if that makes sense.

Peter G. Shaw

[Anonymous]

Posted by Chris Denton on 01/09/2015 03:26:27:

Screwcutting, taping threads, hydraulic profiling, rear mounted tool post, using a file to remove a sharp edge.

Errr, I'm not sure how running the lathe in reverse helps when using a file to deburr an edge, surely it would just rub? Or I have I been doing it all wrong?

When I started using a hydraulic copy unit I was advised to run in the normal direction and have the tool upside down, as all the feeds then work in the familiar way. That way there is less chance of a 'crunch' due to getting the feed direction wrong. Like this:

Andrew

PS: And just to be clear I'm not a professional machinist.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 01/09/2015 08:54:25:

The fact is that 99.999% of threads are fixings, not leadscrews, and need to be free enough to spin up, and worrying about tiny pitch errors is pointless.

Neil

.

… but the remaining 0.001% [ish] is the interesting stuff.

MichaelG.

P.S. … John W1 is quite correct; many of the modern microscopes are using larger threads for the objectives. … That said, the old standard gauges that were commissioned by the RMS are a joy to behold.

Edit: Here is a handy copy of the spec. dimensions.

Edited By Michael Gilligan on 01/09/2015 13:15:28

[AjohnwParticipant @ajohnw51620]

Posted by Andrew Johnston on 01/09/2015 11:25:45:

Posted by Chris Denton on 01/09/2015 03:26:27:

Screwcutting, taping threads, hydraulic profiling, rear mounted tool post, using a file to remove a sharp edge.

Errr, I'm not sure how running the lathe in reverse helps when using a file to deburr an edge, surely it would just rub? Or I have I been doing it all wrong?

When I started using a hydraulic copy unit I was advised to run in the normal direction and have the tool upside down, as all the feeds then work in the familiar way. That way there is less chance of a 'crunch' due to getting the feed direction wrong. Like this:

Andrew

PS: And just to be clear I'm not a professional machinist.

Your comments sound entirely sane to me Andrew – stick with it.

John

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[JonParticipant @jon]

Filing in reverse direction would need to reach over toolpost as if filing right to left unsupported.

Now I could make use of that hydraulic copy attachment it will fit straight on to my Harrison.

Logical to me as well all the feeds don't have to be changed over, no matter how careful its a matter of time before a crash happens. Boring between centres opening up a bore to 22.2mm x 288mm long, problem is cutter has to cut in reverse direction so have to change lead screw direction. Just as well did likewise Friday on an expensive part, forgot direction of lead screw already right up to chuck, luckily set on a fine traverse. Not the first time done it either on same types of parts.

[JasonBModerator @jasonb]

Running backwards means the tang is facing away from you if it should all go pear shaped (file also reversed) No need to reach over the tool post, left hand on handle, right on end of file in the same stance as you would using teh file the right way round.

Edited By JasonB on 01/09/2015 20:36:21

[Anonymous]

Posted by JasonB on 01/09/2015 20:35:34:

Running backwards means the tang is facing away from you if it should all go pear shaped (file also reversed) No need to reach over the tool post, left hand on handle, right on end of file in the same stance as you would using teh file the right way round.

Edited By JasonB on 01/09/2015 20:36:21

Doesn't that mean my left arm will be resting on the lathe chuck? I think I'll stick with running the conventional way. For deburring in the lathe I use a needle file, so no tangs to worry about.

Andrew

[Roger Provins 2Participant @rogerprovins2]

It used to be a sacking offence to use a file without a handle – they're cheap enough.

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