Motor wiring for reversing rotation

only advice i can give you is to pin the chuck or it will come undone in reverce.

all the best..mark

When you say pin it, do you mean to actually permanently modify it in some way?

I am thinking that I need reverse when cutting screw threads so that I can back out before setting a deeper cutting depth and cutting again. Presumably since I will not be cutting on the way out, the chuck is unlikely to come undone?

Cheers Martin

If your motor is like mine (Crompton Parkinson single phase) you only need to reverse the polarity of the starter windings to get it to reverse. The running windings stay the same which ever direction it runs. I'm using a Dayton reversing switch to change the polarity. If the lathe runs the 'wrong' way when using the switch, all you have to do is swap the wires over on the starter windings.

You should NEVER have any load on the chuck (ie, any tool engaged in the work piece) when in reverse, otherwise it will spin the chuck off, which at speed could be very dangerous.

You don't need to reverse the lathe to back off when screwcutting as there should be a lever on the lathe that will reverse the leadscrew and thus changing direction of the saddle.

I have an ML7 which has all the above.

Hope this is of help to you.

from "DAVE"

Unless you are cutting metric threads on an imperial machine in which case you must NEVER take any gear out of the train.

JG

I might be missing something here, but if you use the tumble reverse to reverse lead screw rotation, how do you guarantee that the leadscrew and chuck remain in sync from a tool tip / thread point of view ?...

Chris

Yes, that's exactly the plan. My Myford M-Type is imperial however I am probably likely to want to cut metric threads therefore I thought I had to reverse out of the thread, increase the cut depth a few thou and then cut again. I have actually never cut a thread on a lathe like this so please correct me if I have the wrong end of the stick.

As it happens, the first thread I want to cut is 1/2" UNF and as I understood it, if I am cutting an imperial thread then I could disengage the saddle from the lead screw and as long as I somehow managed to engage the lead screw at the right point I could cut again. Since I don't know how to find the right point again I was just going to back out in reverse.

Martin

It's been some time and am just getting back into machine tool stuff, so someone can correct me if in error, but the way I remember being taught was to take the cut, then wind out the cross slide fast at the end while at the same time stopping the machine. Then, without disengaging the leadscrew, reverse the drive back to the start and take another cut. That way, you maintain the relationship between the work and tool. Of course, you need to be quick to avoid turning down the chuck jaws if the work is close up :-).

Any way that disengages the lead screw means you have to fiddle around finding the start point, not to mention backlash in the gearing etc. Not a recipe for the best results I would think, not to mention the time involved...

Chris

Does an M-type not have a Thread Dial Indicator?

for imperial threads (on an imperial machine) you find the right point to re-engage the lead-screw by using a screw-cutting clock (this has 8 marks around the dial) which fits on the right hand end of the saddle in the 1/4"BSW hole provided by Myford.

If you do't have one already fitted then they are often available on eBay for a few pounds.

It's a long time since I used one (though I do have one on my Super 7) because I work primarily in metric but IIRC you can 'drop in' when the dial aligns with any mark for any tpi divisible by 8 (the native tpi of the lead-screw) but only on certain other marks where this is not the case. I'm sure JS or one of the many other contributors here could give you specific chapter and verse.

JG

Martin Welcome to the Brotherhood! In five years time you'll look back and wish you'd taken up a cheaper pastime, like golf or international ocean racing!

Fortunately you have had the good fortune to come to a place where the likes of Keith (note that 'Jontom' is Keith, not 'Jon') are both able and willing to give you the benefit of a lifetime's professional experience.

To supplement the titbits you may glean in this forum, my own bit of advice is to buy yourself a copy of 'The Amateur's Lathe' by Laurence Sparey - Amazon currently has seventeen listed from £4.76 upwards. see

`Although the style may be a bit dated now, it is still an excellent beginner's guide to using a small lathe. It has a very comprehensive section on screwcutting and I found it to be a very useful part of my toolkit, just as important as the hardware, when I was starting out.

--

Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"

The ancient lathes we used at school had no dial indicators. We were taught to mark spindle (drive plate, chuck, whatever) with a chalk line, and a similar line on the leadscrew, when the halfnut was engaged. Then, waiting until the lines both came up together before reengaging would do the job. I *believe* a similar technique can be used for metric threading with an imperial leadscrew, but can sometimes involve a very long wait I've never tried it, though - anyone else??

Tim

Martin the simple answer to your question is that your method will be fine with any thread you can set up with your gear train. Only two things to ensure, firstly that you retract the tool enough to clear the work and secondly, when you reverse the lathe make sure you wind the saddle back well past the start point so that when the re- positioned tool starts to cut on the second pass all of the backlash has been removed from your system. This method has been used with every success for many years, even without an electrical reverse where the spindle is turned in reverse by hand to wind the saddle back.

As this method can be a little time consuming then other methods were introduced to ensure that the tool/work relationship can be picked up again when broken to move the saddle back to the start. The most common is the Thread Dial Indicator. The method of marking the spindle and leadscrew Tim mentions is a simple (rough and ready) version of the TDI. The number of times the correct relationship between spindle and leadscrew position (to allow halfnut engagement) arises is dependent on the thread being cut and the leadscrew pitch. With a metric thread being cut on an imperial leadscrew this might only happen every 127 turns of the leadscrew, as Tim says a long wait.

The method of reversing the leadscrew can work but does carry a fair bit of risk. As the relationship between our two key elements is broken it is quite possible to allow the geartrain/leadscrew relationship to rotate by a tooth when getting the leadscrew reverse gears to mesh, plenty enough to wreck your thread on the next pass.

The book that Chris recommends is a very good starting point and well worth reading. When I returned to the "practical" activity of using a lathe after many years away I found it a very useful introduction. There are of course other books should you be planning to do a lot of thread cutting. Apart from getting the right geartrain to generate the required pitch and maintaining the work to tool relationship throughout, another issue you will find is how to apply the cut to the required depth and get a good finish on our light lathes. This can be achieved by setting over the topslide and applying the cut at an angle (usually half the thread angle) so that the tool is cutting on one face only thus reducing the tendency for chatter. A very basic introduction to threading on the lathe which shows the use of thread gauges etc to set up the tool and also how to pick up a thread if required is here:

There are many aspects to cutting a successful thread with a single point tool and some preliminary reading to "refresh" the brain is well worthwhile, at least it was for me.

Regards

Keith

When metric threading with an imperial leadscrew, the carriage will re-synchronize with the leadscrew and with the thread being cut every 127 revolutions of the spindle. This can be worked out to inches of travel on the leadscrew and the thread dial can be used to locate the proper point. Sort of un-wieldy and prone to mistakes, but it can be done. Generally easier to just reverse without disconnecting.

Don Young

No, there is no TDI fitted and no obvious 1/4" hole to fit one - perhaps Myford did not add this feature to the Drummond that they inherited.

I think it sounds less complicated to begin with if I just back off the tool and then reverse out. I don't plan on cutting many threads but I have a small pillar drill that needs a new thread for the chuck so I plan on having a go on that - after first doing some practice on scrap bits of course.

Incidentally I have tried swapping around the motor starter winding wires and it does indeed start in reverse so I now just need to rewire the motor with a 4 wire feed and rig up a changover switch to select desired direction of rotation.

I will look at getting the Amateur's lathe, thanks for the link Chris.

Thanks again for all your comments, I feel much more confident now.

Don, I certainly wouldn't argue with your point particularly as I've learnt much from your earlier posts on the subject and I'm also grateful that you pointed out my error quoting leadscrew revolutions instead of spindle.

However I think it is important to point out to those just starting out that the half nuts will in fact also engage at many other positions and merely dis-engaging moving to the right (even if you move a distance which is an integral multiple of the pitch being cut and the leadscrew pitch) and waiting for the half nuts to re-engage thinking that will only be every 127 turns of the spindle will not work. I suppose what I'm saying is you can't use the half nuts engaging to count the spindle revolutions.

Don, I know you didn't say that but I think it might be incorrectly assumed by someone just starting out. Obviously, if you stop the spindle and move the saddle a distance which is an integral multiple of the thread pitch being cut and the leadscrew pitch (ie 4" for a

0.8mm and 5" for a 1mm thread with an 8TPI leadscrew) then re-engage, all will be well.

As you rightly say, in the practical application much easier to leave the half nuts engaged and reverse the machine to wind the saddle back.

Best regards

Keith

I certainly agree with your points. The practical problem is keeping track of everything (spindle revolutions and carriage movement) with the half-nuts dis-engaged. It can be done with the thread dial or other methods such as carriage stops and spindle revolution counters. It might be a help in threading to a shoulder to open the half-nuts and quickly stop the spindle while observing the thread dial. The spindle can then be reversed and the half-nuts closed when the dial returns to the original position. As long as the dial returns to the same position without going around, regardless of any leadscrew or carriage motion, the carriage to leadscrew synchronization will be maintained and everything can be reversed to start a new cut. It is also possible by calculating the proper number of turns, including fractional turns, to find the next sync point on the thread dial at 127 pitches of the work thread.

Of course, re-syncing at 127 pitches only works if using a 127 tooth translation gear and not any of the other approximations. I have tried to find a practical way to make all this work easily but, as we have both said, it remains easier to just keep the half nuts closed until finished.

Don Young