Thread cutting

[jack620]

I once heard that there was a method that can be used when thread cutting say a metric thread on an imperial lathe that does require the lathe to be run in reverse

does or doesn’t require reversing?

It sounds like that setup is just a poor man’s threading dial. If it is, I don’t think it would matter where the saddle was engaged.

[jackaroo]

The dial indicator has to do with the thread on the leadscrew. If the leadscrew is 8tpi then it takes 8 turns on the leadscrew for the dial to move from #1 to #2 on that dial and the carriage will move 1" for that 8 turns if the 1/2-nut is engaged. If you are cutting 8tpi or 16tpi for an instant you can engage anywhere you like. If cutting 13tpi then it has to be exactly the same mark all the time for you to engage the 1/2nut.

To cut metric thread on imperial leadscrew (or vice versa) you will need a 60/127, 80/127, 100/127...etc compound gearset for the conversion, why 127 is important? that is because 1"=25.4mm and 25.4x5 =127 is the smallest whole number you could get...

[Stustoys]

It would probably make sense to wind the saddle back to the same position - possibly use the tailstock as a stop to make sure the saddle stopped it the same spot each time it may have been mentioned at the time but it has been quite a few years. It was told to me by a tool maker many years ago - my first lathe did not have threading dial - he said it works for both native and non native threads, with a native thread I recall he said if the thread you are cutting is a whole number multiple of the lead screw thread then the two alignment marks will line up quite frequently as the lathe runs, if cutting non native threads then the alignment marks will lineup only once every so many turns and you may have to wait up to 10 or 15 seconds (depending on the lathe speed) for the marks to line up.

The way I am picturing it as threads get finer(in relation to the leadscrew) it gets trickier, even worse for non-native. But hey I've only done it once.
Would someone like to check my maths?
Cutting a 8tpi on a 4tpi leadscrew it syncs every second turn of the chuck and the leadscrew is either correct or 180 degs out. Easy.(just an example. you cant drop in half a leadscrew pitch out and cutting 8tpi on a 4tpi you can drop in anywhere right?)
Cutting 3mm pitch on a 4tpi leadscrew it syncs every 20 turns of the chuck.
Cutting 1mm pitch on a 4tpi scyns every 127 turns. The leadscrew error from correct pick up at turn 51 is 0.008 of a turn.

It sounds like that setup is just a poor man’s threading dial. If it is, I don’t think it would matter where the saddle was engaged.

I found it referred to in one of my books and it says this method predates threading dial.
The saddle has to be in the same place, unless you are cutting a thread that doesn't need to be picked up.

[familyguy]

does or doesn’t require reversing?

It sounds like that setup is just a poor man’s threading dial. If it is, I don’t think it would matter where the saddle was engaged.

Yes a typo, thanks for pointing out - I meant to say it does NOT require running the lathe in reverse and I guess it is a form of poor man's threading dial and if indeed it does work then it is an alternative to running an imperial lathe in reverse when metric threading. I should give it a try - mount a felt tip pen in the tool post to see if the spiral mark on the work piece is always in the same place after a few passes.

Without investigating it further I imagine it is like compound indexing - with the lathe running, after a certain number of chuck rotations the chuck and leadscrew will return to their original start positions - this will repeat always after the same number of chuck rotations.

[jack620]

…if indeed it does work then it is an alternative to running an imperial lathe in reverse when metric threading.

Yes it could be, but given the potential for messing it up, I reckon the only reason you would use that method is if your lathe didn’t have reverse. Perhaps in the days before threading dials, lathes didn’t have reverse?

[ajbvicau]

Here is the youtube link to the reverse / upside down threading I mentioned

Threading in reverse and Upside down
https://youtu.be/Z-dqOi_z5bk

what do you think?

cheers
Tony

Baron
thanks for the reply.
I saw a youtube clip on threading with the lathe in reverse and the tool holder upside down. It was quoted as being safer than normal threading due to having removed the risk of crashing into the chuck. I wasnt sure that it could be safely done with a threaded chuck as it might unscrew in reverse unless the chuck was given quite a good locking twist when fitted.

Im curious about the "lathe handle" you mention - what is it?

thanks
Tony

[Ray-s]

[QUOTE]

Yes it could be, but given the potential for messing it up, I reckon the only reason you would use that method is if your lathe didn’t have reverse. Perhaps in the days before threading dials, lathes didn’t have reverse?/QUOTE]

My old Senecca Falls lathe has both issues - no reverse and no threading dial.
I leant to thread on it using marks on the headstock and leadscrew as described. I used an additional mark on the bed or the tailstock to bring the carriage back to the same point.
The technique definitely worked for native imperial threads as I did quite few. I think it worked for the few metric I have done using conversion gears but can't recall for certain. I can't imagine I would have persisted manually winding the carriage back with the half nuts engaged for very long.
Obviously not ideal compared to having a threading dial and reversing capability but it is definitely a valid technique when push comes to shove.
These days having reverse and a threading dial on the Hembrug is something of a luxury.
Ray

[familyguy]

Yes it could be, but given the potential for messing it up, I reckon the only reason you would use that method is if your lathe didn’t have reverse. Perhaps in the days before threading dials, lathes didn’t have reverse?

Yes - as with everything there is a potential for messing it up.
As 16yr old schoolboy I stood and watched as my mate's father (a fitter and turner and working as chief engineer on a tug boat at the time) schooled me on thread cutting on his lathe, he was making a bearing puller, after a number of passes he tried the body of the puller to see if would start on the thread, I leaned over for a look and said 'it looks like there is 2 threads next to each other' - sure enough he had made a nice 2 start thread, he said nothing but stopped work and announced it was time for a drink and he would finish the job later when there were no distractions around, at the time I had no idea if it was right or not I only found out the next day from my mate at school. One would think I learned a lesson from his experience but having messed up a few thread cutting sessions my self, it seems not - most annoying when you've just turned down the last piece of stock you have on hand.

[Ray-s]

Tony,

Back to your last question.

[QUOTE]Here is the youtube link to the reverse / upside down threading I mentioned

Threading in reverse and Upside down
https://youtu.be/Z-dqOi_z5bk

what do you think?

cheers
Tony/QUOTE]

Threading in reverse will definitely reduce your stress level but it would be worth setting up a way to keep your threaded chuck in place when doing this.
Threading the chuck on extra hard to tighten it up is not recommended - they can be very difficult to remove once overtightened.

I recall seeing a set-up where a small recess was cut into the face of the chuck backplate to allow a circular plate with a hole to be fitted between the chuck and the backplate. A threaded rod with a nut fitted was fed through this to the back of the spindle and another plate and nut tightened up to prevent the threaded spindle from undoing.
This set-up blocked the spindle bore of course so wasn't a perfect arrangement but would be OK for shorter pieces being threaded.
Ray

[BaronJ]

Baron
thanks for the reply.
I saw a youtube clip on threading with the lathe in reverse and the tool holder upside down. It was quoted as being safer than normal threading due to having removed the risk of crashing into the chuck. I wasnt sure that it could be safely done with a threaded chuck as it might unscrew in reverse unless the chuck was given quite a good locking twist when fitted.

Im curious about the "lathe handle" you mention - what is it?

thanks
Tony

Hi Tony Guys,

NOTE: For a right hand thread with the chuck turning clockwise looking from the tailstock the tool is not used upside down ! You are simply threading away from the chuck.

This is a picture of my lathe handle.

21022015-01.JPG 21022015-00.JPG

Its simply a device that fits into the lathe spindle and allows it to be rotated !
I made mine using an old washing machine pulley.

21022015-03.JPG 21022015-04.jpg

It is secured in the lathe spindle by an expanding wedge inside a length of tube. The nut on the wheel end draws a threaded rod to pull the wedge shaped cone into the split end of the tube thus expanding it and securing it into the spindle. I also put a handle on the rim of mine to be able to quickly turn it. Great for threading using taps and dies. Also very capable of single point threading if you loosen the drive belt.

A quick point on tumbler gears ! On some lathes, mine included, the two gears are not always the same number of teeth ! So the carriage or crosslide moves at a slightly different speed depending upon direction.

HTH.

[jack620]

NOTE: For a right hand thread with the chuck turning clockwise looking from the tailstock the tool is not used upside down ! You are simply threading away from the chuck.

I think what needs to be clarified is the tool is mounted on opposite side of the work to where it normally is (i.e the back side).

[jackaroo]

here is my setup to thread on reverse

[Stustoys]

A quick point on tumbler gears ! On some lathes, mine included, the two gears are not always the same number of teeth ! So the carriage or crosslide moves at a slightly different speed depending upon direction.

Now that cant be correct, you would require two threading charts.
Idler gear tooth count doesn't matter. Only driver, driven and any compound gears.

[ajbvicau]

So, with tongue in cheek, when you use the big handle you have 1 HP driving the lathe - Human Power that is

The clamping arrangement being similar to how bike handle bars are held in place.

cheers
Tony

Hi Tony Guys,

NOTE: For a right hand thread with the chuck turning clockwise looking from the tailstock the tool is not used upside down ! You are simply threading away from the chuck.

This is a picture of my lathe handle.

21022015-01.JPG 21022015-00.JPG

Its simply a device that fits into the lathe spindle and allows it to be rotated !
I made mine using an old washing machine pulley.

21022015-03.JPG 21022015-04.jpg

It is secured in the lathe spindle by an expanding wedge inside a length of tube. The nut on the wheel end draws a threaded rod to pull the wedge shaped cone into the split end of the tube thus expanding it and securing it into the spindle. I also put a handle on the rim of mine to be able to quickly turn it. Great for threading using taps and dies. Also very capable of single point threading if you loosen the drive belt.

A quick point on tumbler gears ! On some lathes, mine included, the two gears are not always the same number of teeth ! So the carriage or crosslide moves at a slightly different speed depending upon direction.

HTH.

[BaronJ]

Hi Stuart, Guys,

I used to think that ! There is only one tooth difference between the two tumbler gears on my lathe, so it would require 22 or 23 spindle turns to make any real difference. The only nigger in the gear chain is the fine feed cog ! Forgetting to turn it round really messes up the threading !

I've been caught out several times by that one !