How to Calculate Bore Size

[COLIN MARTIN 2Participant @colinmartin2]

Thank you for the info and suggestions. I am side-tracked at the moment because I am making a hand crank for the lathe chuck, so I can cut the threads more safely. It is all good experience and I will keep everyone posted.

 

 

Cheers,

 

Colin

[Dave HalfordParticipant @davehalford22513]

Colin if your 180 has a dial thread indicator they make it very easy.

No reversing the motor or winding back by hand just drop the half nuts in when the indicator points to the correct number, disengage at the end of the cut, wind back with the apron wheel till past the number then engage again when the number passes the marker again. Note the number wheel stops turning when the half nuts are engaged

[HopperParticipant @hopper]

When using your spindle crank handle, either make it a self-disengaging type, or be darn sure to unplug the lathe from the power before fitting the crank handle, or at least hang a big red DO NOT START sign over the on/off switch.  You really, really do not want inadvertently start your lathe up with the handle in place, especially on a small lathe that is maybe not bolted down to a very solid bench. Things get very wild very quickly. (Ask me how I know this!)

 

 

[COLIN MARTIN 2Participant @colinmartin2]

I have a slight problem in that I am cutting Metric threads on an Imperial lathe, so I think that the thread dial will not work?

And yes, the chuck handle will be removable following a pattern that I found on a You Tube video.

 

 

Thank you,

 

Colin

[Howard LewisParticipant @howardlewis46836]

If you are cutting Metric threads on an Imperial lathe, my advice would be not to open the half nuts, but to keep them engaged all the the time.

Having taken a cut, you will know the dial reading, so back off to remove the tool from the job, then wind the lathe in reverse, to bring the tool well clear of the job.

Having moved the Cross Slide forward, again, far enough to take out the backlash, continue moving until the dial reading is as before, and then more for the nex cut.

Since the saddle and tool will be far eniough along the bed to be well clear of the workpiece, winding the lathe forwards again, will take out the backlash betweem half nuts and Leadscrew, so that you pick up where you first started.

Howard

[COLIN MARTIN 2Participant @colinmartin2]

Thank you Howard.

[Dave HalfordParticipant @davehalford22513]

Colin, which tool are you cutting the thread with?

[COLIN MARTIN 2Participant @colinmartin2]

Probably a carbide boring tool.

[COLIN MARTIN 2]

On COLIN MARTIN 2 Said:

I have a slight problem in that I am cutting Metric threads on an Imperial lathe, so I think that the thread dial will not work?

And yes, the chuck handle will be removable following a pattern that I found on a You Tube video.

 

 

Thank you,

 

Colin

I would have thought that if you can set the machine gears to cut a 1.25 pitch the indicator will work as intended

[duncan webster 1Participant @duncanwebster1]

Not if it’s an imperial leadscrew it won’t

[Dave Halford]

Given

On Dave Halford Said:

Colin if your 180 has a dial thread indicator they make it very easy.

No reversing the motor or winding back by hand just drop the half nuts in when the indicator points to the correct number, disengage at the end of the cut, wind back with the apron wheel till past the number then engage again when the number passes the marker again. Note the number wheel stops turning when the half nuts are engaged

A beginner tackling both M62 internal and external threads, might do better not to use a TDI, and doubly not if his lathe in Imperial!

A TDI is way of speeding up threading, and it can go wrong.   The dial keeps track of the leadscrew relative to the spindle, which allows the operator to disengage the cross-slide from the lead-screw, whip the cross-side back just beyond the start point, adjust depth of cut, and then re-engage the cross-slide at the right moment.   Right moment is important:  if the operator misreads the TDI or mistimes re-engaging the half-nuts, then bad things happen!    I suggest using the TDI is advanced thread-cutting, and needs a fair amount of practice.

Instead, there’s no harm in simply stopping the spindle and reversing out.   It takes a little longer but there’s no risk of losing position because the half-nuts aren’t messed with by a trainee.   Even more telling, the stop and reverse method is common on modern lathes, especially on metric machines.  As metric threads are defined in terms of pitch, not turns per, a TDI can’t cover them all, so maybe best avoided entirely.

The advice to angle the cutting tool is suspect for the same reason: an unnecessary complication, that can’t in any case be applied to the internal thread.   My advice is to go straight in, and learn how to angle the tool later.

As an aside, I wonder if it’s time to abandon angling threading tools?   The technique would make enormous sense if this was 1890 and were all using ye olde carbon-steel cutters because these go blunt quickly and benefit greatly from a stress reducing cutting angle.  HSS cutters also benefit, but much less so because they are much tougher.  Angling thread cutters isn’t quite obsolete, but there’s not much need for it.   Carbide goes a step further: threading inserts are formed to go straight in, no messing.

Dave

 

 

 

[duncan webster 1]

On duncan webster 1 Said:

Not if it’s an imperial leadscrew it won’t

Apparently the lathe also covers a number of metric threads with the change gears supplied and yes being a chinese lathe made predominately for the US market it will have a US lead screw thread.

There’s a slightly larger Grizzly version that covers the same threads as the Warco and an inch is 25mm exactly honest, well ish if you squint a bit. The manual does avoid the thread indicator for metric, but surely when the lead screw drives the carriage and the indicator tracks the leadscrew it will also track the thread start as normal.

[COLIN MARTIN 2Participant @colinmartin2]

Thanks for all the info!

 

 

Cheers,

 

Colim

[SillyOldDuffer]

On SillyOldDuffer Said:

Given

On Dave Halford Said:

Colin if your 180 has a dial thread indicator they make it very easy.

No reversing the motor or winding back by hand just drop the half nuts in when the indicator points to the correct number, disengage at the end of the cut, wind back with the apron wheel till past the number then engage again when the number passes the marker again. Note the number wheel stops turning when the half nuts are engaged

Right moment is important:  if the operator misreads the TDI or mistimes re-engaging the half-nuts, then bad things happen!    I suggest using the TDI is advanced thread-cutting, and needs a fair amount of practice.

Instead, there’s no harm in simply stopping the spindle and reversing out.   It takes a little longer but there’s no risk of losing position because the half-nuts aren’t messed with by a trainee.   Even more telling, the stop and reverse method is common on modern lathes, especially on metric machines.  As metric threads are defined in terms of pitch, not turns per, a TDI can’t cover them all, so maybe best avoided entirely.

The advice to angle the cutting tool is suspect for the same reason: an unnecessary complication, that can’t in any case be applied to the internal thread.   My advice is to go straight in, and learn how to angle the tool later.

As an aside, I wonder if it’s time to abandon angling threading tools?   The technique would make enormous sense if this was 1890 and were all using ye olde carbon-steel cutters because these go blunt quickly and benefit greatly from a stress reducing cutting angle.  HSS cutters also benefit, but much less so because they are much tougher.  Angling thread cutters isn’t quite obsolete, but there’s not much need for it.   Carbide goes a step further: threading inserts are formed to go straight in, no messing.

Dave

 

 

 

Sorry to disagree, but using a TDI is threading for dummies (can’t think why I know that.) If you miss time dropping the nuts you try again next time the dial comes round. Bad things only happen if you miss time it and do nothing about it. You can drop the nuts on your chosen number an inch away from the work it doesn’t matter.

You can’t angle thread cutters for internal threads anyway, however you do need to note that you may need 12 passes as having to cut both sides at once can be a 2mm wide cut.

[HopperParticipant @hopper]

By “angling the thread cutter” are you guys referring to setting the topslide at half the thread angle? I can’t see why that could not be done on an internal thread?

Although, I never bother with it myself, internal or external. Much easier to simply leave the topslide in the normal position and advance the topslide by half of the amount you feed the cross slide in (or out for internal thread). Gives a tool path just under 30 degrees, which puts most of the cutting forces on one cutting edge and a light finishing cut on the other. It seems to reduce chatter even with insert tooling on these light hobby lathes.

This method also gives you the option of taking a light clean-up cut along first one flank of the thread, then the other, without increasing thread depth, by advancing/retarding the topslide. Makes final finishing and fitting of the thread to a sample nut etc. much easier.

Pretty sure you can’t use the thread chaser dial on an Imperial leadscrew cutting metric threads. But i have never tried it, as all reputable texts and shop floor custom seem to advise against it. The mathematics involved in proving it one way or the other is way above my pay grade. Getting into Martin Cleeve territory there! (Author of Screwcutting in the Lathe. WPS) I used to know a lot of that sort of stuff but I am nowhere near as smart today as I was when I was 19.

[Dave Halford]

On Dave Halford Said:

On SillyOldDuffer Said:

Given

On Dave Halford Said:

Colin if your 180 has a dial thread indicator they make it very easy.

No reversing the motor or winding back by hand just drop the half nuts in when the indicator points to the correct number, disengage at the end of the cut, wind back with the apron wheel till past the number then engage again when the number passes the marker again. Note the number wheel stops turning when the half nuts are engaged

Right moment is important:  if the operator misreads the TDI or mistimes re-engaging the half-nuts, then bad things happen!    I suggest using the TDI is advanced thread-cutting, and needs a fair amount of practice.

…
Dave

 

 

 

Sorry to disagree, but using a TDI is threading for dummies (can’t think why I know that.)

…

People who have acquired a skill, perhaps lucky enough to start with a natural aptitude, often think what they do is easy.   They forget they had to go through a learning process, and start assuming it’s just ‘common sense’.   Pity the poor beginner: Colin has never cut a thread before, and he only wants two.  Is is really good advice to tell him that a TDI is threading for dummies?   I say not, better for him to keep it simple.

And don’t forget that although using a TDI on an Imperial lathe to cut Imperial threads is straightforward, the same can’t be said of a TDI when cutting metric threads.

Colin, who has an Imperial WM180, asked earlier:  I have a slight problem in that I am cutting Metric threads on an Imperial lathe, so I think that the thread dial will not work?    If using a TDI really is for dummies, Dave will tell us what TDI line or lines will tell Colin when to engage the half-nuts?   I don’t know.  Could be worked out, but I think it’s easier to leave the half-nuts permanently engaged,  and reverse the threading cutter out: that makes the TDI irrelevant.     No problem reversing a WM180, because the backplate is bolted to the spindle, not mounted on a single screw like a Myford.  Finally, as Colin very sensibly intends to hand crank the lathe, there’s no speed advantage in using the TDI, and that means it’s an unnecessary complication at this stage of Colin’s learning.

Dave

[Dave HalfordParticipant @davehalford22513]

Perhaps Colin will be good enough to engage the tdi when he’s finished practising hand cranking the external thread and tell us if it works or takes the thread off on the next cut.

[duncan webster 1Participant @duncanwebster1]

<p style=”text-align: left;”>If you’re cutting a metric thread on an imperial lathe think of if this way. With the tool engaged with the thread and the lathe stationary back off the tool, open the half nuts, advance the saddle by 1″, close the half nuts to re-engage the leadscrew. Provided the Imperial lathe has a whole number of threads per inch, the half nuts will go straight in, but the tool won’t engage with the metric thread. Same applies for any saddle movement of a whole number of leadscrew pitches.</p>
And by the way, you can use angled topslide on an internal thread, put the tool in upside down and work on the far side of the axis.

[HopperParticipant @hopper]

I just checked Martin Cleeve’s book “Screwcutting in the Lathe” and no wonder I cannot remember the math for using the thread chaser dial for cutting metric threads on imperial lathes. It is complicated, full of wheretofores and whyfores, and even by Cleeve’s recommendation,not worth the bother.

You can do it, for certain metric threads over certain lengths, where the number of metric threads per a certain number of inches comes out to a whole thread. But usually that distance is too long to be accommodated on a small lathe. The carriage has to be disengaged after the lathe is stopped and then wound back by exactly the specified number of inches (measured by turns of the chaser dial) and the halfnuts re-engaged before restarting the lathe.

There is also another method for threads under 2″ long, using the lathe in reverse, cutting away from the chuck, and using a carriage stop to place the carriage in the same place again before re-engaging the halfnuts, all with the lathe stopped.

But for general work he strongly recommends simply leaving the nuts engaged, withdrawing the tool and reversing the lathe until the carriage is back to the starting position.

And, no, that is not a problem on Myfords and SouthBend clones with screwed-on chucks. At screwcutting speeds (sub-100rpm) I have never seen one come loose in reverse. Using a tailstock centre probably could provide some extra insurance, just in case.

Cleeve’s issue with the reversing method was the length of time it takes to wait for the carriage to return to the start position on each cut. But he was cutting threads for a living, often in production runs, where time was money. Not the case in the home workshop. Our hobby is a pastime. So if we pass a bit of extra time reversing the lathe and waiting, we have achieved our objective!

[duncan webster 1]

On duncan webster 1 Said:

And by the way, you can use angled topslide on an internal thread, put the tool in upside down and work on the far side of the axis.

Or perhaps angle the topslide the opposite way from when cutting an external thread and keep the tool in the conventional orientation. Watch that chuck/topslide interface though!

I still prefer the method with topslide in normal position and advance it by half the infeed. Much less faff. (But one more thing to think about so perhaps not as easy for beginners?)

[COLIN MARTIN 2Participant @colinmartin2]

Hi All,

At last an update! It has taken me a long time due to the making of the cranking handle and organizing myself, plus other things getting in the way, but I have done it, and to my amazement it all works.

The cranking handle worked like a dream, so thank you for the suggestion, I would never have been able to do the work with power feed, space was too tight, and the handle gave me loads of time without panicking. I made a mistake with the first thread and arranged the change gears wrongly, but a check with the thread gauge showed my error. I think that I also found an error with the Warco threading chart for 1.00 mm threading – the chart shows (for an Imperial 180) 71t driving 40t driving 33t driving 50t on the lead screw, but after double-checking this gave a very coarse thread of about 2.00 mm. Luckily I found an alternative combination on-line from an enthusiast that gave the correct thread. Any suggestions?

The biggest problem is going to be the fit of the thread on the flywheel. I was surprised at how much fiddling it took before the extractor would actually screw onto the threads that I was cutting, and the 1.25 thread that screws onto the flywheel can only be checked in situ when I need to next strip the engine down, so I will need to find a way of picking up on the thread in the chuck after trying the fit in the engine. I was hoping that the threads would fit without much problem, but I would need to be very lucky for that to happen.

Thank you to everyone who helped me with advice and suggestions, it was a steep learning curve and I am only just beginning, but it is very satisfying creating something.

 

 

Colin

[COLIN MARTIN 2]

On COLIN MARTIN 2 Said:

I think that I also found an error with the Warco threading chart for 1.00 mm threading – the chart shows (for an Imperial 180) 71t driving 40t driving 33t driving 50t on the lead screw, but after double-checking this gave a very coarse thread of about 2.00 mm. Luckily I found an alternative combination on-line from an enthusiast that gave the correct thread. Any suggestions?

I think you are getting your drivers and driven mixed up.

There is a fixed 40T driver on the spindle meshing with a driven 71T

The 71T then drives a 40T

Next to the 40 T is a 33T driver

On the leadscrew is a 70T Driven gear

So The maths is

40/71 x 71/40 x33/70

The first two actually cancel each other out as the 71T is just an idler so you just have 33/70 as the gear train ratio

33/70 = 0.471 turns of the lead screw per turn of the chuck.

Taking a 12tpi lead screw 1 turn of that is 1/12 x 25.4 = 2.117mm

So 2.117 x 0.471 = 0.997mm pitch which is about asgood as you will get to 1mm

[COLIN MARTIN 2Participant @colinmartin2]

Thank you,

I have no idea what I did wrong; I checked the set-up again after I found the error and it was right – the top two gears on the outside and the lower on the inside, but I was getting the wrong thread. I must have done something weird. I will have to look again and try to figure out what I did wrong.

 

Thanks,

 

Colin

[COLIN MARTIN 2]

On COLIN MARTIN 2 Said:

Hi All,

At last an update!

Many thanks Colin, not everyone bothers, leaving the rest of us in the dark forever.

The cranking handle worked like a dream, so thank you for the suggestion, I would never have been able to do the work with power feed, space was too tight, and the handle gave me loads of time without panicking.

Handles provide excellent control and I often used one on my mini-lathe.  The disadvantage is they are very slow.  If future project requires a lot of threading and time is short, be aware that a WM180 can be set up to safely cut threads in reverse.  The cutter starts at the headstock and moves away towards the tailstock.  Cuts can be taken at high speed, because – unlike a conventional right to left cut – there’s no risk of crashing into anything at the end.   The cutter overshoots safely and the operator has plenty of time to wake up and stop the lathe!

I made a mistake with the first thread and arranged the change gears wrongly, but a check with the thread gauge showed my error. I think that I also found an error with the Warco threading chart for 1.00 mm threading – the chart shows (for an Imperial 180) 71t driving 40t driving 33t driving 50t on the lead screw, but after double-checking this gave a very coarse thread of about 2.00 mm. Luckily I found an alternative combination on-line from an enthusiast that gave the correct thread. Any suggestions?

See Jason’s answer, who gets ten out ten for explaining the maths!  Again, if a lot of miscellaneous threading is to be done, it’s worth studying the maths (ratios), because the process becomes clear, making it easier to deal with non-standard threads, spot mistakes, and replace missing tables.

The biggest problem is going to be the fit of the thread on the flywheel. I was surprised at how much fiddling it took before the extractor would actually screw onto the threads that I was cutting, and the 1.25 thread that screws onto the flywheel can only be checked in situ when I need to next strip the engine down, so I will need to find a way of picking up on the thread in the chuck after trying the fit in the engine.

Yes, threads have to fitted with more than average accuracy, and it can be fiddly.   Colin has the worst case – he’s fitting a lathe turned thread to mate with one installed on a machine, so it’s necessary to take the new threaded part off the lathe to check it.   And always bad to demount work from a machine mid-job because it’s difficult to reset accurately.  In this case, I suggest#:

1. Always leave the half-nuts engaged – do not use the TDI even if it happens to work on this particular metric pitch.  Not disturbing the half-nuts ensures the spindle and lead-screw remain synchronised.
2. With the work gripped in the chuck, advance the cutter by hand turning the chuck until the cutter just touches the start of the thread.
3. Centre-pop or scratch a line on the outer face of the job, that’s aligned with the cutter.   Also mark the job and chuck jaws with a felt-tip, so that the job can be put back into the chuck in much the same position it came out.
4. Check the centre-pop/scratch and felt-tip alignments agree – close enough.

 

I was hoping that the threads would fit without much problem, but I would need to be very lucky for that to happen.

Thank you to everyone who helped me with advice and suggestions, it was a steep learning curve and I am only just beginning, but it is very satisfying creating something.

Well done you – this is a challenging beginner problem, straight in at the deep-end with the sharks!   There’s much to be said in favour of starting by learning to thread a rod to take a standard nut, say M12 x 1.75.   The standard nut is a portable gauge.  Doing that teaches most of the basics, including gotchas, in a relatively obvious way.  Move on to more difficult problems after the basics have been mastered.   Jumping in at the deep end has a high-failure rate, perhaps convincing a newcomer that it’s all too difficult, or that their tools are no good.   Not so, skills have to be developed: understand, plan, then practice, practice, practice.     My gut feel is that many starting out in the hobby seriously underestimate how much there is to learn, and how long it takes.

 

Colin

Excellent progress,

Cheers,

Dave

# In moderate discomfort this morning, so my brain is somewhat distracted.   If this how to reset advice is wrong, or I’ve missed a step, please explain where I went wrong!

[Howard LewisParticipant @howardlewis46836]

The Mandrel Handle is useful in that it allows threads to be cut upto a shoulder, or down a blind hole without fear of damage to either the tool or the workpiece.

It may be slower than under power, but at much reduced risk; and you have a feel for what is happening!

Glad that you have now got things sorted, as well as gaining some useful experience and a tool that can be used again for other jobs.

Howard

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