Thread dial indicator

[Steve355Participant @steve355]

Hi chaps

i’ve been messing around with learning to cut threads in the last few days. I’ve been getting on okay, but I’m really bored of waiting for the 1 to come around on the thread dial indicator.

I’ve been googling around Trying to find the formula to understand which number I can engage the half nut on for different TPI settings. I’ve been cutting 18 TPI, and it doesn’t seem to matter which number I start on, but then again I can’t get the nut on the thread, so despite the fact it looks okay, maybe I’ve got it wrong.

is there a resource that someone can point me to that would help? I believe my lathe has the same settings as a Myford.

cheers

Steve

[Steve355Participant @steve355]

[Martin ConnellyParticipant @martinconnelly55370]

Message me and I will send a copy of something I wrote about understanding the threading dial (pdf)

Martin C

This is the album with the jpgs of the pages but it is a bit hard to read as a jpg Album thread

Edited By Martin Connelly on 03/11/2021 10:35:56

[Tony Pratt 1Participant @tonypratt1]

IF you have checked the TPI really is 18 & the tool form is correct for the type of thread you are cutting then you will need to go in deeper until the nut goes on, are you using a single point tool or full form insert , even with the inserts sometimes you have to adjust the depth a bit.

Tony

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steve355 on 03/11/2021 10:07:00:

…

I’ve been googling around Trying to find the formula to understand which number I can engage the half nut on for different TPI settings. I’ve been cutting 18 TPI, and it doesn’t seem to matter which number I start on, but then again I can’t get the nut on the thread, so despite the fact it looks okay, maybe I’ve got it wrong.

is there a resource that someone can point me to that would help? I believe my lathe has the same settings as a Myford …

What sort of lathe is it Steve? Dangerous to assume anything, let alone a lathe is the same as another make. It might be, but best to confirm it.

The Thread Indicator shows when the spindle and lead-screw are in the same relative positions. It depends on the change-gear ratio, lead-screw pitch, and the gearing between lead-screw and indicator. As a result, which indicator numbers work vary with TPI. Some will only cut when the indicator = 1, no short-cuts. Easiest way is look them up in a table provided by the manufacturer, and it's often pasted by them to the lathe, or to ask another owner. Confirming yours is the same as a Myford will nail the answer. Otherwise they can be calculated from first principles, which I believe Martin's pdf covers.

Or, as is common practice on metric lathes, back-off the cutter and wind back without disengaging the half-nuts. Slighlty slower than a thread indicator, but it eliminates mistakes.

If the thread indicator is used incorrectly I'd expect the cut thread to be visibly mangled. If not, Tony's comments should help. A lathe-cut thread can be a bit rough straight off the machine and may need a little tidying up before it turns freely. Cone the start and remove any rough edges and loose swarf with a plastic pot scourer. If I have a die of the right size available I always use it to perfect lathe cut threads. The die removes any rubbish and more importantly ensures the thread diameter and depth are both correct: both are tricky to judge by eye. (Looks good, slightly too big…)

Dave

[Oily RagParticipant @oilyrag]

As others have said, a single point ground tool or a non 'full form' lay down insert (for example an insert marked 1.5p to 3p metric ) will leave the crests of the threads incorrectly formed. Running a die down will clean the thread up, or better still, a hand held thread chaser will allow the thread to be cleaned up whilst still in the lathe. Generally inserts for imperial threads will be full form as in 12tpi, 14tpi, etc,. But the single point ground tool will still have the same problem of unformed crests in both metric and imperial.

The great problem is screw cutting metric threads, which due to their definition of 'pitch per individual thread' over the imperial system of 'threads per unit measure', require the TDI to use a selection of gears driven off the leadscrew to correctly synch the spindle and leadscrew. This catches many an unwary soul out!

Martin

[not done it yetParticipant @notdoneityet]

One advantage of single point threading is that the thread will be in perfect alignment with the shaft on which it is cut – especially simple if both the shaft and thread are cut without removal of the piece from the chuck. Not so easy (particularly for the inexperienced) to guarantee if threaded with a die.

Threads are often roughed by single point and finished with a die. The die is not stressed (especially for larger, coarser threads), not worn as quickly, the manual effort is reduced and the complete thread-form should be attainable.

Edited By not done it yet on 03/11/2021 12:16:19

[larry phelan 1Participant @larryphelan1]

Might be worth while checking out Sparey, he explains this very well.

[Pete RimmerParticipant @peterimmer30576]

If you have a imperial leadscrew you can use and mark on the dial to cut even number TPI threads so 18TPI. In fact, if your leadscrew happens to be 6TPI you don't need a dial at all for 18TPI thred it's impossible to get it wrong.

A couple of notes if you are new to screwcutting:

If you have a sharp-pointed vee form tool you'll have to cut deeper than the numbers suggest. The tool should have a flat or radius on it depending on what pitch thread you are cutting so if it has a sharp point the thread will end up shallow if you cut to the depth quoted in the texts.

Some materials like to raise a burr on the OD when cutting a thread. Because you are cutting a vee thread you might not notice that the OD has grown slightly. Stop and measure the OD and if it has grown because of a raised burr run a flat file over it to bring it back down to the proper diameter.

Edited By Pete Rimmer on 03/11/2021 19:01:29

[Howard LewisParticipant @howardlewis46836]

On some lathes, I believe, the gear on the thread indicator dial needs to be changed according to the pitch of the thread being cut.

On other lathes, depending upon the pitch or tpi of the thread being cut, the half nuts can be engaged at other points on the dial, as well as the 1 where they were first engaged..

It all depends upon the machine being used to cut a particular thread..

If all else fails, consult the manual, if you have one!

The infallible way is to withdraw the tool, reverse the traverse, with the half nuts engaged, and reset the tool before taking out the backlash in the leadscrew.

This method is mandatory when cutting Metric threads on a lathe with an Imperial leadscrew, or Imperial threads with a Metric Leadscrew.

But, unless you are on piecework and just a hobbyist, patience is a virtue.

Howard

[duncan webster 1Participant @duncanwebster1]

Posted by Pete Rimmer on 03/11/2021 19:00:43:

If you have a sharp-pointed vee form tool you'll have to cut deeper than the numbers suggest. The tool should have a flat or radius on it depending on what pitch thread you are cutting so if it has a sharp point the thread will end up shallow if you cut to the depth quoted in the texts.

Edited By Pete Rimmer on 03/11/2021 19:01:29

In one of his books Geo Thomas tells you how wide a flat to pout on the end of a screwcutting tool to replicate te radius you should have (whit form at least, I think metric are flat bottom). Question is how do you accurately put a flat on to a width quoted in thous?

[Howard LewisParticipant @howardlewis46836]

Measure O A length and grind to reduce the dimension, based on calculation involving the thread angle?

Just a suggestion.

Howard

[egaParticipant @ega]

Posted by duncan webster on 03/11/2021 23:06:07:

Question is how do you accurately put a flat on to a width quoted in thous?

As I recall, GHT does cover this in his Workshop Manual. I agree it's not easy!

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