Threading thread

[saltytri]

I've studied up on the new lathe and figured out how to cut my first thread:

So, the next adventure is to cut matching threads in a tube. Obviously, I'm trying to learn the skills to do a light from scratch. My question is whether there is a rule of thumb on how to size the major and minor diameters of matching external/internal threads so that they go together properly.

Thanks!
David

 

[precisionworks]

Good looking, especially for the first time.

rule of thumb on how to size the major and minor diameters of matching external/internal threads

Machinery's Handbook, any edition including the ancient ones, has tables of correct dimensions for all the commonly used threads. You'll need either a thread mic or thread measuring wires to hit the minor diameter on outside threads.

If you're doing a one off light, cut the internal thread first. Then it's easy to fit the external thread to that one. Problem is, without cutting to standard dimensions, you will not be able to interchange that part to another light.

 

[gadget_lover]

There are several "types" of thread, and many sizes within those types.

Most threads you will cut will be Unified (UNC) or Metric (ISO). In both cases, the thread height for a particular TPI is the same no matter how big the screw or nut is. There are tables all over the web with that information.

The major -> minor relationship is determined by the choice of thread. You can start with either the major or minor and calculate from there. You pick a major diameter and a thread, then calculate the minor from that.

It's worthwhile to read up on the terminology of the thread profile that you will use. Crest, root, pitch thread height (vs thread depth)....

I have a thread calculator on my web site, but I've never heard of anyone using it regularly. In the last month it got 45 hits. It also calculates major / minor for you. That URL is http://www.tanj.com/cgi-bin/tpi.cgi

Daniel

 

[PhotonFanatic]

Download the threading software from Stellram's website and play with it, along with the Machinery Handbook that Barry suggested. Between the two, you will soon be an expert. :devil:

 

[saltytri]

Thanks, guys! I do have a copy of Machinery's Handbook and have used it to get an understanding of thread forms. With that and Daniel's website, I'm off and running. I'll be gone for the next week but will report in when my matching thread set is figured out and cut. :thinking:

The thread in the photo is a little crude. It wasn't done by measurement so much as by trial and error. I just turned the rod to the correct major diameter, .625", set the compound to 29.5 degrees and started making fine cuts with the compound until I could screw a hardware store nut onto it. I suppose real machinists don't do it this way!

 

[gadget_lover]

I suppose real machinists don't do it this way!

You'd be surprised how many people do it exactly that way. It works.... most of the time. The hidden gotcha (at least for me) was that the threads actually have flat (or nearly flat) crests and roots. When you keep threading it smaller and smaller it till it fits by test fitting you may actually have a sharp thread that does not match well. You may also have a thread that deforms as the crest of the screw mashes into the root of the nut, causing it to lock up badly.

Congrats on the new machine and hope you have fun with it.

Daniel

 

[saltytri]

Daniel, you're absolutely right. I did end up with pointed crests. But is it true that if the goal is to cut matching internal and external threads, this doesn't matter as long as they engage smoothly? As precisionworks pointed out, this means no interchangeability but maybe that's not so important for a one-off flashlight.

Don't get me wrong - I do intend to learn the right way to do it, though.

BTW, Happy Fourth to all! I've been a few other places and know for certain that we're awfully fortunate.

 

[gadget_lover]

You are right, in that it does not matter as much in a one-off light. But....

The sharp crests are fragile and liable to be damaged much easier, making them jam, crossthread, etc. They require a perfect match in order to be tight, so dirt and grit are more likely to be a concern.

I find that with the cut and try method it is much easier to overshoot the desired dimension. It's also easy to get that "perfect fit" where there is zero clearance. When that happens you frequently end up with the two pieces permanently fastened together. Permanently. If you are test fitting and hit any resistance at all, then stop and unscrew it.

Learning to cut threads to the standards has another benefit. It allows you to cut threads to match parts that you can not test fit.

Daniel

 

[65535]

I prefer flat acme threads. The seem to engage smoother because unlike V threads they have pressure points that are head on parallel to the axis of movement rather than at an angle around 60.

Plus I've never cut myself with acme threads, V threads have sliced me good.

 

[BoarHunter]

There are several "types" of thread, and many sizes within those types.

Most threads you will cut will be Unified (UNC) or Metric (ISO). In both cases, the thread height for a particular TPI is the same no matter how big the screw or nut is. There are tables all over the web with that information.

The major -> minor relationship is determined by the choice of thread. You can start with either the major or minor and calculate from there. You pick a major diameter and a thread, then calculate the minor from that.

It's worthwhile to read up on the terminology of the thread profile that you will use. Crest, root, pitch thread height (vs thread depth)....

I have a thread calculator on my web site, but I've never heard of anyone using it regularly. In the last month it got 45 hits. It also calculates major / minor for you. That URL is http://www.tanj.com/cgi-bin/tpi.cgi

Daniel

It is in inch ! When you work in metric with a metric lathe, you don't need all this complication ! :laughing:

 

[precisionworks]

When you work in metric with a metric lathe, you don't need all this complication ! :laughing:

I do miss the ability to cut threads used by 3/4 of all the people in the world, but my South Bend won't go there. I've seen a few made with a Metric lead screw, but have so little call for Metric threading that it isn't worthwhile to buy a second lathe. FWIW, I do some work on Jaguar automobile parts which are threaded ... standard USA tpi & pitch

As Daniel says, you really want to be able to cut threads to a print. If you ever do this for a profit, you will someday get a call, fax or email with a print attached. Then you have to be able to hit the thread spec dead nuts, without the crutch of having a mating part. The talented people here who make the killer Ti parts (like heads, body tubes & tail caps) to fit SureFire E-series have to cut to one standard dimension.

 

[KowShak]

I do some work on Jaguar automobile parts which are threaded ... standard USA tpi & pitch

Everything manufactured in the UK at one point would have imperial measurements, the Whitworth standard originated in the UK. The slow movement from imperial threads to metric happened at about the same time that the british car industry was losing it's independance, there was an interim period where a UK built car would have a mix of metric and imperial nuts and bolts, subsequently european and japanese companies started making cars in the UK and everything went metric. The current situation is that almost everything is metric and imperial spanners in their various incarnations (BSW/BSF/AF) are uncommon. The US car industry is in a similar situation now, so I think that at some point, the US will go metric too, the future will tell whether I'm right or not.

 

[BoarHunter]

Everything manufactured in the UK at point point would have imperial measurements, the Whitworth standard originated in the UK. The slow movement from imperial threads to metric happened at about the same time that the british car industry was losing it's independance, there was an interim period where a UK built car would have a mix of metric and imperial nuts and bolts, subsequently european and japanese companies started making cars in the UK and everything went metric. The current situation is that almost everything is metric and imperial spanners in their various incarnations (BSW/BSF/AF) are uncommon. I think that at some point, the US will go metric too....

The UK autombile industry died partly because of the mess and errors associated with the use of the inch. The UK inch was not even the same as the US, that lead to quite some issues during WW2. It is why they came with the Unified standard.
To this you must add the various company standards !

The US automobile industry has been saved once in the eighties by going metric, but now ?
Others have no choice if they want to export and survive, they must work in metric.
Just look at a catalog of supplies and tools, about two third of them are not required if you stick to ISO.

 

[BoarHunter]

I do miss the ability to cut threads used by 3/4 of all the people in the world, but my South Bend won't go there. I've seen a few made with a Metric lead screw, but have so little call for Metric threading that it isn't worthwhile to buy a second lathe. FWIW, I do some work on Jaguar automobile parts which are threaded ... standard USA tpi & pitch

As Daniel says, you really want to be able to cut threads to a print. If you ever do this for a profit, you will someday get a call, fax or email with a print attached. Then you have to be able to hit the thread spec dead nuts, without the crutch of having a mating part. The talented people here who make the killer Ti parts (like heads, body tubes & tail caps) to fit SureFire E-series have to cut to one standard dimension.

Normally you can get a gear to be able to cut metric screw with an inch leadscrew.

 

[gadget_lover]

The US has gone metric... sort of.

When I pull a screw out of something I often have to guess at the thread. Was it made in the US? Then I start with SAE. Was it import? Then I start with metric. Once I find a matching thread (tpi or pitch) I will then measure the major diameter and see if it's close to any of the standard sizes on my chart.

Occasionally there will be a screw that can be either imperial or metric based on the thread gages. Sometimes the screw is neither! It's quite possible to do a 1.63 mm pitch screw in the ISO profile, just as it's possible to make a 24.2 tpi thread in UNC profile.

In the 1970's I had a lot of british motorcycles. Occasionally I'd find that I needed Metric, Whitworth and SAE tools on the same bike. That was fun to work on.


BTW, BoarHunter, thanks for the heads up. I forgot that the maj/minor calcs on my web page are in inches.


Daniel

 

[gadget_lover]

Normally you can get a gear to be able to cut metric screw with an inch leadscrew.

And even if the pitch is not exactly right, it's close enough for doing short threaded pieces.

And example for the 7x10 lathe crowd: Add a 21 tooth gear to the standard set and you can almost cut a 1.0 mm pitch thread. There are 12 gear combinations that come out to 1.0001 mm but that's close enough for a fairly long thread engagement.

Other pitches are available too. You can almost cut a 1.25 mm pitch (1.2502)

I understand that a 127 tooth gear will give the larger lathes the ability to cut exact metric threads.
Daniel

 

[BoarHunter]

And even if the pitch is not exactly right, it's close enough for doing short threaded pieces.

And example for the 7x10 lathe crowd: Add a 21 tooth gear to the standard set and you can almost cut a 1.0 mm pitch thread. There are 12 gear combinations that come out to 1.0001 mm but that's close enough for a fairly long thread engagement.

Other pitches are available too. You can almost cut a 1.25 mm pitch (1.2502)

I understand that a 127 tooth gear will give the larger lathes the ability to cut exact metric threads.
Daniel

Indeed it is a 127 tooth gear. In fact it is also used to cut inch thread with a metric lead screw. This because the inch in fact is defined as 25.4 mm exactly since end of the 19 century in the US and 1950's for the UK.

The amusing thing is that some US companies had devised their own standard ! Guns mfgd and even Caterpillar for examples ! Although most of Caterpillar is now metric, they kept some of these screws for compatibiliy reasons with their older equipments. Same with John Deere.

 

[KC2IXE]

...snip...The amusing thing is that some US companies had devised their own standard ! ...snip....

Bicycles - many common threads are metric diameter, with inch thread pitches

 

[darkzero]

The US car industry is in a similar situation now, so I think that at some point, the US will go metric too, the future will tell whether I'm right or not.

I wouldn't be surprised. Years ago when working at the shop I remember lots of later GMs with a mix of metric & SAE fastners. I remember the lugs on all the later model GMs were metric. When I first got my own tools I never had the need to buy SAE tools for a while.

 

[saltytri]

As Barry suggested, I cut a thread to spec, not to fit. To make a 20 tpi Surefire 6P tail cap thread, the DOC comes out to .0379. Following advice to cut with the compound set at 29.5 degrees, it took .044 of compound dial movement to reach .0379 of depth perpendicular to the workpiece. It seems that a physical measurement of perpendicular DOC using a dial indicator is necessary to pre-determine how much movement of the compound dial is necessary to get the correct DOC. Am I on the right track? Is there a more convenient way to calculate the correct movement of the compound dial? It's all well and good to have a DRO on the cross slide but it doesn't do anything for me when cutting with the compound!