Looking for thread cutting pointers.

[jhanko]

I'm still learning on the lathe and have a question. UPS dropped off some tools yesterday. I have been practicing internal threading on some scrap aluminum. At first I made a real noob mistake. The finish looked like 60 grit sandpaper. Turned out I had 2 of the gears switched and was cutting at >100 TPI! After a few tries with the gears correct, I ended up with a pretty good thread. Not perfect, but definitely usable. I cut the threads with a 0 degree feed. I haven't tried 29.5 degree yet. Do you think it will work better? How do you guys (or gals) cut your threads? Thanks,

Jeff

 

[gadget_lover]

The 29.5 degree method may work better, but one of it's biggest advantages is that it puts the load on only one side of the cutter instead of both sides. This can reduce chatter and give a better finish. I heard it might make the tool last longer, though I'm not sure why.

The change gears are a hassle, but they add an amazing amount of versatility to the machine. I'd love a QC gear box, and have even considered building one. How's that for "over engineering" ad 7x12?

Your threads look pretty good. Practice WILL make perfect.

Daniel

 

[Mirage_Man]

I use the 29.5 deg. method for all my threading, inside and outside. On aluminum it may not make as much of a difference but in harder materials it does. I say if you learn to do it 29.5 then you'll be set no matter what material you thread. You use the compound for the majority of the depth of cutting then on the last pass you feed in a thou or so on the cross slide to clean up both sides of the V. A machinist buddy of mine taught me this way and it has always produced terrific threads.

 

[wquiles]

I have been cutting threads like Mac taught me - straight, with no 29.5 deg. Works fantastic, at least on Aluminum - I have not tried on steel or Titanium yet

Will

 

[Mirage_Man]

Here are a few good articles.

http://www.metalartspress.com/PDFs/60_degree_threads.pdf

http://www.frappausa.com/PDF/Articles/Machining/Engine Lathe/150235.pdf

http://www.mmsonline.com/articles/010302.html

http://findarticles.com/p/articles/mi_m3101/is_n8_v63/ai_9339164

 

[Anglepoise]

I am a 'Modified Flank Infeed' guy and feel I get a better finish.
I use 29.0° and there is less strain on the cutting edge.

 

[ICUDoc]

Your thread looks fabulous.
I use the 29.5 - it is easy to do and gives a better finish ( with the last past cutting both sides). I have done some 0 degree threading in Al with no probs though.

 

[jhanko]

Thanks for all the suggestions. I have been playing around with different feed angles, but honestly can't see any difference in the result. Then again, I have only tried on aluminum. I think I'm going to stick with the 29.5 degree feed. Almost all of the tutorials I read recommend this way. The one major thing I have discovered is that low speed seems to be causing the slightly rough finish. I was practicing cutting external threads at 100 RPM and was getting the same result as the photo above. I bumped the machine up to 800 RPM and ended up with absolutely beautiful threads with a mirror-like finish. Unfortunately, without a CNC, that speed is impossible when threading internally, especially in a blind hole.

Jeff

 

[Mirage_Man]

I bumped the machine up to 800 RPM and ended up with absolutely beautiful threads with a mirror-like finish. Unfortunately, without a CNC, that speed is impossible when threading internally, especially in a blind hole.

Jeff

Well, I haven't tried it yet myself (might when the new lathe arrives) but for internal threads theoretically you could run the spindle in reverse with the tool on the back side and feed away from the chuck. This of course would require a different threading tool with the insert on the right side of the holder. Set up would also require a little more attention I think.

What depth of cut are you taking on each pass? I can take as many as 5-6 passes (even more to fine tune it) to do a 20tpi thread. If you're going too deep you will tear the threads.

 

[jhanko]

What depth of cut are you taking on each pass? I can take as many as 5-6 passes (even more to fine tune it) to do a 20tpi thread. If you're going too deep you will tear the threads.

I've been cutting 24 TPI and using this as a guide:

 

[gadget_lover]

That's a great chart. I think that it shows the proper infeeds for a UNR full profile bit with the compound set at 29.5 degrees. Is that correct? [edit Nope: just proper depth" ]

I have GOT to get some proper threading inserts.

Daniel

 

[Mirage_Man]

That's a great chart. I think that it shows the proper infeeds for a UNR full profile bit with the compound set at 29.5 degrees. Is that correct?

I have GOT to get some proper threading inserts.

Daniel

I wonder. For instance on an internal thread using my Thinbit partial profile set up I always end up with a slightly deeper reading on the compound than what is stated in that chart. But I guess it depends on the minor diameter I bore to first? I have never used a full profile insert so I have no idea. I use the same inserts for every thread I cut.

 

[VanIsleDSM]

Well, I haven't tried it yet myself (might when the new lathe arrives) but for internal threads theoretically you could run the spindle in reverse with the tool on the back side and feed away from the chuck. This of course would require a different threading tool with the insert on the right side of the holder. Set up would also require a little more attention I think.

What depth of cut are you taking on each pass? I can take as many as 5-6 passes (even more to fine tune it) to do a 20tpi thread. If you're going too deep you will tear the threads.

I'd think it would be preferable for more than just not having to worry about threading into a hole you can't see. Running the spindle in reverse working on the inside of a piece you would be pushing the force away from you as you usually would when working on the outside. I haven't actually done any threading yet myself, but I imagine this would be very helpful when doing some internal threads on a longer piece that was supported by a steady rest, since you'd be pushing the work into the rest and not away from it. Or maybe you can just move it? I dunno.. haven't got one of those yet

 

[gadget_lover]

I wonder. For instance on an internal thread using my Thinbit partial profile set up I always end up with a slightly deeper reading on the compound than what is stated in that chart. But I guess it depends on the minor diameter I bore to first? I have never used a full profile insert so I have no idea. I use the same inserts for every thread I cut.

The difference between a partial profile thread and the full profile is at the tip and the shoulders of the bit.

A partial profile has a sharper tip, where the full profile has a flat one. The sharp tip lets the partial profile approximate the shape of many different threads. If you look at the picture on my web page at http://www.tanj.com/cgi-bin/tpi.cgi , you will see light green triangles at the bottom of the threads. These are left in place by the full profile, but are removed by the partial profile. Since the full profile has a flat tip to match the bottom of the thread, it does not go in as far.

The partial profile has a simple 60 degree shape to it. The full profile includes a shoulder that will carve off the crest of the thread in it's final pass, leaving it nice and flat like the pictures.

With a partial profile bit, it's easy to end up with razor sharp threads if you over shoot your depth. With a full profile the threads will look pretty even if you cut too deep.

The depths generated by my web page, by the way, are for partial profile bits.

Daniel

 

[Torque1st]

I use 29.0° to avoid any errors that could produce a bad finish on one side of the thread.

Make sure your insert is aligned with the work properly.

Cut the thread crest diameter a few thousandths under max on external threads. The amount will vary based on the number of threads per inch. The same technique can be used on internal threads.

Flatten the end of a thread insert to form the flat at the root of the thread if the insert does not have one.

 

[VanIsleDSM]

That's a great chart. I think that it shows the proper infeeds for a UNR full profile bit with the compound set at 29.5 degrees. Is that correct?

I have GOT to get some proper threading inserts.

Daniel

I'm not sure exactly what's going on with that chart, I tried using it my first time threading and I had to cut much deeper using the 29.0 method. Even straight in those numbers don't match up to this other table that I found:

http://www.walteranderson.us/hobbies/metalworking/microlathe/frog/threading.html

 

[gadget_lover]

I'm not sure exactly what's going on with that chart, I tried using it my first time threading and I had to cut much deeper using the 29.0 method. Even straight in those numbers don't match up to this other table that I found:

http://www.walteranderson.us/hobbies/metalworking/microlathe/frog/threading.html

OK, there's alway some confusion...

EDIT: I have no idea why I get so confused about the stupid infeed when set to 29.5 degrees. It has something to do with sine and cosines. Someone will clear it up.

Now the fun part. Which is the correct 29.5? You can set it 29.5 from the compound slide movement or 29.5 from the axis of the piece rotation.

What you want is for the compound slide to move parallel to the right side edge of your tool. That's at an angle that is 29.5 degrees from the axis of rotation. The tool should be mounted 90 degrees from the axis of rotation. On my 7x12 lathe, the built in protractor indicates that it's at 60.5 degrees (measured from parallel to the compound slide movement) when it's properly set up.

Now part two. The chart above is for tools with a specifically blunted tip. If you are using tools with a sharp 60 degree tip, you need to go deeper. The extra depth will cut out material normally left at the bottom of the thread. In the chart from my web site the green triangles are NOT cut out by a full profile threading tool, but they are when using a partial profile or hand ground 60 degree tool with a sharp tip,

The white triangles are there simply to show what the full height of the thread would be if it were not flattened.

So you probably need to use the chart you found, as it is for sharp pointed tools. If you set your compound to 29.5, then double the travel of the compound to get the same depth.

Or at least that's my understanding

Daniel

 

[VanIsleDSM]

I don't think it's twice as much.

For a 24TPI thread the depth on the chart I posted is 0.0316" The chart says for a compound slide set to 30 to advance 0.0365" (it actually says compound at the top and cross-slide for the table, cross-slide must be a typo, should say compound again)

Checking that with the calc seems to work out,

Cos30=0.866, inverse is 1.156 x 0.0316 = 0.0365"

Just happened to notice while calculating that the 0.028" figure on the other graph posted is 0.866 x 0.0316.. like you are magically traveling ~16% further when you're on a 30 degree angle? Seems like they screwed up their math? ..forgot to inverse the Cos30, because you have to travel more on an angle.. not less.

I dunno.. hopefully someone can clear this all up. Need to figure out proper thread depths so I can figure out exactly how big to make the pieces before threading.

 

[gadget_lover]

I don't think it's twice as much.

Yep. You are right. I corrected my post. That will teach me to repeat what I read without verifying my understanding.

I dunno.. hopefully someone can clear this all up. Need to figure out proper thread depths so I can figure out exactly how big to make the pieces before threading.

If you are cutting UNR, the thread depth for internal threads will be shallower than the external threads. Use the formulas from the web page and look closely at the drawings to see where the major and minor diameters are in relation to each other.

In essence, the crest of the nut and the root of the screw will be the minor diameter. The crest of the screw and the root of the nut will be the major diameter.

The external thread on a flashlight body with 1 inch diameter with a 28 tpi will work as follows:

The major diameter is 1.0. This is what you turn the body down to.
External thread depth 0.0271 inch (V * .875) (for a screw)

The minor diameter is .96137 This is what you bore the tailcap to.
Internal thread depth 0.0232 inch (V * .750) (for a nut)

The pitch is: 0.0357 inch per thread
The thread height is: 0.0309 inch (UN and UNR)

This can all be derived from the formulas on the walteranderson web page. Even though the math is simple, I cheat and use my web page.

A good way to practice is to measure a bolt from the store, and create a nut based on the TPI and major diameter. If you understand the process you will be able to make a good fit on the first try.

Daniel

 

[wquiles]

OK, there's alway some confusion...

EDIT: I have no idea why I get so confused about the stupid infeed when set to 29.5 degrees. It has something to do with sine and cosines. Someone will clear it up.

Now the fun part. Which is the correct 29.5? You can set it 29.5 from the compound slide movement or 29.5 from the axis of the piece rotation.

What you want is for the compound slide to move parallel to the right side edge of your tool. That's at an angle that is 29.5 degrees from the axis of rotation. The tool should be mounted 90 degrees from the axis of rotation. On my 7x12 lathe, the built in protractor indicates that it's at 60.5 degrees (measured from parallel to the compound slide movement) when it's properly set up.

Now part two. The chart above is for tools with a specifically blunted tip. If you are using tools with a sharp 60 degree tip, you need to go deeper. The extra depth will cut out material normally left at the bottom of the thread. In the chart from my web site the green triangles are NOT cut out by a full profile threading tool, but they are when using a partial profile or hand ground 60 degree tool with a sharp tip,

The white triangles are there simply to show what the full height of the thread would be if it were not flattened.

So you probably need to use the chart you found, as it is for sharp pointed tools. If you set your compound to 29.5, then double the travel of the compound to get the same depth.

Or at least that's my understanding

Daniel

Daniel,

If cutting external threads with a sharp/pointy 60 deg cutting bit, the white tips on your drawing will be present since the 60 deb cutting bit does not touch them. Now, if you are inserting this "tube" with these external thread into another piece that was cut with a full profile bit (where the green portion is there) - wouldn't this prevent full/ideal thread contact between the two pieces?

Wouldn't it be better then when using a sharp 60 deg cutter for external threads to "somehow" cut those white tips so that this piece will work regardless on how the other internal threads were cut?