# Looking for thread cutting pointers.



## jhanko (Jun 28, 2008)

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


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## gadget_lover (Jun 28, 2008)

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


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## Mirage_Man (Jun 28, 2008)

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.


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## wquiles (Jun 28, 2008)

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


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## Mirage_Man (Jun 28, 2008)

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


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## Anglepoise (Jun 28, 2008)

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.


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## ICUDoc (Jun 29, 2008)

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.


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## jhanko (Jul 1, 2008)

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


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## Mirage_Man (Jul 1, 2008)

JHanko said:


> 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.


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## jhanko (Jul 1, 2008)

Mirage_Man said:


> 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:


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## gadget_lover (Jul 2, 2008)

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


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## Mirage_Man (Jul 2, 2008)

gadget_lover said:


> 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.


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## VanIsleDSM (Jul 2, 2008)

Mirage_Man said:


> 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


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## gadget_lover (Jul 2, 2008)

Mirage_Man said:


> 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


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## Torque1st (Jul 2, 2008)

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.


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## VanIsleDSM (Jul 10, 2008)

gadget_lover said:


> 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


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## gadget_lover (Jul 10, 2008)

VanIsleDSM said:


> 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


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## VanIsleDSM (Jul 10, 2008)

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.


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## gadget_lover (Jul 11, 2008)

VanIsleDSM said:


> 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.



VanIsleDSM said:


> 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


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## wquiles (Jul 11, 2008)

gadget_lover said:


> 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.
> 
> ...



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?


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## gadget_lover (Jul 11, 2008)

When you turn the outside of the screw, you turn it to the major diameter. The thread depth is such that you do not have adjoining threads intersect each other. You are only going to cut in to 7/8ths of the height of the thread. That leaves a flat crest. 

A similar situation happens with female threads. You bore the opening to the minor diameter. This defines the crest of your threads. Then make your threads 6/8ths of the thread height.



This allows a thread cut with a full profile to inter-work with one cut with a partial profile. In both cases the critical dimensions are the same. The thread contact is the same, since the crest of the thread is not supposed to touch the root of the other side anyway. 

The drawback of the sharp V tool is that the threads cut a bit deeper, and so will weaken the material more than a proper UNR thread. I've also heard that the sharp root is a stress point and more likely to fracture. 

Daniel


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## wquiles (Jul 11, 2008)

gadget_lover said:


> When you turn the outside of the screw, you turn it to the major diameter.








But in your diagram, the major diameter does not take into account the white tips of the thread. So if using a 60deg cutter, wouldn't this be a problem since those peaks will be there and they are higher than the major diameter? Is the diagram therefore only valid with a full profile cutter?





gadget_lover said:


> The thread depth is such that you do not have adjoining threads intersect each other. You are only going to cut in to 7/8ths of the height of the thread. That leaves a flat crest.


 There is no flat crest with a pointy 60deg tool. Unless you are talking about a full profile cutter - I am confused


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## gadget_lover (Jul 12, 2008)

The confusion coms from your expectation that you will cut deep enough that the gooves will touch. They do not. You start with the piece at the major diameter. That is the height of the crests.

If you only run the bit in for 7/8 of the thread height as we do with the external thread, the flat space beteen grooves will be 1/8 the pitch (or something like that).

As an alternative, you could turn the piece for an exernal thread to the major diameter + 1/8 theheight of a thread, then cut your thread the full depth of 1 thread height. After you are finished you will have a sharp crest, so you turn down the piece to the major diameter. 

Does that make more sense? I need to find a better way to describe it on my web site.

Daniel


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## VanIsleDSM (Jul 12, 2008)

This is why I just got a 3D program and started modeling stuff.. for all of these little measurements. You obviously want your major diameter of the male piece to be slightly smaller.. and than in itself will stop the thread crests from touching.

I haven't bothered with cutting the internal threads to less of a depth than external and such.. it's just too finicky for me. Maybe after some more experience.. and I can actually see a need for it. For now I'm just going to keep doing what I'm doing. Cutting the male piece just a slight bit smaller, and then cutting threads on both pieces the same, basically just before they form a full V. Doing this everything threads together perfectly and I see no reason to get into all the finicky stuff for now.


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## gadget_lover (Jul 12, 2008)

There are two reasons to 'get finicky'. 

First, when using the cut and try and cut again method, it's really easy to get the parts permanently screwed together. No kidding. If the crest of one thread is almost right you can get it to distort as you screw them together, filling the valley of the other thread and galling. This is, effectively, a pressure weld.

Second, when using the finicky method, you will have parts that are interchangeable. You will even be learning to make parts that work with store bought parts.

The Major and Minor diameters are the height of the flat crests on a UNR thread, so it's not that hard to do.

Dan


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## wquiles (Jul 13, 2008)

Mirage_Man said:


> Here are a few good articles.
> 
> http://www.metalartspress.com/PDFs/60_degree_threads.pdf
> 
> ...



The first 3 of these are excellent, although the first two are even nicer, more detailed. Thanks for sharing these Brian


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## VanIsleDSM (Jul 14, 2008)

I'm still confused.

There seems to be a good amount of conflicting information.

First we have the chart in this thread claiming that 24TPI has an external thread depth of 0.0328

Then I found the other table linked that says 24TPI external is 0.0316, and that you have to advance the cross slide set at 30 degrees by 0.0365 to achieve 0.0316 straight in.

Then reading the second document in the post above, page 11, I see that there's a formula for the thread depth:

Depth = 1/TPI x 0.866

So for 24 TPI you get 0.0365. And if you read further you see they are not talking about cutting 0.0365 on a 30 degree angle, they mean cut that far straight in.

So now I have 3 different tables telling me 3 different things.

..Meanwhile yesterday I had no problem just taking passes until the threads fit.. and they work very well. None of anything I make will ever work with something store bought.. as I don't conform to any certain diameter, I'll put threads on whatever works best.. not 7/8 or some other pre-set form.

That said.. I would still like to be able to cut threads properly and mathematically so they would fit the first time.. but it just doesn't seem possible! Maybe with full profile bits.. I dunno, there just seems to be no solid source of information to make 2 things that will thread together will all of the proper dimensions and formulas.

I think I need to run through an example with some help and then try it to see if it actually works. But I'll start to figure that out tomorrow.. getting too late for math.


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## Anglepoise (Jul 14, 2008)

VanIsleDSM
I am not going to try and answer your post as detailed explanations are not my forte and other members will do a better job.

However I will offer up a tip that has helped me over the years.
I do all my threading using the modified flank infeed. This is Sandvik's fancy way of describing when the compound is set over to 29°.
Now when it is set over , I want an easy way to know exactly how much movement I have into the thread ( 90° to ways ).

I fabricated a simple piece of flat aluminum mounted on the compound and set parallel to the ways. A dial indicator, with magnetic mount, sits on the cross slide and is touching the aluminum bracket.

I zero this out and every time I move the compound in or out ( sliding at 29°), the dial indicator gives me a true depth of cut reading.


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## VanIsleDSM (Jul 14, 2008)

That sounds like a nice addition. I need to get a mill soon so I can start doing things like that.

I think I'm getting it now, I was looking at part of one of the diagrams backward and thought it was for full profile tips with a flat crest. I'm working on a 3D model right now with full formed threads with all the measurements.. see if I get it all right.


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## wquiles (Jul 14, 2008)

Anglepoise said:


> I fabricated a simple piece of flat aluminum mounted on the compound and set parallel to the ways. A dial indicator, with magnetic mount, sits on the cross slide and is touching the aluminum bracket.
> 
> I zero this out and every time I move the compound in or out ( sliding at 29°), the dial indicator gives me a true depth of cut reading.



David - sounds like a nice setup. Do you a picture of this?


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## VanIsleDSM (Jul 14, 2008)

Alright. I didn't just make some pointy threads like my last model, this time I dialed in everything. In one of the tutorials I read to make the actual size of the piece for external threads 0.002" smaller, I imagine that applies to the internal threads aswell, bore it out 0.002" larger, so there's some clearance between the crests and the roots.






And here's the full size, not complete yet. Those are 10440 cells, I already have a really nice efficient buck driver designed to use 6xLi-Ion to power 4 CREE in series. With a 30mm OD this little light will be able to put out ~1000lumen for roughly 45min with the new CREE MC-E overdriven at 1amp per die. Output will be adjustable though. Still trying to figure out the best way to put in some undervoltage protection so I don't drain the 10440s below safe levels, since you can't get protected 10440s.


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## Anglepoise (Jul 14, 2008)

wquiles said:


> David - sounds like a nice setup. Do you a picture of this?





The bracket is attached when needed with a hand tightened wing nut not in picture.


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## wquiles (Jul 15, 2008)

Anglepoise said:


> The bracket is attached when needed with a hand tightened wing nut not in picture.
> (snip pic)


Thanks much. I like how you implemented this - very clever :thumbsup:


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## Mirage_Man (Jul 16, 2008)

Anglepoise said:


> The bracket is attached when needed with a hand tightened wing nut not in picture.



That's really nice to have those t-slots on your cross slide I bet?


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## Torque1st (Jul 16, 2008)

For all threading measurements and diagrams get a Machinery's Handbook available at any good full line bookstore. It also has a ton of information that will help with many fabrication or machining problems.

Amazon


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## wquiles (Jul 16, 2008)

Torque1st said:


> For all threading measurements and diagrams get a Machinery's Handbook available at any good full line bookstore. It also has a ton of information that will help with many fabrication or machining problems.
> 
> Amazon



Thank you - just ordered it. Lots to learn - might as well have a good reference 

Will


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## Anglepoise (Jul 16, 2008)

Mirage_Man said:


> That's really nice to have those t-slots on your cross slide I bet?



Well in theory they look like they might be used allot, but in practice this is just not the case. If I need anything bolted to the cross slide ( milling attachment for example ) , I remove the compound and attach at that point. 

However they are great for sliding on adapters for a magnetic dial indicator etc.


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## Torque1st (Jul 16, 2008)

wquiles said:


> Thank you - just ordered it. Lots to learn - might as well have a good reference
> 
> Will


Enjoy! :thumbsup: 
It is the best reference I know of for a machinist. The big Kennedy machinist chests have a center drawer sized just to fit it. Every machinist I know has one for reference. It was on the required tool list back when I started.


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