# Another threading thread!



## devils4ever (Jul 3, 2019)

So, I plan on cutting my first thread on my mini-lathe and I'm looking for some info. 

I'm making a captured nut puzzle with a 5/8"-11 thread and I'm trying to determine the depth of cut. Since the thread in the middle of the work piece, I can't keep trying a nut to see when the depth is correct.

I've read various websites on cutting threads and their depths, but the info seems confusing. I plan on moving the compound rest to 29.5 or 30 degrees.

Is the formula *0.75/TPI* correct for how far to move the compound rest in? For example, for my thread would it be 0.75/11 = 0.068"?


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## jbobb1 (Jul 3, 2019)

I was given a formula several years ago that was similar (.7426/11).  Never really used it, so I'd suggest making a test piece you can check. Keep in mind, any difference in O.D you're starting with will have an effect on the outcome.


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## T Bredehoft (Jul 3, 2019)

jbobb1 said:


> I'd suggest making a test piece you can check.



Yes, make a short piece of 5/8, chase your threads, check with a nut, record final depth reading on the compound dial, repeat on puzzle piece.


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## higgite (Jul 3, 2019)

When you get close to the correct thread depth, just unscrew the outboard half of the puzzle, leaving the other half in the chuck, and try the nut for fit. If not there yet, screw it back together and make another pass. Rinse and repeat until the nut fits.

29.5 degrees from perpendicular to the work piece is the correct angle for the compound. 60.5 degrees to the work piece. Have fun. That's a fun puzzle to make.

Tom


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## RJSakowski (Jul 3, 2019)

Use pitch wires to determine the pitch diameter.  The root diameter isn't all that critical as long as it is less than the specification.  Check on Wikipedia for thread geometry. https://en.wikipedia.org/wiki/Unified_Thread_Standard.
If you don't have pitch wires, you could easily make an impromptu set from some piano wire.


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## mikey (Jul 3, 2019)

The .750/tpi works. 

A more accurate formula is: *(Major Dia. – Minor Dia.)/2*. This second formula will give you the total depth of cut when feeding straight in with the cross slide and with the compound set at 29.5 degrees. However, most of us feed in with the compound lead screw and this formula will underestimate the actual depth needed when using your compound. To calculate your actual depth required when feeding in with the compound, multiply the results from above formula by *1.1433*.

For example, for your 5/8-11 thread, Dmaj = 0.625"; Dmin = 0.5135 for an 11 pitch thread. So, you have (0.625 - 0.5135)/2 = 0.05575" infeed with the cross slide. You have 0.05575 X 1.1433 = 0.0637" total depth of cut when using the compound feed.

Like all of these formulas, they are estimates that need to be confirmed either with a trial fit as you cut or with thread pitch wires or a thread micrometer. I usually start to check fits about 0.005 - 0.007" before I reach the estimated depth of cut for a class 2 thread. For a class 3 fit, I start checking about 0.010" before. For precision fits, go slow and remember to take a spring pass every pass or two to make sure you don't over shoot.


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## devils4ever (Jul 4, 2019)

Thanks for all the responses.

Tom: I'm reluctant to unscrew the outboard end fearing loss of registration.

mikey: Where does the 1.1433 number come from? 1/cos(29.5)?


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## mikey (Jul 4, 2019)

Yup, the reciprocal of cos 29 degrees. Its been a long time since high school Trig for me.

I hope you realize that formulas are formula; they are gross estimations and in no way precise. This is because there are SOOO many variables to consider when threading. Each thread has a root truncation (P/8), every tool differs in the accuracy to which it is ground or is worn, nobody ever gets their compound at exactly 29.5 degrees and on and on and on. Not to mention that there is a tolerance range for both major diameters and pitch diameters. This thread cutting business, if taken to extremes, can get really complicated.

Personally, when cutting threads I focus on just a few things:

I turn the blank to the Dmaj tolerance range for the class of thread I'm shooting for.
I use a sharp, accurately ground HSS tool most of the time and get it dead on center height and perpendicular to the work.
I calculate an estimated total depth of cut with those formulas but do not trust them to be precise. The precision comes from the operator, not the formula. I start checking pitch diameters early and often and cut until I get the pitch diameter in the range appropriate for that thread. Nothing fancy, nothing anal, just practical.
If you follow all the formulas and calculations for every thread you cut it will take you forever to cut a thread; look in Machinery's Handbook and see what I mean. I suggest you buy some thread pitch wires or a thread micrometer and use the values found in the chart I'm attaching. It will make life a whole lot easier for you.

Find the class of thread you want, then look for the Dmaj range to see the diameter for the blank. Then cut the threads until you measure a pitch diameter that is called out for that thread and you're done. Don't forget to create a thread relief whenever you can so you have a place to end the thread; this relief should be about 2-3 threads wide and about 0.003" - 0.005" deeper than the Dmin diameter. Also include a 30 degree chamfer at the end of the blank so your threads have a clean place to start.

None of this is difficult once you sort out how to do it.


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## MarkM (Jul 4, 2019)

If your feeding in straight with the crossslide it s .6134/tpi.  Having your compound set at 29.5 degrees and feeding with your crosslide has no bearing on the 29.5 degrees.  Your still plunge threading with the crosslide.  
Keep it simple .75/tpi for threading with the compound set on anything less than 30 to 29 degrees.
I tend to differ regarding formulas being correct and using them with a grain of salt.  Carbide threading tooling is very specific with dimensions and if you can grind a tool and are set up or know how to check your flats and such there really isn t much grey area unless you have other issues.
Good to learn both ways for those times you have no choice but to plunge.  Keep it simple!


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## higgite (Jul 4, 2019)

devils4ever said:


> Thanks for all the responses.
> 
> Tom: I'm reluctant to unscrew the outboard end fearing loss of registration.


It shouldn’t lose registration as long as you don’t disturb the the half that’s in the chuck. When you screw the two halves back together, the thread should line right back up. No different than when you have to unscrew the two halves of the finished bolt to put the nut on it. Just make sure the two halves are screwed snugly together before and during threading and don't disturb the half that's in the chuck until you have finished threading.

Tom


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## devils4ever (Jul 7, 2019)

Well, I tried cutting my thread and it was a disaster. The thread was really rough and ragged. It looked like it was torn more than cut. I even tried using another 1/2" piece cut at 13 TPI and I got the same results. Both pieces were 6061 Aluminum. Is that the issue? I have a piece of 12L14 steel I can try.


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## higgite (Jul 7, 2019)

devils4ever said:


> Well, I tried cutting my thread and it was a disaster. The thread was really rough and ragged. It looked like it was torn more than cut. I even tried using another 1/2" piece cut at 13 TPI and I got the same results. Both pieces were 6061 Aluminum. Is that the issue? I have a piece of 12L14 steel I can try.


Are you are advancing the tool with the compound? If so, is the compound angle set at 29-30 degrees from perpendicular to the work piece? Or another way to look at it is 60-61 degrees from parallel to the work piece. Can you post a pic of your setup? Aluminum shouldn't be an issue.

Tom


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## devils4ever (Jul 7, 2019)

Yes, I'm using the compound to advance the tool. I have it set to 29.5 degrees.


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## raven7usa (Jul 7, 2019)

From your picture, you are not at 29 1/2 degrees regardless what the setting tells you. Mine has to be set at 60 1/2 degrees.


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## dpb (Jul 7, 2019)

29-30° from perpendicular to the spindle, not 29-30° from parallel.


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## francist (Jul 7, 2019)

This is one of the best examples of the ever-recurring problem of setting the compound incorrectly, the photography is really perfect. 

Other than a bit more stick-out on the part than is absolutely necessary, your setup looks ok. Except you are actually cutting a 120 degree thread instead of a 60 degree thread. The angle of the compound is way too shallow -- it needs to swing another 30 degrees to get on where you need it. More like 11 and 5 on a clock face instead of 10 and 4. 

Are there graduations on the opposite side of the dial to allow you to set that properly? This happens soooo frequently with new folks setting up for the first time.

-frank


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## devils4ever (Jul 7, 2019)

Ah. Of course. I feel really dumb. The markings on the compound slide reads 0 when parallel to the lathe axis. It should read 90. There is no markings on the back side. I used my machinist's protractor to set it to the correct angle.

I did a test cut and it was much better. I don't think I cut it deep enough since a nut wouldn't go on. I thought it wasn't going on because the major diameter was raised from the thread cutting. So, I switched to a regular cut to smooth it out. Nut still didn't go on. I see a thread mic in my future!


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## francist (Jul 7, 2019)

No need to feel dumb, it's called learning for a reason. There's lots of things to be aware of and try to keep track of when trying something for the first time. Invariably one or two will get away! Next time you won't make that mistake (a different one, perhaps  )

As for the raised burr, I typically run a file over the tops rather than a full cut, but your choice. 

-frank


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## devils4ever (Jul 7, 2019)

I should have used a file like you mentioned. Then, I could have continued cutting the thread deeper. I guess the 0.75/TPI calculation is only a guideline. Learning.....


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## devils4ever (Jul 11, 2019)

Well, I've successfully cut my first thread! It is a 1/2"-13 external thread. I think I made every mistake in the book.

I've learned to not reverse the lathe direction to move the cross slide back into the starting position unless you retract the cross-slide (backlash bad!).

Also, check the feed direction! I tried and tried to get a nut onto the thread to no avail. I bought a set of thread gauge wires (a little tricky to use) to measure the pitch diameter and found my pitch diameter well within spec. I couldn't understand why the nut wouldn't go on. Finally, I realized that I cut a left hand thread! Argh!

In any case, I've learned a lot and thanks to all here.


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## jbobb1 (Jul 11, 2019)

It takes a little " trail and error" but it will come and you'll be an expert!
On a side note, you would have had better results if you used some cutting oil when threading aluminum. It looked like you had a little bit of material build up on your tool bit. Typical with some alum.


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## higgite (Jul 12, 2019)

devils4ever said:


> Also, check the feed direction! I tried and tried to get a nut onto the thread to no avail. I bought a set of thread gauge wires (a little tricky to use) to measure the pitch diameter and found my pitch diameter well within spec. I couldn't understand why the nut wouldn't go on. Finally, I realized that I cut a left hand thread! Argh!


Ha! Didn't notice that in the pic. 

Glad you're getting the hang of it, though.

Tom


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## devils4ever (Jul 18, 2019)

Okay. Now, that I've "mastered" (yeah right!) external threads, I've been thinking about doing an internal thread. 

So, I have a few questions:
1. Does a boring bar give better results than just using a drill bit in the tail stock? Is the boring bar truer? More precise?
2. Do I place the compound at 29.5 degrees as with an external thread, but place it a about 7 o'clock instead?
3. Can I use an internal threading tool as a boring bar? I see no difference except for a deeper reach with the boring bar.

Thanks!


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## jbobb1 (Jul 18, 2019)

1.) Using a boring bar will result in a better outcome, but you could get by using a drill bit. A boring bar is far more precise.
2.) You do set the compound at 29.5 deg. but which direction depends on if you're cutting a left hand or right hand thread.
3.) A boring bar can be used for threading or boring with the correct tooling. A threading tool is just that.


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## devils4ever (Jul 18, 2019)

jbobb1 said:


> 1.) Using a boring bar will result in a better outcome, but you could get by using a drill bit. A boring bar is far more precise.
> 2.) You do set the compound at 29.5 deg. but which direction depends on if you're cutting a left hand or right hand thread.
> 3.) A boring bar can be used for threading or boring with the correct tooling. A threading tool is just that.



2. I'm cutting a right hand thread, so am I correct in setting the compound at 7:00 position?


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## francist (Jul 18, 2019)

Yes. Cutting a right hand thread external the compound typically sits at 11:00-5:00 give or take. Right hand thread internal, swing it across to 1:00-7:00. Your're still cutting the trailing slope of the thread form, just on the inside of the hole.

-frank


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## jbobb1 (Jul 18, 2019)

devils4ever said:


> 2. I'm cutting a right hand thread, so am I correct in setting the compound at 7:00 position?


Correct


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## devils4ever (Jul 18, 2019)

I guess the dial would be _backwards _since I need to retract the tool to cut deeper?


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## jbobb1 (Jul 18, 2019)

devils4ever said:


> I guess the dial would be _backwards _since I need to retract the tool to cut deeper?


Yes


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## jbobb1 (Jul 18, 2019)

Is your internal thread going to be a "through" hole or a "blind" hole?


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## devils4ever (Jul 18, 2019)

Through


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## mikey (Jul 18, 2019)

devils4ever said:


> Okay. Now, that I've "mastered" (yeah right!) external threads, I've been thinking about doing an internal thread.
> 
> So, I have a few questions:
> 1. Does a boring bar give better results than just using a drill bit in the tail stock? Is the boring bar truer? More precise?
> ...



To add on to the other replies:
1. A drill does not create a straight hole, nor is it round or precisely sized. You can bring it to the right size with a reamer but it will very likely not be straight. You will always be better off boring it straight, true and of the size you need.
2. I find it is not necessary to use the compound for internal threading. It is much simpler to set the compound to 90 degrees and cut the thread with the cross slide feed.
3. Some boring bars accept internal threading inserts as well as boring or grooving inserts; the vast majority of bars do not. The bar must be specifically designed for these multiple functions and they tend to be expensive. You can check with your bar maker to see if this ability is an option.

Internal threading is not as easy as external threading because unlike an external threading tool that can be kept short and rigid, an internal threading bar must be extended to accommodate the depth of the hole. Therefore, deflection is going to occur. Most internal threading bars are made of steel and have an extension capability of 4 times the OD of the bar. Keep this in mind; if you must go deeper, switch to carbide shanked threading tools.

Since we typically take light cuts, radial cutting forces tend to be higher and this will add to the deflection caused by the bar extension. Since we cannot use thread wires or a thread pitch mic, most of us will either use a manufactured or shop made go/no-go gauge for the tolerance class we need to gauge progress as the internal thread is cut. If you're just messing around or just need a class 1-2 fit then a hardware store bolt will work as a gauge. It is when you need a precise fit or when you must fit your part to an already made part that things get finicky and an accurate go/no-go gauge is needed.


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## Illinoyance (Jul 24, 2019)

I can tell by the photo your compound is set at the wrong angle.  You want to set it to 59*.  The idea is to have the compound set near parallel with the trailing flank of the tool.  For right hand threads most of the cutting is on the left flank of the tool.  The right flank just gets a skim cut.  That is why the compound is set slightly off parallel with the angle of the thread.  My South Bend compound is at 0* when it is perpendicular to the bed.  On my Nardini it reads 0* when the compound is parallel with the bed.  Unfortunately most instructions for threading fail to recognize that there are two different ways of marking the degrees.


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## Illinoyance (Jul 24, 2019)

Should have read all the responses before posting.  Others have covered the issue very well.


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