Thread depth

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Hukshawn

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How do you calculate the compound depth for a given pitch? Trying to figure out the calculation is kicking my ass.
 
The pitch diameter is the more critical feature. You'll need a set of thread wires and a little patience learning to deal with them, but your threads will turn out more accurately than simply relying on the thread height (or depth as some people call it). Thread height it 1/2 (major actual - minor actual) There are instruments made for this measurement. But as long as your tool radius is correct, hitting the PD is more critical.
 
I use Daryl Holland's method most of the time: 0.75 / TPI. This very closely approximates the required depth of cut using a compound at 29.5. For example, 1/2-13 thread DOC = 0.75 / 13 = 0.0577. I start checking fits early so I don't over-cut.

If using the (Dmaj - Dmin) / 2 method, you get the depth of cut for a straight feed using the cross feed dial. If you multiply the above by 1.1433, you get the depth of cut when using the compound. For the same 1/2-13 thread, (0.500 - 0.4056) / 2 = 0.0472. 0.0472 X 1.1433 = 0.0540".

I find the latter to be a bit more accurate but as Tony says, use the 3-wire method when fit is important. This especially applies to cutting a male thread that must fit into a part that is already threaded with a standard female thread.
 
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I might add that if you have a few sizes that you find yourself cutting fairly often, it would probably be worth it to make a set of soft gages. Then, pretty much you cut your desired major, establish the proper tool radius, and cut until the gage fits.

It gets a little involved when making ring gages, because they are smaller on the PD than the mating plug gage. I think we've touched on this before on the Forum, and maybe an in depth discussion could be started. But basically you have to make two plug gages. One to use as a "set plug'", which has the proper PD for the ring gage, and then the working plug gage, with the high and low limits per MH. In the end, I guess you end up making 4 set plugs (explained if we do a detailed thread) and 2 working plugs, and 2 working rings to have a set. If you're really careful, you can make only 2 set plugs, but one end will actually have 2 sections with different PD's.

This is all part of why hobby machinists cut to fit nuts and mating parts. But for true interchangeability, and known good, compliant threads, you have to go to a little more trouble.
 
Wow... much more complicated than I realized...

My wife is a bit of a university nut and loves math. I was trying to look up formulas but was having a lot of trouble. She kept me up until past midnight cause I couldn't source an answer and she was getting frustrated cause the math wasn't working. Lol.
I found a few handy charts, but the differences between US national threads and unified (etc) has me baffled....
 
I recall doing the 29.5 deg. calculation once or twice and having to go back over the thread with a triangular file to make it work. Typically I now run the threading tool until the thread is starting to look like a "V", then start checking with the mating part if possible. Proof in the pudding is that's where it's going to end up, so it's guaranteed to fit. If it's a standard thread, use a hardware store nut/bolt to check your thread.

The more expensive solution is use a pitch thread micrometer like those pictured below (~$60). They work great and as Tony mentioned above, dials you in dead nuts to the pitch diameter. Not that it's a "best practice", but if I'm cutting multiple threads I zero out the compound after the last finish cut. That way I'm creeping up on "0.000" on the compound instead of "0.047" or something like that.

Bruce

photo 13.jpg photo 15.jpg
 
BGHansen said:
I recall doing the 29.5 deg. calculation once or twice and having to go back over the thread with a triangular file to make it work. Typically I now run the threading tool until the thread is starting to look like a "V", then start checking with the mating part if possible. Proof in the pudding is that's where it's going to end up, so it's guaranteed to fit. If it's a standard thread, use a hardware store nut/bolt to check your thread.

The more expensive solution is use a pitch thread micrometer like those pictured below (~$60). They work great and as Tony mentioned above, dials you in dead nuts to the pitch diameter. Not that it's a "best practice", but if I'm cutting multiple threads I zero out the compound after the last finish cut. That way I'm creeping up on "0.000" on the compound instead of "0.047" or something like that.

Bruce

View attachment 143819 View attachment 143820
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I've seen those and considered them. Seems easier than the wires, perhaps? The wires are much less expensive tho. Albeit, I don't know how to use them...
 
Let's make an example... an I'd like to call on your guys experience.

I've made a few barrel nuts for my spindle for various chucks and collets and thread protectors, so I have my spindle thread down pretty good... however, each time has been guess work...
measuring the major diameter of my spindle, how much less (or more) should be the rule of thumb, if you will, for the minor diameter of the mating part, and vice versa for the opposite? Or, the minor diameter of the male part to the major of the female...

The test fit is fine. But my first attempt at my spindle thread, for example, I had started with an incorrect major diameter and to get the piece to fit I had to keep cutting until it fit, and it made a bit of a mess and I lost my shoulders, etc...

I have an er32 collet nut in the mail arriving any day now. I have the barrel nut ready to have the nut threads cut. But it will need to be very accurate!
How would I go about accurately transferring that female thread to my male barrel nut? What's the preferred method?
 
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