formula for depth of thread?

[cherokeechief79]

my students are making threads in a manual lathe and were asking me how deep to go once the dial is set at 0.

I didn't have a good answer and said I usually bring it close and then thredmic it or see if a ring gage goes on.they were looking for something a little more specific like a formula or something.

any ideas?

we did learn that the flat on the tool should be 1/8 of the pitch.

 

[RLeuschen]

This is a pdf we give the kids that just get out of school and attempt to thread on our manual lathe. Got it somewhere off the internet..

Thread-Calculations.pdf

Other than that short one, we make them break out the machinery handbook...

Other thing to ask, is the lathe cross set on radial or diameter???

[nickbe10]

It does depend on the thread spec. you are using as well.

We use a lot of AS8879 which is an old MIL spec. and has a larger minor on external threads for extra strength.

[cherokeechief79]

thanks...our dials are set to dia

it looks like your chart says the single side depth is .75 of the pitch.

this is what I was looking for if its true.

also we are measuring the thread pitch with different methods.we have a pitch mic,wires and a set of triangles .

I don't have a chart for the triangles and I cant seem to find it on the internet.

does anyone know where I can find the chart?

 

[RLeuschen]

I'll have to look and see if I can find mine and make a copy.

After dropping them (or wires for that matter) into a chip conveyor or a pile of nice stringy chips, i found my pitch mics a much nicer option.

[crazy^millman]

Machinist Handbook defines all threads. Get them in the Habit of looking at the Thread Chart and understand what each dimension on a thread chart means.  Class 1, 2 or 3 have different sizes tolerances for things and then if you start looking at J threads, NPS, NPT and other threads then your students will be better prepared to handle these sitatuions when they get out in the real world and need to cut real parts. 

I was running a shop back around 2004/2005. We had a problem with a 1"-8 Thread done in a 8" deep bore. It was a class 2 Thread and we had a Zero true Position with Max Material Condition on the thread back to a +.000/-.0005 hole that was our primary datum. I had my own Thread Pitch Micrometers at the time(Since Sold them to a shop that needed them) and had the machinist make some custom Go No-Go PD gauges to the high side of the tolerance. I also threw out the tapping and bought an Advent Threadmill and thread milled all the holes to with in -.0015 of the high side of the Class 2 thread tolerance on the 1"-8 thread. On the chart below you see we have .9188 to .9276 tolerance for the PD on an Internal thread. We made all the threads .926 and never had a problem hitting the Zero True Position. We did have a problem with inspection not liking the Go gauge being loose, but the No Gauge never fit. They can read the page below and understand what it all means they will be better prepared for the real world. 

Thread_chart

 

[RLeuschen]

After some search time, I found it. Attached pdf is from an older set that I have, I believe they were Brown/Sharpe but who knows. Depending on the style/mfg of your triangles, this sheet could be incorrect.

Its good that the students are getting multiple ways to check threads. But as Nickbe10 and 5th have stated, a lot depends on the class and spec of the thread when measuring. The root and/or crest of threads can cause more headaches than anyone could believe.

Most asked question I ever get is, "how come my no-go fits, but go will not?".  Most common answer is  " check pitch, then check the root rad/root dia, and crest trunc"  with the handbook values.

Hope this helps,

Rob

thread triangle sheet.pdf

[Rocketmachinist]

I use ME ThreadPal for all the weird sizes. Not too badly priced and just saves me so much time arguing with the inspector.

[cherokeechief79]

thank you all so much for putting the time and effort into this

 

[Rstewart]

Always used these, thought they were a necessity to have in the box.

[crazy^millman]

I haven't look in my Kennedy Tool Box in years. I got a few of those. I have the NPT and ACME thread ones. I wonder how people know what the V notches are for? I have show that to a few graduates in Manufacturing and they gave much a blank look. 

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