Math help needed for threading...

[Machdaddy]

When threading what is the formula for figuring out how much to advance the compound slide? I'm trying to cut a 1/4" 24tpi thread which I know is supposed to be .054 deep but with my compound set at 29 1/2* I don't know how much to advance the compound. I must have been asleep that day 45 years ago in my high school trig class!
Any help would be well, helpful!

 

[cathead]

I use the common sense approach. Generally one needs to make a light
cut of several thousandths and then measure to make sure the pitch is correct.
After that, a medium cut of 8 to 10 thousandths each pass in your case until you get close
to the finish. At that point a little test fitting and measuring and a final light cut.
Of course if the threads were significantly bigger or smaller one would have to
adjust accordingly. In general, I can make threads in a half dozen passes
for small to medium size threads. Threading has a bit of a learning curve to it
so roll up your sleeves and spend some time cutting threads. Very coarse threads
are going to take more passes to get to size and very fine, the opposite. Good luck.

As far as a formula for this, I don't know that there is one.

 

[catsparadise]

With your compound set at 29 1/2 degrees the tool tip will advance cos(29.5) x the advance on the compound scale. So for every 10 thou on the scale your thread will get 8.7 thou deeper. For 0.054 deep threads you'll need to wind the compound in about 0.062.

 

[GK1918]

What you need is eyes and ears instead of math formula stuff. After setup & scratch pass test, its ok to feed 5 or .010
but remember as you go deeper back off the cut somewhat = the deeper you go the tool gets more loaded. so lighten up. As you see the crest starting to form stop and try a test nut if a no go do a few spring passes try nut and sneak up to it until the test nut fits perfectly. And don't look at the clock. One of the best favors you can do for yourself is make
a tread stop for the crosslide whether it be a vise grip C clamp whatever that takes all wasted time returning to '0'.
Another thing I do, if you use a thread dial. Forget about the lines and numbers . Before any cutting start lathe now
engauge half nut (the dial will stop rotating) now mark or put dot on the dial with sharpie. Throw off half nut bring
carriage back look for your sharpie dot> engage. works every time, another mind savor. Another time savor. No need for
a releif cut at the end of thread. Use your eyes at the end of thread quickly release half nut and let it run right there, it will
make a rut like threading up to a shoulder - UNLESS a releif is really needed. Do you need a Nasa fit? Nut goes on but
now we are dealing with tenths: solution valve grinding compound on the threat and hold nut with vise grip & run nut
back and forth under power hopefully you have reverse..... Logic: If one thinks this is time consuming well get in your
car go to hardware store, equals time fuel all for a 50 cent bolt? By the time you get out the driveway I'd have it done on
a lathe of course. Quote from a 1940 South Bend manual "if you are in a hurry you don't belong in a machine shop"
end of quote how true....sa hop this helps. sam

 

[Tozguy]

I agree with post no. 3. It is understood that the compound is set at 29.5 deg from perpendicular to the spindle axis.
Also, because the tops of the finished threads should be rounded or flat, starting diameter of the work might be less than the nominal diameter (less than 1/4'' in this case). So just a caution that calculations might not give you the exact amount of compound travel that is needed to get a good thread fit. I believe in sneaking up on the final dimensions by trial fits.

 

[rgray]

Put your compound mid stroke and zero it. Advance the cross slide and touch the tool to the work.
Move your carriage off the end of the work. Advance the cross slide the depth of thread and lock it in that position.
Now back off the compound move over and touch it to the work and adjust for your first depth of cut and continue cutting till you
reach the previously set zero. you will then be at the depth of cut you set initially.
I usually set zero on the cross slide also and use it to back off for clearance when traversing after a cut.
No trig involved.
Use indicator on cross slide movement set, or know for sure if your dials show .oo1's or if you have a dro your all set.

 

[RJSakowski]

I created a spreadsheet some years ago to calculate the depth of cut along the compound angle based on a 29.5º compound angle for common SAE and metric threads. It also includes calculations for various cutter tip radii. It was posted by another member to a Dropbox account accessible by HM members as I didn't have permission to post an Excel spreadsheet then. Here it is.

 

[cg285]

i have a cheat sheet placard on my chuck guard

 

[mikey]

Mach, cutting threads depends on the class of fit you require. If you simply need to make a one-off and don't care about class then get close as Cathead and the others said and cut until the nut fits. When doing this, it helps to coat the blank with a Sharpie so you can see the thread form better and start test fitting when a small flat appears on the thread. Spring passes help to refine the fit.

On the other hand, you can also cut the thread to a class of fit. An external thread is an "A" thread, while an internal thread is a "B" thread, and there are three general classes - 1, 2 and 3. An external class 1a is a loose fit, a class 2a is the normal fit we see with 90% of manufactured threaded stuff, and class 3a is a close tolerance fit. Cutting an external thread to the tolerances of a class allows any nut of that class to fit. For example, if you cut a class 2a thread then any hardware store nut will likely fit it fine. So, you need to determine what class of fit you are going for and cut it that way. To do this, you need a thread micrometer or a 3-wire set.

While it might seem more cumbersome to do it this way, it is actually simpler for me because I just cut until I get the micrometer reading I want and stop; no test fitting or wondering. On the off chance that it will help, I'm attaching the best threading chart I've seen; it includes all the information you need to cut any class of internal or external thread you need. You do need that 3-wire set or thread mic to use it but you should have these tools anyway if you ever plan to do a precision thread.

If you look at the chart for a 1/4-24 class 2a thread, you will see that you need to cut the blank to an OD of 0.2489 to 0.2417". Use your regular mic to hit somewhere inside this range. Then start thread cutting and start checking the thread pitch with the thread mic or wires when the peaks start to form. You want to cut until the thread pitch falls inside the range of 0.2218 - 0.2181". When you get there, stop and you're done. A thread mic is far faster and simpler to use for this because it is direct reading and simple to use with the part in the lathe. You don't worry about formulas this way and if you hit the pitch diameter range, the thread will fit. You can also use go/no-go gauges but that gets expensive. A 3-wire set (cumbersome) or a thread mic is cheaper. This one will work: http://www.shars.com/0-1-screw-thread-micrometer

I think most of us start out with the formulas and the cut and fit method but eventually we will need to make precision threads for some project and it is a good idea to teach yourself how to produce one.

 

[benmychree]

Yes, I believe that the "seat of the pants" approach is the most practical; just thread it! whether to fit by thread mike or a nut/gage, either is fine, and I believe an undersize OD is a good thing, you don't have to deal with burrs set up by the threading tool to a large extent. I was taught that a setting of 30 degrees on the compound is the proper way, all the journeymen in my apprentice shop used 30 degrees and that was what I was taught in high school and junior college.
Incidentally, it is possible to cut odd threads, such as 1-1/8 - 7 in as few as 5 to 7 cuts; I have done it many times on disc harrow axles; an initial deep cut is made, then on the next cut to the same depth, the half nut is closed in between lines resulting in a double track with the apparent pitch half what the QC is set for (7), you then go back to the numbered line and feed in coarsely for a couple of cuts until the center thread is removed, then a couple of cuts to finish. I was told that this method was used in the oil fields for fast threading; what it does is it makes it possible to take deeper cuts because there are no wide cuts until the finish cuts are made, a appropriate flat on the threading tool makes the initial cuts possible without dulling the point of the tool.