Threading on mini-lathe, what am I missing?

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Skeet_Man

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Second time threading on my mini lathe today.

Problem I'm running in to, both times, is depth of cut being too shallow.

Thread is 1/2-28. Shooting for class 3 fit with major diameter of .498 (confirmed spot on w/ dial micrometer) and target minor diameter of .457

This should give me a depth of cut of .0205" Lathe dials are standard read, so .001 on the dial is .001 in (removing .002 if turning OD).

Threading with the compound set at 29.5*, this means I should read .0235 on the dial to correct for the cosine.

I cut the threads to that, then went in another .001 on the crossfeed to clean up (which theoretically meant I should have been at .456 minor diameter).

The problem I'm running into is I'm still not deep enough to either form a V or screw on a test nut. I even went another .003 or .004 and I'm still coming up shy, so it's like I'm not even close. I touched off, did a scratch pass, ect so the initial depth setting was correct.

I put a dial indicator on the compound, and it's off .001 which i took into consideration so, on paper, my cut was correct if a touch over.

I bought a die as a way to hedge my bets, and ended up using that to finish the thread, but would like to remove that necessity in the future. I realize the huge limitations of my equipment and I may just be running to an inherent issue with the machine, but thought I'd ask to see if someone can catch something I'm missing. Checking with wires would probably be my best bet but I wouldn't use them nearly enough to justify the investment.
 
You could run a 1/2-28 die over the threads ,might be the least expensive accurate way to do this.

Otherwise there are a number of variables that may be difficult to account for: machine rigidity, tool rigidity, springback, depth of cut, tool sharpness, ability to measure precisely, speed of cut. number of passes, experience.
 
If you are using the compound to advance into the cut, remember the cutter will only advance 1/2 of what the dial reads because of the 29.5*
 
If you have flex in your tool, tool post, crossfeed, etc... this can make you come up a few thousandths short when under the load of cutting threads.

Your math looks good. 1/(Cos 29.5) * (desired depth cut of thread) = dial measurement cut on crossfeed. You show 0 .0235 ... whereas I calculate 0.02355, and would round up to 0.0236, and might go deeper by a few thousands If I want it to thread easily. Also, finish matters. If the finish is rough it turns up little tiny burrs, which will make the valley and sides of the threads "bigger" than what your dial indicates.
 
I get 0.0268" estimated DOC with the compound. These estimates for thread depth are exactly that; estimates. Those estimates are just to get you close so you can start checking your pitch diameter to get the fit you need.

Much also depends on the tool, material, rigidity of the tool post/gibs/structure of the lathe, etc. You do not need a huge, rigid lathe to cut precision threads. I can cut Class 3 thread all day on a little Sherline lathe. However, your gibs and everything related to holding the tool rigid must be adjusted well. If I had to guess at the problem, I would guess that the support under the tool is moving.

You did not mention the kind of tool you're using. If you're using carbide then that requires more rigidity in the lathe. If you're using HSS then that requires a small flat at the tip and sharp edges. I am assuming you assured the tool was set perpendicular to the work. It might help to show us a pic of your set up.

Your goal is a Class 3A 1/2-28 thread and you're trying to get that without thread wires or a thread mic but a Class 3A fit requires that you hit a pitch diameter of between 0.4768 to 0.474". Without some way to read the PD, either wires or a mic, you cannot cut a Class 3A thread. And you are not going to get that with a die.

Cutting precision threads is fun, an exercise in precision turning and hitting pitch diameters within a narrow band, but you need the right tools to do it. My suggestion is to buy a thread mic.
 
What is the tip of your cutter look like? Dead sharp or the flat called for by the thread standard?
 
Is your compound for sure set at 29.5* from perpendicular to the work piece or 29.5* from parallel to the spindle axis? It should be the former.

Tom
 
My additional suggestion is to set your compound at 90 and ignore the 29.5 stuff altogether. For 28 tpi it won't matter. Sandvik's Training Handbook Section C4 has an excellent description of the 3 methods, including depth per pass. Only in North America do ppl seem to persist with the 29.5 method, seems EU and elsewhere have moved on from that with manual machines.
 
Pro intent is also with cross feed and not compound

Our 1944 ish 16 in L&S has a locking compound that when locked properly comes OUT 2 turns.

The intent is you sneak up on final cut for work, threading primary function then you activate the lock or limit.

Now you can back off 2 turns and come in on next work.

However, for small or not rigid lathes the 29.5 process is to allow only cutting on one side of the tool.

Less stresses on the system.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 
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