Threading problem

The owners manual for my lathe states that movement of compound is .001" per division on dial, the same for the cross slide. I know that if i dial in .002" on the cross, I am removing .004" of material. So my compound doesnt act in the same way?
 
rightway1974 said:
The owners manual for my lathe states that movement of compound is .001" per division on dial, the same for the cross slide. I know that if i dial in .002" on the cross, I am removing .004" of material. So my compound doesnt act in the same way?
Click to expand...
Yes, it does too.
 
If spec calls for .030 thread depth, then I should feed my compound in steps until it reads .015 on the compound dial. That should put me a few thousandths shy of .030 depth. Then I can make spring passes or very light cuts until satisfied with the thread. Is my process correct?
 
You are trying to cut .030” thread depth, not trying to reduce the workpiece diameter by .030”. If you wanted to reduce the work piece diameter by .030”, then yes, you would crank your compound in .015”. But, that’s not what you are trying to do. Moving the compound .015” only gives you .015” thread depth. You need to move the compound in ~.033" to .034" to get .030" thread depth, assuming your compound is set at a 29 to 30 degree angle.

Tom
 
Thank you for the clarification. I am learning as I go and seem to over think things that should be simple.
 
if you are using your compound only, thread depth is calculated as follows.

depth = cos(compound angle) * compound feed

or you can refactor the equation to be

compound feed = depth / cos(compound angle)

cos(29.5°) = 0.87035569594
cos(30°) = 0.86602540378


The other thing to keep in mind is that that "thread depth" is relative to the the internal threads minor diameter. If its to big or two small, the thread depth has to change. If you look into thread fit classes it might help you out some.
 
Ok, I found a decent webpage.

http://www.amesweb.info/Screws/AsmeUnifiedInchScrewThread.aspx


Pick the class thread you want and the calculator will show you all the dimensions, it even lists all the formulas.

Do keep in mind that most people don't thread this way.

Most people just gauge the fit using a nut or bolt of known quality. If you have to meet a specked fit that's when you get into buying or making gauges.
 
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My compound dials are .002 increment diameter readings. So if my thread depth is .030 for a single side it will be .060 on my dials.

Remember thread spec drawings show 1/2 of thread depth of the diameter.

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3strucking said:
Thank you for the clarification. I am learning as I go and seem to over think things that should be simple.
Click to expand...

Nothing's ever really simple; the usual depth-of-thread is 75%, so a 32 tpi thread isn't 0.031" deeper than the
first touch of the (possibly rounded-tip) threading tool, but could be 0.023" (or anything inbetween).
Internal threads are easiest measured by having the mating part available for cut-and-try.
 
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