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« on: September 18, 2009, 08:49:48 AM »
Threading is a rather complex and interesting machining operation. There are a lot of factors that
come into play when it is done. Material, spindle speed, cutter geometry, cutting fluid, depth of cut
and also type of threading method to be used all come into play and are interelated to each other.
You can get very techie about it and do calculations to see the affects of different combinations. In
fact over the years, it has been studied in depth and testing conducted to try and put threading
material / metal cutting into some good engineering perspective. The SME ( Society of
Manufacturing Engineers ) has done research and many of the insert manufacturers base their
calculations for diffferent tools to maching parameters on the research. All that stuff is fine, but, it is
still subjective in nature, in the sense that, so many considerations come into play when
calculated. That is not to say that one would not benefit greatly by reviewing the information.
As Hood noted the first cut is usualy deeper and you can save time and minimize the number of
remaining passes. Kind of depends on how you want to work and what works for you. You may
want to take a smaller cut just to see that all is working and you are setting a groove for the tool to
follow. Yet , you may want to take a cut that would give you the same material removal rate as the
of all the remaining cuts as that would give you an indication of what how your machine will behave
when cutting the thread. If too deep and just a radial cut you may take the tip off the cutter.
If the material work hardens then you change what you are doing again. Some soft materials are
just as bad or even worst than harder ones.
That a look at the generated code of the wizard and note how the Z and X values change from one
pass to the next. Then change the infeed angle and compare the Z and X moves to the other infeed
angle setting. 30 or 29 infeed angle is good for very fne threads, yet 15 would be a better value for
harder and deeper threads.
Note that as you increse the RPM the stepper acel rate will increase and a good rule of thumb
is to allow 3 to 5 times the depth of cut for the acceleration to take place ( that requires setting the
tool in the Z+ away from the material ). Idealy you would want max power from motor. If you have
a punny lathe with a small motor then speed may become important since you want the the rpm
so it won't slow down, but, it's motor and gearing related. You may want to take a look at my post
on converting a lathe in the members docs as i included some graphs and how rpm, stepper torque,
, motor horsepower and feed rate are related.
Just some practical comments on threading for your consideration,
RICH
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