Screw-cutting with a teach CNC lathe

People ask how easy is it to screwcut with our CNC lathe? Hmm.. I have no idea about other peoples machines, however I screwcut on a Harrison Teach CNC, so I can tell you about it.

This particular machine is a 400T so a bit of a beast, it is a conventional format machine as opposed to slant bed (Google it!) It can take around 1200,00mm between centres and turn a piece of material 400,0mm in diameter. It is equipped with 3 Jaw, 4 jaw and 5C Collet chucks on a D1-6 system and has Fanuc controls.

So reasonably large…

I can operate it in a few ways.

Firstly as a manual machine, the only difference to a more conventional lathe is this is ‘Fly by wire’ so when I operate a hand wheel, instead of turning a lead screw I am effectively turning a potentiometer (Think of a volume knob) that sends a signal to a motor. This is quite an accurate way of operating and I can control the movement to 0.001mm/1 micron (Not a lot) A human hair is around 70 microns.

Secondly, I can draw what I want to turn in a plan elevation with CAD, convert it to a .DXF file and export. Then import it into something else which allows me to chose feed speeds and depth of cut and the tool to use, which in turn is converted to a language the lathe understands. The resulting code is then sent to the lathe and all I have to do is tell it where to start and off it goes (OK, this is the simplified version to save those at the back falling asleep)

Thirdly, I can use the teach CNC side of things at the machine control panel. As an example I can tell it to machine internal and external tapers (Not so easy on a conventional lathe) and screw threads! (yes, I can also program threads via CAD however today it is local to the machine)

So what does a screw thread comprise of? Well a major diameter, a minor diameter and the pitch, or number of threads per inch is about it, here is a picture of what I see on the touch screen.

X1 the major diameter

X2 the minor diameter

T is the tool I am using (I always use 31 for external threads)

P is the pitch, this is a metric thread so 1 is 1mm

A is for angle, 60 degree for metric

L is the length of thread

Z is the overall length of cut. The minus symbol means go towards the chuck.

NS is the number of starts, just one as this is a conventional thread

NP is the number of passes to get to the finished diameter. I usually choose 15.2 with the .2 denoting two ‘spring’ passes where the machine makes a couple of finish diameter passes to allow for any deflection in the work piece.

Once completed I test the fit either with a gauge or the part to be fitted and I work to a 6h tolerance (Quite precise) If the part does not fit I remove 0.1mm from X2 and run the machine again to creep up on the final fit. I would normally expect to be at 16.000mm major and 14.917mm minor diameters.

Yes there is a bit more to it however this gives a good idea of the approach and it certainly takes less to do than write.

Cutting speed? It depends on the material however 600-1000 rev/min is not out of the norm.


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