machining threads using 4th axis

[ncmrfixit]

I've spent about an hour searching this forum and the internet in general for information on using a 4th axis rotary table to machine threads.

In theory it should be possible. Does anyone know of a g-code generator, CAM program, formulas, or code examples for doing this?

The idea would be to mount the stock to be threaded in the a chuck on the 4th axis and then using the appropriately shaped cutting tool, machine the threads. I did find a U-Tube video showing this, but no background on how it was done.

Any hints?

[HuFlungDung]

Its a fairly basic programming job, depending on how many degrees your 4th axis is capable of rotating in one command. There is often an arbitrary limit on how far some controllers will permit a A axis move, for example 9999 degrees.

To cut a single turn of a 10 pitch thread:
G01 X0.100 A360
You can control right or left handedness by the direction of X motion (sign), but for your A axis direction or rotation, you'll probably want to climb mill, and this can be easily modified by the sign of A.

I'd imagine that to be productive, you'd want to use a plain style milling cutter (akin to a slitting saw), and this would require that you have a right angle attachment for a VMC. Your cutter axis would then be horizontal like the workpiece, and almost parallel to it. This right angle head would also need to be rotated slightly around the vertical spindle axis to bring the plane of the cutter to match the helix angle of the thread.

[Alan Wright]

This is easy to do. For shorter parts, with the rotary table horizontal, you can cut a thread with a single point thread mill in the mill spindle. The g-code is quite simple, and I have written a general case g-code generator for same. Easy to do any pitch, diameter, left/right, etc. Basically you just move the rotary axis (A, C, whatever) concurrently with the Z axis, so that the total Z movement corresponds to the thread pitch. For example, if you want to thread a .5 inch piece at 13 tpi for 1 inch, you enter the cut at the bottom (Z=-1) and then do a command such as: G01 A4680 Z0. This will cause 13 rotations of the workpiece while the Z axis moves an inch. Of course, you have to adjust the cut depth for each pass, and it is best to move the cutter along a 60 degree diagonal at depths that give a nearly constant volume of material removal. For me it worked best to rotate fast but cut fairly shallow, requiring a few passes.

When doing this, I found the threads are rough unless I do the last pass at a shallow depth, then a spring pass, and finally a reverse (climbing) pass to clean things up.

It may also be possible to do the same for a longer part in a horizontal mounting with a center support. I had ideas of doing this using a 60 degree cutter in the spindle, but never tried it.

In any case, I used this a lot before I had a CNC lathe. The CNC lathe does not produce any better results, but is easier to set up.