Centroid Community CNC Support Forum

I need to modify a steel part that has gear teeth and a hollow shaft. I need to part it in half, and weld in an extension piece to make the hollow shaft longer, and then clean up the weld.
1. Is it possible to weld while the part is still mounted in the lathe, or will this fry the electronics?
2. If 1 is possible, is it also possible to weld while CNC12 controls the lathe to do a slow spindle rotation? Like 2 rpm or something.

(The lathe is using AcornSix and 0.8 KW AC Servos for Z and X, and 3.5 KW AC Servo for spindle drive )

I would not try it If it was my lathe, but your level of risk acceptance may be different.

It is difficult to ground a rotating part. I am imagining the welder's ground would end up somewhere other than the part. It is possible that the spindle bearings would end up carrying large currents and be pitted or otherwise damaged.

The loop area of your welding current will probably make the largest difference in the potential for damage. For example, a solid ground close to the weld area will minimize large currents outside of the weld. A loose ground or one far away from the part will have welding current attempt to find alternate ground paths, possibly through the CNC components.

The cable shielding, cable routing, machine configuration, etc. will make a major difference in the amount of noise feeding into the system. I don't think anyone can determine the precise effects without running experiments on the machine in question or a clone of it.

1. With control power off, probably nothing will fry. If the welder's ground is not perfect, the prediction is downgraded to "maybe won't fry".
2. Assuming it doesn't fry (question 1 + power on), the drives could get missing or extra step pulses, corrupted encoder data, or other errors. This could result in unexpected motion or stopping due to a fault.

Centroid controls run many plasma cutters and some welders, so my advice may be a bit conservative. However, plasma and welding machines have been tested and troubleshot for the application. You will be finding any weak links yourself since the lathe was not intended for welding.

Just power it down when you weld. Keep the ground on the chuck not the machine so you don't have current going through the bearings. I am guessing you are using the tailstock and chuck for alignment. Just turn the chuck by hand. I would tack it around at a few points then take it out and weld it on the bench.

If you do try this use a braided copper battery ground cable looped around your work piece and clamped with your work lead(ground clamp). That will stop the welding current going through your bearings. I wouldn't have the lathe electronics powered up and weld it out by turning the chuck by hand. I take it you don't want to disturb the part and have to remount the part in the lathe. Make sure you cover all sliding ways and such to protect from the molten spatter.

This is the type of braided ground cable I am referring to in my previous post.

Awesome, thanks for all the great insights.
I only need to tack it in while held in the chuck and tailstock, and I'm using a Kemppi minArc MIG welder, which is relatively controllable.
I'll give it a careful go.

Johannes, are you in South Africa? I ask because I noticed the glasses in your picture are upside down so I figured you are on the other side of the planet! I love all my Kemppi welding equipment.