Siemens 828D, how to compensate C axis feed to keep surface speed constant when engra

Hello!
I need to engrave on my lathe, Emco E45 with live tooling, on a complex surface: it’s not flat, it’s not a constant cylinder of known diameter, we can think of it as a cone-like thing.
With a CAM I can export an engraving file that I simply execute within the shop turn.
The machine handles nicely the X, Z and C axis with an incredibly smooth and clean result.
The problem is that I have to keep the feed VERY low because the exported file is a series of micro segments (no G3 or G2) each with X,Z,C and F parameters, just like this:
G1 X10Z7C20 F50
G1 X11Z6C15 F50
And so on (I made the numbers up).

The problem is that when I move on X and Z the feed is in fact 50mm per minute while when C axis is involved 50 represents degrees per minute.
This means that the surface speed when C is rotating depends on the diameter I’m at in that moment.
So, if I’m drawing a straight line on the C axis at diameter, say, 10mm, I will engrave at speed X, if I do it at diameter 100mm, I will engrave at speed 10X, and this will bring to a broken tool.

Is there a way inside the Siemens controller to compensate the angular speed depending on the diameter?

Thanks a lot!


Inviato dal mio iPhone utilizzando Tapatalk Pro

If i understood correctly, not sure if works on 828D, but you might be looking for fgroup.

fgroup(c3,x1,z1) for example.

Thanks!
I’ve tried with your syntax and it doesn’t accept it, seems like only the C axis has to be put inside the brackets.
Anyway, once I put FGROUP (C) before the movements the C axis finally doesn’t more super fast but now the problem is the Z axis (I’m on a lathe). When the direction is mostly on the Z axis the feed is now much faster than when it rotates the C axis.
I’ve reversed the problem...
Here’s my program:






Inviato dal mio iPhone utilizzando Tapatalk Pro

Depends on the axis of machine, since some machines can have c1-c4, and x1,x2,z1,z2, use the axis of your machine.

fgroup (c,z,x)

(page 355)
https://cache.industry.siemens.com/dl/files/316/29064316/att_96041/v1/FB1_0310_en_en-US.pdf

Ok, so if I got it right I should write FGROUP (C1, Z1, X) as I have 3 C axis, two Z (second is sub spindle) and only one X
But before I should also write FGREF [C1]=28
If my biggest diameter of the piece is 28mm
Is that right?


Inviato dal mio iPhone utilizzando Tapatalk Pro

Haven't used fgref personally. If your spindle number is 1 where you are holding your part during engraving, then yes. I'm not familiar with emco machines, so i don't know how it numbers its spindles.