Rapid speed issue

[spumco]

During the last two machining sessions I noticed that the rapid speed during linking moves was randomly higher than the maximum axis velocity settings. This was random behavior, not every time the G-code file contained a G0 command.

For example, my X&Y axis velocity settings are 200 units (inches). But at least three times the axis was commanded to move at speeds greater than that - the feed was reported as 257(IPM) in one case, and 239 in another. Most of the time it rapids right at 200, but it's gone from 210 to 275 during these events.

I can hear & see the axis moving faster than normal during these events, and I'm not losing steps. I only noticed it on longer linking moves (greater than 3" in length) as the the mill can't get up to max speed on short moves.

Does not happen - as far as I can tell - with G1 feed moves; only on X, Y, or X/Y G0 commands. Nor does it appear to happen on the Z axis.

UC400ETH, PMDX-126 BOB, Windows 8.1, UCCNC version 1.2101.

Profile attached in case that's helpful.

Any ideas what might cause this?

[cncdrive]

Feedrate is the travel rate of the machine on the movement vector, so ofcourse the feedrate (Fact) with G0 can be higher than what you set for the individual axis.
For example you make a move X=0 Y=0 and G0 X10Y10 and both of your axes velocities are setup to e.g. 100 units/min.
The above g-code is a 45° diagonal move for the simplicity of the explanation.
With the rapid G0 move when both axes reach the 100 units/min max. velocity then the feedrate on the movement vector (the 45°path) is not 100, but 100x1.414 = 141.4 units/min.
The reason for this is that the movement vector is longer than the separate X and Y axis vectors and that rapid feedrate G0 command means that all axes can move upto it's own max. velocity setting.
So, it is normal if the Feedrate is higher with G0s than the velocity of the individual axes when more than one axis is moved. So, the faster feedrate with G0 does not mean that your individual axes running faster than what you setup...

[spumco]

Very helpful. If I understand your formula, a 45 degree diagonal move will result in the highest travel rate, and with my particular settings I'd see a maximum of ~282 units/min.

Explains why I observed (up to) a 275 IPM move, but nothing higher than that.

Thanks again for the usual superb support!

-Ralph

[cncdrive]

Ralph,

I'm glad that you found my explanation good.
Yes, exactly, if the velocity of both XY is 200 and the movement is made on the XY axes then the highest possible feedrate is at 45° and it is then about 282 units per min. It is simple Pythagoras theorem calculation.
c^2 = a^2 + b^2 = 200^2 + 200^2 ~=282