[cheetah650]

our vertical machining center(built in 1981) is consistantly off measure parts. We are machining bores by circular interpolation and diameters are smaller than commanded. i had to program 91.6 mm to get 80 mm holes. whats more, if a play with feedrate, diameter i get also changes. i am disabling cutter diameter compensation on the control by g40. measures on the z axis are accurate. what can i look at?

thank you

[cnc2149]

The actual tool path is off from the programmed path by the position error due to servo lag and acc/dec delays. The amount of servo lag (or sometimes referred to as following error) is proportional to feedrate and inversely proportional to position loop gain. If you double the feedrate in a linear move, you will double the servo lag. Using a higher position gain setting will result in lower servo lag so there is a closer match to the programmed and actual tool path.

FANUC gives the formula for radial error of an arc in their operator manuals. I can’t easily type in the formula in this post with a simple text editor due to the use of exponents.


Example 1:

I’ve calculated a sample radial error using the following values:

Programmed Diameter = 91.6 mm
Programmed Feedrate = 500 mm/min
Position Gain parameter = 3000 (or 30/sec)

However, negating the effects of any acc/dec settings and assuming you are simply operating on the position loop gain curve, the radial error would be 3 mm.


Example 2:

I can rewrite the formula and solve for feedrate based on a given radial error. In your case of 5.8 mm of radial error, I compute a feedrate of 695 mm/min (assuming a position gain of 30/sec and negating acc/dec effects.


So, it is likely that your machine is working as per spec. The 6M uses analog servo amps and you do not have the ability to use feed forward as you would with a modern CNC. You are limited on how high you can increase the position loop gain as it involves overall servo tuning for the sweet spot between servo response and stability. If in doubt as to the condition of your servo system, you may want to get a serviceman that knows something about servo tuning to assist. The quick solution is to drop the programmed feedrate during circular interpolation.

[cheetah650]

thank you for your valuable help. i was expecting that with most cnc systems following errors are on the order of hundreths even at rapid feedrates becuse error compensating algorithms involbe both proportional and integral factors, which works to eliminate steady state error. it sounds like this goes only true for digital systems. anyway, can play with gain pots, if there are any, on servo drives, or can i change position error gain parameters on the control?

[cnc2149]

You would have to increase the position gain parameters for X and Y. I don't have a 6M manual so you would have to find the exact parameter in your manual. You always want to have balanced position loop gains on contouring axes so they should always be set to the same value. In order to prove that position gain is the reason for the path error, try dropping the position loop gain and making a new cut to see if the path error has increased.

The gain pots on the servo amplifier affect the velocity loop gain. Adjusting them alone will not lessen the following error, but it may be necessary to boost the velocity loop gain to support the higher position loop gain since the velocity loop must be 4 to 5 times hotter than the outer (position) loop. Normally, the best way to tune a servo is to use a chart recorder or dual channel storage scope and monitor the tach signals while making adjustments. Also, if you have a Renishaw Ball-bar, you can quantify the servo response and identify issues that would affect servo performance in this (older) machine.