spindle sync trouble

I am having an issue with the synchronized motion using G76 and G33. I have the speed connected for manual control. When issuing G76 and G33 commands I note that changing the speed has no effect on the Z axis speed. Does LinuxCNC monitor the shaft encoder for synchronized movement or assume the stated spindle speed? If the shaft encoder is monitored, why don't I see Z axis speed change with shaft speed?

Hi

You will have to expand upon what you are doing and why you want to manually change speed during the cutting of a thread,
or select a speed which is different from the one in your code?

Also upon what encoder you have and how it is connected.

The Z feed IS synchronised to spindle speed, but how well this is done depends upon what spindle encoder you have.

A Z index is basically good for positional use, getting the thread started in the right place each time, but you cannot cut good threads without at least an A pulse.
It is this that allows the adjustment of feed to match the fluctuations of spindle speed caused by tool drag etc

The optimum number of pulses and the accuracy you can get in speed matching, is largely governed by your ability to read them.
With a hardware encoder interface, you could go very high, but with the software encoders, anything much above 200 tends to quickly go haywire as speed increases,
because they cannot keep up.

regards

I built a low resolution shaft encoder. Here is a screen from HAL scope.



I am using manual speed control because I haven't gotten the VFD hooked up yet.

I don't want to change speed during the cutting of a thread. I only did that to test the system during an air cut. It should have compensated the feed rate, but the feed rate was unaffected.

andypugh wrote:

G33 and G76 use spindle _position_ not speed. (motion.spindle-revs). So manually changing the speed will have no effect unless that also changes the rate at which the encoder position feedback changes.

I don't think I followed that. The shaft encoder is physically connected to the shaft, so changing the speed in any way would also change the rate at which the encoder position feedback changes - right? We are talking about A and sync pulses here aren't we? What did I miss?

Thanks Andy. Yes, I have a pot tied to the VFD for speed control.

About maxing out - I don't think so. When I have the spindle speed turned up high LinuxCNC won't take a pass. I assume that this is because it cannot make the pass at the maximum Z travel speed (though I am guessing). That leads me to think it IS monitoring the spindle speed and matching the Z speed (actually following the surface I know). However, when I have the speed set low enough to take passes I can change the spindle speed around and the Z feed rate does not change. It does not even change between passes. If it is 83 (mm) it remains at 83.

As you can see from the HAL Scope capture I posted, LinuxCNC clearly sees both A phase and Z phase. I know this works for other people, so I am pretty sure the mistake is in something I have done. I just can't figure out what.

Andy totally rocks!

I got to step 2 and stopped the spindle in the middle of a thread run. As I approached 0 the Z speed dropped off and stopped. I ran the speed up and down in the very low ranges and the Z speed (feedrate) followed the surface. Without a readout I just had the spindle speed too high before. I had assumed incorrectly that when the machine took a run at the threads it meant that it could match the feedrate to the spindle speed. It couldn't keep up at that speed. Andy had considered that possibility earlier. He was right! It works perfectly but at a lower speed than I had "eyeballed" with my manual speed control tied across the VFD.

SOLVED (thank you Andy Pugh)