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Jerky movement

Posted: Fri Jul 25, 2025 7:23 am
by Farnsbarns
Hi all,

I'm new here, and new to CNC, and new to any type of milling. Cue lots of broken cutters, etc, I'm sure.

I converted an old Bridgeport mill to CNC using Acorn and Centroid CNC12. I created some models in fusion. I created tool paths with advice from a few people and today I have made my first basic part (just an odd shaped block with some holes). I have done the first 2 operations which are clearing from top, flipping the stock, re-probing and clearing the bottom. It has gone very well. The part is within the desired tolerance with a reasonable finish despite very little dialling in.

However, I notice that the mill movement is slightly jerky, feeding a little bit, stopping, feeding again in ~0.5 second pulses. If I do a dry run with no stock the jerking stops.

I am using very cautious feed rates and step overs at the moment (100-200mm/min and 1.2mm step over with 5mm step downs. The stock is ally and I'm using a 12mm 3 flute cutter.

My acceleration was set to 0.5, tried 0.3 and it was worse. 0.6 seems about the same and if I try to set it any slower Centroid CNC says this is atypical and suggests I don't.

Can anyone offer any advice or insight?

Thanks in advance.

Re: Jerky movement

Posted: Fri Jul 25, 2025 8:44 am
by CentroidFrog
Hello, and welcome to the forums!
Please post a current report for us from your machine.
Here is how to do that:

Re: Jerky movement

Posted: Fri Jul 25, 2025 8:54 am
by Farnsbarns
Hi CentroidFrog,

Thank you so much for taking the time to read. Here's the report...

Re: Jerky movement

Posted: Fri Jul 25, 2025 9:25 am
by Farnsbarns
Worth adding, I did a dry run with the spindle running (not controlled by the CNC) in case the spindle motor was causing any interference but it still ran smoothly.

Re: Jerky movement

Posted: Fri Jul 25, 2025 11:33 am
by Chaz
Hi, what is the size of the motors on the X and Y axis? Ive done a few Bridgeport Interacts and they typically wanted 750W servos or similar. If you have run with the spindle off and its smooth, is that not your problem?

Re: Jerky movement

Posted: Fri Jul 25, 2025 1:22 pm
by Farnsbarns
Sadly I'm not going to be able to get the motor details until Monday now but I can say they're very beefy steppers and my brother has used the same on his similar Bridgeport. I will look for some pics. Maybe someone else can identify them from that. I am reasonably confident the motor spec isn't the issue.

Re: Jerky movement

Posted: Fri Jul 25, 2025 1:25 pm
by Farnsbarns
PXL_20240924_145601565.jpg
There we are. If anyone recognises these, otherwise I will get the details. (As I say, fairly confident they're up to it with my brother having used them and not getting the same result). I feel it must be something in my tool paths or something in my centroid settings.

Re: Jerky movement

Posted: Fri Jul 25, 2025 1:29 pm
by Farnsbarns
I should have said earlier, it only seems to pulse when X and Y are moving. Where only 1 axis moves there is no pulsing.

Re: Jerky movement

Posted: Fri Jul 25, 2025 1:34 pm
by Farnsbarns
Chaz wrote: Fri Jul 25, 2025 11:33 am Hi, what is the size of the motors on the X and Y axis? Ive done a few Bridgeport Interacts and they typically wanted 750W servos or similar. If you have run with the spindle off and its smooth, is that not your problem?
Oh, and no, it's smooth on a dry run whether the spindle is running or not. Put stock in the vice and the pulsing returns when X and Y move concurrently.

Re: Jerky movement

Posted: Fri Jul 25, 2025 1:58 pm
by centroid467
When cutting stock, single axis moves are smooth? Only multiple axis moves have this issue?

What are you using for a power supply for these steppers? Output voltage and current rating and pictures would be helpful.


What are the stepper drives?

Also, just for reference, could you please also post the offending gcode file?