Y axis Skewing badly at higher speeds

Hello everyone,

I have just bought my Longmill mk1, I bought it preowned. The previous owner had changed out the board for a mach3 but gave me all the original parts.

As I’m very new to the CNC world I wanted to keep it simple and use all the original parts so followed the assembly instructions.

My issue seems to be when my machine is running too fast. I am using Gsender, on normal and precise the machine runs smoothly, when in “rapid” (4000mm/min) it skews badly, makes some horrible noises and crashes. The same happens when I run a job.

I have restored the firmware to default on gsender but I still have the problem. I’m using Carbide Create to build my guide.

I don’t think there is any issue with the construction as it runs smoothly at slower speeds and have checked the obvious (leadscrew, squareness, delrin and bearings)

Thanks in advance for any help, this community has already been an amazing resource

@Marchant232 Welcome to the group, Chris. Unfortunately, I don’t have an instant solution to your problem, but I expect that others here will. You’ve already looked into the usual suspects, so I admit to being at a loss. Since you can run at lower speeds, continue to do that and have fun. With luck, someone will chime in with an answer.

Thinking while typing, I have one question: is it always the same Y axis that is “slow”? If so, try reversing the plugs on the long mill controller. Is the same axis the slow one, or does the slow one change to the other y axis?


Did the previous owner change the stepper motors that you know of?

Have you verified that eeprom settings match Sienci’s current settings?


Does anyone run the LM with G0 at 4000mm/min? :thinking:
I think I altered the maximum values to 3000mm/min.

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@JHahn FWIW, Jannik, I run some bits and profiles at 4000. For example, when I am doing picture frames with a tapered ball nose bit that is .03 at the tip and using a stepover of 10% and it is taking off a few thousands on each pass, I run as fast as the machine will go. These things take 10 hours as it is. I’ll do whatever is safe and efficient to shorten that. :grinning:

I actually have the same exact problem and it happens (nearly) at the exact same spot every time. I reversed the wires and the problem reversed itself to the other side which suggest a motor or electrical issue. I checked all the things to check in the computer housing and everything is normal (actually you need a magnifying glass. I question whether if it is just a 1/2 a hair off would that make it do this?) and all the switches to check are correct.
So, now I just make sure I don’t go fast and just listen to the musical notes it makes while I’m cutting.

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Marchant232 did you ever solve this??

@iwannaapple FWIW, if the problem moves from one side to the other when you exchange the Y axis cables, the problem is not the motors.