Hello Forum, I hope someone has seen this before or has a clue, because I’m stumped.
New Elite Journeyman, just got it a couple of weeks ago. PwnCNC 80mm spindle and VFD. So far I’ve mainly been surfacing boards and slabs but I’m getting very odd results. It seems the Z axis will step up or down at certain locations, quite suddenly. Here’s a picture of today’s result, this is doing a standard back and forth path:
At first I though the bit was slipping…but the beginning of each pass is very accurate. If the bit was slipping each pass would be at some random height.
I thought maybe I had a loose coupling since i installed the braking Z motor, but it seems solid. Also, like the bit if the coupling was slipping I wouldn’t expect to see such an exact match between the start of each pass.
I thought maybe wiring, but it’s all brand new and looks it; all the harnesses & wire runs seem normal with no kinks. I tested for continuity back to the Masso board and it all checks out.
It’s odd that it’s jumping up and down by what appears to be a consistent amount. Todays operation failed with an X motor alarm, doubtless my 2" surfacing bit was dropped back into this slab of cherry suddenly and was overloaded.
I had a look at your file and description of what is happening.
I use a different spindle - 2.2Kw 80 mm, and have open loop 3Nm steppers run by a Masso G3 on a Woodworker X50.
A few thoughts…
You are running at 5000 RPM, which in my opinion is very low, even for spindles that claim to be able to machine at those speeds. I believe you will have very little torque.
You are using a very large diameter surfacing bit which may add to the challenge.
Perhaps given the above, and the rigidity of the CNC, a higher RPM and shallow depth of cut - feed rate to give appropriate chip load - may help.
Are the problems occurring perhaps when there is a grain or density change in the slab you are surfacing?
Given the above ensuring minimal stick out and proper collet tightening will be important.
This is true. On my spindle, a 2.2 kW 6000–24000 rpm spindle, the manufacturer tells in the manual to program a lower speed limit of 6000 rpm in the VFD. The constant torque range of a typical 24000 rpm spindle begins only at 6000 rpm:
Also when milling wood, you mill at high speeds. This is because the problem with wood is heat. In this popular jazz bass video, everything is milled at 24,000 rpm: DIY High-End Jazz-Bass-Guitar produced with the CNC machine – Youtube. Of course with a large bit diameter you would choose a lower speed, but you’d have to stay inside the constant torque range of your spindle.
Hmm, interesting point on RPM. I’ll try something higher.
These passes are very light, taking 1/16 or less, and the feed rate is slow at 40 ipm.
There does not appear to be a related change in material density or features.
I’ve checked the bit and collet a gazillion times. I’ve had this issue with my current surfacing bit and with a smaller 1/2" mortising bit with a 1/4" shaft.
there can be many causes for loosing steps. EMI is one of them. I noticed the PwnCNC VFD housing is neither made of metal nor is the VFD enclosed into a steel control cabinet, despite the fact that a VFD is a device intended to be installed in a VFD control cabinet (which serves as faraday cage). Also all Onefinity cables are not shielded. In industry, all motor and signal cables of a CNC machine are shielded and grounded to prevent EMI.
