Weird issue tramming, spoil surfacing - hive mind help?

So I recently upgraded my xz plates to aluminum and recut my yz plates, put it all back together. Machine is quite rigid now, which is great. I also added some lights and a switch to the gantry bottom, and realigned everything. Hard to believe I made it tbh, thanks Ryan & co!

I lollygagged on resurfacing the spoilboard until yesterday. Using a 1" surfacing bit, 75% stepover, .5 mm doc back and forth on x 3/4" at a time. No problems there. The issue is really strange to me though. On positive x, the cut is deep, but on xmax to xmin the cut is shallow. Like somehow the router is higher on the x pass back to zero than zero to xmax. This is more pronounced in the middle than on the sides nearest gantry plates. Since I had taken everything apart recently I did end finding some things loosened that I tightened up, but then ran the spoil program again and it seemed to just move the problem more towards xmin.

I don’t think the router is out of tram in the xz plane, so I am quite confused. yz is probably out of tram, but I wouldnt expect to see this. Quite confused at the moment. Any suggestions?

I don’t know how else to describe this, other than pictures. tldr, cuts deep going left to right, shallow right to left.

new gantry lights :slight_smile: I really dont like 6500k though. Probably going to put in a pot to make them dim a bit more.

Do you have anything dragging on the gantry? If something was pulling when it moved one way, but not when it moved the other way, that could cause it.

I would suggest checking the core bearings for any movement or looseness. Any slop could give you variations when travelling in opposite directions

Also you may want to search the forum for tramming. Any angular offset from perpendicular (in all directions) of the router may cause grooves. I have seen suggestions to use a layer or three of masking tape to adjust the router in the core to compensate.

I had the same issue when surfacing my MPCNC with a 1" facing bit. I assumed my router or gantry was flexing from conventional milling in one direction, and climb milling the other. Maybe an unequal force on the gantry from each alternating pass?

Ended up redoing the surfacing gcode to cut from left-to-right, travel right-to-left, then start a new left-to-right cut. Not sure if this is a proper solution though.

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I ended up doing this, but I am worried that the cuts will be deeper going the other way while cutting. No real way I can think of to check this other than trial and error. It seems to me that there is some flex in my gantry. It also may be that the MDF spoilboard isn’t held down sufficiently in the middle, maybe some humidity or something causing it to bow up. Still I don’t understand why almost a 1mm difference in right to left vs left to right - problem solved, I guess, just want to understand it. With my plastic cutting, the tolerances are fairly low so I need to figure this out before I start boring through $100 sheets of plastic.