stiffness measurements cnc mk3

[routercnc]

Hi Neale,

However, it's difficult to know where "acceptable" is on that kind of scale, which is why seeing some measured numbers helps. Is your machine currently usable? Can you relate the stiffness numbers you have obtained to actual cutting speed, DOC, surface finish, etc, results?

I've been cutting out a few 20mm thick upgrade parts (will post in the build log soon) and have taken some photos of the edges of one of them. These were cut using the following:
6mm carbide 2 flute
1.0mm DOC
600mm/min feedrate
occasional spray of duckoil

cutting in the X direction
(there was a fair bit of what sounds like cutter resonance in this direction)


cutting in the Y direction
(much less chatter/resonance sounds in this direction)



My X & Y axes are currently 1000N/mm and 1250N/mm respectively at 50mm Z extension (measured on the collet nut). But these parts were cut at a longer reach, closer to 100mm as they were almost on the bed so the actual stiffness for these cuts would be lower.

I don't have an airline at home so can't clear the cutter path and suspect some of marks are chip re-cutting damage. But some look like cutter resonance as they are so regular. This suggests if you regularly want to cut a lot of aluminium, and want a nice finish, then you need a stiffer machine than my mk3 machine, perhaps more like 2000-3000N/mm in X&Y @ 50mm Z.


For the latest part, the Z axis motor mount plate, I followed the cutter with the workshop vacuum cleaner to remove as many chips as possible and got a better finish in terms of surface nicks etc. It's better but the resonance type stripes are still present although in this photo you can't see them that well. Here is the start of the job pocketing for the stepper motor:



Hope this helps.