[Paparashie]

Hey all!

I'm struggling for a very long time now with my homemade wooden CNC machine.

In short: when contour cutting sheets of plywood or hardboard (4-6mm) it will always leave some rough chattered side surface. This only happens to certain side of the stock.

The issue: Cutting straight, single axis, lines leaves a perfect smooth finish. The issue is when cutting diagonal/arcs.
When cutting a circle for example, where the grains are going from left to right (nr 1&3 on first image), the chatter accurs from 12 to 3 on a clock and from 6 to 9. *Rotating the plywood 90 deg mirrors the chatter in de opposite sides.

I made about 40 circles with different speeds, feed, RPM and different tools.

Things i tried:
- Checking for backlash: basically no backlash
- Using 3mm to 6mm 4f carbide flat end mill
- Using 3mm 1f straight flute
- Using 3mm 2f spiral downcut end mill
- Screwing + double sided tape the stock
- Using tool with minimum stickout *(10mm)
- Conventional or climb milling with 0,5-1 mm stock to leave and then climb or conv. milling the meat + finishing pass climb and/or conv. I tried every combination possible.
- Spoilboard is flattend
- Cutting full depth*
- Cutting 1mm per pass
- Beefing up the arms that hold the y carrier
- Tramming

I've been looking on a lot of forums discussing schatter issues. My issue lies only on certain edges, the opposite side of de grain seems clean. This is the part that really confuses me and i really have no idea what to try next.

I recently tried some lettering in MDF. even this gave me poor results when carving diagonal.

I have really no where else to look. I’ve already spend 30+ hours to track this issue down without any significant improvement. Any help would be much appreciated!

HGR20 linear rails
SFU1605 ballscrew with BK BF
1/32 microstepping
Makita rt0700c router
Rough cut 1500 mm/m
Finishing cut 200 or 500 mm/m
Fusion360 for modeling and generating gcode*
Using ramp to enter stock*
Router is for about 98% square to the spoilboard

[dalrun]

What you're showing is tearout and it can happen anytime the bit cuts diagonally across the grain. With plywood circles, each ply has two (opposing) quarters most likely to show tearout. The pictured Meranti/luan is cheap, soft, and prone to tearout. I usually rough with a two flute straight bit and take an ~.5mm wide full depth finishing pass with a compression bit to clean things up. I always use two flute bits on wood, four flutes are not suitable and single flutes are only okay for roughing. I haven't analyzed or experimented much with feeds and speeds but ~1800 mm/min, ~18k rpm and ~1/2 bit diameter depth per pass work well for me. If anything, I speed up mm/min and rpm for the finish passes, too slow and you're just slapping the wood and dulling the bit. Climb cutting can help prevent tearout if the machine is stiff enough (mine isn't), but a shearing bit (compression, upcut or downcut) that's nice and sharp and moving at a suitable speed is more important.