Bumpy edges on cut

[Hammersmith]

I have recently acquired a CNC router but the edges of cut plastic (acrylic) are not smooth. This only happens on curved shapes. In other words when cutting on the Y axis or the X axis by itself the cut is fine. However when the X and Y axis work together there are bumps on the cut face. Having read the other threads in this section I cant help but wonder if its due the size of the sprokets I have on the rack or the size of the cog I have on the shaft of micro stepper motor. The sytem is rack and pinion and it advances 5 inches per revolution of the motor. What should I ideally have ?

Any help greatly appreciated


Hammersmith

[SignSolutions]

I had this issue before and it was due to the driver not being correct. On the screen it would look correct, but the G-code was created and processed by the software driver, bumpy curves resulted.

If your router has an on-board shape utility (cutting rectangles, circles, etc. without the need for loading a .dxf or similar) I'd give that a shot to rule this out.

Keith Watkins
Sign Solutions

[ger21]

Most rack and pinion users get 1" of travel per motor revolution.

On the curves with bumps, are they G2 and G3 moves, or a series of G1 moves.

[Hammersmith]

Im not sure what you mean by G1 G2 or G3. To help illustrate the problem I have traced around a letter G that I cut on the machine.

Would the fact that I get nearly 7" of travel per rotaion amplify my problem ?

[jharvey407]

Do you have a picture of the bumby edges? I am having the exact same problem.

I am cutting a plastic called 'Starboard'. The problem is the worst at 120 ipm, so I am wondering if it is a problem with tool chatter caused by resonance.

I'll try to post a picture in a day or two.

[rock4xfab]

sounds like a backlash issue seen in older knee mills when the screws start to wear out.

[schbrownie]

It could be in your program.... If you move in the x axis a 100 ipm and it's ok, then if you move in the y axis at 100 ipm and it's ok you're still going 100 ipm. HOWEVER, if you move in both the x and y axis at 100 ipm (at the same time) then your endmill (or tool) is actually seeing about 140 ipm! You may need to slow down your feed rate during these XY moves.

(note, speeds made up for easy math)

[Torsten]

Im not sure what you mean by G1 G2 or G3. To help illustrate the problem I have traced around a letter G that I cut on the machine.

Would the fact that I get nearly 7" of travel per rotaion amplify my problem ?

Actually it dose look bumpy even in your Picture.
Is this a DXF file you are driving your Toolpath with?
I sugest you take a close look, zoom in on the areas in question and check the quality of the geometry.
If possible convert any line geometry on those curves to circular arc's, before creating the toolpath.

[lawrencex2]

sounds like a backlash issue seen in older knee mills when the screws start to wear out.

I second that.. I also had this problem with my mill. Every time I cut a circle it came out realy bad. Once I tighted the screws on my nut I could get realy nice round circles.

The sytem is rack and pinion and it advances 5 inches per revolution of the motor.

with such a large movement with one revolution this could also be your problem depending how many micro steps you are taking.