Parts not sizing up

[lonnieh]

Hi all. I am working on a JGRO style build. To speed things up and to have immediate access to a CNC machine, I went ahead and purchased a Zenbot 1216 (edit: Didn't buy it on eBay.) Pretty nice machine for a newbie. However, I am running in a problem, I believe with my RotoZip. I can't seem to get parts to size up correctly as they have been drawn.

Here is what gets me. So I drew a 1"x1.5" rectangle and generated some G-Code with CamBam. When I run the program, I end up with a 0.9725"x1.468"x0.75" rectangle (cut from a sheet of 3/4" HDPE.)

I've been reading up on router runout and it seems like this could be related. I'm baffled, any ideas?

FYI, I am using EMC2 for step generation. I am pretty sure I have everything set up correctly. I am using a config file sent to me by Zenbotcnc.com.

[ger21]

Are you using a 1/16" bit?

[lonnieh]

No sir, I am using a 1/4" bit.

[chester1957]

From my experiance run out will cause an inside cut being oversize, as the cutter will cut larger than 1/4". I would check backlash. I made a quick reducer for running 1/8" bits in 1/4" collet and ended up with some run out that caused inside cuts to be too large and outside cuts too small. I adjusted cutter size in cambam from .125 to .135 and parts came out right on. This is only a band-aid till I can get better tooling.

[wendtmk]

Correct tool offset?

Mark

[lonnieh]

@wendtmk: I have CamBam set up to use a 1/4" bit, I'm not sure if there are any other configurations for offsets. I am using the free version and not the plus.

@chester1957: Thanks for the info, I'll have to give that a try.

[ger21]

Are you're steps per unit set correctly? If you do a G1 X1, does it move 1"?

[lonnieh]

Yeah I believe so. I popped an ink pen into the 1/8 collet and drew some things out and measured them with my calipers. Everything looked pretty much dead on. I did notice that my RotoZip (a Rebel) wasn't the tool that the holder was designed for (a RZ10). I trimmed some of the ribs off and now it fits a bit tighter, so it is very possible that the bit wasn't true.

I'm going to try a bit lighter load tomorrow. I live in an apartment building so I have to wait for the neighbors to leave