Accuracy

[paul60]

i have been cutting cabinet panels 787 x 609 mm
when i measure diagonal i am out by 2mm
what is the best procedure to go about solving this problem.
thanks for any help
paul

[Brady Watson]

It sounds to me like your X car is not square. Searching this forum will return several threads on how to square it again.

-B

[harryball]

Yes, Brady is right, check your X car square. Also, run a light material like foam or 1/4" luan at a slower speed where you know there will be no deflection of the gantry or Z axis. If you get a good square cut you could be experiencing some kind of deflection of the bit or Z assembly while under pressure.

Robert

[paul60]

thanks for the help.
what i did was to cut a square 3mm into a piece of plywood and that came out perfect. so there must be
some deflection can this be a collet problem?
i am using a 3/8 upspiral bit with a 1/2 to 3/8 collet. the bit i used is new.
thanks paul

[Brady Watson]

Paul,
I'm not sure what feedrate & stepdown you are using with that 3/8" bit, but you might want to try reducing the stepdown AND feedrate & see if that helps. If it does, you are getting bit deflection. Slow things down and see how it does.

-B

[paul60]

my feedrate is between 1.5 and 2 ips
in .75 ply 3 passes at 20k
thanks paul

[harryball]

Paul, I'm by no means an expert and I've been learning at warp speed... though some days I feel about as dumb as a brick. I'm curious about your issue because I've seen similar issues myself and have made corrections as I go to work them out. I've found that most of the time I'm the problem. I'll share a few things I have learned that might help...

Chip loading is a big deal, and bit geometry has a big impact. After this first shopbot camp I learned I was running the wrong bit for the cutting I was doing. I was running a 1/4" 2 flute upsprial that will cut plywood. Looking in my newly obtained Onsrud catalog, that bit has a chipload of .05 to .06. in hard wood and is not even listed on the soft plywood list. I was running at 18,000 RPM at 5 ips with this bit based on some numbers I'd looked up on my own with some educated guessing. It was running and cutting but my parts were about 1/32" out of size and had less than perfect edges.

After talking with Scott from Onsrud and doing some research I found I needed to run a 60-111 1/4" single flute bit. My speeds are the same, but the chipload range of .015-.017 is correct for this new bit. When I pulled my first parts off I noticed my parts were now oversized! Then I remembered I'd cheated on my part file while running the previous bit. All fixed now.

Chiploading is not as complicated as it seems at first, it's all about getting the bit through the material at the "right" speed. If you don't run in the bit's sweet spot you can get excessive burning or bit deflection or force on the gantry.

Your cutting speeds certainly don't seem excessive. But it's something you might want to investigate further. The bit I was running at 5 ips should have been running at about 2 ips for the material. If I was in hard plywood that would be even slower.

I guess the moral is that the bit geometry and number of cutting edges can make a huge difference. More than I ever expected. It seems counter intuitive to me that a single flute bit can run faster than a 2 flute bit. But it's all about what the bit geometry is designed to do.

With excess force to the gantry the X car and/or Y car can torque as well as the Z axis. Making sure things are tight and your V wheels are tight and properly engaged will help.

You may have known all this, but it's a new lesson learned for me and I figured it couldn't hurt to share.

Robert

[Brady Watson]

Making sure things are tight and your V wheels are tight and properly engaged will help.

Yes...especially the v-roller bearings on the Z axis if you didn't loosen up the Y-car before install the Z axis & you got some spread...or your T-rail has worn & the v-rollers need adjustment.

-B

[paul60]

Thanks again for your help the bit i am using for plywood cutting [ pine plywood] is made by courmatt
3/8 upspiral 9cu-375-2 i wrote to them asking for
the recommended cutting speed http://www.courmatt.com/
i will also check on the v rollers to see if there is any play my bot is a prt standard 8 months old
what would you say is the tolerance acceptance lets say for a piece 780x780 mm
thanks paul