Dimensional Accuracy

[jamesmcbennett]

(New Shopbot owner and new to CNC. Background in design.)

I am trying to get my CNC to make an accurate profile cut, but can't figure out what is wrong whether it is geometry or software settings or machine setup.

I took the images below which tell some of the story.
http://imgur.com/a/IQpIg

Basically the problem is....
18.0 mm in rhino = 16.14 mm when CNC'd
18.5 mm in rhino = 16.32 mm when CNC'd
19.0 mm in rhino = 16.56 mm when CNC'd
20.0 mm in rhino = 17.35 mm when CNC'd

Also I need to cut 20mm down to get guarantee cutting through the 18mm plywood of which the cut into spoil board is less than 1mm deep.

[jerry_stanek]

Have you checked your bit to see if it is under sized.

[khaos]

Have you checked your bit to see if it is under sized.

X2! bit size seems like a great idea more than an actual thing for some bit manufacturers.

[adrianm]

What are the dimensions of the pieces when they are in VCarve? Same as Rhino?

Moving from one piece of software to another there is always a risk of one or other not converting properly. Best to stick to one wherever possible.

Easiest thing to do would be to draw some simple shapes in VCarve and then cut those. That will eliminate the Rhino step and let you see if the issue is with your ShopBot setup or Rhino exporting.

Another basic test worth doing is to tell the machine to move 6" and then verify that it has actually moved 6".

The quality of the cuts look very bad so my gut feeling would be something is loose on the machine.

[steve_g]

James...
Is this a new machine? Something’s not right... is the part being held securely while being cut?
SG

[gc3]

looks like material moved while cutting, i would use a 3mm end mill lhf on something this small cutting slow

[MogulTx]

Something else to consider: What size are the lines? In other words, when the part was drawn in Rhino, what width were the lines being drawn and were they interpreted with a dimensional width when brought into the Partworks/Aspire?

Whenever I draw something, I do it in another program ( similar to Rhino but much less costly, and one with which I am familiar), then bring it into Aspire. That program ( any CAD program) can allow you to create lines of a specific width. If you have lines with width and which you are telling the program to cut inside of, maybe the program is doing exactly what you are asking.

My problem with that theory is that your first example is rather mis-shapen. The cuts are very irregular. Whatever the machine does, it should do with "purpose" and precision... and that photo does not look like precise cuts

To check out this possibility: Do a dimensional check of the inside of the lines in your Part Works/ Aspire program, to see what it is that it is "seeing".

Next: the first cuts are not looing precise....So could you have loose pinions? (check by: Turn on ShopBot and then push and pull on the y axis carriage and see if you get any slop in the X or y. If so, you need to address that slop. ANY movement needs to be removed by pressing the pinions back into its rack and securing. (assuming PRS or PRS Alpha) Pinions may also be worn out, depending on age/use.

Those are my two guesses and potential ways to check out. I hope you sold the mystery quickly. Good luck.

Monty

[jerry_stanek]

What Shopbot do you have size and model

[jamesmcbennett]

Wow. lots of great responses! Thanks for the support.

Jerry/Khaos: I'm not sure how bit sizes are meant to be. It is certainly 0.25 inch at top of bit on the smooth part, but the flute part is at its widest 0.22 inch tapering slightly as the flute appears. Not sure if that right or wrong? Shopbot is brand new PRS Alpha 96 x 48, 4HP Spindle. Also I am using it in millimetres.

Steve_G: Brand new machine. Just got it. Bit is very securely held.

GC3: Don't yet have a 3mm bit, but maybe should get one. Just have the Shopbot Starter kit at moment. Also. I don't have suction, but the kink on each of the edges was done on every corner, not just the last one to be cut. Given it did 4 passes which the wood wasn't loose on the first three passes, but the kink was in all of that passes rules out looseness being the main contributing factor.

AdrianM: I checked that dimensions in VCarvePro are the same as in Rhino as 18mm,18.5mm, 19mm and 20mm. I will try draw something in VCarve and cut that to see if it behaves differently now as well as doing a 6" test.

MogulTx: Not sure if I am understanding question correctly on line-width, but rhino has 0 thickness to lines unlike Autocad that can specify line-thickness. Will check the pinion test, but this is a brand new machine, machine is PRS Alpha 96 x 48, 4HP Spindle.

Will be back in 2hrs or so having done tests above and provide more info.

[jamesmcbennett]

Posted on Reddit too. Someone noticed the dimensions cut are all between 87%-89% of the specified dimension. Not sure what this means, but likely something there....