I’m pretty new to CNC routing, but not new to 3D parts modeling.
I’ve made several items over the course of the past month, (how long I’ve had my Pro XXL up and running), with no problems other than a couple silly mistakes on my part. Most of my items I didn’t check for perfect dimensional accuracy since it didn’t matter on them. Well I’m building some parts using a combo of CNC routing and 3D printing so accuracy is a must. Today I had my CNC make a rectangle that was supposed to be 577.85mm wide on X axis, it only ended up being 570mm wide. I can’t say if Y axis is accurate due to having the bit purposely exit the wood. I know some software has allowances to dial in accuracy but not sure since I’m designing with Fusion360 and moving with Carbide motion. Any thoughts?
First step would be to calibrate for belt stretch:
https://my.carbide3d.com/faq/belt-stretch/
As @mhotchin noted, if using a machine w/ belt-drive axes they will need to be calibrated for belt-stretch.
If you have a lead-screw or ball-screw axis, it should be very close if not dead-on.
Another consideration is tool deflection.
Naturally all of this assumes that the tool is correctly entered/selected and toolpaths are appropriate to the material/part.
I would suggest making a simple test part such as:
in both Carbide Create and Fusion 360 and cutting it in a stable material such as MDF.
If both are wrong, then we can help you w/ the Carbide Create file, and that should provide insights to fixing the Fusion 360. If only the Fusion 360 file is off you’ll need to contact Autodesk.
hmmm the thought of belt stretch never entered my mind. With belts these long I could easily see a quarter inch over 2 foot, thanks!
Very good idea using both softwares to make the same part. Tool deflection in this shouldn’t be noticable I wouldn’t think. I’m running 2000mm a minute at only 2mm cut depth cutting MDF.
Thanks for the ideas! Will cut a matching pair tomorrow.
For an alternative approach to belt stretch see:
7mm seems way off for belt stretch unless they are so loose i would be surprised the machine is moving. Even on ye olde S3 XXL my belt stretch never came out to more than ~1.2mm across the whole machine envelope.
Some ideas:
1 - Often times F360 defaults to “Stock to leave” on some toolpaths. Buggs the crap outta me. 
Give that a check.
2 - Did you use a roughing/finishing strategy?
3 - Did the material move on you? Workholding has a huge effect on accuracy.
Stock to leave was unchecked.
I didnt use a roughing cut, just checked ‘multiple depths’ like I generally do for simple shapes.
Material definitely didn’t move.
The next step would be a mechanical check then.
Belt tightness
Pulley set screws
And just because I’ve fat- fingers a dimension (or 50) in my time, verify that you have the dimensions correct.
7mm is a ton to be off and I just haven’t seen one be that far off without any of those issues present.
What actual tool path did you use in Fusion360? Did you do Trace toolpath (or another that follows a vector) but not move it to the outside (so the center of the tool followed the line rather than the edge).?
Did you run a 1/4 inch diameter bit? Moving on the center line vs outside the line would give you that difference (or at least most of it). You could make a cut using same parameters but a much smaller rectangle. If the error is the same it is not the belts, if the error is proportional to the distance it could be the belts.
If it is an error following the center vs the edge that would put the error from belt stretch more on the order of 7.85 mm - 6.35 mm or 1.50 mm which over 570 mm is a bit more believable.
John
Used a quarter inch bit and toolpath is correct, I went back and ran simulation in Fusion multiple times lol. And the more I thought about it that IS a massive amount of belt stretch so I can’t imagine that playing into it much.
Checked all my puleys today real quick after I saw your message and all seem well. Kinda fingered belts and they don’t SEEM to be loose.
Checked and rechecked dimensions soooooooo many time so far so I do know they’re accurate.
So I only had a couple minutes to spare today so I checked pulleys and verified they’re tight and wiggled belts by hand. To ME they felt ever so slightly loose. The belt tension was set exactly as stated in directions. But being brand new belts maybe they stretch a tad.
Tomorrow is going to be a horribly busy day as well. I think before I do anything I’m going to give a good mechanical once over and retension belts again like in the manual just to be 100%
Hopefully thursday I can do my test cuts and measurements and dig in a little deeper.
Thanks for all the ideas.
Terry
I go thru my machine every three weeks. Every nut bolt and screw. Good cleaning and lube.
Good Luck
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