CNC - calibration issues for inlay work

[David Falkner]

Let's see if I can explain what I just tested - Just a few minutes ago I cut two slots 1/8" deep with this 0.123" bit in hard Maple, one with the grain and one across the grain. Now, I don't have a good way to measure that slot other than my dial calipers so I used a 1/8" carbide bit shank, measured it to be 0.124", and it fits in the slot with the grain with a little wiggle room as I would expect. On the slot across the grain it fits like a glove.

At this point I used feeler gauges and set beside the shank to press both into the slot with the grain. At first I tried 0.0015" and it's loose so I tried 0.002" and it's a good fit. I tried 0.003" and I can't get the two into the slot without tapping it in with something harder than my finger and at that point I would probably be compressing wood fibers. With the 0.002" feeler gauge the slot with the grain fits like the slot across the grain. So that tells me this 0.123" diameter bit is cutting a slot about 0.126" wide with the grain and right at 0.124" across the grain and that's about what I would expect.

Also, I just tested backlash and it is between 0.0015" and 0.002". I have a Harbor Freight magnetic base that's just so-so at best but a good dial indicator and I did it a dozen times. I got 0.0025" one time but all the other times were less so maybe that's the mediocre base.

I don't think it's a measurement or machine issue but rather something to do with Fusion 360 and the way it's telling the CNC to cut. But I will find it, sooner or later, I will find it!! LOL!

David

[David Buchhauser]

Post you g-code file from Fusion 360 for a 1" round pocket and I will run it on one of my machines and find out. Or email the file to desert.hybrids@cox.net.
What Fusion 360 post processor are you using?
Thanks,
David

[Brad Shipton]

Open the gcode, and find a couple of known dimensions. You should be able to find some dimensions and do a little math to see if the code is correct. I doubt the gcode is the problem unless you have been testing new things recently. Been there, done that.

[David Buchhauser]

Or better yet, put you file in the Autodesk public folder (to share) and send me a link. I would rather take a look a your Fusion 360 cad/cam file.
David

[Kevin Jenness]

David, do you get the same results on end grain vs side grain regardless of how the pieces are oriented on the machine? If so it would seem the issue is in the mechanics rather than the g-code.

[David Falkner]

Ok, found it!!! At least, my tests are now working as designed. I'll try some more complex shapes later.

Here's what I found - I recall reading somewhere that there's a difference between 2D and 3D Contour in Fusion 360 and that 3D is more a finishing profile. But 2D works for what I have needed about 99% of the time and that's what I use. Plus, you can have tabs in 2D but not 3D. I'm not certain where I read/heard that but I couldn't find this again in a quick search so I'll look later. Backlash has been discussed amongst the folks I queried on this so I tested that, as well. It was very minor - 0.001" to 0.002".

Inlay - For the inlay piece I used 2D Contour and my standard climb cut with 0.005" Radial Stock to Leave for the rough pass followed by 2D Contour conventional cut to remove the final 0.005". The inlays, while slightly off, have not been the issue; the pockets were.

Pocket - I created a profile for 2D Pocket climb cut to clear the inlay pockets with Stock to Leave set to 0.005" Radial. I followed that with a 3D Contour conventional cut and no Stock to Leave plus selected Repeat Finishing Pass to clean up the sidewall. This makes the cutter go around the sidewall twice, so even if the 0.005" clean-up pass had any deflection the second pass around should take care of that.

The pockets now measure what I have specified in F360, or as close as I am able to measure. The important thing is that now the inlay pieces fit with no problem. I even placed my 1" round gauge bar in the 1" pocket and it fit (snug, but it fit).

1" gauge bar in pocket -
008 - 1 inch gauge bar fits into 1 inch hole.jpg

Testing backlash -
009 - Testing backlash.jpg

Fusion 360 measurement for random double curve I drew for test -
010 - Fusion 360 measurements.jpg

Actual measurement -
011 - Actual measurement.jpg

All pieces fit as needed, no forcing, not a sloppy fit, just right for glue -
012 - All pieces fit after using 3D contour for final cut.jpg

Thanks to all for your help and suggestions! These are just test files but I certainly don't mind sharing them if you want to dive into the settings.
David

[Ted Reischl]

Ah yes, a tad spindle runout, a bit of tool runout, tool not exactly to size, a wee bit of gantry deflection,a mite of backlash. . . . yup, like they say in government, a billion here, a billion there and pretty soon we are talking about real money. . . .

I have a hard time believing that any motion measured with a tape measure is declared to be "exact" or "perfect".

[David Buchhauser]

Ah yes, a tad spindle runout, a bit of tool runout, tool not exactly to size, a wee bit of gantry deflection,a mite of backlash. . . . yup, like they say in government, a billion here, a billion there and pretty soon we are talking about real money. . . .

I have a hard time believing that any motion measured with a tape measure is declared to be "exact" or "perfect".

+1
Yes Ted - well said!!
David

[Ted Reischl]

Here is some food for thought:

"Units is Units".

What I mean by that is if you use a tape measure to set up a cnc machine it is going to use the units displayed on the tape measure. If you cut something on the machine and then measure it with the same tape measure it will be "perfect". Only problem is that it may not relate to the rest of the world. That is why there are precision measuring devices that are calibrated to a known standard that everyone uses.

So, ya got your machine calibrated to the tape measure which is your known "standard" which are notoriously NOT very accurate when compared to the real standard. The tool you put in the spindle IS manufactured to the known standard. When measuring a big part cut the tape measure does not show .010 of an inch discrepancy, heck the tick marks themselves are .030 wide. But cut a small part and then measure it with a more precise instrument like a dial caliper and voila, sure enough, errors show up. Suddenly that .010 difference looks like the grand canyon.

The way I set up my machine was to use a dial indicator on a magnetic stand. It has two inches of travel and reads in .001 increments. So when I command 1.000 movement that is what the dial had better read or I need to do some tuning. Importantly all these measurements when setting up should be done from the same approach direction to eliminate introducing backlash issues. Backlash needs to be dealt with separately.

[Jim Becker]

Lots of great discussion here.

One thing...not a great idea to use a tape measure for anything that needs to be "precise". Steel rules and calipers, my friend...

[David Falkner]

I agree that tape measures shouldn't be used as a standard but how else can I measure 48" without spending a boatload of money? Nearly everything inside of 6" gets measured with my dial indicator or one of my Starrett rules. I can't think of anything I've done/cut/built in the 'tape measure' range where a tape wasn't a sufficient tool for measuring.

And fwiw, when I did my major calibration last year I fixed one of the tape measures to the spoilboard (taped it down with about 10 pieces of blue tape) and with the spindle off and using a 60° V-bit I scribed a line on a mark, 4" if I recall, and then moved the spindle 48" by command line to 52" and scribed another mark. Both marks fell on the very edge of the tick marks. Tape tolerances aside I think that's pretty good. When I place a dial indicator on the gantry and move it 1" by command line both the DRO and the dial indicator show that it moved 1.00".

I am fully on the side of accuracy and do the best with the tools I have on hand. I just left our monthly Woodworking Club meeting and had a 'discussion' with a friend who was shocked that I use dial calipers for my woodworking. He's of the opinion that 1/32" is close enough and to me, as I told him, that's like leaving a barn door open!

Yes, good discussion here!
David

[Mark Bolton]

Great work on solving your issue. Tape measure or not, who cares if your happy.

[David Falkner]

Great work on solving your issue. Tape measure or not, who cares if your happy.

Exactly, Mark! LOL!

******************************
We've been gone all day down to Natchitoches to tour a shop and a couple of galleries as part of our Woodworking Club we started a little over a year ago. Saw some really cool shop designed and built equipment, too - old iron!

Anyway, now that we're back home I headed out to the shop to do a more complex inlay with my newly discovered technique. This is a treble clef about 7" tall and a lot going on for an inlay. I cut the treble clef and promptly broke it in one place so ignore that. I figured it would suffice for my test. I cut the pocket just like I did on the earlier simple test - 2D Pocket to clear followed by 3D Contour with two passes around the sidewall. It was snug but fit, so I did another pass on the sidewall with Stock to Leave set at -0.001" and now it fits just fine.

Since I don't want to break it again I didn't press it into place fully but it does fit with enough room for glue.

002 - Treble clef, scale.jpg

001 - Treble clef.jpg

I'll do some others later.
David

[Jim Becker]

I agree that tape measures shouldn't be used as a standard but how else can I measure 48" without spending a boatload of money?

https://www.leevalley.com/en-ca/shop...etmakers-rules . up to 36" - I have the set

https://www.woodcraft.com/products/w...netmakers-rule 40" (I also have this one)

https://smile.amazon.com/Johnson-Lev...0607188&sr=8-4 48" aluminum rule (just from a search)

[David Falkner]

So what happens if I order the 48" rule and my tape matches it perfectly...?

Btw, I have a 24" Starrett rule but I may look into getting one of these 48" models.

Thanks, Jim!
David