# Calibration question



## difalkner (Jan 3, 2012)

My calibration still isn’t as close as I know it can be so I created a file today for some testing and calibration. This is a busy piece but served its purpose. Here’s what I cut with a 1/4" two-flute downcut spiral bit running 18k rpm and 125 ipm. The depth of cut is 0.1875" and it is cut with one pass leaving 0.005" on the side walls for a final clean up cut to get the fuzz off. The depth needed to be just deep enough to allow my dial calipers in for good measurement. 









I started on the bottom left, ‘Before’, and made my first cut. You can see it was out +0.005” on Y and -0.010” on X. I changed the settings in Mach4 and then ran the top left. Ignore the diagonals, those are to ensure I didn’t get too close to the screws. The circled dimensions are exactly as they’re supposed to be. I then ran the top right and it’s exact on Y and out +0.005” on X. The middle right was next and you can see it is exact on Y but out -0.002” on X. Bottom right is exact on X and +0.005” on Y (I wrote it wrong on the piece). The circle is very close, as well.

However, all of the inside circles are 0.015” to 0.025” undersize and I don’t get that. The outside squares, rectangles, and circles are close enough to be acceptable. I think the variance is due to the material which is a sort of soft Purebond plywood from Home Depot. If I was cutting hardwood I think the tolerances would be much closer.

So how do I get the inside circles to cut accurately? This makes it very difficult when I’m doing inlays and inserts, fitting dowels or bolts in holes, etc. I can’t change settings now or the other measurements will be off and I don’t want that. Is it something not set correctly in Fusion 360? Something in Mach4? It's not mechanical; the machine is very rigid and tight and if it was mechanical the squares, rectangles, and outside circles would be off, as well.

Thanks!

David


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## BalloonEngineer (Mar 27, 2009)

First, check the “inside” ears of your calipers - use them to check something of known size - I use a 1-2-3 block, or compare to a a micrometer if you have one. Need to eliminate possibility of measurement issues, especially if it is only being seen on internal measurements only. It’s also fairly easy to not find the true diameter when the sharp edges of the calipers could dig in to the soft wood. Try cutting .500 circles and see how Router bit shafts fit. 

Second, how does the “calibrated” steps/inch match the calculated value’s based on the rack and pinion and microstepping settings? This should be directly calculateable number, not something that needs to be arrived at experimentally. The grade of rack and pinion used on this caliber of machine could have an error of + or - .001/12 inches, and that is as close as you’ll ever get, although this is far less than your measured discrepancies.

I would also repeat your test using a more rigid material (phenolic, acrylic, etc), and carefully check your actual bit diameter. If the bit diameter is off, and you correct outside dimensions, the inside will show double the bit diameter error. Also check climb versus conventional cut holes of the same size.


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## difalkner (Jan 3, 2012)

I just got in from church and checked the bit - it is 0.250" on the nose. If I really work at it I can make it be 0.2495" at the very tip of the bit but for all practical purposes, and for this test, it's a 1/4" bit.

I know using hardwood or acrylic would be better but until I get this a lot closer I don't want to waste good materials. Because the X and Y are very close now I need to figure out what's going on with the inside dimensions. I can slow the feed rate but I don't really think that's the issue. And going from conventional to climb (or vice versa) won't amount to the 0.020" and greater errors I'm seeing. And if the X and Y are as close as they are I don't think any error in the R&P factors in on the inside dimensions.

When you suggested measuring the bit I was hoping that would be the solution, and I actually thought about doing that beforehand, but the bit isn't very old and I figured it would be pretty close.

I have Forstner bits of the size holes I cut and I can't get those bits into the holes. I can accurately measure the Forstner bits and most are about 0.010" undersize but even at that they won't fit into the holes I'm boring, so that tells me my inside measurements aren't too far off.

David


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## MEBCWD (Jan 14, 2012)

My initial thought was bit size and I'm still thinking along those same lines. Part of it may be in the material and like Richard says it might be better to use something more rigid for alignment testing. Also I would make all the test cut files the same so you are not trying to compare apples and oranges to walnuts and basketballs. Maybe an outside square, an inside square, an outside circle and an inside circle, like the picture shows and use it for all test cuts.

To my way of thinking if you make adjustments to the measurements until the *error* in the cut is equal both inside and outside then you have the perfect alignment and that would all be based on a bit with unknown measurement. It will be setting the center of the cutter an equal distance from the cut edges both inside and outside. Then it would not make any difference what size bit you use because it is based on the center of the bit, if entered in the database correctly then all cuts would be correct. A test cut with any bit would let you know what you need to use for bit size to make each cut with that bit right on the money.


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## MEBCWD (Jan 14, 2012)

Look at what you will have after the test cuts. A set of coasters, a set of rings for a ring toss game and some square picture frames with round holes and square frames with square holes.


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## MEBCWD (Jan 14, 2012)

Just to make that a little clearer here is a picture. Even if your bit measures correctly and your cuts are off this will correct the problem.


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## 4DThinker (Feb 16, 2014)

What CNC do you own? The error may be to a little backlash, some flex/play in the Z axis, etc.. If either the direction of cut may pull the bit left or right as it goes forward. perpendicular to the play in the system.


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## MEBCWD (Jan 14, 2012)

One more thought. You are running Mach 4 so if the bit is actually 0.25 then you could make your adjustments to correct the error in the configuration settings in the software and not the tool database.


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## difalkner (Jan 3, 2012)

4DThinker said:


> What CNC do you own? The error may be to a little backlash, some flex/play in the Z axis, etc.. If either the direction of cut may pull the bit left or right as it goes forward. perpendicular to the play in the system.


Saturn 2x4 - you didn't see the build thread? :wink:

I doubt seriously there's any deflection, 4D. The finishing pass is only taking 0.005" off the side wall and the material isn't that hard. I'm going to say on a machine this rigid there would be no deflection on such a light pass. Backlash doesn't make sense to me, either. Several of the outside cuts are exact and a few are less than a couple of thousandths out. If it was backlash everything would be out to a similar degree and repeatability wouldn't be nearly as good as it is. 

But the inside dimensions are out as much as 0.025". Yet I can hit 'Go to Zero' and run each of these profiles again and not take even the tiniest bit of new material off. That tells me the machine is rigid, repeatable, and accurate as set up. But something is telling it to cut inside dimensions undersize, whether that's something I did or am doing in Fusion 360 I just don't know.



MEBCWD said:


> One more thought. You are running Mach 4 so if the bit is actually 0.25 then you could make your adjustments to correct the error in the configuration settings in the software and not the tool database.


The bit is actually 0.25", Mike. Where would I make the config settings change? I guess I'm not familiar with that. We just got in from church so no time to try it again tonight but I may try your suggestion a few posts earlier on what to cut and how to do it.

Thanks, Guys!
David


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## MEBCWD (Jan 14, 2012)

Here is a PDF of the M4 manual. The calibration procedure starts on page 21.


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## difalkner (Jan 3, 2012)

MEBCWD said:


> Here is a PDF of the M4 manual. The calibration procedure starts on page 21.


Thanks, Mike! I'm just seeing this, guess I missed it when you posted this nearly a week ago.

David


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## difalkner (Jan 3, 2012)

*Update *- I planned to cut the stripes plaque but decided instead to test the machine to its full capacity of 26" x 50" and calibrate to that instead of the way I did it the other day using partial travel. So, now I am comfortable that when I plug in a dimension I will get that dimension, at least as close as I can measure.

So here's the *new *issue - gantry squareness. 

Let me see if I can explain this properly:
1. Using Fusion 360 I generated a toolpath for a 23” square and then an offset 1/32” to the inside. My spoilboard is only 24" wide and I wanted the cut inside the edge of the MDF for obvious reasons.
2. I set a depth of 1/32” with a 60° V-bit and cut the square into the spoilboard.
3. I then measured the inside points at the corners (I put a black dot on the inside point to see it better).
4. When I measured across the diagonals I found one is 32 7/16" and the other is 1/32” shorter, might be slightly less than that by 0.010" or so which makes it about 0.020" off.
5. The proper measurement should be 32.439” which is close enough to 32 7/16” (32.4375”) and given the distance that’s as close as I can measure anyway.
6. *What now?* Is this within tolerance or should I expect closer? I rarely cut anything that large so the error of <1/32" in a 32" diagonal may just be as good as it's going to get - or is it?

I realize the actual measurement is not relevant, just that the diagonals are the same measured with the same device. I’m glad it’s actually very close to what Fusion 360 says it should be, though, and might be right on though I can’t measure to a couple of thousandths over that distance.

Tomorrow I'm going to extend that out to the maximum I can engrave on the spoilboard and that will be right at 23" x 47". I mounted the spoilboard back a couple of inches from the front and that means I can't engrave the full 50" capacity. Once I cut those rectangles I'll check the diagonals on a far greater distance and see how close they are.

Here are a few photos to support my description above - 
























David


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## MEBCWD (Jan 14, 2012)

You should be able to adjust your homing switches on the Y axis to square the machine.


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## difalkner (Jan 3, 2012)

I only have proximity sensors on the left side (X-), none on the right side (X+). I'm heading to the machine now to run the larger sampling and see what I get there and I'll report back.

David


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## difalkner (Jan 3, 2012)

I just ran the calibration rectangle at twice the length of the square for a greater distance - 23" x 46". The results are that it is double the error of the 23" square. That was a little under 1/32" off and this is a tiny bit under 1/16" off on the diagonals.

Fusion 360 says the diagonal should be 51.43" and I'm measuring 51 7/16" (51.4375") - close enough to 7/16". The other diagonal is measuring 51 1/2" or a tiny bit under but very slight. So for all practical purposes I can call it 1/16" greater distance.

I was told on Sawmill Creek that I can fix this with autocorrect in the software. Any of you gurus know how? I haven't gotten a response there yet. I'm running Mach4 with ESS if that helps.

This graphic may help –








David


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## MEBCWD (Jan 14, 2012)

David do you have homing switches? If you do then all you do to adjust the squareness of X/Y is adjust one of your Y axis homing switches. This lets that side move to change the angle of the Y axis to the X axis to make it 90 degrees. This will adjust the diagonals where they are even like they should be. You probably should do the calibration first so your measurements are correct.

If your cuts are too long or short then follow the procedure starting on page 21 in the manual I posted to calibrate the length of cut. This makes your cut lengths correct.

Here are two videos that might make it clearer.


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## difalkner (Jan 3, 2012)

Here's an update - Yesterday I loosened and/or removed the 24 fasteners required to square the gantry at least 5 times before I got it dead perfect. You can see in the spoilboard how many rectangles I cut so I could measure the diagonals. I cut multiple rectangles of 23x46, 22x44, and 20x40 and kept sneaking up on 'perfect' until I finally got it to where no matter what size rectangle or square I cut the diagonals are identical. At least, they're as identical as I can see with 3.0 reading glasses and getting right down on the tape to compare each measurement. 

At one point the diagonals different by 3/16" on the 23x46 rectangle. And I don't have to be concerned with temp and humidity because the shop is climate controlled 24/7 but that aside, neither temp or humidity will have enough effect on MDF in the span of a few minutes to cause a change. I'm comfortable enough with my measurements and methodology at this point to say this is as close in calibration as I can get this CNC machine.

I also upgraded the drive spindles to the latest ball and needle bearing versions with hardened pinions and the better GT belt.

You can see here how many times I cut for these tests (patience is one of my strong points - LOL!) -
















David


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