Here’s MY spreadsheet for doing this calibration, and it works fine for me. I did mine with a 2" square because thats the scale of stuff I need the precision for. I’ve checked against 10" items, and it’s close enough. They are belts. They don’t stretch in exactly the same way all along their length, so there will be -some- variance, but on average, it shouldn’t cause any major issues.
Before you start, check that your v-wheels are adjusted properly, and all the set screws are tight and in place on the flats of the shafts.
Remember, the value you are setting is the number of steps per unit of length. You would expect that if it cuts too short, the value is too small. If it cuts too long, the value is too big. In theory, you could do this over and over and adjust the value a little at a time (0.1?) and home in on it, but it would get really old really fast. You can do it in one pass with a little calculation.
I used HDPE for this because it was stable, and I was tired of messing with MDF. It doesn’t matter much what you use, something hard enough to be easy to repeatedly measure is good, but soft enough you can cut it in one pass is nice.
Bill, here’s step…
Cut a square. I’ll leave that part to you. Something small enough to fit in your calipers. 2-4" is reasonable, bigger is better than smaller.
Measure X (USE CALIPERS!!! Do not attempt to do this with a ruler.)
Using the spreadsheet, enter the X and Y values, the size of the square you intended to cut, and the $100, $101 values you CURRENTLY have set.
The bottom of the sheet will show you what to set the values to. Set the values via the MDI screen in CM.
Cut a square.
Measure X (USE CALIPERS)
Close enough? No: Enter the values from step 5 in the top part of the spreadsheet for the “Current” settings, and go to step 4. Yes: Stop.
You can do this with Z, but you need to cut a stair step and that’s a little more involved. The spreadsheet process is the same.
You shouldn’t need to do this over and over to refine it. Belts do stretch over time, so you should expect to do this periodically, but it shouldn’t take more than cutting a square once and adjusting each time. I keep a label on my machine for the last time I did this so I don’t forget (shows date of calibration, and date of belt install)
Thanks for the reply. That’s one of the article I was referring to in my post.
It states… "Convert it into a ratio by dividing the measured distance by the expected distance, multiply the number of steps by this ratio, and update the number of steps in Grbl with the new value. If the left edge to left edge of two holes which were expected to be 75mm apart measures as 74.97mm, one would calculate:
75mm ÷ 74.97mm = 1.00040016 × 40 = 40.0160064
The calculation is correct, but the original instruction is reversed.
I’ve checked and my V-wheels are good. I’m going to re-check my belts. I’ve only got a total of about 3 or 4 actual hours of cutting on my machine, so there shouldn’t be that much wear on the belts.
I’ve tightened them to what I think is the right tension. But, reading posts regarding belts, you keep hearing “Tight, but not too tight”. One persons Tight might be another persons too tight and visa versa.
Can’t thank you enough for your reply and spreadsheet. I’ve been doing those calculations with pencil and paper for hours. I don’t know why I didn’t think of loading up MS Works Spreadsheet and plugging in the formulas myself. Frustration mostly I suppose.
I ran some of the measurements I’ve made through your sheet and found that I was doing the calculations correctly. However, the adjustments to $100 & 101 were not producing consistent results. I’m going to go back and re-check the belts this weekend and then run the figures through the sheet again.
Consistency problems are going to be caused by something being loose. ie. You have backlash somewhere. The big culprit is always the set screws on the pulley’s not being a) firmly seated b) not having one of the screws on a shaft flat. V-wheels if overtightened can develop flats that will cause some consistency issues. V-wheels that aren’t tight enough will cause play in the system. Another one to check is just plain screws everywhere.
The belt stretch process will get you to round round things, and straight lines that are the right length, but can’t adjust square. That has so be dealt with in the chassis. You should be able to put a good square onto the square you cut and see if things are square, and which direction they need to go. I think getting things square is the hardest part, and requires a lot of patience. Then again, it may be “square enough” already, and you’re good to go.
Oddly the XL assembly instructions are missing the squaring section:
Squaring Gantry to Front/Rear Plates
1.Loosen all of the screws that hold the gantry together (4 on each side), these should still be loose from the initial assembly.
2.Loosen the screws that hold the Y axis rails in place (16 total). These should also still be loose from the initial assembly.
3.Slide the gantry to the front, so both Y plates are touching the front plate.
4.While holding the gantry against the front of the machine tighten the front of the Y rails (8 total)
5.Now - systematically begin tightening the 8 bolts on the gantry. Work your way from left to right, going back and forth in a X pattern (similar to tightening the wheel of a car).
6.After the gantry has been secured, slide the gantry to the rear of the machine and tighten the 8 screws while keeping the gantry pressed against the rear plate
A quick first pass at determining runout is to simply cut a slot and measure it and compare it to the endmill diameter.
Alternately, cut a 4" square out of the material in question, then measure both the square and the resulting hole — half the difference is the effective cutting width*, less the endmill diameter is runout.
Alternately, when cutting using the testing technique the Precise Bits folks advocate:
Over the past four days I’ve taken my machine back down to the original waste board and started from scratch trying to square and calibrate things. I found that the base was an 1/8th out of square. Loosening the four corners and putting some tension here and there I managed to get that down to a 1/16th which I feel is about as good as it’s gonna get.
Even with the base square, the right Y-Rail touched the back of the machine a little less than 1/8th inch before the left causing the X-Axis to be very slightly out of square with the Y-rails. Loosening,adjusting and tightening both sides of the Gantry pulled it into perfect square.
A test cut showed that I had no Runout. Running the butt end of an endmill through a few test cuts was snug everywhere.
Though my belts were what I called tight, I tightened them about two belt teeth. That made them slightly more taut and I believe they probably needed that.
Thanks to Stacy @bonch. On a related topic, she included a video showing a pretty neat way to test your stepper motors to see if they were producing what you were requesting. Using a 4 foot metal ruler with MM markings and an flat sided engraving bit. I sent my gantry on a 750 mm run in the X direction and then on the Y. I couldn’t believe the results. 750 mm away from the starting point on both tests, the blade stopped on the exact mm marking it should have.
With everything tightened down I ran a test cut on a 5" circle profile. Calipers showed readings of 4.998 in one directions and 5.001 in the other. A square profile test showed similar readings within the same range of .003 to .005.
Though I’ll have to start over, maybe now I can get back to @garyl 's clock project.
Thanks again to all.
Edited to add the results of the stepper motor test with the metal ruler.
Just remember, part of the frustration is working out the kinks. Part of the education is working out the kink. As you use the machine you will become more and more knowledgeable of what causes what problems. At some point, we will put together a troubleshooting manual with pictures and explanations. But when will we have time
I have to agree with you. Didn’t really want to take up the new waste board and loosen all those parts, but the whole experience, including all the thread reading, was worth it in the end. I learned a number of things. Especially the effects of one part on another.
I don’t know exactly which change or adjustment or combination thereof actually produced the great final results. But, at this point I’m happy to just work with what I’ve got.
Though I haven’t experienced any problems with the Z-Axis, I still aim to check it soon as well. Now that I’ve put in another new waste board and received a new 3/4" flat endmill, I’m also going to take a shot at tramming.
All of my results were achieved with $100 & 101 set back to 40 steps and the video you referred to for checking the steppers just put an exclamation point on everything.