Machine accuracy

I just received my 12x30 and am setting it up. I have it mounted on a torsion box that is nice and ridged. I was doing some testing for square and I noticed that when I jog 600mm I get around 602mm. Everything is nice and snug and it homes back to the Mark I made at zero. I am using a 30 degree engraving bit to plunge a tiny mark to measure off with a steel rule.
Is 2mm over 600 as good as I am going to get? Anything I should be doing with setup parameters everything is as per the assembly instructions.

@daniel.kerr Is if off in X, also? If I understand correctly, you jog Y in one direction for 600mm in the software, but when you measure the actual distance, it is 602mm. When you jog back in the opposite direction for 600mm in the software, it returns to the starting point. So, again, it is actually moving 602mm. Correct?

If you start in the opposite direction,does it do the same thing?

What software are you using to send the manual jog command?

I am using UGSPlatform to jog it manually. so far I have just been measuring the x travel as it’s the longest axis ( 12x30 machine) yes when I home it it comes back to zero in the same spot. It doesn’t appear to matter if I do it in one big step or many little ones and if i go a step past to 700 and come back it finds the same point so not likely a backlash issue I’m thinking.

You can calibrate the stepper motors to tell it how many steps are required to move 1mm.

Maybe google it, but you should be able to do it with UGCS. I’ve only done it for 3D printers, not a CNC, but should be similar. Commands $100 and $101 for X and Y axis.

Moving back to your zero is a good sign though. Means it isn’t losing any steps.

@daniel.kerr Sorry, Daniel. Somehow I had it in my head that the 12 x 30 machine was longer in Y than in X.

So, back to my questions. If you jog in X+ manually for 600mm, it really moves 602mm. If you jog back manually - not home it - does it end up at the same point. Meaning, is it off in both directions.

What about jogging in Y?

Hi Daniel,

While it is implied in the questions others asked, I don’t think anyone explicitly said this - No, I would not say that being out 2mm over 600mm is normal for the Longmill. I have a 30x30 not a 12x30 but I know of no reason that would be different.

It is also important to note that tapes and rules come in varying degrees of accuracy. You might gain more comfort using a digital caliper (available for $15-$25 at places like Canadian Tire or Harbor Freight) and doing some calibration cuts.

I used a descending size of squares with circles in them (touching the four edges) and then a caliper to determine the accuracy of the cut compared to the source file. So a 400mm square with a 400mm diameter circle in it, a 200mm square, a 100mm square and a 50mm square (all with circles in them). You could repeat this in various areas of the bed if you think the problem is at one extension of the axis versus another.

I did a lot of fiddling to try and get my machine right when I first built it and had some frustrations. Ultimately, when the discovery by Sienci that a recent batch of V-wheels was not meeting spec arose, I went and checked one side of my XZ and discovered a single wheel (that had been giving me trouble and got damaged) was NOT the same size as the others. I had purchased spares so I replaced it and the machine has been much quieter and consistent since then. I’m not suggesting you have this problem (doesn’t sound like it at all), just sharing the detail.

Try the circle/square idea in some decent MDF (you don’t have to go deep either, maybe 4mm to give the caliper something to grab on reliably) and check it. Once I made sure everything was tightened properly I was getting accuracy at the .1mm level.

I did find, after my initial build and scratching my head, that slowly double checking all the “tune-up” items that are detailed on this link REALLY helped:


Thanks for the feedback @jwoody18, @gwilki @drrkdrrk. I havent had a chance to get back out to the machine yet but i will definitely do some more testing. the testing started just by putting 4 dots at the machine extents to try and confirm my rails are square i did a 800x300 square and measured the diagonals and it was sub 1mm so that is good so any issue i have is scaled on both axis. i will use some different measuring methods with different sizes to try and confirm a true measurement.
I haven’t even put down my wasteboard yet i am just plotting on the plywood torsion box to try and confirm trueness first.

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I would recommend double checking the tightness of all the items in the maintenance link before you do any other stuff. I felt like I chased things for a while before everything was “dialed in” and those variances caused me to get variances in my calibration readings too. Even when I had it for a bit it would drift out. Carefully checking the anti-backlash blocks and getting rid of whip in the lead screws and adjusting and re-adjusting the wheels and the grip on the rails made all the difference.


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@gwilki, @drrkdrrk Finally got back home and into the shop, Did some more jogging around in UGS and the error was consistantly .25mm per 100mm so at 500mm it was 1.25 out measured with every different rule i have. and with calipers at the 100mm step size.
I had seen a youtube vid on how to calibrate step size and i did this. at $=199.5 instead of 200 its bang on.
Y direction doesnt seem to need calibrating its much closer to being as measured.
I am going to draw up a design to run that i can measure the actual output of some outside profile cuts to see how this translates in the actual cutting.

This is very interesting news. I wonder if this is manufacturing variations in the motors or if others would find they have the same situation?