Scaling issues - pieces not fitting together

I’m having scaling issues - my guess is that I’ve always had this issue but it’s only noticeable with larger pieces, such as the one I tried to cut today.

I’m making model buildings out of MDF. My workflow is fusion 360 > export DXF > lay up in inkscape > save svg > import into lightburn > cut.

I’ve taken some photos and screenshots to demonstrate what is going on:

The piece on the left is around 0.25mm taller than it should be and the piece on the right is around 0.25mm shorter than it should be (my calipers aren’t big enough to measure the heights). This means that they won’t tesselate on the corners like they should. The first image in the imgur link gives the best demonstration of this. At the bottom the bricks line up and can tesselate but at the top there’s a difference of around 0.5mm so they cannot join.

I’ve tried cutting with 0.265 line width and 0.01 line width, this made no difference. I’ve also tried putting small gaps in the lines so they’re not complete closed shapes, this has made no difference either.

I’m using a generic 80W co2 laser.

I’d really appreciate some help figuring this out.

We’re they cut at the same time in the same orientation or was one of them cut rotated 90 degrees?

Yep, the piece on the right was @ 90 degrees.

You probably need to adjust the step size (length): the distance the laser head moves with each pulse of the stepper.

Also, make sure that your belts are properly tensioned - not too tight. It seems that in general, people adjust their belts way too tight.

Do you know how to adjust the step size? Here is a bad way of doing it if you are using a Ruida controller;

The reason it is bad, is because it does not take into account the kerf. It will work great for cutting something of the same material of the same dimensions as you use for calibrating but when you cut something smaller or larger it will be off.

The right way to do it is to use math and calculate what the step size should be. Reading the existing step size will be a good clue. Let me know if you need some help doing it this way.

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Cheers Rich!

I’ll have a go at cutting them both in the same orientation. If the issue goes away I’ll go through what you’ve said re stepper step size.

I haven’t had to account for kerf yet. The width of the cut has marginal for what I’ve been doing.

I’ll definitely ask you a few questions before I go ahead adjusting the step size, I’d hate to make it worse than it already is.

I have found that when cutting box joints that the material removed in the kerf works out to be just right for easy assembly and allows space for glue. Usually quarter inch thick material and parts that are up to five feet long.

About adjusting step size, as long as you keep track of the original settings you’ll be okay. For that matter it is a good idea to make a backup of your machine settings in case you need to replace your controller.

Gotcha. Thanks very much for taking the time to reply.

Adjusting the step lengths isn’t hard - you cut something of known size, then measure from the inside on one side to the outside on the other (this accounts for kerf). Then you simply adjust the step size by the ratio of what you cut vs what it was supposed to be. Running larger cuts makes this more accurate.

For example, cut a 200 x 20 mm rectangle. Measure the result as noted above. If the cut came out to 200.7 mm, your step size is too small, IE the controller is emitting too many steps. You would either decrease the step count per mm, multiplying by 200/200.7, or increase the step length by multiplying by 200.7/200, whichever is appropriate for your machine.

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How is this done?

You could measure the width of the hole you cut out and them measure the piece that you cut out of that hole and subtract the two to find the kerf. Although, it is tough to get a perfect measurement considering that there is usually some angle to laser cuts.

I’m not sure why this is a thing with lasers. If you use this cut and measure method of figuring out the step length then how can you be sure that there isn’t some other issue contributing to the error?

If you use simple math and apply it to your machine you can figure out exactly what the step length should be. Then, if there is some inaccuracy left, it is being caused by something else.

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Re-orientating the second peice did the trick! Thanks Rich. I’ll have a crack at adjusting the step size tomorrow. Thanks guys.

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Belts do stretch over time, and most people don’t have a clue what the tooth count of their pinions or pitch of their belts is, or what micro-step settings they have on their motor drivers.

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Heya Rich,

I’m having some non-software related issues that I’m not quite sure where to post about. You seem quite knowledgeable, could you possibly give me some feedback?

I’m having trouble with the vertical alignment of mirror 3. I’ve read everything online that I can find. My guess is that the nozzle/lens holder may be be bent as when I fire the laser without the lens/nozzle in place I can get it perfectly aligned but when I put the lens in it’s way off. If I try to calibrate mirror 3 to offset what’s going on with the lens in place the laser beam hits the inside of the nozzle before I can get it centered.

If you’ve got the time I’ll take some pictures of what I’m talking about so it makes more sense.

(Not being able to get the vertical alignment spot on is meaning my cut edges aren’t square . Before I played with it only two cut edges weren’t square but now I’ve played with it all 4 are off, but to a lesser extent. It would be nice to get my edges as square as possible.)

That’s why I suggest “proper” tensioning of belts, most people tend to over tighten them. I think if a belt is stretched to the point where it has lost accuracy then it has also not stretched evenly along its length.

The people that are not willing to learn how to count the teeth on pinions or figure out their micro-stepping are not the same people that are striving for the best possible accuracy. For them, the cut and measure method will do just fine.

Not exactly sure what you are describing. I look forward to the pictures.

Probably best to start a new thread.