K40 Accuracy? -- 6" drawn rectangle is cut 1/16" too long

My first post here–

I’ve been using my K40 for about two years with no issues. But today I discovered it’s quite inaccurate, at least in the X-axis.

I was just trying to cut a 6" x 3/4" piece of acrylic to use as a spacer for attaching pcbs to an aluminum strip. The spacing of the pcbs is critical, since IR sensors must align very accurately to holes in a tubular enclosure.

I discovered that the rectangle was actually 6 1/16" ,not 6". I can do better on my table saw!

The beam is focused, line width is hairline. Using CorelLaser.

Is this all I should expect, or is something wrong?

Thanks!

Hi Dave and welcome!

The accuracies of the K40 should be much better than 1/16" in my experience.

With CorelDraw line width should be set to 0.01mm or smaller or else you get a double cut as the beam will cut both sides of the line. Hairline is bigger than 0.01mm. However this would make it slightly smaller.

Might want to check the belt tension and condition of the belt.

What version of CorelDraw are you using?

Line widths were set to hair line.

CorelDraw 2018.

Same drawing printed on paper is more accurate than my ability to measure, so it seems a K40 issue, not something in the design.

I can see belt tension causing the rectangle to be too small (slipping, missing steps), but not the other way around. Right?

Thanks!

Printing is different than coreldraw generating G code. Can you provide a copy of the coreldraw file?

Here’s a simplified .cdr, with just the rectangle.

What I cut was a layer in a bigger design–I’ll head to the workshop and use this .cdr to make sure I get the same thing (6 1/16" long).

Thanks!

rectangle.cdr (13.5 KB)

I’ve always used hairline in all my designs (cutting, not engraving!) and have never seen lines double cut.

Interesting, they must have fixed that long time bug then.

I’ll take a look at your file later.

Thanks,

I confirmed that the simple rectangle cuts too long – 6 1/16".

I also rotated the rectangle 90 degrees and cut again. Same issue in the y axis.

I read somewhere that someone had a similar issue when he upgraded belts. But, I’ve never done that.

I can’t find the post–I’ll keep looking. But, he was being told to change the number of steps per unit distance (somehow) and calibrate against a precise distance. I’m thinking I can’t do that in CorelLaser anywhere.

Assuming this is a scaling issue, the inaccuracy could have always been there and I wouldn’t have noticed. It’s quite obvious now, since I have a CNC holes drilled into an aluminum tube that must align with pcbs taped to an aluminum strip. I was just cutting acrylic rectangles to help accurately space the pcbs on the aluminum strip.

The holes in the CNC’ed tubes (3" apart) are as accurate as I can measure using calipers. But, holes in 6" (or longer) acrylic cut on the K40 are off. And, I can measure the length error in 6" as 1/16" (as I mentioned previously)

https://www.reddit.com/r/lasercutting/comments/7v877w/k40_xaxis_accuracy_issue/

Turns out the belts are compatible, but the old one was stretched out just enough to cause a new one to appear inaccurate.

Belt stretch?

I suppose if the belt has stretched, the distance for each step would be longer. But, this K40 hasn’t had heavy use, so I’d think that would be a very common problem.

Is inaccuracy due to belt stretching a typical issue?

If I scale by 98.6%, the length is as accurate as I can measure.

Hole spacings inside the rectangles match the aluminum tube holes now too.

This will work for this project. Usually, absolute size isn’t important for my projects, but a 1.4% error seems way larger than I’d expect.

Unfortuanlty I am unable to open that file because I’m using CD X8 (2016).

Seems that would indicate a file problem if the inaccuracy just changes based orientation?

I don’t think I’ve really run into that issue before other than missed steps. Does the accuracy vary with the size of the box?

The only scaling issues I’ve really seen are if you change steps / pix anywhere else other than in CorelDraw Settings. Sometime you also get weird things that happen depending on what you have “Cutting Data” set to in the settings menu. You could post a screenshot of your settings and of the cut manager window.

Seems that would indicate a file problem if the inaccuracy just changes based orientation?

No. The issue is the same in both the x and y axis. (I said same issue in the y axis). Which indicates it is NOT a file problem. Again, the file (.cdr) prints accurately to a printer.

You mean in CorelLaser settings? I didn’t see anything there about steps/pix, but I’ll check again.

Does the accuracy vary with the size of the box?

Everything scales with size. When I multiply lengths by .986, they cut accurately.

Not sure then. Things may behave differently with a newer version of coreldraw. Here are some settings from corellaser I’ve used in the past in case it helps.

image

image

Just keep in mind that printing to a printer is different then creating and sending G code to a CNC machine even if it’s based on the same file. I’ve seen CorelLaser generate some weird G code before for things that print fine. :slight_smile:

data%20setting engraving%20machine%20properties

Lots of differences.

At least one seems significant–cutting data file. You have EMF while mine is configured for PLT.

I’d love to see (English) documentation on what all the settings mean.

Thanks!

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I’ve found in the past that EMF generally gives the least issues. Sometimes imported svg cut files have been problematic and believe I’ve instead used the PLT instead.

I’ve seen something before. I’ll have to look for it.

All the rest of your settings look fine.