Accuracy VS step / mm

I’m preparing and ramping up my knowledge prior to attempting to machine my costly stocks :wink: Accuracy is very important for what I’m doing (guitars), so I’ve been running some tests. I was initially off about 0.5mm on a 200mm line, on both X and Y (haven’t tested for Z yet).

I found that by adjusting X to 100.19 steps/mm and Y to 100.20 steps/mm, I’m getting pretty spot on results.

Is that a “good” way to achieve the accuracy I’m looking for, or is that just a band-aid for something else that would need adjustment on my machine (belt tension, squareness of the corners, gantry, etc) ?

What kind of accuracy do you guys get in general? Thanks!

200mm seems kinda small, if you are going for accuracy make the largest cut possible. 0.05mm (that is 99.975%) in wood is actually pretty insane. The discrepancy could be compression from your bit of the fibers or humidity. The only way to actually know if you have an issue is if that same cut is 0.15mm off over a 600mm cut and I still wouldn’t call that much of an issue.

At the same time if you actually want more than that, you might be wasting your time.


Thanks for your valued opinion, but you didn’t answer the actual question: is step/mm adjustment an accepted way of gaining accuracy, or is it just meant to stay static and never be touched?

The above tests were done with a pen, btw (using your holder, too). I can figure that I won’t get as accurate in wood, but even if it’s not as accurate doesn’t mean it’s not worth doing to get it more accurate.

I have it in there. Double the distance and see if the error doubles or stays the same. That will tell you/us what the issue is. An error or slop.

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Thanks, good idea, I’ll test that. But still: is it something (the step/mm adjustment thing) that people usually do in order to get more accuracy?

BTW, sorry for the confusion, but the offset I got on my 200mm tests was 0.5mm, not 0.05mm :open_mouth: (corrected my original post, too). I can see how I would be crazy to want to achieve more accuracy than 0.05mm, but for 0.5mm over 200mm, I think that’s something that needs to be done, especially considering what my end goals are. Thanks!

Do the test so I can actually help you here. There are two possibilities. Simple test takes a few minutes. While you are at it draw the largest square you can and make sure your machine is actually square.

No the steps should not have to be adjusted, it is a set number. On 3D printers the extruder is variable do to depth of the teeth on the extruder. No adjustments should need to be made otherwise. Kinda why I keep asking for the other test to find out if there is an actual issue.

So, do you trust your measuring tool? Well, you shouldn’t :wink: I became suspicious after drawing a point within ~1cm of the min and max of X and Y and then drawing a few test gcodes to check:

The plastic ruler and the tape measures seem to agree:

So basically, I’ll have to re-remeasure everything again, to be sure; I will probably do that tomorrow or thursday. But even if the ruler isn’t accurate versus other rulers, it’s still useful by itself: I was able to measure that every side was within ~0.2mm of it’s opposite. Diagonals are within ~0.5mm of each other. Is that too much?

My test gcode still doesn’t draw accurately:

Those straight lines look suspicious versus the ruler, let’s try to check:

Looks like it’s out of squareness (the top is shifted to the left, if that makes sense)? Can’t be 100% considering it’s a drawing on some thin paper; maybe it stretches too much while drawing. I’ll have to find a method to double-check squareness a lot more accurately while adjusting, as I feel I’m blind while doing it currently. Any suggestion?

Edit: just so you know that I started by doing my homework :wink: The dots were drawn at 736mm x 609mm :

Maybe do some test cuts, start learning the CAM and bit selection. Break it in a bit, tune it up after a few hours use and check again.

Pictures of your machine? Pics of the pen and it mounted? You are close enough to where it could be a lot of things. Ziptie tension and length. If the pen is a solid pen or one of the clicky ones with 2mm of slop. Ball point vs ink. Poorly mounted because of some rubbery grip or trying to tape it instead of some cable ties. Moving too fast. Or maybe you have a 18" Z axis.

The straight edge check isn’t the best way to use that straight edge. If your line is not straight your rails are bent, but that is very rare.

Really it is easy to just draw a large rectangle measure the diagonals. Your dots are within 0.5mm over 700mm…can it be better probably but you will also have to make a much smaller dot with much less pressure so the grain of the wood below does not influence it. But 0.5/700 on the diagonal is extremely good for most any project that is not getting sent to NASA. I also think if you need better you will have to get a set of larger calipers, as you just found out accuracy is dependent on your ability to check with certainty.


Thanks; will post pics of the machine tonight. However I don’t see how it could be the pen or it’s mount, considering that my test draws lines going in the perpendicular direction of what I’m measuring (eg, when measuring X, the line that mark the spot is travelling in Y only), and the pen lifts between each lines. If the pen isn’t straight, it shouldn’t matter, as mostly the offset would be the same between each lines, thus negating it (?).

I will probably print this in order to help me see if everything is square, altough this won’t help me for the diagonals. A built-in slot in the top of Top[M]_CornerC.STL for a tape measure’s grip would be very nice, maybe I’ll try to do that if this doesn’t already exists.

Some obligatory machine pics :wink:


While drawing the test pattern:


Here are the measurements results:

Some obligatory machine pics :wink:


While drawing the test pattern:


Here are the measurements results:

I will double-check the measurements for the 736mm lengths and the diagonals tonight. In the meantime, it looks like we have a parallelogram situation:

Okay so without the dual end stops the diagonal measurements will change every single time you use the machine as they are dependent on how well you square it be fore you hit start. You are close, close enough for most but yes it can easily be better.

You can not use a clicky pen for measurements as it has slop in the tip, like spindle runout. Easily 0.5mm some much more.

I understand you want perfection but you would be much better off chasing these numbers with dual endstops so it is at least repeatable. Everything you have done here is a one off. Every time you start your machine it is in a different home. Try to draw that test pattern again on top of it and see where it is in relation to the one you already have. Without dual endstop these sort of tests are a waste of time.

Ah, ok. Would have been nice to know that doing such tests were a waste of time without dual endstops before you advised to execute those tests :wink:

Off to figuring out what I need in order to implement dual end stops, then. A new board (I got the mini-rambo, so no go) and the actual stops+wire, I guess.


You didn’t ask, tell me what you had, or post a picture no way for me to know. Sorry, Next time I will lead with more questions, or you can lead with more details. No big deal, no we both learned something new.

No way for me at this point to know what information might be crucial or what might be uterly dumb. That’s why I said I’m gathering knowledge in the OP here, and why I’m here asking basic questions here about my machine in general :wink:

Thanks for helping. I will be ordering a mega+ramps+drivers, will probably get them tomorrow. I’ll probably have a mini-rambo for sale :wink:

Are you sure you need to spend more money? The numbers you are getting are great. With a few actual uses it could be better. What sort of thing do you plan on making that need that kind of accuracy?

Spent ~50$ canadian (~37usd) for the new parts, not too bad. Getting 50$ back in accuracy shouldn’t be a problem, I think. I want to use the machine mainly to produce my electric guitars; accuracy is very important (otherwise you can’t hope to have a correctly intonated instrument) as well as repeatability, obviously. Always dealing with sub-mm measurements making them.

Well glad I never got around to trying to make one then. I would have messed that all up. I didn’t think the body did much if anything on an electric. I was going to do all sorts of weird carvings. The first year I was pretty serious about doing it, kept hitting road blocks and put it at the end of the list of dream projects. I figured the neck was the only thing that had any critical dimensions at all and I planned on buying that.

You’re correct about the neck being the most dimensionally critical and generally the hardest to make (every dimension on it is important, really), but some elements of the body are crucial too; the neck pocket must be perfect and the location of the bridge too. Aesthetically if the ferrules are off when using through-body wiring it’s also very displeasing, and you don’t have much play there.

I prefer to start with a high goal and see what happens; as someone I respect once told me, you don’t have any chance of attaining a goal you haven’t set. When you know where you’re going, the road seems clearer. I find that most people are just happy discovering where the road gets them. That’s not how I do stuff.

I’m not in a hurry, although I’m advancing fast currently. I do plan on breaking in the machine and my knowledge by doing other projects, too. I already made some pretty nice bas relief in soft wood, I’m not completely setup-centric :wink:

BTW, I just stumbled on this thread of yours; it feels like I’m walking in a pretty fresh track of yours.Thanks for your help and for an amazing machine.