Adjusting sizes for print tolerances

I have a magnet that is 0.125" (1/8") in diameter, and0.25" (1/4") long, I know these dimensions because I measured them, so how do adjust the dimension in lay out the print to hopefully get it right the first time? To put it another way, what is the resolution of a poly printer? Test prints work, but are very wasteful and time consuming. If I print a 0.5" peg what is its true diameter likely to be? Conversely, If I create a 0.5" hole, same question? Some general rules of thumb would be very useful.

I’ve always wondered that as well. Most of the time it has been where I needed a hole of a specific size for say a screw and it always comes off of the printer too small. It is easy enough to fix with a drill bit but it is annoying. For your own gratification I’d suggest creating a 1.0" or 20 mm cube with a 0.5" or 5mm square hole in the center in Tinkercad then print and measure it. This will give you the size of adjustments needed…

I typically use a 0.25 mm gap. That seems to work well with the PolyPrinters when printing ABS.

approximately: 0.5 in + 0.25 mm + 0.25 mm
approximately: 0.5 in - 0.25 mm - 0.25 mm

The 0.25 mm varies by…

  • Filament (not extruding well means less “overage”)
  • Z-axis zero (too close to the bed means more overage)
  • Nozzle quality (partially clogged means less overage)
  • Infill (higher infill results in more overage)

If the plan is to “bury” the magnet then acetone (with some filament dissolved) is very helpful.

“Ooze” also varies somewhat by Brand, so you really need to test before committing. Make a low rectangular prism with a number of holes of varying diameters and test. If you plan to glue the magnet in place, ever-so-slightly oversized will allow room for the glue. I would not recommend heat-setting the magnets in place, as the Curie point for RE magnets is rather low and heating them to press-fit them could cause them to lose their magnetism.

If doing screw holes, you can make them self tapping by giving the hole an asterisk-shaped cross-section rather than round. The screw threads will grab onto the spikes from the star-shaped opening and give good grab w/out tearing into the main body of the part.

In ABS you might want to scale the dimensions up by atleast 1.02 to account for shrinkage.

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So .25mm is the resolution of the printer or9.48 milinches? Their will be a test print, hope it works. I plan on super glue to mount it. So the magnet hole will be 0.25"-2(0.00948") or 0.23", since the hole will be smaller than I am aiming for I will turn the sign around an use 0.25+2(.0948") = approx. 0.27" hole.

I love easy to test hypothesis, Plus if it works first time it will work for future 3D drawings too.

Doing math can be interesting. 0.125"+.00984"+.00984"=0.14468" . That seems off , but for now I’ll go with it. I may try several sizes to test things out.Say 0.125"+.00984 = .13484" Sometimes math just doesn’t give you the answers you want, no matter how you fudge the answers.

The printers are designed to eliminate that concern. When I did bother trying to get the most accuracy possible the scaling I used was significantly smaller than 1.02.

@Wendy, a chamfer / fillet (rounded edge) is a very handy addition.

I suppose the geometry of the part you’re making is relevant too, but I don’t know how to account for it.

When I’ve needed very precise holes, I’ve just printed the hole a bit smaller than necessary, then used the soldering gun to heat up the side of the nail near the tip of the pin/screw/nail as I press it in. The plastic just melts out of the way, giving you a super-precise, snug fit. I’d just heat up a 1/8 in nail and press it in while heating the shaft with the tip of the soldering iron. Just don’t make the tip too hot or you’ll burn the plastic, which is bad.

Another alternative, although an expensive one, is to melt in a press-fit nut. See

Good luck.

@Wendy, the resolution of the PolyPrinter is finer than that: you are conflating two different issues.

The proposed adjustments suggested above are experimental results based more on the ooze characteristics of the plastic used than on the movement resolution of the printer.

The resolution is the smallest distance one can move the printer head. This is determined by the stepper motor steps per revolution (typically 200 with 1/8 or 1/16 microsteps possible) and the size of the geared pulley on the stepper (# of teeth and tooth size). The Z axis is dependent on the steppers and the screw pitch.

For the PolyPrinters, this works out to:

  • 0.0125 mm per microstep (12.5 microns) in X and Y


  • 0.05 mm per full step (50 microns) in Z

The adjustment for hole size needs to consider the physical characteristics of the plastic being used. The ABS will ooze and settle a little bit after being extruded: if there is open space next to the filament (e.g. a hole), it will spread in that direction. The fan blowing on the extruded plastic is designed to minimize this effect, but a little ooze is good so that the filament will bind with the already laid down plastic next to it. This becomes a bigger issue when using high-ooze filaments like NinjaFlex (which requires a special extruder on the PolyPrinter due to its squishiness).

Also, the nozzle diameter (which drives the extrusion width) is a factor in design. For the PolyPrinters, they use a 0.35mm extrusion width, so making anything skinnier than that will likely cause problems. Using whole multiples of this (0.35, 0.70, 1.05) for skinny areas will improve the output. Thus, making a narrow wall 1.05mm thick will work better than 1.0mm. Similarly, making one 0.9mm thick will likely end up with two 0.35mm shells with a 0.2mm gap in between (or slightly less after oozing).


It is understood the hole will be smaller than I want, using the resolution of the printer to creat a fudge factor seem like a logical place to start. So the stepping motor resolution is 012.5um X/Y and 50um for z? Ihave trouble believing no one has run these experiments. I think they are worth doing.So I will start out with a 0.125 hole and use the following suppositions and see how it works out.

0.125+.0049+0.0049 =.1345 or 0.125+.0049 =0.130 .

I will try to have a good rule of thumb when I finish this project.

So if the nozzle extrudes 0.35mm of liquid plastic, what is a good guesstimate of the plastic spread after it settles? Little by little we can make the so called negligible factors less so. Electronics has similar problem with parasitic capacitance and inductance Again I’m not try to catch anyone out, just trying to understand…So if I assume a spread to 0.4mm then use .2mm for the spread numbers and see if it works.

Just to add a data point in here:

A few weeks ago I modeled a part as a duplicate of a small PP/30GF piece I couldn’t easily source, and printed it in ABS. The part was very roughly 40mm x 25mm, measured out to 0.1mm precision (and what I subjectively believe to be high accuracy, though I did “armchair-evaluate” accuracy by doing about a bazillion re-measurements). The modeled part, printed with no scaling, was practically spot on, and none of the measured dimensions were off by more than 0.1mm again.

I mention this largely to say that the suggested minimum 1.02x scaling to account for shrinkage may be overshooting in the other direction, depending on the model in question.

If you set the precision in Kisslicer to “0”, you get +/- 10 um repeatedly on those PolyPrinters.

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Thanks for the advise guys. For now I’v decided to go with the 2% option, if the parts don’t mesh I’ll do 2% again. I’ll post the results here when I have them.Alignment pegs are0.25". Adjusted dimensions: Pegs = 0.245" Holes = 0.255" Magnet holes are 0.125", 0.121.275, and 0.130 Respectively.

Latch Test (22.9 KB)

Experimental observations:

As near as I can tell a 3D printed rod is0.0003" bigger than the stl files is programed for.

Holes come out about 0.0003" smaller.

A tight fitting 3D printed dowel is0.0005" offset from its matching hole.

In making a box / lid 0.001" or 0.002" is not enough, while 0.003" is way too tight. I’m going to try 0.005" offset to see if it is a good loose fit,easy to pull off an put on.

I will come up with a personal rule set which I will publish when I think it is ready. A lot of what I said above will be included.

What are you measuring with that gives you three ten-thousandths accuracy?

I apologize, I realized I was spouting nonsense after I stated designing my latest brain fart. I will try to get my act together when I get around to really organizing my numbers. The z axis resolution of a poly printer is 0.0001". Sketchup, my current CAD tool of choice goes down to 0.000001", which is nonsense of course. Fun to play with though.

My measurements are based on a magnet that is 1/8" dia. (.125) and some test prints to see what hole sizes would work. Part of it is making parts that are suppost to mesh with each other.

Some of this is the rantings of a brain damaged survivor, but I’m getting some good data if I can restrain my enthusiasm a bit.

Ispend the week frantically designing ideas to polyprint 3D when they let me out of my cage on Sat. trying to understand some of my various failures during the week. I will attempt to share what I think I have learned on DMS talk.

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The Polyprinters have a 0.05 mm Z resolution, which is closer to 0.002” (20X coarser than you stated).

X and Y absolute nominal resolution is 0.0125mm or about 0.0005”. These are based on the stepper microstep sizes and the X/Y gear/belt sizes and the Z rod thread pitch sizes.

In practice, wobble and ooze lower these theoretical resolutions.

You sadly can’t get machine-shop precision out of a computer controller hot glue gun.


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