Nickel Calibration Test

I'm fairly new to the 3D printing world, and I just had one question.

What exactly is a nickel calibration test? How does it determine which axis on the bed is better?

This:
[www.thingiverse.com]

if you aren't american, you'll need to tweak the design to use one of your local coins. Theres a parametric verstion here:
[www.thingiverse.com]

Edited 1 time(s). Last edit at 07/31/2012 05:17PM by Andrew Smith.

AFAIK the nickel test is not in any way valid to calibrate your machine.

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I agree with Sublime. Because of the way curves are produced on these types of machines, (read Nopeheads work on poly holes) the hole for the nickel will likely be printed smaller than the dimensions laid out in the design. This would not be an accurate way to calculate the number of steps needed.

There are plenty of other much more accurate calibration STL's on thingiverse.

Well,

It is a bit vanilla, but Spacxula's calibration set is where I started

Spacexula's Calibration Set



I like to use the 20mm block; I just print the first layer to make sure my "width over thickness" is on the money.

Maybe others would like to post their favorite calibration items here

I designed the "nickel test" and the "better nickel test" as a real way to test my prints. If I have an stl with 21.21 mm internal diameter and it print it a nickel should fit inside. That is the only point of the test. To make sure the parts you print are accurate to what you want to make. Plus you get to see which axis is making the circles, oval as well.

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While I don't have a nickel in front of me to measure, if 21.21mm is the actual size of the nickel, the hole will likely come out undersized on an accurately calibrated printer. This is due to the way these machines actually print holes. See Polyholes article for an explanation.

If you then based your steps/mm on an undersized hole i.e.: The hole was supposed to come out 21.21 and only came out 20.9 then you will end up printing objects slightly larger than intended. This may not be noticable on smaller sized objects, but printing something larger may yield measurements off by a few mm.

Not knowing what the actual diameter of a nickel is, I can't tell you if this is the case or not. If the hole in the STL was designed with polyhole shrinkage in mind, then it would be a fine test.

I happen to have a coffee can full (pocket change is annoying when it begins to accumulate and wears holes in my pants pockets) ... So...I just measured about two dozen to see if it confirms the expectations.

the results ranged from 21.15m to 21.28mm. My caliper was calibrated in March of 2010, so the absolute measurement may be off by a few hundredths, but the variance can't be accounted by anything other than coin variance.

Therefore....it all depends on the age and wear of the nickel... as well where on the rim of the coin one measures (most of the coins I measured vary by several hundredths when rotating the coins within my digital caliper.) . And interestingly enough, it also *seems to depend on the specific year of mfg. (I noticed that all of the 1999 coins I measured were of the widest dimension)

Conclusion:
If you're going to use the test using two places of accuracy (ie,, 21.XXmm) , I'd recommend dedicating a single coin for the purpose, carefully measuring it with an accurate dial or digital caliper, and marking it so it doesn't end up being money.

I wouldn't kick the test out of bed, so to speak, since it's a logical method, but do remember there are limits and make accomodations.

Edited 2 time(s). Last edit at 08/13/2012 08:28PM by xiando.

I agree that basing calibration off of a known standard is a good idea. I think that the polyhole "shrinkage" that I mentioned earlier makes it tough to design such a test around a circular object. You would be better off making a 21.xx square hole and made sure the coin fit in there.

Take a look at Prusa's configurate.scad in the Prusa I2 git hub zip. It'll show how he over sizes the holes in order to make them the needed size. i.e. m8 = 9mm

I just finished calibrating my Mendelmax 1.5. I will print out the nickel test and see what the hole measures.

Strangely enough, I have printed this out probably 50 times before, but never put a coin in it. I always used it to check for backlash in each of my printers.

Just printed out the better nickel test stl. Here are my measurements:

X slot: 21.21
Y slot: 21.22
Circle Diameter X axis: 21.08
Circle Diameter Y axis: 21.01

The nickel that I found has a diameter of 21.20 and would just barely fit through both of the slots and would not fit the hole.

I then made an STL via OpenSCAD of a framed cube with interior wall lengths of 21.22 (to fit my nickel a little better):

The resulting measurements:

X: 21.22
Y: 21.23

And the nickel fit perfectly inside of the resulting cube.

There is definitely a place for calibration stl based off of an object with a known size, but a circle isn't the best way to do it.

Probably not, although if one knows the dimensions and uses the same object every time (not just the same object *type, ie any old nickel), and as you noted, uses a square print instead of a round hole to calibrate, it might not be so bad. It kinda depends on how accurate one really needs it in the first place. At a certain point, it starts to tread on the areas of bearing performance, the type of drive belt or screw, etc.

One other point. A larger printed object is a better calibrator anyway, since it increases the statistical accuracy. I haven't completed my machine yet, but when I do, aside from the initial rough setup, my "real" cals will be done with full or near full-bed square prints and accurate measures (my digital caliper only has a 150mm spread, but I have a few very nice machinists rules too.)

Edited 2 time(s). Last edit at 08/13/2012 11:21PM by xiando.

The size of an object is made up of the distance the head travels plus the filament width, holes are minus the filament width. So calibrating with a single measurement can be misleading. It needs at least two distances in each axis.

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