M3 Bolt Length Measurement Gauge

Never use the wrong bolt again! 😀

I built my MK3S printer from a kit, and then upgraded it to the MK3S+, and there are a lot of different length M3 bolts used. Using the wrong length bolt will sometimes work, only to find out a couple of hours later the length you need is missing and going back and disassembling to find where the mistaken length was used. My nemesis was the 18mm and 20mm long bolts, these are visually very close in my opinion and easy to mix up. The full size printed images on the parts bags are great, but recently  when rebuilding my E-axis to diagnose a troubling “X axis error” problem, you don't have the bags around to help keep them sorted. After using my caliper to double check bolt lengths (slowly!) I decided once my printer was back up and running I'd make a tool that would solve this problem for me once and for all. The result is this model, the M3 Bolt gauge.

 The Gauge

The role of any gauge is to show a measurement of the variable of interest, in this case the length of an M3 bolt. The design requirement was that it be easy to read, quick to use, and not take up a lot of space. 

This design ended up being a 60 x 60 mm square which can measure the length of every M3 bolt on Prusa supplied cheat sheet. In the picture there are two different prints (black/yellow and silver/black) showing my nemesis bolt lengths 😀

The Design

For my printer which only has the 0.040 hot end nozzle, the smallest feature I can reliably resolve in PLA is 0.5mm, so this design had to work within that constraint. As the shorter bolt lengths (4, 5, and 6 mm) are all 1 mm apart, the 0.5 mm indicator lines have 0.5 between them of background. The lettering is raised 0.75 mm but could probably be only 0.5 mm tall and still work.

I did a lettering test using different fonts (Segoe UI, SwissBLK, and Tahoma) to identify which was the most legible. I settled on Tahoma with a 5.64 mm font height. This keeps the smallest features of the letters at 0.5 mm or better. 

Because the letters were so “tall”, they could not simply line up 1:1 with the indicator lines. The workaround for that was to have the indicator lines bend and then terminate at the appropriate text label.  This is a bigger problem when the indicators are 1 mm apart and not a problem when they are 10 mm apart like the 30 mm and 40 mm indicator lines are. 

I'm not at happy with the descriptor ('M3 Bolt Gauge') in the lower right corner, I'm thinking I should have put a rounded rectangle around it to help it stand out.

The only thing that really makes this “M3” is that the groove down the center is 3 mm wide, and there is a cutout at the top that allows the bolt head to sit flush there. An M2 and M4 version would be easily made by changing those features. If you do modify this for M4 and M5 bolts you will want to make the back plate a bit thicker, it really does need to completely  hold the bolt if it is to maintain accuracy.

Finally I added the round hole in the top so that I could hang this on a hook and not lose it in the mess on my desk.

The Print

This isn't a “structural” or “load bearing” design so you can print it in any rigid material you want. That said, you may find as I have that “stringy” materials like PETG will yield less optimal results. Since it is important to be dimensionally accurate, ABS might not be a good choice due to shrinkage. 

The provided G-code is for Prusament PLA and has the code for the filament change when it goes to do the lettering. The lettering takes make 10cm worth of Filament so you don't need a second spool, if you have a PLA sample it could work easily. 

There are both .3MF and .STL versions of the model to support most slicers. The CAD files are in TurboCAD 3D which is not very common so I also uploaded a STEP version of the file which most CAD programs can import.

Final Thoughts

From the picture you can see two copies I've printed out. The first was the silver/black one and since the bolts are silverish I thought better contrast would be better and then printed out the black/yellow one. Good contrast and I can tell at a glance how long a bolt is. If you do print this out, please upload a picture of your make! I would love to see different color combinations.