Petri Plate Marker Jig

Microbiologists and/or plant biologists often need to perform competition assays, chemotaxis assays, or any related assay where they need to mark a Petri plate at specific intervals and then place organisms on those specific intervals.

This process can often take a long time with just a straightedge. It can be difficult for the user to hold the straightedge completely still while marking the plate, or difficult to even make nice and proper perpendicular lines. Imagine all of this hassle multiplied by dozens/hundreds of Petri plates. Imagine the amount of user experimental error introduced by all of this. Sounds like a job for an underpaid and overworked undergrad, right? What if you could make their/your life easier?

This jig was created with user sanity (even though you'd have to use FreeCAD) and reproducibility in mind.

 

The FCStd file is parameterized. It comes standard for a “100mm” Petri dish to create 4 lines with 4 hashes every 10 mm. 

You need 3 lines with 5 hashes every 6 mm? I gotchu. Just change hashLineSets to 3, numHashes to 5, and distBetweenHash to 6.

You need 8 lines with 2 hashes every 20 mm?
Same thing, change hashLineSets to 8, numHashes to 2, and distBetweenHash to 20.

Just want the lines and no hashes? We'd have to slightly modify the file to remove the hashes without breaking the rest because FreeCAD is silly.


SHA256 checksum for the 4×4 (4 lines with 4 hashes) FCStd file:
ede92e756b89ff27d6e34c3cca2902a84510ed99de483de72fb6fd56794ff384
(Keep in mind that this checksum will change if you alter the file.

Future plans:

• Recreate model in a more user-friendly CAD software.
   
• Make chamfer on walls to allow Petri plate to slip on and come off easier. Or just add multiplier to truePlateDiam.

Known limitations:

• The hashes can kinda snag your marker as you're going. So it might not always be the prettiest marking, but I've seen worse with a straightedge. This can also be a limitation of the 3D printer used. Resin printers can make for cleaner lines, but I personally don't think it's worth the hassle of cleaning up the print(s).
   
• Without the chamfer mentioned in “future plans” above, it can be difficult to remove the jig from the Petri plate. This can defeat the whole purpose of the jig and make it more likely for you to contaminate your plates. I need to find a way to create that chamfer without breaking the rest of the model when changing parameters.
   
• Short distBetweenHash will result in the hashes crossing over each other with certain hashLineSets. For example, a distBetweenHash of 5 and a hashLineSets of 5 will result in the first hashes of each line crossing over each other, breaking the chamfer that allows for the marker to make better contact with the plate.
   
• Obviously if you make the lineLength too long for the diameter of the Petri plate you're working with, you're going to break the model.