PLAb: Variable Width Slit and Knife Edge (60mm Cage)

Summary

PLAb System and Example Use Cases

A component of the growing PLAb Collection of 3D printable optomechanics. This is a variable width optical slit mechanism with independent control of each side. It leverages a print-in-place flexure mechanism with standard hardware and utility knife blades. When printed with a single side, it can be used as a knife edge. The mechanism can be mounted vertically or horizontally to crop down a light source with hard edges or cut-off the image for a schlieren system. 

In addition to mounting on the PLAb Posts, the optical slit is compatible with standard M6, 1/4-20, M4, or 8-32 threaded optical posts. Additionally, it can be mounted to a 60mm optical cage system for maximum flexibility. Please share what you build with PLAb as Makes here on Printables. If you have an issue, need a customized version for your application, or just have a suggestion please drop a comment or reach out to me via my profile page.

License and Support

If you find the system useful, please share it with your colleagues. If it makes your lab easier or cheaper to setup, perhaps Buy Me a Coffee so I can continue to develop the system and keep it free and openly available under the Open Community License (OCL) with the General Attribution (GAtt) Plugin. 

Printing and Assembly

Print Setting Recommendations

The system is designed to be as simple to print as possible. No supports, no special materials, no slicer tricks. I started developing the system on a Prusa Mk3s and am now using a Snapmaker U1 and Prusa XL. General printing guidelines are as follows:

• The flexure mechanism is designed for a 0.2 mm layer height on a 0.4mm nozzle

• The mechanism is optimized first for PLA

• Tougher filaments like PETG or PCTG can be used but will result in a softer flexure mechanism.

• Use three perimeters with five top and bottom layers for best results.

• Infill is not critical here. I usually use 10% Adaptive Cubic.

• Print orientation should have all nut pockets facing upwards. An example of good part orientation is shown in the slicer image below.

Parts List

• 4x M3 Flat Nuts

• 2x M3 30mm Socket Cap Screws

• 4x M3 6mm Socket Cap Screws

• 6x M3 Flat Washers

• 1x 9mm Standard Duty Segmented Utility Blade

  • I use Olfa ABB Black Tool Steel Blades since they are less reflective than standard steel blades

• (Optional) 2x M3 Flat Nut Thumb Turn and 2x M3 Socket Cap Screw Thumb Turns

  • Such as the Handles for M3 Bolts and Screws by RemiRafael

• (Optional) 4x M3 6mm Socket Cap or Set Screws for 60mm Cage System

Assembly

• Insert M3 flat nuts into the two vertically oriented nut pockets in the frame.

• Place a nut and flat washer on the 30mm M3 screw.

  • Add the optional thumb turns if desired

• Flip the frame over and insert four M3 flat nuts into the back of the moving tabs.

  • Take care not to twist the flexure mechanism while doing so.

  • You may need to thread and screw through and use the leverage of the screw to fully insert them into the slots as it is an interference fit.

• Snap on of the segmented utility knife blades in half along the scored line

  • WARNING: Wear eye protection when snapping the blades.

  • WARNING: These blades are very sharp. Take care not cut yourself. I recommend dulling the blades on some sand paper for extra safety

• Place the un-sharpened, back side of the blade flush against the raised edge on the front of the flexure tab. Ensure that it sits fully flush and the tips of the blade do not interfere with the frame.

• Secure the blade down with two M3 6mm screws and flat washers. 

• If using a variable slit model (as opposed to a knife edge model), repeat on tab with the second half of the blade.

• If you are using a cage system, directly thread M3 screws into the corner holes next to each of the 6mm through holes in the frame.

• Mount to the desired post and install for use.

Usage

• The 30mm control screws through the side of the frame to push each side of the slit or the knife edge into the desired location.

• The maximum extension for each side is 3mm enabling a slit width of 0mm to 6mm wide.

• Lock the blade by tightening the loose nut on the control screw against the outside of the frame.

• Store with the control screws retracted and the mechanism uncompressed to avoid permanent deformation of the mechanism.

WARNINGS

• Blades are sharp. Take care not to cut yourself when adjusting or mounting

• If using the slit with a laser light source, take care to control reflections. Even with the blackened ABB blades, laser reflections can blind you as easily as a direct beam

• With high-intensity light sources, the blades can locally heat up. Take care to avoid burns.

Updates

2026-09-03 1.1 More Rigidity

The thickness of the flexure legs has been doubled to improve the stiffness of the mechanism. This reduces creep when the flexure is left stressed for an extended period and produces more reliable behavior between PLA and PETG versions. The difference between 1.0 on the left and 1.1 on the right is shown below.