Precision Reloading Concentricity / Runout Gauge

This design is better than most commercial solutions as it uses a far more accurate test indicator (each hash mark is 0.0005") and easy to estimate down to 0.0001" vs dial indicators which are really only good to 0.001" and due to their plunger design far more likely to "bounce" which will invalidate the reading.  The design also allows for measuring both the OD and ID of the neck, while most designs only support OD.

Astute readers will no doubt notice this design is very similar to one that costs about $330 USD and is CNC'd aluminum.  I'm able to closely match the precision of that far more expensive solution via the use of precision ball bearings (I recommend grade G25 or better) which are inexpensive and easy to find on Amazon, McMaster-Carr, etc.
 

Includes mounting points to mount to an Inline Fabrication quick change mount, or just use a c-clamp to mount it to your table.  Options are endless really. :) 

Parts

• 1x – 200-300mm long 2040 V slot extrusion (not for T slot extrusions)
• 8x – 6mm x 3mm magnets (actual size 6mm diameter x 2.75mm high)
• 4x – ¼" or 6mm steel/chrome (NOT stainless steel!!) ball bearings, rated G25 or better
• 4x – M3 stainless steel washers
• 1x – 5mm stainless steel dowel 20-50mm long
• 6x – M5-16mm BHCS
• 2x – M3-12mm SHCS
• 2x – M3 x 4mm long heat set inserts (Voron spec)
• 6x – M5 T nuts
• 1x – #10-22 thumb screw (I prefer nylon, but metal is fine if you're careful to not damage the indicator mounting shaft)
• 1x – #10-22 tap or metal screw if you're using a nylon thumb screw
• A quality test indicator gauge
• Crazy Glue/Cyanoacrylate
• Purple (preferred) or blue Loctite

Optional Parts for Inline Precision Mount:

• 2x – ¼"-20 1" long SHCS
• 4x – M5 T nuts
• 4x – M5-16mm BHCS

Printing

Print all parts in the orientation provided.  No supports are required.   Since we need these parts to be very stiff and I use heat set inserts in the design, I recommend:

• 5 perimeters
• 4 top/bottom layers
• 40% infill

I use ABS, but PLA should work as well as long as you don't keep it in your car on a hot summer day or other hot place which may cause PLA to deform due to it's relatively low glass transition temperature.  I don't recommend PETG since it is less stiff and we care about rigidity.

As with any functional print, ensuring your printer is well calibrated is important for best results.  I use: https://www.printables.com/model/758730-voron-design-calibration-cube

Assembly

1. Insert 2 magnets in each of the two holes near the edge of the brass mount.  Be sure that the polarity is opposite for each stack so they aren't fighting each other.  See photo for details.
2. Use Crazy or CA glue to glue a M3 washer to the top with the smooth side up.  Since most washers are stamped, there tends to be a burr on one side which must be facing down for best results.  Be sure to only use glue on the outside edge (don't let any get in the center hole). 
3. After the glue has dried, place a steel ball bearing in the center of each washer. 
4. Use a M5-16mm bolt and T nut to install both brass mounts in the front channel of the 2040 extrusion.
5. Use a soldering iron to install the two M3 heat set inserts.  See CAD images for location.  It's important that the top edge of the insert is both square to the surface of the part and either flush or slightly below the surface.
6. Use a #10-22 tap or metal screw to chase the threads in the test indicator holder if you are using a nylon thumb screw
7. Use the M3-12mm bolts to assemble the indicator arm.  Use a drop of purple (or blue) loctite so the bolt doesn't loosen as you rotate the pieces.  Adjust the tension so the parts move smoothly, but require intent to move.  Too loose and your readings will be worthless.  Too tight and you'll have a hard time making the fine adjustments necessary to line up the test indicator.
8. Use 2x M5-16mm bolts and 2x T nuts to install the indicator arm assembly to the rear extrusion channel.  The arm should be to the back.
9. Optional: Install the two Inline Fabrication brackets to the front channel with 2x M5-16mm bolts and 2x T nuts and use ¼"-20 bolts and the stock wing nuts to mount to the Inline Fabrication top plate/riser.
10. Insert the 5mm stainless steel dowel in one of the center holes of the brass mounts.
11. Insert the test indicator in the arm and tighten the thumb screw to hold it in place.

Note:  The bearings should firmly locate into the center.  If you are finding the bearings fall out, you are either using to much pressure while measuring (which will invalidate the reading) or the magnets were not oriented correctly.