One Day Build: Strong Indicator Arm

For a while now I have been using one of these ubiquitous Swiss made indicator arms. It works okay but I've never been fully happy with it. They have too many joints and too much wiggle for precision work. Even on the best units I have seen problems with the indicator alignment when indicating small holes. The dovetail isn't perfectly radial with the stem and it side loads the contact point.

I wanted a better, simpler solution. I came up with this single pivot arm inspired by some cheap indicator arms I've seen. The clamp plates are hardened A2 and the shank is 60 case Thomson rod. The spherical surface on the clamp plates were surfaced on a CNC and match the spherical indents on the shank.

The screw is kinda cool. Its a standard SCHS with a little plastic knurled knob pressed on. I didn't know these existed until one of the toolmakers I work with bought some a job. The clamp gives just enough friction on the joint that it wont be knocked out of place, but can easily be adjusted by hand.

The little spring on the front of the clamp is just there to hold the clamp plates on the shank and the dovetail open when the indicator is not on the arm. Right from the start the radial alignment wasn't perfect. This this design, that alignment is easily adjusted by grinding the faces of the clamp plates to affect the angle the indicator is held at. With a bit of fiddling I got the contact tip to less than 0.25mm radial misalignment. 

This isn't a perfect solution for every application but it works perfect for any time you need an indicator in a mill spindle. Going forward I would make a second one of these with a 10mm reduced shank for using in my small drill chuck.

Example of the suspect indicator arm. Photo from wttool.com