Squareness comparator

Video to go along with the project:
Squareness comparator
(Link opens on Youtube in new window.)

For measuring squareness, I always wanted something better and more convenient than a surface gage with a large bearing ball.
This is not a unique desire, a lot of people build devices like that.

Tom Lipton has a relatively long four-part writeup about a squareness comparator that he designed and built:

(Links open in seperate window)

It has some very elegant design features that I want to use, but mine will be smaller and use a dial test indicator. I prefer DTIs over normal indicators for most uses, they have less measuring force and are generaly more accurate.

Brian on Instagram ( Toolndie7: https://www.instagram.com/toolndie7/ – Link opens in seperate window) showed a very nice squareness comparator, that way pretty much spot on, what I wanted.

I decided to draw something similar up with my own twist:

Squaring the stock on the milling machine, clamped directly to the table. This is from a time where I was not to lazy to pull the vise off the table.
Today, I would do whatever possible to leave the vise on the mill.

The rotary table is used to mill the rounded design features.

As a side note, all the milling was done dry – The stock I had on hand was D3 toolsteel, which is not exactly pleasant to tools. Almost all milling was done with a 8mm fourflute carbide endmill that has a roughing geometry. Those endmills are very robust and cut with very small effort.

Instead of machining the feet as an integral part of the baseplate, I decided to bore pockets, press in some 10mm bearing balls and surfacegrind them flat.

Squaring the stock for the vertical slider and machining the necessary features:

The baseplate got a small pocket bored and faced to locate the vertical column. The column is a piece of 20mm hardened and ground linear shaft.

The piece that mounts to the front and holds the DTI needs a tiny 5mm 60° dovetail. I ground a matching singleflute cutter out of an old endmill for that task:

The finished slider for the column and the DTI holder will be connected by a piece of spring steel, which forms a flexture (acting like a hinge), that has no play or backlash and will be used for fine adjust.

The last major thing left to machine is the bumper with the radius front:

The bumper was machined from the same piece of D3 toolsteel. After hardening to ~50..55HRC it was surfaceground and hardmilled for final precision:

The finished squareness comparator: