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Identifying a Combination Square Accessory

M.B. Naegle

Feb 7, 2011
Conroe, TX USA
Curious if anyone recognizes this combination square accessory if it's factory made, or if it's shop made what was their intent?

The combination square is a 6" blade and basic head. The blade is made by Union Tool Co. Out of Orange Mass. marked #4. The head I assume is also Union, but unmarked and could also be a Lufkin. The part I want to pick your brains about is the angled piece with them. It clamps flush behind the blade and has cut outs for the head on either end. There are no identifying marks on it but it looks like it could have been factory made. One end of the blade is broken over one of the cutouts behind it and it dosn't look intentional. The angle is 60 degrees. The assembly does not lay flat as the blade clamp is a little proud on the back and the angled bar is thicker than the squares head.

Could this be some kind of lathe compound or cross slide gage? Something to set a repeatable stop? It seems excessive for such. The initials "DEH" are scratched into the heads paint.
I looked a little closer at it and it also has "Irwin Spalding" faintly etched in the blade, who was an old school Aerospace machinist my Dad learned from and bought his tools and machines. He had a 9" South Bend, 11" Rockwell, and Cincinnati 2L horizontal mill in his garage that he used to make aircraft gauge parts at home after he retired. Not sure how those machines or old aircraft parts could be related to this combo squares job.
I'll take a guess. They would have a reference surface to the side they would sit the angle on, and this would let the square reach in and over a different surface. I agree it looks shop made. Somebody had a unique problem. The notches on the ends would allow reaching in from either side by turning it around.
I wonder if it could have been for setting a mechanical punch-press? The angle could have been to fit into the vertical slideway on one of those machines and often setting the head's height is an application where you need to be exact, but not "thousandths exact". That could also explain the broken bit off the blade. I can only think if it was intentional, it would have been cut clean, but it looks to me like it was an accident, but impacted harder than what would happen dropping it, and I can see that happening if they had set it down in a die-set while making adjustments and neglected to clear it before making a test strike.