Wednesday, April 3, 2013

Squareness Comparator Gage Finale, finally

I finally felt bad enough to go ahead and really finish the squareness comparator I started a few months ago. Part of the reason is I'm close to heat treating a couple of 8620 cylindrical squares that will be used with the comparator to check squareness in the shop to very close limits. There will be a separate article on the cylindrical square fabrication. It all needs to be done to make a nice article about it.
The first step toward completion was to break down and buy a few carbide balls for the wear surfaces on the bottom of the gage. These are standard McMaster Carr items. In fact everything on the gage was sourced from our friends in yellow and green. These ball were then pressed into holes in the bottom plate and then ground flat into a good size bearing spot.
The carbide balls are .375 diameter and the press fit was about .002 on the diameter. With a ball you have very little diameter in contact with the hole so they can use more interference and still be pressed together easily without having to bore the holes to a .0002 tolerance. If the rest pads were cylindrical I would have only used .0002 interference for the amount of axial engagement.
The Bridgeport quill and a good chuck make a great precision arbor press. If I need to remove them for some reason I can drill from the other side and punch them out. There is plenty of meat left on the balls for several lifetimes of re-surfacing for even the fussiest toolmaker.
I started to grind the balls flat on the surface grinder but it was taking too long with all the traversing across the part. I took the base out and roughed them on the carbide tool grinder until I was happy with the spot size then came back and finish ground the flats all smooth and coplanar. They don't need to be ground for function specifically but its nicer on the surface plate with larger flats.
The finished base plate of the comparator gage.
Close up of the wear feet and spot size.
Finally finished. It took a couple of months of walking by the thing every day to make me finish it.
I did make one modification that is not on the drawings I posted on the blog. I was using it one day for a squaring job and I realized that the indicator wouldn't go low enough for the part I was checking. I looked at the indicator bracket and saw a simple solution to  allow the indicator mount to drop another inch lower.
By flipping the indicator mount over the offset goes down lowering the indicator stylus. The only thing I needed to do was cut a relief into the mount for the back cap of the indicator. It was literally a five minute job. Depending on the indicator you use it may not need this relief cut.
So that concludes the squareness comparator gage. Sorry it took so long to get to the end.

Thanks for looking.

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