Comparator part III, the guts

Steel, stainless steel, aluminum, brass and plastic.

I destroyed a lot brain cells designing this one, mainly trying to figure out a half nut quick release. I didn’t so you gotta wind, wind, wind to get from one end to the other. Fortunately, it runs pretty freely, so it spins easily when slapped.
I couldn’t find a comparator with features I wanted (or they were too rich for my wallet), but some old vernier height gages would work nicely and are inexpensive on ebay. The one feature I really wanted was a knob to turn to adjust height as I tend to move things herky-jerky and slamming a precision instrument like an indicator into something is painful. I couldn’t find an microscope dual gear sets and a dual adjusting stack (as on lots of height gages – slide to rough height, lock one screw, use fine adjust, lock second screw) would be too tall. Hence the internal screw.

I used the comparator a bunch the last two days working on my mill vise, and I gotta say, moving the head gets tedious. Gotta figure out a quick release positioner. The ability to quickly change indicators is really nice as is the ability move them in and out. The low down isn’t very low so I have a “S” bend rod that drops the indicator (when I can’t use a tall dial indicator, like when I need to use a test indicator (pictured above) to check a keyway a half inch off the deck).

The center tube is stainless seamless bored for roller blade bearings. The center rod is stainless 1/4×20 all thread. The top nut is loctited and the bottom nut has Vibra-Tite (the red stuff in the image below), which is like liquid Nyloc. Worked real well in this case and is stronger than the LocTite, opps. The rod should have been a left hand thread but I didn’t figure that out until I used it.

comparator-bottom.The base is a three piece weld up, the bottom is two chunks of 2″x1/2″ with a chunk of 2″ rod welded on top. Then machined flat and bored for a press fit of the center tube. I was so pleased with my weld I left it (rather than grind a radius tool). The machining was done on the lathe using the three jaw holding the spud to flatten the bottom and the four jaw to flatten the top and bore the hole (indicating off the spud).
Over to the mill to cut a lot of funky little angles and radii. There is a cutout on the bottom for the bumper. The threaded holes for feets. All holes are 10×32. The outer edges are cosmetic so I just cut to layout lines.

The collar runs on a brass tee nut that was supposed to be a button release half nut.  I made several half nuts worked well but I couldn’t figure a pop open/close mechanism. The nut is keyed to the 1/4″ slot in the tube and is a good thread so there is very little backlash (the copious backlash seen in some of the videos was the bottom nut unscrewing, which it did before I glued it on).  The sides are drilled for a 1/4″ rod that holds an indicator. The rod is secured with a nut that grabs it as a knob is turned.

The tube was supposed to be a press fit but I got it a bit tight and it became a sledge fit, which in turn made the bearing slip fit a tight press fit. Not looking forward to changing that one.

The feet are 10×32 flat head screws. Originally, I was going to use high speed steel studs, some blown up tooling scotched that. Then ball bearings pressed into screws, just didn’t seem right. These flat heads work just fine, the top of the screws slide on the plate and raise it a bit.comparator-front

The bumper is a piece of cold rolled welded to O-1 with a 8″ radius and heat treated. It is used when comparing faces and isn’t when comparing heights. I got the radius from Tom Lipton’s comparator build. I wanted to cut the radius on the lathe but didn’t want to build a fixture so I just used a cardboard template, hacksaw and disc sander, it doesn’t need to be perfect.

You can use either test or dial indicators. Although I don’t think they make a dial indicator short enough to work with the bumper.


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2 Responses to Comparator part III, the guts

  1. Nicholas says:

    I have really enjoyed this series of articles on your comparator and its use. Awesome!

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