A Poor Man's
Quill Position Digital Readout

Here is my implementation of a digital readout (DRO) on the quill of my Sharp milling machine. The set-up has been trouble-free and entirely satisfactory.

Some folks said they couldn't see the need for such a DRO. Well, after having gotten accustomed to one on another machine, I was lost without depth indication, especially when counterboring or countersinking for flathead screws.

When I bought the Sharp I thought it didn't deserve cobbled-up accessories; but, after spotting the $200-plus price tag of a ready-made DRO, I threw all my fancy notions out the window, grabbed a spare $19 six-inch digital caliper, found a scrap of -inch aluminum plate and set to work.

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Here is my pride and joy, a fully loaded Sharp mill that I purchased at the late Bob Washburn's shop auction in 2009. Bob and his wife, Frances, published the Strictly IC magazine for a number of years.

An off-topic note: Take a close look at the Off-Forward-Reverse switch plate. Spindle rotation of Bridgeport mills and their clones reverse when in the low speed range. When low is selected, that means that when forward spindle rotation is desired, the motor switch must be placed in "Reverse." Confusing, eh? That's why I slip on a "correct" switch plate whenever using low range. It snaps in place and stays put without fasteners.

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Here is a closer look at the DRO. Note that all the sharp and pointed parts of the caliper have been removed. They no longer serve any useful purpose; and, had they remained in place they would have been a constant safety hazard.

The stainless steel used to construct these calipers is very hard and it work-hardens at the slightest excuse; however, a Dremel cut-off wheel makes quick work of removing unwanted pieces.

An off-topic note: Take a look at the quill locking lever to the bottom-right of the DRO. It is held in the "Off" position by a -inch rare earth magnet that is attached to its tip with a piece of heat-shrink tubing. (I tried fastening it with epoxy, but it did not bond very well.)

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I milled that scrap of -inch aluminum to fit the flat above the depth stop assembly and attached it with 4-40 X -inch button-head screws. I used the aluminum adapter as a drilling and tapping guide, ensuring accurate placement and perpendicularity.

The stationary jaw of the caliper attaches to the aluminum with a 10-32 X -inch button-head screw. I drilled the 3/16-inch holes in the caliper with a solid carbide spade bit. Don't even think of trying it with HSS tooling.

Note the hole at the top of the caliper beam. At first, I planned to cut the jaws off the caliper, altogether, making the installation resemble a factory-made DRO. Thus, the unused holes. (I also drilled and countersunk one behind the electronic module.)

After seeing that central mounting would place the whole assembly too close to the fine downfeed handwheel, I decided to use jaw mounts, after all.

As a matter of interest, I drilled those unused holes for 4-40 screws. Not having that size carbide drill, I finally discovered a use for my Harbor Freight carbide micro drills. After drilling pilot holes with one of them, I enlarged them with diamond-coated burrs.

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This close-up shot shows my buggered-up expanding-plug attachment to the movable jaw of the caliper. To me, it seemed to be an easier approach than the diagonal screw system Mitutoyo uses for their DRO.

First, I dismantled the depth stop assembly and removed the power downfeed trip ring. The hole in the side of ring is only there to allow access to the attachment cap screw so it need not be perfectly formed. Of course, a rough hole will not do for an accurate mounting of the DRO, so I chucked it in a four-jaw and cleaned it up.

Then, I machined a brass plug that was an easy slip fit in the trip ring's side hole. The plug was drilled and tapped to accept a brass 1/8-inch pipe plug. With it snugged into the brass adaptor-plug, I drilled and tapped it 10-32.

Then I slit the side of the adapter plug open with a jeweler's saw. The rest is obvious, except for the buggering of the adapter. I cut flats on it for a wrench; but, the wrench slipped while tapping for the pipe plug.

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Here is another view of the expanding plug. Before installing the screw that connects the movable jaw, I attached a test indicator to monitor the fore-and-aft position of the jaw. Thus, with careful installation and adjustment of the screw, I was assured that it was not placing the jaw into a bind.

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There's nothing fancy, here. The bottom bracket serves mostly to keep something from snagging the bottom end of the caliper. It is fastened with 4-40 screws, same as the top. The clamp is attached with 2-56 screws and does not grip the beam tightly. I don't know if differential thermal expansion would ever be an issue, but just in case, provisions have been made for it.

The quill moves five inches whereas the caliper allows for six. Rather than have that extra inch dangling below, surgery by Dremel lopped it off in short order.

The small pocket to the left of the beam makes room for the slider at the extreme bottom of its movement. I've checked for lost motion and there is none. By comparing DRO movement against that of a dial indicator, the readings are consistently identical.

This was a fun exercise. Seeing as how I'm the only one who will be seeing it I didn't worry much about cosmetics. I hope you don't mind.

By-the-way, this is such a simple affair it did not require any drawings. You won't need any, either.

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Revised -- 11/27/09