By Jose F. Rodriguez

The use of the drill press as other than a drilling tool is a subject dear to my heart as I have been forced by financial situations to ask it to perform double duty as a light milling machine as well as its normal function of drilling. X and Y axis movements are all performed with a heavy duty machining slide table that although of Asian origin, performs extremely well. It is bolted to the base of the drill press to provide the maximum rigidness during operation. Adjusting the height of the cutters in relationship to the surfaces being milled or the depth of the holes drilled was originally supposed to be done with the traditional side mounted screw with double nut arrangement and a simple wobbly pointer running against a stick on scale in inches. Yes, very coarse adjustments are indeed possible but any serious kind of work involving finishing cuts removing a few thousands are nearly impossible to do this way. You could I suppose, install a dial indicator on some sort of multi jointed contraption to take depth readings against the chuck, quill or directly off the workpiece. I almost decided to do just that until all of a sudden I had the following brain storm.

As I was drooling over a recent advertizement on a precision high speed micro drill press, I noticed that the little gem of a unit came with a dial indicator factory mounted on top of the head stock so its plunger could directly read the height of the top end of a non rotating part of the spindle protruding through the drive belt cover. I did not want to move my only good dial indicator for permanent duty on the drill press so I came up with the idea of a graduated dial on a threaded 1/4-20 rod that would become part of the upper end of the spindle. The top surface of the drive pulley was faced off to provide a trued bearing surface for the dial. The theory for this goes as follows. The rod which is now an integral part of the spindle will rotate at the same rate as the pulley. As I lower the quill, the unit will stop as the bottom of the dial contacts the top of the drive pulley. Since their rotations are synchronous, they mesh perfectly. To drill a hole to a depth of say .375", you simply lower the quill until the tip of the drill bit just touches the surface of the work and at this time you would lock the spindle nut to hold it in place. Turn the dial down on the rod until it is bearing on top of the drive pulley. There is a witness mark on the pulley which should pointing to one of the engraved marks on the dial. All you have to do is to unscrew the dial .375" by directly reading from the marks on it. Just like advancing the milling table or cross slide on a lathe. Now you can unlock the quill, raise it and proceed to drill the hole being sure that when the dial bottom contacts the top of the pulley, the drill has entered the work .375" give or take a couple of thousands. The same would apply when milling, specially after just completing a cut and you want to lower the tool for the next pass of 10 to 15 thou or maybe just take a light finishing cut of a couple of thou. Just unscrew the dial the chosen amount, lower the quill and lock it. You are now ready to take the second pass.

Let's begin the actual work needed to produce the positioning dial. The first thing to do is to remove the quill and spindle unit off the head stock. It's actually a lot easier than it sounds as the unit comes off in on one piece after loosening and removing the return spring assembly and removing the alignment bolt that mates with the vertical slot on the side of the quill. Once these two items are out of the way, the quill handle pulls right out of the headstock and the quill / pindle unit drops out the bottom. I proceeded to thoroughly clean the spindle assembly and check it for wear and other possible problems. The bearings seemed to be nice and tight and all I did was to clean them up. The vertical alignment slot on the right side of the quill was full of small dents and other small surface blemishes so the thing to do was to get that slot back in shape again. Some careful work with a square riffler file and the surfaces of the slot were brought back to a nice smooth even finish. The locking screw tip was the culprit for all the damage so that was machined to a nicely rounded contour so it would minimize any future blemishes but still allowing it to lock the quill in line.

The quill and spindle was chucked on the lathe and an end hole was drilled through the top end of spindle to about 1" in depth and then threaded to 1/4-20. A length or corresponding threaded rod was cut to 4-1/2" and screwed, aligned concentrically with the spindle axis and secured in place with Loctite At this point I re-installed the spindle/quill back into the head stock and replaced the drill handle. A 1/4-20 threaded dial was made out of a piece of scrap aluminum as follows. It began as a slice of 1-1/2" diameter rod and after facing both ends and turning the outside to true it up. I drilled and reamed it to 3/8". A pair of bronze bushings were made with 1/4-20 threaded centers. The bushings have a 1/2" diameter shoulder and the a .376" body to give a nice press fit into the dial center holes. This allows them to be adjusted by turning against eachother to adjust how tight they run on the threaded rod. Backlash can be reduced to nothing in this fashion. The fifty engravings are done with a lathe pulley installed dividing plate and a pointed tool turned on its side. A depth stop rod on the headstock of the lathe controls how far the carriage advances to cut the any of the engraving lines. By using the depth stop rod, all of the lines will be cut at exactly the same length. The lines representing each .005" divisions are set to be cut somewhat longer to differentiate them from the single thousand inch lines. I cut each line twice to a final depth of .006" but the five thousands lines are cut to .010" in depth. Now that the dial is finished, I just screw it down onto the rod so it is bearing against the top of the drive pulley. To lower the quill .500" from the top position just unscrew the dial ten times or the equivalent of .500". Remember that we are using a 20 pitch screw which wasn't chosen just out of the blue as this would be the only way to be able to make a fifty division dial. Other posibilities could have been a 10 or 40 pitch acrew to give dials with 100 and 25 divisions. I chose the 20 pitch thread as I happen to have a bunch on hand. Besides, this size threaded rod can be found at just about any hardware store. Actual tests for accuracy of the dial were done with a dial indicator in the chuck to take direct readings to compare with those set with the dial. Everything was right on throughout a 2" range which is the limit of my dial indicator. So I am very happy with my my little contraption to solve the old problem of tool depth positioning in a drill press.

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