Another tool I have wanted is an optical center punch. I have the plans for one in an old edition, Aug 2016, of "Model Engineers' Workshop" out of the UK. The article with plans is titled "Build the Twist n' Punch" and was written by Mogens Kilde. It looks like a fun tool to make and use, while being reasonably straightforward. A perfect project, while I continue to learn how to use the new laser engraver. A photo from the printed article shows the parts and the assembled rotating optical center punch.
The plans are in metric, which would not be a problem except for the drilled and reamed holes as I don't own metric drills or reamers. One hole for the detent ball is 5.5 mm or within 0.0023" of 7/32". The holes for the punch and the optics are both 8 mm or 0.0024 over 5/16". A final hole in the bottom of the insert, 8.5 mm, is 0.0026" bigger than a "Q" drill. The countersink for the punch hole is 12 mm, but this can be bored to size. Consequently, the drilled holes can be managed with my non-metric drills.
The lathe was completely lubricated yesterday, so it is ready to turn. Cutting will begin tomorrow on the four parts, 2, 5, 6, and 7, that are machined on the lathe. Two lengths of 1 5/8" round steel bar were cut with the bandsaw, 1 11/16" and 1 3/4" long. The first is for the inner part of the device and the second is for the outer and cut long to hold in the chuck.
This morning the garage was 80°, but cooled down to 75° after opening the garage doors and turning on a fan. Machining commenced. The inner body was reduced to the desired 30 mm after facing. The DRO in metric made it trivial to get the length correct, whereas the diameter required conversion. (A quarter of a millimeter is very close to ten thou.)
The outside corner was chamfered and the part was reversed in the three-jaw chuck. The upper part was faced to length and then reduced to diameter.A large chamfer was cut on this part with the compound set to 45°. A knurl was then put on the center of the top. Knurling turned out okay after I solved all of the issues. The biggest was that the QCTP would turn during knurling as pressure was applied to the cross slide. I might fix this at some point by gluing some rubber gasket material or sandpaper to the bottom of the QCTP.
The outer body was begun by reducing 1 1/8" of the round bar to 36 mm. The outer edge was lightly filed. This part was also knurled with the same rotating QCTP as above. The cylinder was center drilled and drilled up to 3/4". At this point drilling was replaced with boring. The hole was bored to a depth of about 7/8", leaving a drill tip shaped hole in the center. Boring was continued until close to the correct size and then care was taken to hit the target. This was achieved as the sliding fit between the two body parts was only hampered by the buildup of air pressure in the cylinder! The bottom of the cylinder was then flattened to the desired depth. By this point the temperature was now over 80° so machining was stopped at this juncture.
Two days later the morning was free and a cool 70°. The outer body was held between vee blocks and the wide end cut off with the band saw. After chucking the part in the three jaw the end was faced to slightly over length, ~0.4 mm. I was worried that the tiny indentation left by the drill in the center would leave a hole if I made the bottom of this part too thin, so it is 1.4 mm thick instead of 1.0.
Drilling the two holes through the inner body was tackled next. The plan calls for 8 mm reamed holes. This will be closely approximated with 5/16" holes for which I own a reamer. The holes need to be drilled 10 mm each side of center, so setting the center of the part directly under spindle center is critical. Any error here directly translates to twice the error in the punch mark location. Time to make a fixture for the dial test indicator.
With the fixture complete the inner body was held in the mill vise and centered under the spindle using the new fixture as seen in the first photo below. The y-axis was locked and the x-axis was offset 0.3975". The spot drill was followed by first a 7/32" drill and then an M drill. The hole was reamed 5/16". The reamer had to be shortened to accomplish this in the mill. This was easily done by hacksawing off about 1.5". The x-axis was then offset 2 X 0.3975" in the opposite direction, where the second hole was spotted, drilled and reamed.
The second hole still aligned with the spindle needs to opened for the spring 0.592" deep, ideally with a flat bottom. A 1/2" end mill could be used to accomplish this, but that would leave the hole 0.040" larger than the spring. A drill could be used, but leaves an angled hole bottom. Boring is the answer, especially after drilling out most of the recess. Drilling did not work as the drills are too long for the mill. Boring the entirety of the recess also did not work as the boring bar is slightly too large for the hole and the amount of material to get started was too much for the small mill. This will need to be completed in the drill press or lathe.
In the meantime the hole for the detent ball and spring was drilled. The part with two rods through the holes, one a 5/16" punch and the other a 5/16" piece of brass tubing, was held in the vise with both rods resting on the top of the vise ensuring the spindle was at right angles to the plane defined by the two drilled holes. The spotting drill was followed by a 7/32" drill to a depth of 0.59". The 7/32" ball drops right in with no play. The photo below shows the inner body at this stage.
After much hemming and hawing the hole was just opened with a 15/32" drill to a depth of slightly over 9/16". The inner body was then returned to the milling vise on two rods inserted in the holes. The small end was located and the part moved 0.256" along the y-axis. After locating the side with the 0.200" diameter edge finder the part was moved away from the spindle 0.150". Replacing the edge finder with a 1/2" end mill set its edge right against the part. The light port was milled 0.118" deep in increments of 0.010" with cutting proceeding down the side of the part. The part was deburred and is shown completed in the photo below.
The outer body was treated similarly. The spindle was aligned 0.315" from the bottom and 1/4" from the outside. Milling went twice as deep in this case and in increments of 0.015". Before removing the part from the mill vise, the spindle was aligned with the center-line of the part and 0.315" in from the top edge. A 0.120" hole, #32 drill, was drilled through both sides of the cylinder. After removing burrs from the three holes the part was slid over the inner body with the spring and detent ball inserted, producing a perfect match as shown in the photo below.
The punch was made next in the South Bend lathe using collets. A 3" length of 1/2" drill rod was held in a collet and the exposed end was faced and cut to a point, 106° included angle. A little less than 1 1/2" was reduced to a sliding fit, 5/16", in the inner body. After hacksawing off about 3/4" the part was turned end for end and placed in a 5/16" collet and the larger end was reduced to 15/32" and faced to length. A 45° chamfer was cut on the end. The part did not fit, as I should have realized, the drilled hole was not quite concentric with the reamed hole. Consequently, the "head" was further reduced to fit. The pointy end of the punch was heated to cherry red with a propane torch and then quenched in oil. After cooling it was tested with a file, which did not leave a mark. The punch is seen in the two photos below.
The finish on the drill rod as cut was quite poor until the speed was greatly increased by switching pulleys. A green Scotch Brite pad was used to give the punch a nice polish prior to hardening.
With the detent in place and the light port aligned with the punch side, the punch was given a zetz with the brass hammer. The punch mark can be seen in the bottom of the outer body in the photo below. (The ball is there so it won't be lost.) The next morning the final hole in the outer body was drilled, first with a 1/16" drill and then with a 5/16" drill, followed by an 11/16" drill. The latter was used after discovering the hole did not quite align with the punch, my drilling prowess with the drill press is minimal.
Only the optics remain. Acrylic round bar was ordered from Online Metals. A 3" length of 1/2" round bar was cut from stock with a hacksaw. The part held in a collet was faced and then the face was sanded with 600, 1000, 1500, and 2000 grit sandpaper. This was followed by polishing with a fabric wheel charged with blue rouge. After much effort a compass point was reasonably centered on the lathe axis, using a scrap of 1/8" aluminum and six layers of sandpaper. The point was a tad low, just about right actually, to produce a tiny circle when pressed against the spinning acrylic. The setup and the result can be seen in the two photos below.
About 7/8" was reduced to 0.310" for a sliding fit in the inner body's reamed hole. The part was pulled further out of the collet and the diameter was reduced similarly for the length shown in the plans. This reduced section was sanded and polished as above. The part was removed from the collet and most of the excess was cut off with a hacksaw. The part was returned to a 5/16" collet to face the head to length and reduce its diameter to 0.4". The optics after polishing is seen in the first photo below, the second shows the part at this stage.
The radius cutter was set up on the Sherline lathe with the tool bit set to cut a 0.5" radius. The end of the shaft was cut in small passes to produce the curve. It was then sanded from 600 to 2000 grit. The entire piece was polished with blue rouge on a polishing wheel on the polisher. The photo below shows the nice polish on the radiused end.
The last work on the part was gluing on a circle of 600 grit sandpaper with wood glue and cutting the hole, done with the drill press. A drop of oil was applied to the mating surfaces of the inner and outer bodies before assembly. The first picture below shows the sandpaper glued to the bottom. The second shows the completed optical center punch.
