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Posted 20 hours ago

Fowler 52-500-075-0, On-Mark Optical Center Punch

£8.495£16.99Clearance
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ZTS2023
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About this deal

My approach to making an optical punch is to make the base first and ream the holes so they are smooth and identical in size. Then, the optic is made and the diameter carefully reduced to fit the base. The punch is then made and adjusted to fit the base. Any looseness in the fit will affect the accuracy of the unit in use so it is worthwhile to get a reasonably precise fit - but not so tight that the unit moves as you swap the optic and punch. It is also possible to make the optic first and fit the base holes to the optic but it is generally more difficult to get the right size reamer than to turn the OD to fit a reamed hole; only minor adjustment of the optic OD is possible using fine sandpaper. The optic and punch are made from 1/2" diameter material so the base should be about 1.125" diameter or more to accomodate them. The height should be 1" to 1.2" or so. The holes should be far enough apart to allow the punch and optic to be inserted without interfering with each other. Building an optical punch is a little harder than it looks, but not a lot. The main difficulty is making the optic; some trial and error established a technique which works pretty well for machining and polishing the optic. Some parts of this technique still aren't perfect so there is room for improvement - if you find a better way, please let me know. The base holds the optic and punch for storage plus it is used to position the unit in use. I used plastic for the base but brass or aluminum would work well also. Plastic is difficult to ream to a specific size in one pass so it takes some extra fiddling to get the optic to fit the base properly. Tired of getting his centerpunches thereabouts but not quite there, [Uri] decided something had to be done. A common tool to solve this problem is the optical centerpunch, but models on sale were just a little too pricy for something so basic. Instead, [Uri] elected to build his own.

The punch is made from 1/2" drill rod, oil or water hardening. The height of the holder has been adjusted to match the length of the optic so the diameter and length of the small section on the punch body must be adjusted to match - relatively easy because steel is more predictable than plastic. Harlan used a file to produce the optic for his punch. My experience is that a fairly accurate spherical surface is needed or the view through the optic will be distorted. The optic for the punch is usable even with distortion because the distortion is symmetrical about the center but I find it disconcerting because the magnification of the circular target changes if your eye moves off the lens axis. I have tried to correct zones on an optic with a file and been unsuccessful: the resulting surface was rough and difficult to polish and/or the view through the resulting lens was distorted. Similar temperature cautions apply to polishing: 500rpm or less with WD-40 on 1500 grit paper converts the turned finish to a coarsely polished optic in a minute or two. Wipe the carbide paper to remove the plastic dust/WD-40 and add more WD-40 occasionally. Meguiar's polish for clear plastic on a linen cloth completes polishing in another minute or two. Always ensure there is some liquid (WD-40 for sandpaper, Meguiars or equivalent with cloth) to avoid melting the surface accidentally. It isn't always easy to obtain an optical polish so some trial and error may be needed; sometimes I end up re-doing the lens with the ball attachment if I get over-enthused polishing and cause surface distortion from heat. The lens may end up a little shorter than expected if this happens a couple of times but it doesn't seem to matter with most of the simple optics made using this technique. Adjust the height of the base so the flat surface of the fully inserted optic is close enough for a clear view of the surface it is on while keeping the end of the optic clear of the surface so it doesn't get scratched. Glue a piece of super-fine sandpaper to the bottom of the base and use an X-Acto knife to trim excess sandpaper away. The sandpaper keeps the unit from sliding on smooth surfaces as you swap the optic out and the punch in. An alternative is to make a groove in the bottom of the base and fit an O-ring to provide traction without scratching. Reverse the optic in the collet and use the ball/sphere attachment to make the spherical magnifying lens. A radius of about 13/16" works well for an optic of 1.5" to 2" length; the focus should be about 1/16" beyond the flat end. I tried a smaller radius to get more magnification but it didn't work because the focus was inside the lens. As noted earlier, I cut from the outside edge of the lens toward the center and move the cutter away from the lens when returning it to the outside edge. Polishing is as described above.Gloves are an over-reaction. Brass is mostly copper. Copper itself is somewhat toxic. The green corrosion often seen on brass is copper sulfate. Also somewhat poisonous. Common free-machining brass has around 3% lead in it. Sometimes other metals like arsenic are in the mix. Lead is toxic and causes neurological problems. California (naturally) has restrictions on leaded brass. But, I’ve spent a lifetime up to my armpits in copper, lead, arsenic, and other stuff regarded as toxic. I’ve always lived in houses with copper pipe soldered with lead-based solder. I’m apparently not dead yet and possibly not unreasonably crazy. Wash your hands before you eat. Don’t breathe solder fumes or machining dust. Don’t don’t worry about it. Everything is toxic in sufficient quantity, including water., oxygen, bacon, etc. The holes can be made on the drill press or on the lathe in the 4 jaw. They should be drilled slightly undersize, reamed to 3/8" diameter and chamfered; the reamer provides a smooth bore so the optic and punch slide in and out smoothly. Face the flat end (bottom) of the optic first, polish it as described above, then turn the 3/8" section of the optic to fit the base using low RPM and lubricant. Most any tool can be used to rough the 3/8 section but the cutoff tool works well to finish it because it has little rake; take light cuts and use generous lube. Trial fit the base onto the optic (leaving it in the collet) and adjust the diameter as necessary for a good fit. I polish the 3/8 section to help it slide smoothly into the base.

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