FWIW, I was digging through the web for an engraving tool for scribing markings on finished parts, I found a number of tools which seemed to me shockingly expensive for what was involved. So I spent 30 minutes saving a couple of hundred beans, and now have a very reliable tool for the CNC mill.
The diamond points are sold on the usual sites - they come in a 90 and 120 degree tip, with a slightly undersized 1/8" shank. Cost was around $25 for two points which will last a lifetime. I prefer the 90 degree which seems to cut more cleanly in steel. The 120 degree point worked better in soft ductile materials such as brass.
Body is 3/8" 12L14. The magnet is 1/4x1/2 from Lee Valley. Spring is 3/16"x1/2" and quite stiff. 1/8" steel ball to pack under the M8 set screw.
Here's the napkin sketch. I drilled 7mm for the M8 setscrew, then used an F drill (0.257") to provide a little clearance for the magnet. The edges were chamfered and the part reversed in the chuck. The hole for the diamond tip was drilled and reamed to match the shank which measured 3.15mm. This last part is critical because the bit must be a snug sliding fit in the body. If there is lateral slop, the tool will simply make a mess of the work.
Here's a design engraved in steel. The lines are less than 0.1mm across and very clean. When cutting acrylic, the edges of the lines were not as crisp due the brittleness of the plastic which left a rough edge. Cutting speed 200mm/min. The Z height of the tool compresses the spring after the tip makes contact. I found about -0.2mm applied enough force to cleanly mark the steel.
Don
The diamond points are sold on the usual sites - they come in a 90 and 120 degree tip, with a slightly undersized 1/8" shank. Cost was around $25 for two points which will last a lifetime. I prefer the 90 degree which seems to cut more cleanly in steel. The 120 degree point worked better in soft ductile materials such as brass.
Body is 3/8" 12L14. The magnet is 1/4x1/2 from Lee Valley. Spring is 3/16"x1/2" and quite stiff. 1/8" steel ball to pack under the M8 set screw.
Here's the napkin sketch. I drilled 7mm for the M8 setscrew, then used an F drill (0.257") to provide a little clearance for the magnet. The edges were chamfered and the part reversed in the chuck. The hole for the diamond tip was drilled and reamed to match the shank which measured 3.15mm. This last part is critical because the bit must be a snug sliding fit in the body. If there is lateral slop, the tool will simply make a mess of the work.
Here's a design engraved in steel. The lines are less than 0.1mm across and very clean. When cutting acrylic, the edges of the lines were not as crisp due the brittleness of the plastic which left a rough edge. Cutting speed 200mm/min. The Z height of the tool compresses the spring after the tip makes contact. I found about -0.2mm applied enough force to cleanly mark the steel.
Don
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