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- Jul 2, 2014
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- 7,594
I recently helped a friend install magnetic scales on his new to him lathe, and had to drill and tap holes in cast iron for the small mounting screws. The job came out fine, but I was less than happy with the quality and consistency of the work I did with drilling and tapping holes while holding the correct sizes and getting a good thread fits and accurate hole locations.
So, now I need to install DRO scales on my Millrite mill, 3 axes, all holes will be in cast iron, and all holes will be blind. The hardware all uses M4 x .0.7 screws (or I suppose I could change to another size, like 8-32.) All the holes will be drilled using hand held power drill motors. I have quite a few, and can hold whatever RPM range I want to. I have drill bits in all sizes imaginable, with backups, but only half millimeter increments in ISO sizes. I only have one M4 x 0.7 mm tap, a 40-50 year old unused Craftsman Kromedge carbon steel plug tap. Those are good taps, but I probably should have at least one more on hand for the job, and a bottoming tap may also turn out useful. The results do not need to be to high tolerance for this job, but I would really like to achieve good fits, consistently, and no breakage. I will definitely be hand tapping and taking my time.
Suggested taps for the job?
Tap drill size choice beyond the usual charts? 3.3mm (.1299") is the usual global recommendation, but other choices might be #30 (.1285") or #29 (.1360") or maybe others. I have lots of #29 and #30 on hand. One of my charts shows the preferred drill as #36, but that is definitely not correct, probably a typo for #30.
Specific advice for getting hole placement where I want it, considering hand held drill, awkward locations, hard spots in the iron, etc.?
I know for sure I will be making a drill guide to match the chosen tap drill size for keeping the drill normal to the surface while drilling the holes after starting them.
Other suggestions?
Thanks!
So, now I need to install DRO scales on my Millrite mill, 3 axes, all holes will be in cast iron, and all holes will be blind. The hardware all uses M4 x .0.7 screws (or I suppose I could change to another size, like 8-32.) All the holes will be drilled using hand held power drill motors. I have quite a few, and can hold whatever RPM range I want to. I have drill bits in all sizes imaginable, with backups, but only half millimeter increments in ISO sizes. I only have one M4 x 0.7 mm tap, a 40-50 year old unused Craftsman Kromedge carbon steel plug tap. Those are good taps, but I probably should have at least one more on hand for the job, and a bottoming tap may also turn out useful. The results do not need to be to high tolerance for this job, but I would really like to achieve good fits, consistently, and no breakage. I will definitely be hand tapping and taking my time.
Suggested taps for the job?
Tap drill size choice beyond the usual charts? 3.3mm (.1299") is the usual global recommendation, but other choices might be #30 (.1285") or #29 (.1360") or maybe others. I have lots of #29 and #30 on hand. One of my charts shows the preferred drill as #36, but that is definitely not correct, probably a typo for #30.
Specific advice for getting hole placement where I want it, considering hand held drill, awkward locations, hard spots in the iron, etc.?
I know for sure I will be making a drill guide to match the chosen tap drill size for keeping the drill normal to the surface while drilling the holes after starting them.
Other suggestions?
Thanks!