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Drilling accurate holes in Aluminium

I have a one-off job to put some extra 3/8" UNC threads in an expensive marine item - so want to do the best I can in preparing for this

Task is to drill a small number of 5/16" blinds holes (3/4" deep) in

6061 T6 Aluminium

Recc Cutting speeds using this table (and calculator)

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Shows as 7334 rpm, my bench drill which has Max of 2580 rpm so assume I should go with that. I'll use appropriate cutting fluid ....

I have always in past when accuracy required - drilled undersize (by about 1/16 on this size hole) - then opened out hole to final size on a second past with the 5/16 drill. Almost using final drill as a reamer.

I was recently berated for this with advice that I should just feed in final size drill and do it all in one pass ... better accuracy.

Welcome advice on best approach.

Reply to
rick
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If you have an accurately ground 5/16 drill then just use the one drill. Especially if it is a split point drill. I use in my shop split point drills as much as possible. Not only do they self center they also require less pressure and cut to size. Eric

Reply to
etpm

No just standard twist drills

Reply to
rick

Are they any good? Drill a test hole in another piece of aluminum. And you don't need to have the drill spining 2850 RPM. 7334 is just how fast you CAN run it, and what would be used in production with flood coolant. You can spin it at 1000 RPM and it will cut just fine. The most important thing for drills is to have the end ground equally. If off center the drill will cut oversize. If you drill a pilot hole this can help the drill cut on size. If you are drilling a large hole it does help to drill a smaller hole that is slightly larger than the web of the drill. This will greatly reduce the tool pressure required to push the drill through the work. Eric

Reply to
etpm

Thanks ... I won't bother with higher rpm I have a new 5/16" drill 'hope' (assume?) ground correctly. I'll put a pilot hole in first using a new drill and then open up with the 5/16"

As I also have to tap each hole once I have drilled hole, without moving piece will swap drill out for tap, and put a live centre in the drill press so the point will fit the 'dimple' in the end of the tap. That way I should get thread at exactly plumb.

Reply to
Rick Hughes

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