How to? Help

Hello folks, I'm new to the site/forum and a friend of mine who is an experienced model engineer and he has asked me to find the solution to a problem he has given me . The problem is that I want to drill a 26mm hole in a block of aluminium but the largest size drill bit is 24mm, how can the 26mm be drilled accurately and to the correct size an shape only using a milling machine ? He knows the answer but want me to find the solution and for the life of me I can't figure it out, Any thoughts?

kind regards

jacob

This is an issue frequently encountered. I drill to largest size I can then bore the rest with a boring head I have.

On a hobby machine such a boring head is useful but a fiddle as you have to stop the machine and set a new cut after every new traverse through the material thickness, works well enough though.

Bill D.

There is another way that I was taught many years ago as an apprentice but it would help if you could sharpen drills by hand and that is to offset the centre of the drill bit when sharpening so that drill will drill oversize, this can only be done if your looking for not to big a hole.

Martin P

The critical words are accurately and to the correct size.Only two methods will tick all the boxes, a reamer and boring with a boring head.

However if you want to be pedantic with your friend tell him to hold the block in the spindle and use a cheap lathe boring bar stuck up vertical in the vise, Cut is applied by moving one of the slides.

..and if you don't have a boring head or reamer and have to buy stuff then buy a reduced shank 26mm drill bit

Actually another way to do it that was demonstarted by toms techniques was to regrind a fly cutter to act as a boring bar. My smallest flycutter is 1 in diameter - so j..u..s..t doable. Or make a smaller flycutter or a one-off boring tool.

Theoretically, for accuracy and rigidity a microbore boring bar is probably one of your better options. At 24-26mm diameter you should be able to cover the 2mm range required.

More rigid than some of the lighter boring heads around, but in practice most would finish with one as these kits which are more affordable and easier to get hold of.

Seconded. Without the proper kit, a 12mm bar with a cross-hole, tool bit and a pinch screw. Clumsy, tricky but cheap and simple. It's no jig borer but it could be decent with care.

I take it from what Jacob says, the job is to be done with the tool at hand, a 24 mm drill, to drill 26 mm. Resharpen with the point 1 mm off centre. The piece will need to be clamped in a vice, and the vice allowed to float.

Or, use the drill as a boring bar in the lathe, first drill 24 mm.  Maybe no lathe.

Ian S C

Edited By Ian S C on 05/03/2015 08:44:44