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Posted by on October 3, 2006, 2:55 am
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>> I need to drill a (fairly) accurate 5mm hole in aluminium to hold a 5mm
>> silver steel rod, which will be acting as a pivot. I thought silver
>> steel was fairly accurately cut to diameter, so I first drilled a 4.9
>> mm hole in the aluminium as I read that drill bits tend to cut
>> oversize. The s.s. wouldn't fit, wouldn't even enter the hole. So I
>> opened the hole out to 5mm. Still wouldn't enter (and a micrometer
>> check on the s.s. showed it as about 4.97 mm). A tiny touch of filing,
>> still no fit. In the end I had to use a 5.1mm drill before I could get
>> the s.s. through. What gives? Should I invest in a 5mm reamer?
>>
>Drills tend to make a triangular hole, the points of the triangle will be
>slightly over 5mm, the flats slightly under, so the rod will be getting
>blocked by the flats.
>
>Ream or (as suggested by someone else) make a 'd' bit from the silver steel.
>I have made quite a few d bits, following curly's instructions, and they
>work pretty well.
>
D bits for drilling accurate flat bottomed holes need to
flatted to few thou over half diameter and carefully relieved on
the noncutting side of the end face.
For a hole that is already near final size the quick and
dirty method is to replace the half diameter flat with a simple
45 deg cut across the end face. This face must have a dead sharp
edge but, for a few cuts in light alloy, the silver steel does
not have to be hardened.
Jim
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> Forgot to say - the 5mm silver steel entered a 5mm hole in a drill gauge
> plate very snugly and cleanly.