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How to enlarge a hole and keep it perfectly concentric?

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RogerS

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I've got a drilling jig that is designed to use the 15mm Trend guide bush. Sod's Law dictated that said bush would snap off.

Option 1 is to buy another set from Trend..for £30 :shock: Option 2 is to bu a metal 15mm bush....only they don't do one.

Option 3 is the cheapest and that is to enlarge to holes to take the 16mm guide bush from the set. Only problem is I can't see how I can ensure that the enlarged hole is going to be perfectly concentric (tolerance 0.1mm or better) with the original hole. If I had a metal lathe I'd turn up a bit of steel that was 15mm at one end and 10mm at the other, stick it in the chuck of my drill press, align the jig with the 15mm end, clamp everything down real good, replace the steel with a Forstner bit and drill the new hole. But I don't have a metal lathe. My woodworking lathe is buried beneath a load of stuff and not easily getatable plus I'd need to sharpen the gouges .....
 

andycktm

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i thought for a moment you said 0.01 mm
then i realised you said 0.1
i was going to say do you work for bnf. :lol: :lol:
 

TheTiddles

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A reamer surely? Problem is, if you haven't got one it'll be a whole lot more than £30

Aidan
 

RogerS

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Thanks chaps...a step drill it is. Not come across these before.

A tapered reamer or big countersink wouldn't be practical in the long run as Id end up with only a point contact with the guide bush...which would wear quite quickly. A vertical surface is what I'm after.
 

RogerS

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9fingers":3t8xx9jw said:
The idea of a countersink was as a centering device. Do really think I am that thick Roger??
Certainly not, Bob. Was not my intention to infer that you were. My misunderstanding.
 

Spindle

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Hi

Unless the material is thin you will have difficulty finding a step drill that will afford enough depth before it increases in diameter to the next size. I also think you will struggle to find one with 1mm increments. Good luck though.

Regards Mick
 

RogerS

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Good point, Mick. Luckily I am going for 18mm which is the last diameter and so I should be able to drill down as far as I want to.
 
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