Centre finding small holes

Hi Folks - I have a 5mm diameter hole in a 10mm thick flat mild steel part.

I Need to drill it out to 7mm. I have DRO on the mill and have often indicated larger holes (for centre finding) using my 4mm edge finder, however on such a small hole the finder virtually fills the hole and it is difficult to tell when it meets the edge as the hole constrains the throw. I am sure someone must know a cunning trick on how to centre small holes?

Martin

put a short length of 5mm bar in the chuck, enter it into the hole, clamp down, swap to 7mm drill and away you go.

Or if you're crude like me, just put the 7mm drill in the chuck and hold the workpiece by hand!

Loosen the clamps securing the plate to the packing above the table surface, fit a 5mm rod/drill into the chuck/collet, move table to align rod onto the hole, bring spindle and rod down into the hole. tighten clamps to secure the plate, raise spindle and fit your 7mm drill and drill the larger hole.

Emgee

Duncan beat me to it, must speed up my typing !!!!!!

Edited By Emgee on 10/10/2015 20:36:50

Why not work from the edges of the plate?

I have a number of bits of silver steel with a 60 deg cone turned on the end for just this sort of thing -same idea as Duncan and emgee, but it pulls the work on centre, and one size works for a range of diameters. A lathe centre in the mill spindle does the same thing, just a bit more faff swapping things over.

Robin

Turn up a a button say 3/8" dia with a 5 mm dia spigot to fit hole. Part off to leave say 1/4" then on the milling M/C
ass button into plate clamped on bed then you can clock button with dti in chuck when on center remove clock and button fit 7 mm drill . Job done . I have many like this they come in handy

Why would this be better than just fitting a 5mm bar into the chuck and using that to centre as suggested earlier? To my naive mind it just seems like a convoluted way of achieving the same end, with more potential error. But I stand to be corrected! I often miss stuff!

Robin

The button method suggested by Nobby removes the potential error from concentricity in the chuck, but a 7mm drill will follow the 5mm hole anyway, so unless you were going to bore the hole it probably wouldn't achieve better results

The button method would be useful if the part was large/heavy, or if there were other reasons why it had to remain clamped firmly to the table throughout.

Why not just use a dial indicator on the hole? If you put say a 6mm slot drill through when the hole is lined up then follow it through with the drill.

Some terrific ideas. Spoilt for choice!

I think I will try the 60 degree cone method and then clamp the part down, Put a button / bar in and clock with my edge finder and DRO - should be interesting.

The hole edge might have a very slight amount of uneven wear on it - but using the cone should get me very quickly into more or less the correct position, the button/ bar should allow me to do any correction needed.

Many thanks folks.

Martin

Thanks for the explanation of Nobby's method Duncan - I knew I was missing something, I understand now - I think!

Robin.