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Hi Wise Metalworking Guru's

I have a 15mm hole in an alloy plate (about 8mm thick) I wish to enlarge this hole to 19mm so I can tap for 1/2" BSP, however I don't want to just enlarge the hole but to do it off centre as per attached diagram.  I have the tap(s) but not the correct size drill, so I have to buy something.  The Alloy plate is an awkward shape so I cant mount on the lathe and bore it, so I'm going to have to use the big drill press which is fairly sturdy and I have a 360 movement vice on it so I can get it in the correct position and lock it, but I know that ordinary drill have a nasty habit of wanting to centre on an existing hole if they can, so I was wondering as I want to keep the bit as fixed as possible should I go for something like this.

https://www.ebay.co.uk/itm/HSS-Blacksmiths-Drill-Bit-Reduced-Shank-Drill-Bits-14mm-to-25mm-Steel-Drilling/401142942842?ssPageName=STRK%3AMEBIDX%3AIT&var=670688525016&_trksid=p2055119.m1438.l2649

or a milling bit like this

 https://www.ebay.co.uk/itm/3-4-19-05mm-HSS-4-FLUTE-AUTOLOCK-END-MILL-EUROPA-TOOL-CLARKSON-5072010480-96/272108814761?hash=item3f5af301a9:g:chgAAOSwX~dWnOfr

Alan

5ab0fd2408847_enlarginghole.thumb.jpg.76266cbf48a2978307ca17750fefd26d.jpg

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Ah about the same price as the milling bit, and with a 19mm shank would easily go in my press chuck, as I also have a couple more 19mm holes to drill in the alloy it might do everything I need.

Alan

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Well digging through an old box that probably hasn't been opened in 20+years I found a 19mm drill (well given age of it almost certainly a 3/4") it had a morse taper, but I have various old taper holders, so I just machined a rough looking one round so it would go in the Pillar Drill Chuck, and with everything locked as tight as it would go, I gave it a go, and it worked perfectly as you can see from attached shot.  I think it only worked because it was Alloy, if it had been steel I think the tug of the existing hole would have been too strong, but anyway Job Done, maybe not a master piece of engineering, but will do me. You can see why I needed to drill off centre.

Alan

5ab14c1f7c2df_halfbsptap(Large).thumb.jpg.98e681ab745daaa06fdbbff4aa191fdd.jpg

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Glad you've done it!

Me, I'd have used a die-grinder, with a burr, not a stone.

Or, saw cuts radially from existing to desired radius, as many as possible, then a round file.

John

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You got away with it with existing in hand tooling, resourcefulness in the right application for the moment . Sometimes you just have to make do with what you have. A drill can be ground flat bottom like an endmill and can be used carefully to "move" an existing hole going to a larger diameter, carefully being key.

Cheers Tim

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3 hours ago, TIMSGT6 X 5 said:

A drill can be ground flat bottom like an endmill and can be used carefully to "move" an existing hole going to a larger diameter.

 

Funny you should say that, I did that to a large drill about two years ago when I wanted to mill a channel in the surface of a metal plate and didn't have either the right tools or time to take it somewhere, it worked well, but never had the nerve to admit to such rough engineering before! :biggrin:

Alan

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