Hole drilling problem

I had 8 off 15mm holes to drill in a 1.125" thick disc of mild steel on an 80 mm PCD. I have a 4" chuck mounted on my 6" Vertex rotary table and there was a 60mm recess in the back of my disc so the chuck was wound out to grip. The whole plot is mounted on the Bridge port, the 40mm offset dialled in and drilling commenced using a centre drill to start with, a couple of intermediate size drillings finally finishing off with a brand new 15mm drill. The indexing is done with one of those electronic thingummy bobby things, the name of which currently eludes me.

I then make the drilling bush to 15mm - .001ish and lo and behold they won't fully go home! Entered from the side that the drill entered they will go in 3/4 of the way, to the point of jamming they slide in nicely. Entered from the other side they bind 1/4 of the way in. Oh bugger. I have resolved the problem with an adjustable reamer so the job is done.

But......what went wrong. Any suggestions?

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On or around 20 Mar 2007 02:19:10 -0700, "Anzaniste" enlightened us thusly:

drills are bastards. 3-flute ones make better holes, but not that good, especially in thin material, and they need a pilot hole and are more of a tart to sharpen correctly.

Mind, make sure your brand new drill *is* sharpened correctly - I had a brand-new 20mm one the other day which was so badly sharpened that it drilled a hole about 21mm. mind, it wasn't the best quality.

for accurate fit, you need a reamer.

which brings me neatly to something I was wondering... I've got a 20mm reamer, to get accurately round 20mm holes. How much material is it reasonable to expect a reamer to take out? Obviously, the hole you start with has to be under 20mm (in this case), otherwise the reamer doesn't do anything, but how much is acceptable?

Reply to
Austin Shackles

"Austin Shackles" wrote in message news: snipped-for-privacy@4ax.com...

About 10 thou for a reamer that size. It isn't very critical as long as there's enough material for the reamer to cut rather than just rub. You could probably get away with less if the hole is accurately drilled or double drilled.

Reply to
Dave Baker

Nothing went 'wrong'. It's a drill not a precision tool. It isn't meant to cut to an exact tolerance. That's what a reamer's for. Also all drills tend to wander about a bit. If you really needed a true straight hole to an exact size you should have drilled to about 0.25mm under and reamed to size.

Reply to
Dave Baker


drills tend

Or he could have bored it in his vertical mill as it was already set up


Reply to
Andrew Mawson

Much harder to work to a tolerance though, especially a small hole that long and a lot slower than drilling and reaming.

Reply to
Dave Baker

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