How do you drill a flat bottom hole?

I have to drill a 37/64" flat bottom hole in the end of a 2 1/2" long piece of 1 1/16"AF hex bar that is chucked in the lathe, to a depth of

0.600" +0,-1/64 and then thread it 5/8-18. (I'll be making about a dozen parts).

I will be using a drill bit with a 118° point.

The issue is: how to get the bottom flat and at the exact depth.

Pete Stanaitis


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End mill.

Reply to
Pete C.

Drill so the point is just shy of full depth, regrind your drill like this

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finish to depth.

Reply to
Ned Simmons

Brad point drill bit (will leave a small center divot from the point).

Or use a boring bar.

Reply to
Paul Hovnanian P.E.

To get the exact depth, you can always drill the hole with the flat bottom, measure the depth, and then trim the hex bar until the hole is the exact depth. Does require starting with stock longer than 2 1/2 inch long and trimming both ends. One end to get the hole the exact depth and the other end to end up with a 2 1/2 inch long part.

=20 Dan

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A 37/64 end mill might be a bit difficult to find.

If it has to be threaded clear to the bottom, I think the best approach would be to drill deeper, thread to depth and then insert some 37/64" dia plugs with Loctite.

Reply to
Don Foreman

That's just tap drill size. The next smaller size end mill should flatten the bottom just fine and the tap isn't going to fully bottom anyway.

Reply to
Pete C.

spaco on Wed, 17 Nov 2010 09:15:20 -0600 typed in rec.crafts.metalworking the following:

Flat Bottom Girls - err drills.

Use an end mill, or boring bar. Drill a 'pilot hole' smaller than final size needed, say .500 dia , where depth of the hole is measured to the of the drill. Then use a boring bar to "clean up" the last

0.039" (diameter of final hole - drill size)/2. You could thread it the last bit, I'd have to look up the min/max values for a 5/8-18 thread.

at least that is what comes to mind. I'm sure there are easier (and more complicated) means

Like using a 9/16 (36/64") drill and a 37/64" end mill to clean up.

Or drill a sequence of .125 holes in a circle pattern, and knock the center part out. Then weld the hole section back onto the stock, and finish truing the hole.

tschus pyotr

Reply to
pyotr filipivich

Instead of putting the drill in the tailstock, mount it like a boring bar, use the carriage feed dial to measure depth from first contact to your target depth as you advance the drill.

Then using a small endmill or a real boring bar, touch bottom in the hole at the center, and bore from center to the sidewall. There may be some problem if the flat has to be dimple-free in the center, though.

You might also consider an endmill in the tailstock for the finish (but the diameter and depthfinding are trickier).

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Why not run the hole slightly deeper and focus on the flat bottom first. Then measure the depth after facing off the end. It's more accurate to use the compound infeed to remove the correct amount to stay within depth tolerance, eih? The end needs to be faced off it second op. Then do the other end as was mentioned here earlier to get overall length. phil k.

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Phil Kangas

Ned Simmons wrote in rec.crafts.metalworking on Wed, 17 Nov 2010 10:37:41 -0500:

That's how we do it at work. Excellent pics Ned.

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That'll get you one mil accuracy; the original poster was talking about 1/64 (16 mils).

I'm a big fan of metal removal with a twist drill, it's SO quick and easy. Not so accurate, with a tailstock, though.

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Two flute endmill just long enough to get it done. This assumes your tailstock is perfectly centered and doesn't droop like many old machines will.

As far as depth, I've been considering a clamp on collar for my tail stock quill that I can use a long travel dial indicator against to see depth of drilling.

Picking up the surface is easy, just run the tool lightly against the stock and zero the indicator.

If you don't want to invest in the endmill, then flat grind a drill bit to take out the cusp after conventional drilling.


-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

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I have been doing this for shallow blind holes with 10-32 thread. Flat bottom was needed to get the extra threads in.

Michael Koblic, Campbell River, BC

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