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View Full Version : ON Topic :) enlarging hole in thin sheet



darryl
10-25-2009, 10:22 PM
Often enough I run into this where I have a thin sheet or a disc and I need to enlarge a center hole. Usually, if there's many to do, I set something up on a faceplate to mount the pieces to and use a boring tool to do the job. It usually results in a good sized burr on both sides which is often a pain to deal with on thin material. I'm talking about sheet that's maybe anywhere from .020 up to maybe .075 or so.

My guess is that a sharp cutter is a good start, possibly custom ground to give proper relief angles for the size hole that's required. Maybe a certain nose radius- any ideas?

Jim Shaper
10-25-2009, 11:54 PM
I think the boring bar is part of the problem. Too much surface area on the cut.

Instead, try using a backing plate of MDF or plastic, then use a cutter with a neutral or slightly negative rake and positive relief (like a parting tool, only narrower) and plunge into the work close to your desired dia.

beanbag
10-26-2009, 12:13 AM
What size holes?
I would plunge with a 4 flute end mill.

darryl
10-26-2009, 12:27 AM
Thanks Jim. I'm using an mdf backing right now, but I'm not plunging into the surface, I'm enlarging from the inside of the hole. That's something I could try, especially since I need to go from a 3/4 inch hole to about 1 1/2 inch.

That gives me another idea- if I make the backing plate from steel and bore a recess into it the desired size of the hole I need, I could possibly make up a cutter that would shear the hole. This would be a round cutter and would roll as the spindle turned. It would be something that I'd mount to the crosslide and adjust to touch the inside of the recess. I'd press it into the sheet material mounted over the recess and have the spindle turning at low speed. Hmm. It would be fast- one rotation and the hole is done. The shape of the cutter would be critical- it would have to be at an angle so it would touch the inside of the recess (the die I suppose) over a bit of area so it could shear.

Hmm. Probably too much work to set up for the limited number of pieces I'd do of any particular size. I'm going to try grinding a cutter to plunge into the face and see how that goes.

PixMan
10-26-2009, 12:54 AM
Trapanning tool, the thinner the better.

lakeside53
10-26-2009, 01:15 AM
Unibit.... can't live without them..

darryl
10-26-2009, 02:13 AM
Trepanning tool- yes. That's what I just did, grind one up to suit this kind of job. It's working pretty well and isn't dishing the material much if at all. I ground it to a thin section, with clearance on the three sides and a bit of top rake towards the spindle axis. I realized after that I didn't need clearance on the inside, not much anyway, but it doesn't matter that it's there. I can see that I need to grind under a bit more so the bottom of the cutter can clear the edge of the hole a bit more, but for the diameter I'm doing right now, it's fine.

What's really tedious is mounting and unmounting these discs I'm working on. Four screws into the mdf with the backing washer that clamps down on the discs. Step one, step two, step three, screw, screw, screw, screw. Turn the disc. Unscrew, unscrew, unscrew, unscrew, pry. Back to step one, etc.

I've used Unibits and sometimes there's nothing better for the job. I think at this diameter, 1 1/2 inch, and thinness of material, .030, a Unibit would probably tear and distort the material.

lakeside53
10-26-2009, 02:24 AM
Try a unibit... (real one, not a cheap india/chinese knock-off). I've enlarged big holes in material thinner than 30 thou with excellent results. I used a couple of pieces of double sided tape and a backing support (mdf). Use a drill press and drill the backing at the same time if you're worried.

Jim Shaper
10-26-2009, 03:26 AM
Have you thought about doing them on a rotary table?

Make a fixture that holds the OD below the surface (assuming the OD is consistent enough to get your hole aligned accurately with such a jig) and then use toggle clamps to retain the disk in the counter bore. A conventional roughing pass and climb finish pass milling should give you perfect edges.

You could also plunge it with a 1.5" end mill, but I'm not sure what your tolerance allows for.

Peter S
10-26-2009, 07:02 AM
An ideal tool for deburring holes (especially large holes) in sheetmetal is the Noga-type hand-held tool with the correct tip. I can't imagine life without these tools, especially for de-burring bores on the lathe, but also unbeatable for de-burring large-ish holes or slots, edges etc. in any shaped piece of sheetmetal off the lathe or mill.

We recently had to enlarge holes in about 50 x 1.6mm sheetmetal components. Holes needed to be enlarged from about 90mm to 120mm diameter. Set up in mill with fixed stops to locate job, and quick-release drilling clamps (Scan-grip) to hold the job. Used holesaw without its pilot and cutting oil. Took about 2-3 minutes to load, holesaw and deburr each one.

Other ideas: Screws for clamps sound tedious and slow, but try using battery driver, and also having something that swings aside that does the clamping so you are not having to remove screws each time, e.g. large washers with one side cut off.

Black_Moons
10-26-2009, 10:22 AM
If the holes size isent very critical, holesaws do an OK job at enlargeing holes if you have a backing for the pilot bit to sink into, or use it on the mill/drill press. a flycutter ground/setup as a 1 tooth hole saw might produce decent accurate results.

Duffy
10-26-2009, 11:24 AM
Lee Valley Tools lists a hole saw mandrel for "enlarging Misteaks." basically, it is a mandrel that takes two hole saws-the one too small and the next try. If you made a dummy for the inner unit to fit the hole, and ground all the set from a saw of the correct diameter, would that not provide a clean hole with a tidy edge? Used with a backing with the correct recess on your face plate, and it would center the work also. Duffy