Precision(?) Hole Drilling in Cold Rolled Mild Steel

[Bill Adamsen]

I'm evaluating a project which would require drilling several hundred 3/4" holes and then reaming the holes to "slip-fit" cold rolled steel 3/4" OD tubing. If it were five holes I wouldn't think twice, and would do it myself ... though the same questions would apply.

What process steps should I use for creating these holes? Assume: measure, punch, pilot/counter, drill, ream, deburr. The holes will be drilled through 2-1/2" rectangular tubing with a .120" wall (holes - thru type - both sides of tube). Reaming I've never done. Does the implied reaming require I use a bit smaller than 3/4"? How much smaller (47/64"?) and what type of reamer works well for this application? How many bits and reamers will I burn through? Do I get HSS or Cobalt or a specific brand?

I have a reasonably good drill press (Clausing 15" with VS) but this is more metal drilling than I've done at one time. Do I outsource to a machine shop?

Didn't want to put this in the general woodworking forum because it seemed more specific to metalworking. I just hope some folks can point me in the right direction.

[Mike Heidrick]

You priced a 3/4" carbide reamer? Job better pay a LOT.

[Bill Adamsen]

Mike: I see 3/4" carbide chucking reamers on ebay for $20 to $50 ... which is almost inconsequential. I see these 3/4" adjustable hand reamers at <$10 which is truly inconsequential. If I knew the right process and tool requirements I could better perform the cost benefit analysis (CBA).

[Mike Heidrick]

You want a chucking reamer if you are using it in a press. Your linked adjustable reamer is meant for a tap handle or a press with a tap reamer chuck (square).

Use drills to 18.5mm. Then use the reamer.

[Ryan Baker]

Doing hundreds of these holes is going to be a huge task. I would seriously be considering farming this out to some automated equipment.

Getting consistent slip fit holes is not simple to do unless your material is very consistent. The first thing you need to do is measure (with a micrometer) the actual size of the tubing in question and see how much it varies. Do not assume that 3/4" is exactly 0.750. To get a good slip fit, thousandths of an inch matter. Only once you know the actual size of the tubing, then you need to find a reamer about .001 larger than the tube. Run as slow as you can (certainly below 100 rpm, preferably more like 50 rpm) with the reamer, using plenty of oil, to avoid burning up the reamer too fast. You will probably wear out at least a few HSS reamers for this. Carbide will last much longer, but won't have nearly as much choice of size. Forget about adjustable reamers. The drill hole will be fine at 1/64 under, but you don't want to take more than that with the reamer. But check that your setup is not drilling significantly oversize. (You will know if your reamer isn't taking anything out.) Set up some suitable stops and clamps so that you don't have to measure and mark every piece. You will probably have to starter-drill, or at least center-drill before the big drill.

I would take a look at a Hougen Rota-broach of the proper size for this. That would tear through this material, probably would last if run slowly, wouldn't need centering or pre-drilling, and may well be accurate enough to not need reaming (depending on your needs).

[Larry Frank]

Have you considered a hydraulic hole punch? It might be easier. You did not say what thickness the steel is but I think drill thin steel is not easy especially a large hole.