Accurate counersinking?

Tapped holes are not good for locating. If you can, use a pair of holes with dowel pins to locate and one or more tapped holes plus clearance in the pass-through hole to clamp. Make a drilling jig to set spacings and keep the bits from wandering. Use cutting fluid.

On Thu, 26 Oct 2006 21:22:50 +1000, with neither quill nor qualm, Jordan quickly quoth:

I can think of 5 possible ways:

1) Try using a piloted counter-sink/bore, Jordan. 2) Or drill the countersink first, then drill the through-hole, then tap the body. 3) Clamp the assembly in the drill press and drill the hole. Without moving the assembly, swap drill bits for the countersink and bore it. (You need a good bit of quill travel for that in most cases.) 4) Drill the hole and countersink in the plate, clean it up, and mount it on the body. Use a transfer punch to line up the body hole and mark it. Remove the plate, drill the hole, and tap the body. 5) Drill 2 holes through the plate and 1/4" into the body. Pin those with taper pins. Now your assembly lines up always! You can now drill your through hole and countersink it, then tap the body.

On Thu, 26 Oct 2006 12:30:04 GMT, with neither quill nor qualm, Fred R quickly quoth:

82 degree flathead screws and countersinks really help alignment, though.

I don't seem to have this problem. I clamp the plate in the mill vise and pick up the edge, and work from a single reference edge. Similar for the object body. Are you perchance doing this work by hand on a drill press?

GWE

Yes, drill press. I tried to do it, unsuccessfully, by drilling through both cover and body with 1/16 inch drill as a pilot. It wasn't enough to keep things true. I can see that a mill, or dowelling/taper-pinning would work, but thought there might be a way to avoid that. I have a late brainwave: make the cover larger than finished size, screw to body, trim down excess. Thanks all for your advices!

Jordan

In case it matters (like, if there isn't enough room for dowels), you can also get shoulder bolts; it has a threaded end, a dowel section (the shoulder) and a head. So the bolt insertion fits a dowel all in one unit. The critical drilling operation is to the diameter of the shoulder, of course.

And other alignment tricks include center-drilling a conical dimple and dropping a ball bearing into it; the corresponding dent in the second plate can either be another cone-shaped hole, OR you can just press it hard and get a hemispherical dimple (works best in aluminum).

Tubular dowels are often used in moldbases.

A nice bit of appropriate diameter tubing could work if you're not so fussy.

Best regards, Spehro Pefhany

Threaded fasteners aren't meant for providing precision location, although you see them trying to be used for that all the time. If you need something precisely located and removable, drill, ream and dowel pin the sucker. THEN put your threaded fasteners in. Precision-ground dowel pins are a staple item, MSC, McMaster-Carr, etc. all stock them. For a more popularized treatment, see Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook.

For countersinking, a piloted countersink helps a lot, along with a rigid setup. A vertical mill provides a precision stop, too, if you have access to such.

Stan

What I used to do back when I only had a drill press, was to cut out the parts slightly oversize, tack weld them together on the ends, clamp them to the DP table and drill them. Now your holes may not be in a precise pattern, but they match. Then cut the pieces apart, sand the edges to size, and go ahead and tap the one piece, and open out the other piece's holes to clearance and then countersink. The clearance and countersink I'd do with the piece just held on the DP table so it could move slightly, allowing the tool to "find" the hole center.

GWE

That's pretty much the way I figured could work - make oversized first, csk, trim off excess. But is welding necessary? If one csk is done at a time, and screwed in place before moving on to the next one, you'd get enough control I think? Jordan

Sorry I should have made it clearer at the start - the locating tendency of the csk screws is an unwanted effect. I'm using them to get a flush finish of the screw head to the plate I want to fasten to a housing body, both of which were already finished to the same size. I found out that it's not easy to guarantee it'll all end up in alignment, as the csk dictates the position, and it's not easy to control using basic methods. Jordan

What type of c'sink are you using? Try a Ford uniflute, run it slowly, and you shouldn't have any trouble keeping the countersunk holes concentric with your pilot hole.

Weldon c'sinks are pretty good, too, but don't have nearly as wide a range as the Fords. See the Series 67 on the link above.

Ned Simmons

I was using a single-fluted HSS countersinking bit, not Ford but similar. I don't think I have decent enough machinery - too light.