Countersink Inside a tube

I needed to countersink a hole inside the case of a model engine so that I could fit a bracket. The only way I could think of doing it was to make an extension for the countersink with a small drive shaft - by default the countersink head is bigger in diameter than the hole it is countersinking. Even with holes opposite each other this would be a difficult problem to solve.

Attachment 34338

in action

Attachment 34339

or nearly in action as I ended up driving it with a small hadnheld electric drill.

There are tools made to countersink the otherwise inaccessible back side of a hole but such tools would have been expensive overkill for your problem.

Your solution of constructing the needed tool inside the obstruction is an ingenious technique that should be in everyone's mental quiver of unusual solutions. Good on you.

I wonder if this could be done with something like the mechanism used in expanding anchors. Slide the tool into the hole, turn a screw to expand the end, and then back out as the reverse countersink is cut. Then turn the screw the other way to collapse the cutter and retract the tool. Not a lot of room for the mechanism.

As long as we are cutting AL, it could be made from mild steel. Beyond that, use oil or water hardened drill rod.

Rick

Quote:

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Originally Posted by rgsparber

I wonder if this could be done with something like the mechanism used in expanding anchors. Slide the tool into the hole, turn a screw to expand the end, and then back out as the reverse countersink is cut. Then turn the screw the other way to collapse the cutter and retract the tool. Not a lot of room for the mechanism.

As long as we are cutting AL, it could be made from mild steel. Beyond that, use oil or water hardened drill rod.

Rick

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Most of the commercial tools I've seen pictured do essentially what you suggest. Actually, I think the physics of the situation demand that sort of solution. But, as I said, they would be expensive overkill if you have some access to the backside of the hole as was the case with the OP.

Another possible technique is a cutter with an inverted cone shape, i.e., the cutting end flares outward from the shaft at the tip, major diameter at the very end of the tool (picture a router dovetail bit, only much smaller). If the diameter of the cutter is small enough to pass through the hole and the angle of the cone matches the required countersink angle, then insert tool through hole and move around in small circle to cut the countersink. Recenter to withdraw from hole.

The geometry works but I sure wouldn't want to be the guy tasked to make it work in practice.

Quote:

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Originally Posted by mklotz

There are tools made to countersink the otherwise inaccessible back side of a hole but such tools would have been expensive overkill for your problem.

Your solution of constructing the needed tool inside the obstruction is an ingenious technique that should be in everyone's mental quiver of unusual solutions. Good on you.

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Thanks, the thing is when you need a tool like this you need it fairly urgently and hence making it was a good option

Cheers, Nigel

Just a thought experiment - picture a countersink with the shank coming out the end where the point normally is. Slip in from the back side, chuck into your drill, and pull as it turns.

Rick

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I see countersinks that slide over a drill bit so you can drill the hole and countersink in one operation. Replace the drill bit with a rod and you have a “reverse countersink “. Feed the tool in from the back side, chuck the rod in a drill, pull trigger while pulling away from the surface.

Rick

Anybody fancy building and sharing some of the different countersink options?

Quote:

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Originally Posted by mklotz

There are tools made to countersink the otherwise inaccessible back side of a hole but such tools would have been expensive overkill for your problem.

Your solution of constructing the needed tool inside the obstruction is an ingenious technique that should be in everyone's mental quiver of unusual solutions. Good on you.

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At my workplace, we use tungsten carbide cutters and die grinder.
Like this:

Attachment 34436

Also ones that are shaped like car valve.
Those are cheap and easy solution.

What Marv comes closest to describing in commercial version are a notched shaft corresponding to hole diameter, and a cutter with a female opposite of the notch; basically a bayonet fitting. They are called back countersinks, in concert with back spotfacers and counterbores. Very common in certain castings, and lots of aircraft forgings. These are important when a fastener needs a flat seat close to a wall, impeded by a fillet or radius.

They run easily in a sturdy drill press or milling machine, and a quill stop makes depth control very simple.