Button, button

[cfmorrill]

Here's a couple photos depicting a deceptively simply jig that can eliminate one module of cutouts in Daniel's new scanner design, speed assembly, and lead to greater accuracy. You do, however, need two things:

1) A drill press.

2) A 7/8 inch diameter forstner drill bit.

The jig consists of a shouldered plywood button machined instead of the fixture module. Lacking the skills to program one I whipped one out on my lathe. Here is what it looks like:



The button is machined to about .872 in diameter for a quarter inch of its length, then to just about .003 smaller than the pass through hole that connects the ball bearing counterbores for the remaining half inch or so. I think that hole is about .680 inches. Check your plans. The button then gets deeply counterbored, at least .250 down to accept a sheet metal screw, then drilled with a 3/16nth clearance hole straight through. The accuracy of these last two is not particularly important. They just allow the button to be bolted to a ply or particleboard drill press table.

Here is how it works:

Delete machining the fixture module. Delete machining all bearing pockets and just leave the smaller, inner pass through holes when you cnc your parts. Do cnc this small button. Obviously leave tabs so it doesn't fly across the room. Don't worry about the 3/16 clearance hole.

With CNC complete, check to make sure the .680 diameter or whatever just barely slips in those inner pass through holes. if too large, drill the 3/16 clearance hole now, put a bolt through, and chuck the bolt in your drill press, sand it down so it fits.

Put a ply or particleboard or something you don't care about table on your drill press and screw it down. Chuck up the forstner bit. Drill a 7/8 hole just a bit deeper than 1/4 inch. Put the button in wide side down. Drill the 3/16 clearance hole. Screw the button down with a sheet metal screw or something, taking care to ensure that your counterbore is deep enough so that the tip of the forstner bit doesn't contact the screw hole when you drill the bearing pockets.

Take one of the parts that needs a bearing pocket and slip it over the button. Hold it down and drill .230 down for the bearing pocket on one side with the 7/8 bit. Flip the part, repeat. Yes, skateboard bearings are 22 mm, which translates to something like .866 inches. This does in fact mean that the bearings will have .011 inches slop in the hole. I don't care and gently suggest you don't either.

If this small fixture works correctly you can drill all bearing pocket counterbores in about two minutes. You will also find them almost perfectly concentric. If they are slightly off, the .011 slop will solve the problem.

In this picture, I have just drilled a bearing pocket:



Works great. Perhaps one of you computer jockies could draw one up in a .dxf file and try it out?

Regards, Charles

[dpc]

That's certainly a nice jig you've made there that will ensure that the counterbore made with the forstner bit will align with what was made on the other side by the CNC router.

Let me ask what might be perceived as a dumb question. I realize why you're not doing the counterbores on the backside of the sheet with the CNC router, but why not drill all the counterbores (front & back) on the drill press?

Just make an 'L' or 'U' shaped jig, run the radiused end of the arm into the jig so that it's positioned correctly under the chuck center, make one counterbore with your forstner bit, flip the arm over, and make the other on the backside. What's to gain by doing all of the counterbores on one side of the arms on the CNC router then having to come back and bore the backsides on the drill press?

[cfmorrill]

Exactly! The button fees you having to do any counterbores on the cnc router. You just bore what I'm calling the "pass through" holes on the router. When the button indexes on the pass through holes, then each counterbore is exactly concentric with its partner on the opposite side. Counterbore, flip, counterbore, no fences, no jigs, done.

Put another way, the button is indexing on the pass through hole, not one counterbore or the other.

Here's an even easier way: just make yourself a small plywood square with slightly rounded corners to fit smartly in the pass through hole. Screw it down to a plywood board, taking care that the screw is set real deep so when you counterbore for the bearings, the forstner doesn't hit it. Loosely clamp the plywoood board to the drill press table, taking care to center the little plywood scrap underneath the 7/8th forstner bit as best you can. Place a part needing a counterbore on the square button. Gently drill down a bit, if it doesn't look quite right, bang the board back, forward, right or left so it does. When your counterbore is centered, reef on the clamps.

Then it's counterbore, flip, counterbore, done.

cheers, Charles

[aeturneus]

Finally understand what this is about - for confirmation, with this method it's unnecessary to cut any of the "fixtures" drawings from GaryK's release?

[cfmorrill]

Right, you've got it.

Charles