An Extra Finger

Every now and then I get a job in that requires me to tap or drill a shitload of holes. Frequently the parts I'm tapping are fairly small and holding them becomes a real chore. I've made various fixtures to keep them on the table in such a way that they don't jump up in the air on the return stroke of the tapping head, but these jigs have all been special made for a part with a specific thickness. What I needed was something that would hold any part, regardless of its thickness, to the table, as close as possible to the tap. What I came up with is this little pneumatic holder.

I'd seen something like this device in a woodworking catalog once, but the cost was pretty horrible. I built my version for about $30.- in pneumatic parts and a few odd bits from the scrap bin.

A part is first positioned against stops (bits of wood or barstock clamped to the table) on the right side and behind the part, so that it is lined up for the tapping operation. Once these values are locked in, so to speak, I flick a switch to turn on the compressed air that runs the gadget. Once a part is located I start the motor and begin to lower the quill. As soon as it starts descending a button is released on a pneumatic switch and a little holder snaps down onto the top of the part. Pressure in the cylinder doesn't need to be very high (I'm using about 60psi) and it can be overcome with the leverage I get with the feed handle, so tapping into the part isn't a problem. Retracting the tap is no longer a problem; the part stays put until the tap is well clear and the quill is fully retracted.

Photo #1: Overall view of the device. The air cylinder has a 4 in. stroke; the rod is threaded #10-32 and the little "finger" at the end is 3/8in. square and maybe 3/4in. long.
Photo #2: View of the finger in the retracted position. By careful design it's possible to get the end of the finger within about 1/2in. of the centerline of the tap.
Photo #3: The operating tackle includes a stop bar with vertical adjustment (that fastens onto the side of the drill press) and the switch it contacts. When the quill is lowered the switch goes along for the ride, leaving the stop bar behind and activating the cylinder. The slot on the right side of the plate allows me to change the location of the cylinder and its finger, so I can accomodate different tap sizes and different part geometries.
Photo #4: The mounting clamp. I made the plate and clamp from the same bit of 1/2in. thick aluminum. After boring a hole to fit around the quill, I drilled and tapped the end 1/4"-20. After sawing it apart I bandsawed the contour to fit between quill and threaded depth stop, ground it smooth and bolted the pieces back together.

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