[ATM] re: Locating holes

["Ellen Mackenzie" <mackenzieconsulting@sympatico.ca>]

I think the point of tolerancing should enter into the placement of the
holes. I would think that hole position has a lot more latitude than is
ascribed. We definitely don't need these holes drilled on an NC jig boring
machine. Hole position within 1/16 of an inch should be more than accurate
enough.
	First, stainless steel is a bad choice because of its high
coefficient of thermal expansion mild steel will do.
	Secondly, draw out your triangle on a piece of ordinary paper, draw
the intersection of the hole centers using a ruler, compass and sharp
pencil.
	Thirdly, with scissors cut out your triangle shape and with ordinary
rubber cement for Office Depot etc. glue the paper to the triangle, when the
glue had dried after about ten minutes use a long sharp center punch to
locate your hole positions
	Fourth, if you don't have a drill press, remove the paper and drill
a 1/8 inch hole at the punch locations. I recommend you use a 135 degree
split point stub drill to do the job. This is your drill template.
	Fifth, take each additional triangle and with a pair of vise grips
clamp it to your template and drill another three holes.
	Lastly, open all the holes to your finished size or if you are
really fussy you can open the holes out to 1/16 inch undersize and ream the
holes to finished size
	A note about drilling: 300 rpm is fast enough and apply enough
pressure to see a long chip curl up from the drill flutes. Also backing up
your sheet metal part with a piece of wet wood will prevent the heat build
up generated when drilling thin materials. Run your drill right through to
the shank to assure the drill has removed the last bit of burring. A light
touch on each side of the hole with a countersink will clean up the burr and
upset on the exit and entry respectively.
	I think you will find the hole positions are accurate enough for the
application. I have used this system often on printed circuit boards when I
only need 20 or so pieces. If you want to get fancy you can draw your
triangular shape on the computer and print it out.

Ellen


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