Drilling a precise pattern manually

[mmarshall]

I have a manual mill and lathe--can't quite afford CNC just yet.

I'm making what is a re-usable rocket motor. Basically it's just a 1.5" diameter 304 stainless steel tube with machined nozzle in one end and a plugged bulkhead in the other.

OK, I've turned a bulkhead that plugs the front end of the tube. I need to drill 6 holes around the tube's circumference, for screws going into the bulkhead to hold it in place under pressure.

Is there a computer program that will print a precise template onto a piece of paper which I can tape to the tube to drill through?

I want to be able to replicate the hole pattern on more than one tube so that they all fit the same bulkhead precisely.

Thanks

[Peter.]

Simple maths will do it.

1.5" tube & .003" paper = 1.506" diameter.

Circumference is Pi*D so 3.142 x 1.506 = 4.732"

4.732" / 6 = 0.789"

So take a strip of paper, mark two lines on it 4.732" apart then make five more lines in between these at 0.789" apart. Cut it off at the end two lines and wrap it around the tube, hold in place with sellotape. Spot it on the marks.

Of course if you want to re-produce the holes exactly in many tubes you should make a drilling jig.

[Lew Hartswick]

If the holes are radial on the tube, a CNC isn't going to do you any better than a manual mill.
What you need is a "spindex" like fixture and just index the holes. Can do as many as you
like and they will all be interchangable. The Spindex (RTM) uses 5C collets and they only go
up to 1 inch but PERHAPS with a machined collet and hold by the open end you could do it.
...lew...

[mmarshall]

Thanks guys for your posts.

I do know how to do the math, I just thought that being able to print it on a computer program would be the best way to make the pattern very precise.

I'm going to try to use Photoshop to make a pattern. I'll post how that goes.

I'm only going to do 3 or 4 tubes, so I'm not so sure any major investment would be worthwhile.

[mklotz]

Bore out a piece of hex stock to fit the tube. Fit with a setscrew to lock the tube in place. Use it to index the tube while drilling the holes.

Or, better yet, drill guide holes in each face of the hex stock, lock to tube with setscrew and drill through guide holes into the tube.

[Michael_Moore]

There's a good chance that Marv Klotz has written a program that does what you want:

http://www.myvirtualnetwork.com/mklotz/

cheers,
Michael