Jig making 101...I need help!

[myxpykalix]

This seems so simple i don't know why i'm having trouble. I want to make a jig out of wood to hold the pvc pipe between centers.
I have a 4" inside diamter piece of pvc pipe. It has a 1/4" thick wall and the outside diameter is 4 1/2"
What i want to do is take a 1/4" endmill and cut a groove 1/4 deep in the round wooden jig so that the pvc pipe will fit in it, then run a toolpath to cut it out of the material (the 5.5 cut). For some reason i can't seem to do this correctly.
Tell me if this is correct. In PW I draw a 4" circle. Using the 2D profiling/profile side/ I tick outside, using a 1/4" endmill. That should give me a 1/4" groove with a center of 4" right?.....NOT! Its too small
I then drew a 4" circle and a 4 1/4" circle and told it to macine inside the vectors, that was too big.
I tried to do a machine along the vectors making the circle 4 1/8" (compensating for 1/2 diameter of tool) but that didn't work either.

So what i want is a jig with outside diameter of 5.5 and a groove 1/4" deep to hold a piece of pvc that is 4" inside diamter, 1/4" thick wall with a 4 1/2" outside diamter of the pipe.
What am i doing wrong? should i be cutting on the vectors/inside/outside?

6840

[frank_hav]

Jack I think you need to "POCKET" between the 4 & 4.5 diameters. Then profile outside the 5.5 diameter. My guess if I'm understanding what you want correctly.

Frank

[cbradshaw]

Jack,

If you used profile cut to the outside that should have worked. Just asking, did you check to see if you selected the right tool from the database before cutting?

Carroll

[myxpykalix]

yes i used a 1/4" endmill. So let me ask, if i make a 4" circle, use a profiling toolpath and tell it to cut outside the vector then it should cut a circle the width of the bit. That kerf that it cuts will be at the 4 1/4" space on the board right?

[jsfrost]

Jack,

How much error do you have?

Im principle, the outside profile of a 4 inch diameter circle or several other equivalent choices should do what you want, but the bit, the Bot, and the pipe all have tolerances. And, as Carol suggests, the tool definition could be wrong. The Pipe is likely not even precisely round. To get a consistant fit for multiple pieces of "4 inch" pipes, you may a need a slightly oversize slot (done with two passes) or a big hammer. I am dumb enough to have overlooked undersize bits on several ocasions.

Jim

[Brady Watson]

Frank had it right when he said 'pocket' the area between the vectors. In SB/PW language, that means, Area Clear. If you want to add a little give for tube distortion etc, use NEGATIVE allowance. I would shoot for -.01" allowance, which will result in both the inner circle being .01 smaller than the vector and the outer boundary .01 larger than the outer vector. Just enter it in the box when you calculate the toolpath.

-B