When trying to machine or manufacture something, building a jig to hold something in position is almost always a good idea. I have been struggling with how to improve the accuracy of position of a piece of acetal inside a piece of square aluminum tube. The problem is that the acetal sometimes ends up a fraction of an inch too far out or too far in. So I took a piece of acetal already at the right depth and angle in an aluminum tube and put that against the workpiece in a vise. Then I use a smaller piece of aluminum tube behind the acetal to tap it against the opposing jig. It is then exactly in position. Because the aluminum tube into which the acetal goes has a 1/4"-20 hole in it, a thumbscrew locks the smaller aluminium tube in position against the rear of the acetal piece. Now I can drill through the square aluminum tube into the acetal and be sure that it is exactly matched up with the end of the aluminum tube where it meets the jig.