Workshop/Tools/Parts

[Cory]

Originally Posted by techhelpbb

Why? A lathe or mill that is properly trammed should be setting the standard for machined surfaces both parallel and at a sever. A ground surface is a level above that for flatness.

I would think the only way that is true is if your cut is very close to target material size leaving nothing to further true-up.

We've been known to rough drive train plates on the CNC plasma cutter because it's fast but the cuts are nasty. We then clean up on the mill.

Important for a number of reasons. Minimizing stock size needed for a part. Minimizing variation in size from part to part, which is more important for CNC since you're taking a fixed number of passes and you don't want to program for the length of Part A then find out Part B is .125" taller (an exaggeration, but it can happen). An easier scenario that will screw everything up is a part that has left and right edges that make it a parallelogram, due to non straight cuts, which can no longer act as registers for a fixed stop on the vise, since your origin is moving dramatically with each part change.

Sometimes based on the size stock you have on hand, you need to hold the cut faces in a vise, not the stock edges. The more parallel they are to each other the easier that is. Sometimes you cut bar stock and you need to stand it up on the edge that was cut. Again, the more perpendicular to the stock edges, the better.

When using the lathe sometimes you'll have pieces that are too large in diameter to hold with jaws in their standard configuration and you end up flipping the jaws and holding the part on the stepped portion of the jaws. If your bottom surface sitting against the step on the jaw isn't flat you end up with a lot of runout on the part, which is a PITA. Likewise if you're cutting something too short to be clamped by the full length of the jaws you don't get proper self centering of the part and you need to stick parallels or something else against the face of the chuck and seat your part flat against them. If the part doesn't have a flat face you get the same runout problem.