Toying with 3d machining - a few basic questions

[GITRDUN]

Been machining for almost 20yrs and now trying my hand at some basic 3d milling just to try something new. Im 3d milling some plastic right now and not getting the surface results im after. I am ball milling a varying radiused outer surface with a finish DOC of .0002" for finish pass after a rough pass leaving .01". The finish just isnt smooth, machine lines are rough. Only place where the finish is like glass is on a steep radius that gets cut with the side of the ball, the rest of the part thats cut with more of the lower end of the ball is not good. Im climb milling on the surface with a chip load of .004" at 4500RPM on a 1/2" ball. My toolpath is basicly making a spiraling cut all the way around the part and then stepping down Z-.0002" and making the next cut around.

So would i get a better finish using a zig zag style toolpath on X or Y and using Z axis to do the 3d machining, versus spiraling type machining with XY moves at a fixed Z depth per pass? Or are there some other 3d basics that im just overlooking? What are a few basics that are must knows to get started with 3d machining?

[mcphill]

DOC of 0.0002" seems way too small. You are probably burnishing more than cutting.

What stepover are you using? Stepover is much more critical than DOC. A stepover of .0141 inches will give an RA of 100 with a 0.5" BEM, but you need the DOC to be enough to generate a decent chip. You also should use an endmill made for plastic, look at Onsrud.

[GITRDUN]

The step over amount on X and Y is determined by the parts radius. The software basicly machines slices of the part every .0002" deep or whatever amount is specified. Thats why im wondering if a different toolpath aproach such as straight X planar slice moves with a fixed Y step over per pass would result in a better finish or not.

[mcphill]

What CAM software are you using? Not being able to define stepover is a pretty big omission...

[GITRDUN]

After thinking about it i think thats the obvious problem. I was just thinking all wrong. Rather than having a XY step over i am using a tool path with a Z step down. Thus my XY step over is controlled by the parts surface dimensions. After useing a slice planar tool path i can control the X step over amount and use Y to zig zag back and forth over the part, the Z axis is moved up and down as the part surface changes. With a 1/2" ball tool and a .004" step over the scallup is .0002". I was trying to machine by level with a Z step down per pass at .0002" which took 8.5hrs to machine. After simulating with the slice planar tool path and a X step over of .004" the same part machines in about 2hrs and im sure the finish will be much better.

Im using Bobcad V23. I know its not the Ferrari of cad-cam but for the price i am amazed what can be done with it in the right hands. I cant drop thousands of dollars on software to just play with.

[mcphill]

I am using V23 as well. Sounds like you are on the right path now.