I want to get into CNC machining and I have some elementary questions:

A 3 axis CNC mill will obviously only mill a part from the top, with the stock material secured against the table in some way.

If I'm machining a part with a flat back, how do I machine all the way through in some areas so the part can be detached? I assume you leave tabs so the part remains held while machining and then cut them manually? But how do you machine through the edges with out hitting the table? Do you put some hopefully flat sacrificial material under the part?

I guess the other possibility is to turn over the part, carefully align it and mill from the other side. However this might be a waste of time if that side is flat. Also the previously milled face will not be flat, and would be difficult to secure in a level position.

I've searched the web for answers, and I find articles typically showing the machining the outline of a part from the top, but they don't answer the questions.

Any advice would be appreciated.

Also, is there a CNC book that starts with the basics?

- Chris

Chris,

You've nailed one of the difficult areas of machining. Not only does this problem apply to cnc but to many types of manual machining as well.

If you want to cut right through, you would definitely need a sacrificial spacer. MDF is a pretty decent product so far as being quite flat and parallel sided. And, its cheap. So this makes a good spacer material.

Care needs to be taken that any cutouts do not get completely cut loose with the cutter still in the cut. This may force you to plan a method to hold the scrap area, by, for example, drilling a hole (or holes) through the workpiece to fasten down the loose cutouts. Or, you can turn such areas completely into chips, too.

I think you could find some texts used in machine shop courses in some technical schools. If you have no metalworking experience, a course outlining the methods used in manual machine operation would make a sound foundation for you to work from. CNC machining practice is based on manual machining methods. The computers involved permit you to undertake difficult toolpaths and to attain repetitive part consistency. But, cnc is no magic bullet so far as the basic principles of workholding, feeds and speeds, material machining characteristics, tool types, etc.

Chris,
Many parts must be flipped over to machine the opposite side. Plan your job so that you can locate the second side on a fixture (or fixed stops). The surfaces you use to locate the second operation should be ones milled in the first operation.

Sometimes this can require milling some temporary surfaces that will be removed in later operation.
Bill

The best way to mill the underside is to machine setup blocks with protrusions on which the flat (hopefully) machined surfaces will rest. You don't have to make a complete "negative" of your part, 4 -5 "pads" will do, depending on the size of the part. You can design the blocks along with your part in your CAD program.

No, if the underside is flat this is indeed a waste of time. You could use a sacrifisial base or put the part on parallels in the vise, secure the vise and remove the parallels. Keep two things in mind:
1. Tighten the vise as much as you would if you had the finished part in it, otherwise you might crash it when material has been removed from the center.
2. As mentioned before, no loose cutouts. You can leave thin connecting tabs and cut & file them off later. I've left aluminum tabs so thin I could cut them with a hobby knife and still worked fine.

HTH, Nikolas

Thanks! Great tips.