Newbie fixturing questions

[sp1nm0nkey]

Sorry for the super basic question but I'm just starting with my cnc mill and I'm not sure how fixturing changes. On my manual mill, if I were making a rectangular piece I'd pick an end mill of suitable length, turn it up, mill two sides and ref one to the back jaw, face the top and cut one remaining side, face the bottom and cut the other remaining side. This doesn't seem practical for production runs... Is the idea to keep the piece far enough above the vice that I can face it and get all four sides, flip it and face the other side and mill off any overhang?

[Geof]

.....This doesn't seem practical for production runs... Is the idea to keep the piece far enough above the vice that I can face it and get all four sides, flip it and face the other side and mill off any overhang?

More or less, but an astonishing number of people seem to do it the long way even in production runs.

My company does medium scale production of our own products and for parts that are machined from rectangular bar stock many times out approach is to cut the stock to a length that is enough for anything up to 10 parts depending on the size of the part and the machine it will be done on.

The stock profile is slightly larger than the finished part profile plus a little bit extra for gripping in the vise and two or three vises are used. Because the stock is gripped over a long distance it is quite secure even when the vise only holds about 0.10" so the feeds can be fast with good depth of cut. If the same feeds and cut depth where used on a single part it would rip out of the vise.

The first operation does all the top, side and end machining. The stock is long enough to allow enough room for a cutter to pass between adjacent parts; this consumes a bit of stock but saves time so it is worth it.

Depending on the part after the first operation the partly finished parts may be left connected by tabs, cut into pairs or cut into individual parts for the final operation which many times is done with custom vise jaws which fit the profile of the part. Sometimes the second operation is a custome fixture.

Often the second operation is just machining off the surplus material that was used for gripping in the vise and chamfering edges. Sometimes when there are cross holes or side features that cannot be machined during the first operation in the vise the parts will be held in a custom fixture on a rotating holder to allow these to be done.

The second operation separates the parts and for many parts only two operations are needed to go from a length of stock to several completely finished parts coming off the machine.