A fixture can be a big help when you're machining an oddly shaped part, or a bunch of parts at once. For my MDX-15 vise project, I need to be able to hold a thin aluminum section rigidly while I machine it in order to get the nicest surface finish. To help me achieve that, I designed a set of fixtures.
The first one holds raw stock and ends with the top profile of the vise roughed out. It uses Mightee Bite clamps to hold the stock in place during machining, and has a precise reference hole for locating the work coordinate system.
A matching fixture accepts the half-machined part, holding it on a set of three bosses that mate with the features on the vise. This ensures a tight hold on the part while I machine the bottom flat, which I hope will eliminate the vibrations I had in the prototype part.
Machining these is taking some time away from modifying the vise itself, but I think it's worthwhile. One of the things I'm learning is how finicky our machine is. Since the axes can't travel very far, if your vise is off by more than an inch or so, you can have a lot of trouble making a large part. I spent a lot of time moving the vise and re-indicating it this week. The results, however, are pretty good.
For these parts, I re-calculated my feeds and speeds based on the previous experience from the vise. This time around, I ran the face mill much slower and at a higher torque load, and while the results sure look good, I'm not sure that they're so much better that I'm willing to take the hit on the cycle time. I still had some areas that had some imperfections when using my 21mm insert mill; these may be addressed by using polished inserts.
So far I've made two fixture blanks, and hopefully I'll have the features on one of them machined this week.