More rigid vise!

Sure enough, when I mounted my DTI to the table and measured the rigidity of the vise by pushing against it, I was able to move it about 5 thous each way simply by giving it a firm push. Not a very scientific push but enough to see that my method of raising it up on parallels wasn't the cleverest idea I've ever had.

So I repositioned them at 45 degrees or so, giving better rigidity in both axes. Still not the ideal solution but the best I can do in the circumstances. I could do with a large lump of cast iron or a box fixture, whatever they are called. The result is less than half a thou movement when I push haard against the vise. Again, hardly scientific but clearly an order or magnitude better.

 And of course I had to square it up again:

I also modified the CAD and CAM files to account for the fact that the stock is now smaller (by about 3mm in X and Y) and also to give roughing and finishing operations for both the facing and contouring, still using the same 50mm Alphamill.

This time, it's taking a 1.5mm skim off the top and 2mm off the sides, with 0.2mm left for finishing passes. The suggested default for "stock to leave" was 0.1mm but that seemed a bit thin. Having said that, the finishing cut seems heavier than I'd expected. I wonder if there is enough backlash or compliance to be causing the work to end up more oversize after roughing than planned. I will need to do some tests to investigate...

Much better finish this time not surprisingly.

The cutter is cutting about 0.66mm undersize. It's supposed to be 43mm x 92mm, so for now I have edited the tool library to correct its diameter dimension to 49.34mm. I haven't sussed out tool tables, cutter length compensation etc yet. The surface finish is better but still some way from showcase stuff. I probably need to look again at feeds and speeds but this tool should be capable of good results.

It has to be said, I've been pushing the material removal rate (MMR), which is hardly the way to get perfect finishes on an old machine. Having said that, the surface speeds are quite a bit lower than recommended.

Most of the main manufacturers provide online setup tools that will help you to determine feeds and speeds for their products. I have a snazzy carbide tool from Sandvik that cries out to be snapped by an inexperienced user like myself. The recommended feeds and speeds are much higher than my machine can handle but they give an idea of what can be achieved.

Here's Sandvik's online tool I used to find the recommended setup for their R215.36-12060-AC26L cutter.