Thanks for the pointers to sources, with luck I won't need them (I may have enough bits to last) but will keep them in mind if needed.
As I mentioned in my earlier post, I've kept the RPMs and feed rate the same, but have gone to just .040" depth of cut per pass. Since I did that I've cut more than I had done when I started this thread, and only broken one bit.
That one was at least partly my fault - my current technque is to cut a pass with the power feed, disconnect it, and manually return to the start point. Then I drop down another .040", re-engage the feed and make another pass. I do the return fairly quickly, and as I was doing it I apparently snagged a burr on the end of the cut and popped the bit... Now I do the return more slowly, especially when re-entering the cut part of the channel.
I'd love to be able to cut in both directions, but I don't trust the position dials on this POS mill, and because of the flood cooling I'm using I can't really see the position of the bit easily. I'm afraid if I cut my way back, I would either snap a bit when I got to the end of the channel, or mill out through the side of the block...
This is a little slower but safer, as my end point is the 1/2" wide cross channel already milled so I can stop anywhere in it.
LR - I have a few problems with my toolholding - given the increasing frustration I'm going through with this POS machine, I'm reluctant to spend very much money on tooling that I would probably sell with it if I put it on E-bay (increasingly likely!) The machine has an MT-3 taper which makes stuff hard to find and expensive compared to R-8. Currently I have 3/16", 1/4" and 3/8" bitholders, and an NR-40 collet chuck with collets for 1/2", 5/8", 3/4", 7/8" and 1" bits (the 3/8" came with the machine, the rest I picked up at a used equipment place) I also have an MT-2 collet chuck (Harbour Freight, unsure of taper) with collets for 3/8-5/8, and smaller sizes on backorder. I have a used tang style MT-2 / MT-3 adapter that I need to drill out so I can use a drawbolt on it, and mount the MT-2 chuck.
JF - I am currently using 2-flute Cobalt Import bits, 3/16" shank, 1/8" diameter, with different LOC's ranging from 1/8" to 3/4". (My target channel depth is about 1/2")
The machine I'm using is a Smithy brand, model Midas 1720CNC. (The CNC means that you can get a CNC kit to put on the machine, but I don't have it.) This is a combination Mill / Lathe, which I still think is a nice idea, but the implementation sucks big time. I've run into machine induced problems just about every time I've tried to use it - for more details on my tales of woe see this thread...
Help, My copper bar is not flat
I am not using cutting oil as such, rather I'm using a pump fed stream of 'soluble oil' and water coolant, which is supposed to both lubricate and cool the workpeice. It also does a fairly decent job of getting rid of chips. I've found that if I aim the jet right at the mill bit, I tend to get alot of spray, so instead I just keep a slow but steady stream running into one end of the block, so that it runs down the channels at a pretty good rate. It seems to work as a coolant, I've never felt the bits or block get more than slightly warm.
The only trouble is that the coolant solution is an opaque blue/white, and when it overflows I can't really see what I'm cutting.
Gooserider