I picked-up a 1989 series I manual Bridgeport a handful of months ago. My intention was to help me machine parts for a custom loudspeaker I have designed.
Shops in the area would either not quote my parts or wanted in the ballpark of $800 to $2500 to do the work, I figured I could pick up a used mill and learn to do the work myself - saving a significant amount of money in the long run.
The material I am working with is 2" x 6" hot rolled bar stock. An L shaped channel is cut into the workpiece and all sides are faced, my tolerances are fairly wide at +/- 0.010" thus I wasn't too worried about trying this myself.
For one loudspeaker I need to make around 15-20 of these L shaped sections. Each one requires the removal of around 1.75 to 2.5 cu. in. of metal. I started machining with low quality 0.5" HSS tooling, and after several nights learned a few things:
1. The 1/2" diameter endmill would loose its edge in around 45 minutes - if I needed to buy an endmill every 45 min it won't turn-out cheaper to machine this myself
2. I started using 3-in-1 oil as a cutting fluid, turns out I'm allergic to it - my hands turned bright red and swelled-up about 2x their normal size.
3. I estimated 120 machine hours at the current rate I was removing metal.
After this I learned it is possible to not use coolants with many forms of carbide tooling. I did a fair amount of research and found a TMX 1-1/4" indexable carbide insert endmill (directly fits into an R8 taper). The indexable takes APXT-1604 (grade NCM325) inserts designed for steel. This tool worked much better than the HSS tooling I started out with, however I chipped the inserts after running the following settings:
1200 RPM (~400 FPM)
0.020" depth of cut
Feed: somewhere between 10-20 in. / min
Total runtime before edge failure: 1.5 hrs
I indexed the inserts to their second side and found some more tech info on the TMX website and ran the inserts closer to their rated speed:
2200 RPM (~600 FPM)
0.020" depth of cut
Feed: ~ 20 in/min
Now the endmill chatters easily and I'm limited to taking passes at 1/2 width. The mill head makes a bit of noise at this speed, but I don't know if this is normal or unexpected as I don't have anything to compare my mill to. I'm also not sure how much pressure I'm supposed to apply to the cut - do I let the machine do all the work and crank the cross slide just to keep up or do I need to apply constant pressure to keep the chip size right? Did I go down the wrong path with an indexable carbide endmill - should I consider going back to HSS, but how do I solve the sharpening and speed (of metal removal) problem? Is my old bridgeport not up to the task of reliably handling carbide tooling?
Any recommendations for changing my setup, feeds/speeds, tooling, or suggestions for improvements to my machine (just don't tell me I need to rebuild the head) would be greatly appreciated.