Lathe Observations

[the penguin]

I went to work at 2:30 am  and got home at 4:30 pm, long day. I still had some shop time, I got 8 parts that have to go out the door.

So I had some lathe time to finish up , I talked to Ken (Pixman) last night, having a bad time get a acceptable finish on a piece of 1-1/2" ground and polished shafting. As I was trying to face the end of the shaft, I was tearing the metal, not cutting it. I finally gave up for the night, got my 3-1/2 hours of sleep and then went to work.

So today with  a hopefully somewhat clearer mind, I picked up where I left off last night. I had a 10" piece of scrap shafting, chucked it up and started to do some research. I checked tool height, my compound setting and started to play with the feeds and speeds. I finally good a very good result, finished off the job, so I can now relax.

I rechucked the shaft chunk, I took out the majority of the tools I got set up and compared the tools at various feed and speeds, some of the results surprised me. By far the best looking finish came with the Aloris #16 tool with a TPG-321, but the max depth of cut was down to about .020" -.030", above that the quality dropped off drastically. 

The CCMT 32.51, insert gave a very good finish, but again depth of cut was somewhat limited. 

The tools using the CNMG and WNMG inserts, had an acceptable, though slightly rougher finish, but the depth of cut was incredible,  I was taking  cuts over .120" with ease and the finish seemed the same regardless of depth of cut.

I also ran a couple of Grizzly marketed tools and few other cheapies and got somewhat  variable results, but these were used just to check, some I haven't used in years.

Obviously these observation are done by a boiler technician, who owns a few machine tools, not a machinist, who knows, I still might be doing something completely wrong. This was also, only facing the end of the shaft, I didn't do any turning. Who knows, maybe I just need more sleep.

[PixMan]

Negative rake inserts always seem to prefer a depth of cut equal to or greater than the nose radius, and feed rates that would make a screw-down or top-clamped (positive rake) insert blush.

When finish with a negative rake insert is troublesome, chipbreaker (top form geometry) is often the solution. High positive geometry can help by reducing cutting pressure while doing more shear action than displacing the material. Note that the actual sharpness of the edge can matter as well. Many positive rake inserts have a ground periphery rather than "as sintered" that is found on a CNMG. A sharp, ground edge reduces cutting pressure even more. Lastly...approach angle. The obtuse angle (100º) angle of those CCMT and CNMG inserts, presented to the face of a part is stronger and further reduces cutting pressure by chip thinning action. You both have that type of toolholder, no better way to use up the corners of inserts which you can find in my carbide scrap bin. ;)

Of course with carbide tooling the rigidity of the workpiece and tooling is paramount to good performance. Weakness is flexing and a chatter, both of which can be detrimental to tool life of carbide.