Tips On Turning Hard Steel On Lathe With Cemented Carbide Toolbits

Oops, OP provided hardness information while I was fooling around somewhere else. Not HSS territory at all - need good carbide insert tooling, as Tom mentioned previously, rather than the $.90 brazed carbide throw-aways.
 
When I'm cutting to a shoulder like that I normally grind nose of the tool to an 80 to 85 degree angle, they normally come out of the box at 90 degrees and while I'm at it I normally add some front and side clearance angle. It has also been my experience that the cheap AR bits that I buy won't even cut out of the box they have to be ground to be useful at all.
 
I've always referred to those as "tool blanks". It is true that they might cut as purchased, but nearly every case proves some grinding and or honing improves things immensely.

For what you are doing, my tool of choice would be a 80° diamond inserted tool. The most common holder gives you a -5° lead angle while still providing 5° clearance on the trailing edge. That tool will present a robust cutting edge but retain front clearance for facing. I use that profile more than any other for general purpose turning, up to and including a shoulder. Plus I have a inserted milling cutter that uses the 100° corners, so I get more use from that style insert than most. A turning tool is available to use that corner also, but it has a low lead angle and cannot turn up to a square shoulder. It's great for roughing, especially if you can run off the end of the part.

Generally speaking, I dislike triangular inserts, except for threading, either laydown, or on-edge.
 
I would use a parting tool to cut the diameter at the shoulder down to the desired OD, then turn the rest down from there. A carbide parting tool would be best but I bet a sharp cobalt cutter will do it without any problems.
 
T Bredehoft said:
In my humble opinion, cemented carbide tools are not satisfactory for serious machining. They once were all that was available, but soon were replaced with index-able inserts of varying shapes. Use a triangular insert with a small radius and undercut the shoulder to eliminate the radius.
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All carbide tools are sintered (or cemented, grains of carbide bound together by a third material under heat and pressure in a mold.), inserts, drills, endmills and the like. I believe that you are confusing cemented with brazed tools, which is sintered carbide brazed to steel tool shanks.
See here http://todaysmachiningworld.com/magazine/how-it-works-making-tungsten-carbide-cutting-tools/
 
Yes, I was confusing cementing and brazing. Its brazed carbide tooling Henry Ford used when he first machined his V8 engines in 1932.
 
I also dislike brazed carbide tooling, have never found it to work well without careful preperation yet comes in handy when you need to grind a form tool for use on a manual lathe.
 
As I understand it, you have a 5/8 shaft with the last inch dropping to 3/8 forming a shoulder. You want to push the shoulder back .500" for a total length of the 3/8" portion of 1.5". I would set a carriage stop .005 short of where you want the shoulder. Then machine .020 at a time by hand feeding. Spindle should be at least 1000 rpm. When you get to the last cut (.020 or less) readjust the carriage stop to get the last .005 at the shoulder. Feed the last cut in by hand and when you hit the stop back the tool out towards the OD with the cross feed cleaning up the shoulder on the way out. While backing out the cross slide with your right hand, keep the carriage hard against the stop with your left hand on the carriage wheel.
 
if it's one of those 90deg brazed carbide bits, most likely what's happening is that as you hit the shoulder your cutter is jammed into the corner. Instead of cutting on one edge, it digs in and cuts on both, springs back and chips the tip. Either use a non-90deg cutter or angle the cutter so that there's some trailing edge relief, then use a parting tool or LH 80deg cutter to "face" off that shoulder.
 
You are leaving too much on the shoulder - leave .005" max. After your finish pass on the 3/8 diameter, manually move the carriage to the finished shoulder position, lock the carriage then clean up the shoulder by manually feeding the cutter out of the part.
 
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