Help/Ideas for turning 2⅛" 303 stainless with low power

OCJohn

OCJohn

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I'm pretty new at this stuff. But I've been happy with the results I've gotten turning 304SS in sizes under an inch. So I felt emboldened So when I found a 4' length of 303 round bar at the local scrap yard... But the larger diameter (even in a more machinable alloy) is not nearly as cooperative. So I'm asking for help...

I would appreciate input on how to approach this and improve/optimize my cut. I also need a good starting point from which to test ideas. With little experience, I lack instincts about which variables to test/change first. Feeds, speeds and DOC are the easiest changes to try. But without knowing if I'm starting with the 'right' tool/geometry, it's hard to know if I'm just spinning my wheels...

The setup/modifications:
  • HF/Seig C2/3 with OEM 250w motor (for now)
  • 3:1 underdrive pulley kit for extra torque
  • "tuned" brass saddle clamp plates
  • "tuned" cross slide ways & gib
  • solid toolpost block (compound slide eliminator)
The generic Chinese DMCT inserts that came with the holders have done the best, but that's not saying much. They produce a rough, grooved finish at .0020" DOC/1000rpm and don't seem to improve much with less DOC or slower speeds.

Cutters 1 & 2 seem to have more good finish potential, but with so little power they are prone to chatter or stopping the spindle at anything close to a productive cut. They do "okay" in low gear between 250-290RPM, but hand feeding with .0015-20" DOC is like watching corn grow.

TLDR:
  1. Should I be looking at ground DMCT bits for aluminum that some have said would well on underpowered machines? If so, do you have a specific recommendation? (I get a little overwhelmed by the zillions of insert choices...)
  2. Are their "off the menu" HSS tool grinds better suited to low-power machines? The recommended geometry for steel and SS in the Engineers Black Book seem like they are better suited to bigger machines than mine...
Any and all suggestions appreciated!

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Forgot to mention, also using a 4-jaw and live center.
 

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Even 303 is still stainless. I find it likes a DOC closer to .020 before it starts playing nice. You need to peel off the material thick enough that you get "below" the work hardening under the cutting point, if that makes sense.

If you're low on power, try the HSS tools (#1 and #2) instead. Set your feed for about 1/2 the tool radius, and back off the feed rate if you start to stall.
 
I've never used a machine of that size/power BUT my experience with SS tells me you are in for some issues. The problems I've had with SS is it work hardens almost instantly, if the cut heats the surface. There may be better solutions but mine has always been to cut aggressively. Figure out your depths of cut so you don't end up with "finishing" passes that are shallow.

My best guess for your use is to use the sharpest tool you've got at the highest feed rate you can manage then with the deepest cut the machine can power. Don't pause with the tool touching the work. Once the SS has work hardened your machine can't likely do anything with it.

Others with very small lathes may have found a solution.
 
I have machined tons of 303 SS and never noted any tendency to work harden, it is very little more difficult to machine than CRS; I have always used TPG inserts with a small radius for most all machining, used on holders with adjustable chip breakers. Granted, I do this on indusrtial sized machines with coolant, but just the same, the metal itself machines easily, particularly unlike 304.
 
benmychree said:
I have machined tons of 303 SS and never noted any tendency to work harden, it is very little more difficult to machine than CRS; I have always used TPG inserts with a small radius for most all machining, used on holders with adjustable chip breakers. Granted, I do this on indusrtial sized machines with coolant, but just the same, the metal itself machines easily, particularly unlike 304.
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I'm with John here, 303 doesn't have a tendency to work harden, maybe simply because it cuts so easily. However, I think it machines better than CRS. Given the choice, I'll take 303 every time.
 
Without flood coolant in 303SS I would run about 200 SFM (about 300 RPM on 2.5'' material). My go to insert for SS is the Mitsubishi DiaEdge DCMT xx.51 MV where xx is the size, I assume 21 for your tool holder. Your feed rate at 0.0015 should be good for finishing at about 0.005 DOC. If your machine is rigid enough, a bit more aggressive on the roughing cuts would be in order.

I have cut about 1/4 mile of 303 SS in the last 6 months or so and these inserts have proved to do a nice job.
 
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