South Bend 9 lathe still struggles with parting

I have a South Bend 10K with a 3/4 hp motor and have no problem with a .020 DoC in 1018 steel using a sharp HSS tool.
Are you using carbide or HSS?

I am using HSS. Cleveland Mo-max. I don't think my motor is 3/4 HP. I cannot see any markings. I'll have to get under the table and check. My lathe will do 0.020" DoC in mild steel.
 
I don't worry about the chuck unscrewing when working on small stuff. When I'm holding bigger diameter stuff in the 3 Jaw I use a T-shaped "nut" that slips inside the chuck. The crossbar of the T bears against the backplate and the base is screwed into the collet drawbar. Because the thread pitches of the drawbar and spindle nose are different, there is no way the chuck can unscrew.
This is very interesting to me because I am parting challenged and I would love to be able to part in reverse. My thought is to use a LH thread drawbar. That way the T-shaped nut would tighten against the chuck if the chuck tried to unscrew.
 
I am using HSS. Cleveland Mo-max. I don't think my motor is 3/4 HP. I cannot see any markings. I'll have to get under the table and check. My lathe will do 0.020" DoC in mild steel.
Sorry I hijacked your thread a bit replying to @Nyala as they were saying that they can barely part because of a 1/2hp motor.
 
I had tons of parting issues with my 10" Logan, until I got the blade type carbide insert tool. This solved most problems, and the rest pretty much went away when I got the 14" Takisawa lathe. Wow, now I can part off 3" steel rounds without too much fear...set the speed where the cut temperature looks under control, lots of cutting fluid, not too aggressive a feed, around 0.003" per revolution, and it generally works. Occasionally I mess up by failing to get an exactly perpendicular blade on a deep cut and I have broken a couple of inserts like this. I count this as a user error.

I buy the cheap blades with 10 inserts of unknown quality. They work OK. Someday, maybe with my virus check, I'll get a brand name tool and inserts, and compare. Maybe. Or buy food, have to see. :)
 
This is very interesting to me because I am parting challenged and I would love to be able to part in reverse. My thought is to use a LH thread drawbar. That way the T-shaped nut would tighten against the chuck if the chuck tried to unscrew.
Don't worry about using a left hand thread. Just as long as the two threads are of different pitch and the finer pitch is on the drawbar. If the chuck tries to unscrew it just tightens the T nut against the backplate and presto chango the unscrewing doesn't happen.
 
I have two RH thread drawbars for the spindle on my lathe for holding 3C collets. One is a 3/8 external thread. I'll have to measure the TPI. The other has a very fine internal thread. I'll make a T nut for the 3/8 drawbar. Be a good learning experience. I will make the nut with a MT3 taper to fit the spindle and with a collar to fit the bore on my chucks.
 
I guess I am the only lathe operator who has never had problems parting off.

My philosophy is slower (RPMs) and more cutting oil with hand operated feed rate listening to the machine while I part-off. Slower with hand controlled feed rate allows one to take enough bite on the metal without over stressing the tool (face).

2-years into my 12×36 and I am still using the original sharp edge on my parting tool with hundreds if not thousands of part-off events.
My parting tool is HSS t-shaped and about 3/32" wide.
While I part mostly aluminum, the blade has also parted useful amounts of mild steel, some 304 stainless, and a couple of pieces of Titanium.

I'm in the same boat. Set lathe to 100 rpm and I like using the power cross feed. Drip a bit of oil and let the machine do its job. 3/32 parting blades held in a cheap AXA parting holder. Works fine for steel, aluminum, stainless, etc. I only have issues on poorly supported or thin and hollow parts.
 
That is not completely true. I experienced the "chatter at normal speed but too slow of feed". I would suspect that no less than 1/2 recommended speed is appropriate for parting. The problem with too slow is that feed becomes tricky, even with nerves of steel. Higher speeds give you more leeway in the feed rate. As I recall, I was parting aluminum (3/4" at 750RPM). I got a lot of chatter until I increased the feed rate. The noise changed to a smooth hiss, ribbons started flying off, and the finish was excellent, even to a professional machinist who I showed this to. But timidity does have its advantages. I am pretty smooth, and I can smoothly force the parting tool into the part until the lathe crashes. I only did this once, though. Ok, twice. So "feed faster" is not a hard and fast rule.

the opposite actually - the slower you go, the slower you need to feed. Seriously mate, I'm working with a lathe that makes yours look like an American Pacemaker. 1/2 of normal turning feed is too fast. I can happily turn 1" steel at 6-700rpm, but I have to drop down to 100-120rpm for parting. Works beautifully, nice little curls, never snapped a blade. You can get away with an awful lot when working with aluminium that you can't when you're working with steel.
 
the opposite actually - the slower you go, the slower you need to feed. Seriously mate, I'm working with a lathe that makes yours look like an American Pacemaker. 1/2 of normal turning feed is too fast. I can happily turn 1" steel at 6-700rpm, but I have to drop down to 100-120rpm for parting. Works beautifully, nice little curls, never snapped a blade. You can get away with an awful lot when working with aluminium that you can't when you're working with steel.

Maybe you are right, Matt. I just put a 5/8" mild steel bar into the chuck and tried it at 250 RPM. It parted just fine with no chatter. lBut, it was slow going and no curled ribbon chips. The lathe was noticeably struggling and it sounded like the blade might catch if I pushed it too hard. I estimated about 0.001 per rev. But the piece was clean, and no drama.

I have a strong feeling that the lathe is underpowered. I looked at the motor, and it is 1 HP. Should be enough. I puled out the underdrive belt tensioner and found that the turnbuckle was tightened as far as it would go *on one side*. That means that it was bottomed out. Also, the cotter key was missing. I made a new one and centered the turnbuckle. It seems that I am getting more tension on the flat belt now. I put a torque wrench in the chuck and it showed about 5 foot pounds before the fix and 10 foot pounds after. 10 foot pounds at 250 RPM is 0.47 HP, so that should be enough. I still feel it is marginal.

How wide (thick) is your parting tool?
 
it's a little counterintuitive, but if you think about it in terms of chip load per rev it makes more sense. If the rpm is low, the feed is slower or the chip load would be too high, conversely it allows you to keep the chip load high enough to avoid rubbing or chatter. Also gives you more margin for error - it's easier to speed up or slow down feed speed in response to cutting conditions when you're not turning the handle as fast as you can.

I typically use a 0.04" T-type blade for small stuff and the next size up (it's about twice as wide, around 2mm) for large stuff as it can tolerate greater stick out without flexing. Both from Shars many many moons ago. The thicker one has to run slower as the amount of metal removed per rev is higher.
 
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