South Bend 9 lathe still struggles with parting

The first one was too much rake on the parting tool. ... Also, tightening the gib screws for both cross slide and compound helps.

A P-type parting tool is flat on top so it has zero rake. Are you using a different tool in the above statement or are you talking about the relief angle, the angle under the tool tip? Also, are you using a QCTP with the tool holder that holds the blade at an upward angle? If so, that gives you positive rake and can definitely dig in if the lathe is not rigid.

Another problem I had was not feeding aggressively enough. ... You have to listen to what the machine is telling you and "do the right thing."

The feed when parting manually is actually about feel rather than sound. You want to feel a slight resistance to the feed. When you get it right you have to maintain it all the way in. This applies to all materials. The resistance is lighter at higher speeds and harder at lower speeds.

So, the time came to part a larger steel bar (about 1.5"). Slow feed caused chatter, so time to crank it faster! This didn't work. The blade jammed, and in a scary way. Finally, the belt slipped. It appears that there wasn't enough torque, so I used the back gear. This gave plenty of torque, but the feed became very fiddly. I could only feed slowly, else the blade would catch and stall the lathe. I was afraid of breaking something! This problem was probably due to insufficient power. I could part by feeding very slowly with little crumb like chips. This was no good, but as soon as I tried feeding faster it would jam again. The job could be done this way, but it would take forever, and also be risky. I didn't want to damage anything.

This experience reminded me of a parting lesson that I saw at Techshop. The instructor told the students to part at minimum speed, which on the 14x40 lathe is pretty slow. Feed slowly and carefully, barely getting a chip. This took a long time to part, but at least he said it was safe. These 14x40 lathes had a lot of power, and parting was easy at almost full speed for the material, but this was an intriguing technique. It seems that it does have its place with an underpowered lathe.

You should be able to part most materials at 1/2 - 2/3 turning speed. For your 1.5" bar, that would be between 125-200 rpm or so. As I mentioned, feed gets easier at higher speeds. My Super 11 will part mild steel at 200 rpm with no issues, and my little Sherline lathe with a rear mounted parting tool will part a 1" mild steel bar at about 1200 rpm. The speed helps.

Could my parting blade be too thick?

All else being good, a P2 blade should part most materials on a 9" lathe. Parting tools are form tools; the wider they are, the more resistance you will have and the more power and rigidity the tool will require to function for any given material. In my opinion, it is best to use the narrowest blade that will work with your machine. I use a P1-N blade (0.040" wide)on my Sherline to part almost anything that fits in the lathe and have no problems parting at speeds that are usually 2-3 times normal turning speeds, and that includes steel up to about 1.25" OD.

I do not like the tool holders that angle the tool upwards. In my opinion, a horizontal tool holder is better because it allows the tool to perform like what it is - a form tool with zero rake.

I suspect the issues you're having may be due to multiple factors. The gibs and fits of the cross slide and compound are always suspect. This is why guys with plinths have fewer issues when parting - no gibs, just solid material under the tool post. Getting the tip of the tool exactly on center is important; not close but dead on. The tool aligned perpendicular to the work is important, too. What is not as critical is overhang; a P-type blade can stick out and still maintain rigidity if all other factors are good.

I would look really hard at movement at the tool due to movement of the cross slide and compound. Put a turning tool in your tool post and lock it down. Put a dial indicator on top of the tool bit and try moving the cross slide and compound up and down with your hands to see how much movement there is. If it is excessive, like more than a thou, then you have some issues to sort out.
 
Also something I have found interesting and may point to the need for a new cross slide nut.

If I turn the compound to 0* and use it instead of the cross slide to do the parting cut, it goes much better. When I had it apart when I got it the compound nut and screw were very tight fitting where the cross slide nut had noticeable play on the screw. Maybe I will take this time off to get another set and see if that helps.

Do you lock down the compound when just using the cross slide. This may be where your lack of stiffness lies.
 
I had trouble using a parting off tool until I made a special holder for my QCTP that holds the cutoff blade upside down. Then I run the lathe in reverse. Because this forces the tip of the tool to flex away from the work when under load it completely gets rid of the problem of digging in.

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.
 
I too own a SB 9" lathe, a Model C. It is powered by a 1/2 HP motor, which I attribute most problems to.

I recently parted off a piece of 1.5" aluminum. It took 20 minutes, using a 3/32" parting tool. Any attempt to adjust the feed resulted in bringing the lathe to a complete stop. It didn't matter if I adjusted the RPM higher or lower. I had to use a feed of about .004" and wait for it to complete the cut before feeding in another .004", all the while flooding it with oil.

I rarely knurl anything, but when I try, I use my clamp-on knurling tool and I can bring the lathe to a complete stop, if not careful. The best knurl that I usually get is basically a roughed up surface. Try to do more and the lathe stops. Flooding with oil doesn't help.

My standard feed cuts are basically .010, and maybe .015 if I'm lucky, otherwise I can bring the lathe to another full stop. I recently turned a piece of 2" steel to a major diameter of 1.75" for about 1/2" and a minor diameter of 1/4" for a 1/4-20 thread for about 1/2". It took about 2 hours.

I've thought about moving up to a 1HP motor but I'm not sure that the bearings would hold up to more power, so I just suffer through.
 
Made a solid plinth for my 9x20 and all my parting woes vanished.
Also this solved the chatter when it reappeared
check out post 29
Rigidity is the key.
 
I too own a SB 9" lathe, a Model C. It is powered by a 1/2 HP motor, which I attribute most problems to.

I recently parted off a piece of 1.5" aluminum. It took 20 minutes, using a 3/32" parting tool. Any attempt to adjust the feed resulted in bringing the lathe to a complete stop. It didn't matter if I adjusted the RPM higher or lower. I had to use a feed of about .004" and wait for it to complete the cut before feeding in another .004", all the while flooding it with oil.

I rarely knurl anything, but when I try, I use my clamp-on knurling tool and I can bring the lathe to a complete stop, if not careful. The best knurl that I usually get is basically a roughed up surface. Try to do more and the lathe stops. Flooding with oil doesn't help.

My standard feed cuts are basically .010, and maybe .015 if I'm lucky, otherwise I can bring the lathe to another full stop. I recently turned a piece of 2" steel to a major diameter of 1.75" for about 1/2" and a minor diameter of 1/4" for a 1/4-20 thread for about 1/2". It took about 2 hours.

I've thought about moving up to a 1HP motor but I'm not sure that the bearings would hold up to more power, so I just suffer through.

@Nyala 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?


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Also something I have found interesting and may point to the need for a new cross slide nut.

If I turn the compound to 0* and use it instead of the cross slide to do the parting cut, it goes much better. When I had it apart when I got it the compound nut and screw were very tight fitting where the cross slide nut had noticeable play on the screw. Maybe I will take this time off to get another set and see if that helps.

This is indeed interesting. I'll have to give this a try. It may give me the impetus to replace the cross feed nut. Although I am afraid that this may not be enough, since the screw is also pretty worn. The compound is pretty tight, though.
 
the fact that you said you then put it into back gear told me alot. You're parting too fast for the material and blade thickness. Start off at your slowest possible speed, use some thinner stock (1/2-1") and get a feel for things. If you get alot of chatter on first contact, you're going too fast.

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.
 
I had trouble using a parting off tool until I made a special holder for my QCTP that holds the cutoff blade upside down. Then I run the lathe in reverse. Because this forces the tip of the tool to flex away from the work when under load it completely gets rid of the problem of digging in.

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.

That does work. I tried this on a lathe with a camlock chuck, and it worked great. But I am afraid of unscrewing my chuck. I have seen several "experts" post that beginners are overly afraid of the chuck unscrewing and that it never happens. It happened to me once, however, and it was extremely scary. It looked like the chuck was moving in slow motion, but there was nothing I could do to stop it. I couldn't even manage to reach the switch. I was lucky that I didn't lose any teeth, and realize that I can't believe everything I read on the web.
 
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