South Bend 9 lathe still struggles with parting

ericc

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I have had a South Bend 9 lathe for a while, and I am still having difficulties with parting. I think I know what the problem is. Actually, there were lots of problems. The first one was too much rake on the parting tool. The cross slide has too much backlash and the tool gets sucked in, causing a crash. A lot of internet posts say that backlash does not matter as long as you wind it all out, but that is not quite correct. Also, tightening the gib screws for both cross slide and compound helps.

Another problem I had was not feeding aggressively enough. I developed a timid habit after reading an internet post that suggested increasing the feed rate at the first sign of chatter. This is true. Kind of. The first time I tried this, I snapped the tip off a parting blade. It seems that sometimes this is good advice and sometimes not. You have to listen to what the machine is telling you and "do the right thing." The second time I did this, I was using a homemade parting tool holder with a trimmed down blade that was for a larger lathe. This time, the blade had zero rake, due to previous bad experiences. The material was aluminum, and as soon as I touched the bar with the tool, there was a bunch of scary chatter. I doubled the feed rate and the chatter instantly stopped. Not only that, a beautiful curled ribbon chip was produced, and the tool sailed right through the bar. The finish was beautiful, and I saved the piece that I parted off.

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.

Could my parting blade be too thick? I have a grooving tool which is thinner, and has no trouble with steel. The parting tool is 0.089", which is similar to the Empire P-2 blades which seem to be the correct size for a South Bend 9. I have a China-made HSS tool, but it looks and feels cheap, and I'm afraid to use it. It measures 0.053" thick on the top and 0.054" on the bottom. It has a slight taper like the old fashioned blades. My large 7/8" blades start at 0.125 on the top and are 0.090" in the middle and thinnest at the bottom. This Chinese tool is thicker at the bottom. But it is thinner, Could 3/32" (Empire size) be a little thick for the South Bend on steel? For aluminum, I think I have it down. Easy and smooth with the 0.089" blade.
 
I have noticed parting actually works better when I use higher spindle RPM than lower. The only thing I can figure is that at a higher spindle speed feed rates are reduced as a function of inches per revolution. If I slow down I have all sorts of issues but they seem to go away at higher speeds.

One thing I do notice at the higher speed is increased heat build up so it is important to lube well or you can fry a HSS parting tool in short order.
 
In my opinion, 1/8" is too wide for a small lathe, 3/32" is probably the best to use. I can't imagine that a parting tool thicker at the bottom could possibly work, even it it was one thou thicker at the top, it would surely bind in the cut; the best type is the T type, such as the P-2 blade that you mention. One thing that I have seen that makes for parting troubles is when a QC tool post is used and it is located on the compound so that it overhangs the parting tool to the left, over the edge of the cross slide, the tendency being to allow the tool to bind in the cut by leaning over due to any looseness in the slides. This I found in a lathe that I bought some time back, a 9" Monarch in like new condition, now when I part on it, I move toe tool post to the right so that the tool is over the cross slide; problem goes away. With my 19" Regal LeBlond, no problem to part. You pointed out that some tools you used had back rake at the point; if they do, expect hogging in, they should have very little or no back rake. Besides that, maybe its time to fix the backlash problem ----
 
I have noticed parting actually works better when I use higher spindle RPM than lower. The only thing I can figure is that at a higher spindle speed feed rates are reduced as a function of inches per revolution. If I slow down I have all sorts of issues but they seem to go away at higher speeds.

One thing I do notice at the higher speed is increased heat build up so it is important to lube well or you can fry a HSS parting tool in short order.
I quite agree with you in that parting is usually best done at higher speeds, and yes, lube is a necessity, whether it be coolant or cutting oil. Of course the (higher) speed should be within recommended cutting speeds for the material being worked.
 
I have had a South Bend 9 lathe for a while, and I am still having difficulties with parting. I think I know what the problem is. Actually, there were lots of problems. The first one was too much rake on the parting tool. The cross slide has too much backlash and the tool gets sucked in, causing a crash. A lot of internet posts say that backlash does not matter as long as you wind it all out, but that is not quite correct. Also, tightening the gib screws for both cross slide and compound helps.

I Changed the cross feed nut on my sb9. Backlash went to zero and parting is easy. You can fine them on ebay mine was $39.00 and very easy to change.


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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.
 
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.
 
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.
 
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