Cutoff Tool

Here's my set up. I increased the relief below the cutting edge on the blade just to see if it made a change... not much different. As you can see, I'm slightly above center here. I completed this cut adjusted slightly below center which seemed to help. The tool is T shaped and 3/32" wide. I read some comment that said wide is better so maybe I'll jump up to 1/8 or 5/32. I had a brassed carbide insert blade used with a tool post I made but broke the carbide off both ends. This picture is at the beginning of this thread. WP_20160221_002.jpg WP_20160221_003.jpg WP_20160221_004.jpg

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Here's a short little video
 

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I've considered going in from the back side but need to give this more thought. It would limit the diameter I can part but I'm not sure how much. Worth investigating and trying for sure. Ultimately I think my Logan 200 is a little flimsy. It's perfect for most things I do but for serious production work I'd want to get something much more solid.

I think the next step is carbide inserts or perhaps cutting from the back.

Parting from the rear is the same as parting from the front, except that it works much, much better. You can part anything that fits on your lathe, provided your parting tool is long enough. The key problem is going to be mounting such a tool because it requires T-slots on the cross slide. There was a post recently that pointed to a company that sells rough castings so guys with older lathes can put a T-slotted slide on their lathes. Parting from the rear works so well that if I had issues I would seriously consider modding my cross slide.

I've used a rear mounted parting tool on my Sherline for well over a decade. True, some think its a toy and can't run with the big dogs but I've parted materials up to 1-1/2" over the cross slide at high speeds without a hitch, from brass to 4340. I typically run at speeds somewhere around 2-3 times normal turning speeds for the material without a catch, stutter or hint of chatter. I use only P-1-N tools on this lathe, which is 0.040" wide, and there is very little force required to feed the tool into the work. I don't even pay much attention to stick out because it isn't critical with a rear mounted tool. I don't lock my slides or adjust my gibs. (I went to this tool because this lathe chatters when parting from the front in harder materials. However, if I am careful with alignment I have parted 2" OD steel from the front with this lathe.)

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This is a groove being cut in mild steel at 1200 rpm. Note the stick out of the blade - still couldn't get it to chatter.

I also have an 11" lathe that parts from the front without any chatter so maybe this might help. I typically use a P-1 or P2 blade on it and very little feed force is required for it to cut cleanly at whatever turning speed I'm using. In contrast, parting with a GTN-3 blade does require more feed force but not a whole lot more if my turning speed is high enough. I like sharp tools but my parting tools seem to not be fussy about this; they get sharpened maybe once every few months (or more :frown:).

Parting from the front does require me to do some things. The tool has to be absolutely vertical in the tool holder and then rigidly held. The tip has to be on center; I check it with a tool, not my eyeball. The blade has to be perpendicular to the work; I align the blade on the face of the chuck because its handy. I prefer a 7-8 degree relief angle at the front of the tool; this allows it to cut cleanly without rubbing or sacrificing edge support. When parting from the front I do pay some attention to stickout but only casually. I typically do not lock anything and feed manually so I can feel the tool cutting. Again, very little force is used once it starts to cut and I maintain a constant positive feel as I go in. Finally, I relax. If I get tense I cannot feel the tool cutting and that affects the cut. Stupid? Maybe ...

I will admit that my Emco 11" lathe is in good shape and not worn excessively. If it were old and loose then things might be different. I have had this lathe chatter a few times. Typically, the tool is not on center or perpendicular to the work. Once I fix that I can get on with it.

Hope this helps.
 
Great post mike! Hey... it sounds like parting from the back is the way to go... at least on my lathe. Take a look at the picture I just attached. I can do it without T slots although my diameter is limits to about 1-3/4" diameter. I don't like the limit but in most cases this is more than enough. I'll have to make a tool post but I set the one I made for the front side (complete with broken carbide tip) on the compound just to check clearances. Now I'm thinking about threading from the back also, for the same reasons, although I have had fairly good luck threading I do get occasional chatter on large threads.

Thanks for the suggestions!

Mark
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I just realized i could get a bit more (an easy 2 inch) by turning the compound around.

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Mike... you are talking about normal rotation right... with the tool mounted up side down?
 
Dang, you're right - you can mount a rear tool post! Okay, my bad.

Yes, the lathe rotation is normal/towards you but the tool is mounted upside down. If you do this be very accurate when milling the slot for the tool as this sets your cutter height. The bottom of the slot must be on the dead center of the spindle. To do this I turned a piece of brass (any material will do) and took skim cuts until I got a very clean finish. Then I used a height gauge to check the distance from the cross slide where the tool mounts to the top of the work piece and subtracted half the diameter to get the spindle center height. Mic the OD of the work piece to be as accurate as you can.

I use the same principle to make a simple tool height gauge. It gets used a lot!

I'm not sure about threading from the rear - never tried it - but a rear mounted scissors knurling tool works better for me back there, too.

I know my last post seemed to go against what many of the other contributors said. I did not mean to imply they are wrong; only that my experience is different. If I offended anyone, I apologize and offer that it was not on purpose.
 
I don't think you offended anyone. I know for a fact there are many that struggle with parting on a lathe. I even watched a video on youtube by Tubalcain where he said, "I've avoided making a video about parting on a lathe because it is something I struggle with".
 
a trick to try is to invert the cutoff tool, run the lathe in reverse, and part from the back side of the work.

With the tool inverted and behind the work, the spindle should be turning naturally. Many lathes don't have the capability to mount a tool behind the work, but this sure is the best way to cut off work in a lathe.

I build an inverted tool to hold in my four tool holder in front of the work, but I have to be careful not to unscrew the chuck when parting off.
 
Reversing with a inverted tool would be a great way to part off but I'd also be worried about my screw on chuck.
 
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