Cutoff Tool

bah, yes, yes, chunks of cemented carbide brazed onto steel shanks.
 
Upon reading the Kyocera parting tool literature that I posted above a few things pique my interest. I have never read the booklets that come with the tools that I use because I am lazy and they are not my tools.

They advise setting the tool above the center line , I suspect that this is to account for a certain amount of tool movement even if you are running the coolest Swiss lathe.
 
No experience with cemented carbide, but I've had very good results with both the ISCAR's style GTN3/4 replaceable carbide inserts and 5/8" wide HSS cut off blades. Key things are getting the most rigid set up possible and a sharp edge on the HSS blade (with tapered sides). I always follow Tom's Techniques tip of touching up the HSS blade on the grinder prior to cutting.

Bruce

edit: 5/8" wide blade is 5/8" tall x 1/8" wide x 4-5" long.
 
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I think above centerline might also have to do with the way the inserts are ground. The 1/2" AXA holder I have holds the blade at an angle which has the same effect as running above center. I suspect this helps in two areas, first it changing the pressure angle on the compound (although not a much) and second it adds rake to the top of the tool without grinding... although the grind angle on the front of the blade needs to be more extreme. I suspect this is why new blades come with opposing ends sharpened at different angles, one for a level tool holder and one that holds the blade at an angle.
 
Hi Mork,

Parting is probably the one operation that generates the most grief and discussion. It looks so easy and fantastic when it works well that it seems like a "no brainer", however many people have problems getting to the point of it working well. I sure did.

Personally, for me, the two suggestions that helped the most were:
1) use a wider parting tool. This seemed backwards to me; I figured the wider tool would mean more force trying to flex everything, but I now believe that the wider tool is simply more stiff
2) remove any looseness/slack: lock the carriage, top slide, anything you do not need for this operation. (Same on the mill; if you get chatter lock the gibs for the feed directions you are not currently using!)

There are many great suggestions in these two threads:
http://www.hobby-machinist.com/threads/parting-for-idiots.32938/
http://hobby-machinist.com/threads/parting-difficulty.35356/

-brino
 
Thanks for the links... interesting stuff. Toward the end of the second link they showed various ways to support the blade holder which was interesting. Somebody made the comment that chatter is your lathe saying, "I'm not solid" which is undoubtedly a true statement. I tried locking the carraige and this had no effect... probably a good idea in general but it didn't help the chatter at all. Now the other end of my brassed carbide cutoff tool is missing... I finished the test cut with the AXA tool post (with a HSS blade set at an angle). Next step is probably a wider cutoff tool either HSS or maybe buy a blade with the changeable insert.
 
something else to check is the tension of the gibs to the dovetails in both the saddle and compound rest.
wear or maladjustment in these areas will make your parting suffer.

a trick to try is to invert the cutoff tool, run the lathe in reverse, and part from the back side of the work.
this may help if your lathe light or worn:)

P.S.
the suggestion of being above center greatly reduces the tools tendency to pull the work into the blade.
the work has a hard time climbing over the blade and therefore has less of a chance of breakage.
 
The dovetails are about as tight as I can get them without restricting movements. They are slightly tight in some areas which obviously means there is some wear and some movement. It's an issue I'm sure but something I need to work around. In my opinion, above center will help but if you are only very slightly above. More than slight and the undercut on the blade has to be ground significantly. I have tried various heights and always end up going back to center. It always works the best for me. 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.
 
a picture of your set up would help. I have a much smaller and more flexible lathe, yet I can part steel without chatter. Chatter is either your tool set up flexing and springing back or your work flexing and springing back (or a combination of both). There are lots of things you can do to minimise both, but without seeing the set up that's causing the problems we may be directing you to fix something that isn't causing the problem.
 
Lock the carriage down (otherwise you will get movement that makes your part somewhat bowl-like.) More importantly, drive the compound rest all the way forward on its screw prior to parting. That was the trick for me (an actual machinist from work told me that). Not only does it stiffen things up, but there is less of a chance the tool will grab the workpiece (especially brass) and pull the tip under the part (due to backlash slop).

Rick
 
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