Another Parting off Thread

[erikmannie]

Try it, you'll like it. Keep up with cutting fluid. It works, you have the rigidity, make it square, and on center. 36 is a little slow, depending on diameter, you've been doing some big stuff.

I will try it. I am going to start with the slowest feed available.

36 RPM was for 8-10” pipe. I should have probably mentioned that.

 

[Aukai]

That will give you the surface speed. If your hand feeding, if it grumbles at real slow feed, feed faster until it starts shaking. You can see if slow, or fast makes a difference. I was surprised at the difference with the hand feed, slow was chattering, I cranked on it hard, and it started curling. The 1228 did not like that at all, the tool post would rattle so bad it would release.

 

[mikey]

It might help to consider what a parting tool is. All parting tools are form tools. Some are sharp, like HSS tools, and some are not so sharp, like inserts. Some have zero rake, some have scoops on top, some inserts have complex shapes. Regardless, they are all form tools SO all else being equal (enough power and rigidity, perpendicular orientation to the work, tip on center height) you have to feed the tool into the work at a rate that is commensurate with the speed. Unlike many form tools, a parting tool's cutting area does not change as you feed in so feed rate is simpler to manage. About the only complication to this is that when you reach the inner 1/3 or so, you have to slow the feed a bit because surface speed drops significantly that far in.

Like all form tools, surface area matters. The wider the tool, the greater the tangential cutting forces will be and the slower you will be able to feed. For me, a P1-N or P-1 blade is sufficient for almost all the parting I have to do on either of my lathes for most materials. For me at least, the wider the blade the more problems it creates. I have carbide parting tools but rarely use them, mostly because they tend to be too wide and therefore require more feed force and that takes away from me being able to feel the tool cutting.

I have read that you have to use a lot of force when feeding a parting tool. This has not been my experience parting from the front or the rear. Rather, I have found that feeding so there is a slight resistance to the feed is sufficient to keep the tool cutting continuously. Yes, this is when feeding manually and is how I prefer to part. This same positive resistance to feed is the same resistance I am looking for with any cutting tool when I feed it manually, be it a lathe tool or an end mill. I'm sure older hands than I know exactly what I'm talking about here.

Sure, power feed works but you will break more tools with it.

I also think that too slow a speed is detrimental to parting. For me, parting speeds are determined by how well I can keep up with the feed. As long as I can feel that positive push back from the tool, I don't worry much about the specific speed I'm at. When parting from the rear on my Sherline lathe, I am often turning at 2-3 times normal turning speeds; somewhere in the 1100-1200 rpm range in mild steel and maybe a bit faster in softer materials. On my Super 11 when parting from the front, I am going somewhere near 800-1,000 rpm for the same materials. I may slow things down a little with larger work pieces but I never have chatter or a dig in on my Sherline and can recall maybe once having chatter on my Super 11.

I think sharp tools in general are important but I think my parting tools get sharpened maybe once or twice per year. I may take a few honing strokes across the top once in a while but the blade is taken to the grinder only occasionally.

I guess what I'm saying is to worry less about speeds and focus more on feel. If it chatters, do the usual thing by slowing the speed or even better, increase the feed rate to keep up with the cut. Once you develop that feel for a tool that is cutting efficiently you will be fine. That feel will be the same when turning, boring, drilling, reaming, feeding an end mill and so on.

 

[Mitch Alsup]

Just a follow up for everyone who offered me some guidance: I think I found the issue, I noticed the parting tool would almost swivel when it was mounted in the tool holder. I checked the bottom of the tool holder (where the tool mounts in the holder) against a straight edge and realized it was bowed (crowned) in the middle.

Perhaps this gets to the point of why I haven't had any parting off problems. My parting blade is a few thou smaller than what the holder can grasp, so to solve this problem I made a short shim and placed it at the front of the holder......