Negative Rake Cutters

[comerrm]

I've been reading alot of the posts on this topic but cannot completely grasp all of the uses for it. From what I have gathered, negative rake is good for roughing. I have been messing around alot lately with some larger HSS tool bits to make it easier to figure out what works well with certain materials. I mostly work with 6061 Al and 4130/40 Steel. Is a negative rake cutter good for either of these applications, and if so what kind of angle should I grind? Any help would be appreciated.
-Rob Comer



US Government Philosophy: "If it ain't broke, fix it till it is"

[SGW]

My knowledge of the subject is sketchy, but my understanding is that, unless you have a MASSIVE machine -- and probably nothing you could fit in your basement would qualify -- the machine won't have the rigidity, or the horsepower, required to be able to handle a negative rake cutter effectively.

For home shop use, IMO the most practical toolbits for a home shop are HSS, ground to appropriate positive rake angles for the material being machined.

[lazlo]

Negative rake tools are meant for high-volume production environments. The big advantage of negative rake inserts is much higher edge strength since there's no relief angle under the cutting edge. They're also more economical since you can flip the inserts over and get twice the number of cutting edges (since 0° relief angle means straight sides on the insert).

The downside for HSM'ers is that negative rake tooling requires more horsepower, and a more rigid machine. The classic analogy is using a snow shovel: with the sharp edge up, you're using a positive cutting edge to slice through the snow. With the shovel upside down, you're plowing the snow with a negative cutting rake.

For HSM'er, positive rake inserts have a sharp relief angle, requiring a lot less horsepower, and generating less chatter. But the cutting edges are more fragile, and you get half the number of cutting edges (since the relief angle means sloping sides).

[comerrm]

Thanks for the input, I guess that probrobly doesn't apply to my personal lathe. At home I have a 13" SB, but here at school I have access to some 5 hp Leblonds and a 14C Monarch. Maybe those machines could handle it, its not necessary for me to be doing rapid production. I just wonered because we get stock donated to our Formula SAE race team, but often they come in sizes bigger than what we need: IE Turning a piece of 3" 6061 to make a 1.5" piece, just want to get things done.
-Rob Comer

[rwf71]

In the shop where I work we use mostly neg. rake inserts & can flip them over to take advantage of cutting edges on both sides of the insert, BUT the holders we use place the insert at a "nose down" angle so they still work & cut like a pos. rake insert.
I don't have a pic & wouldn't know how to post it anyhow but I hope what I said makes sense to you. A neg. insert can act like a pos. when it's not mounted "flat" to the toolholder centerline. Rick

[riceone]

Now I know what I have. Last box of inserts I got from Enco were the same on all sides and carbide. Did know they were different but did not know what I had. They will cut on any side. I have been using them on a 13x40 Jet with no trouble. riceone.

[Davyboy]

I agree with above comments about inserted lathe tools. The only time I've ground a negative edge on brazed tool was for very hard, usually heat treated or welded material. For 4140, a good chip breaker (groove) can help control and steer the chips as they come off the tool. I've cut .200 or more per side with a brazed tool ground with a chip breaker, on a 12" LeBlond. Aluminum also seems to need a positive rake chip breaker to turn chips over and break them for roughing. Alum likes to stick to the tool if pressure builds up. I haven't used high speed tool alot.
Good luck, Davyboy

[jimsehr]

I think it is worth trying a insert holder and neg inserts. Just to learn how they
work in your shop. [ Look for an ebay deal .] I like positive cutting on my light duty Logan. But I sometimes use neg inserts when taking light cuts like
facing or light turning. With 5 hp machine neg inserts will work fine. And it sure sure makes it easy for a Home Shop Machinist who has a hard time hand grinding tools. Lots of neg inserts have a positive rake top and you can get 6 fresh sharp edges from 1 insert.
Jim

[lazlo]

At home I have a 13" SB, but here at school I have access to some 5 hp Leblonds and a 14C Monarch.

You won't have any problem with negative inserts on most materials with those machines. There just isn't much of an advantage to the HSM'er over positive rake inserts.

I use CNMG's (80° negative rake inserts) on my Clausing 5914, but that's only because they're probably the most popular turning insert in history, so you can find them on Ebay for 50 cents to a dollar a piece (for name brands).

But on 4140, especially if it's heat treat, or if I'm looking for a really fine finish, I really like TPG (positive) inserts.

[wierdscience]

Neg inserts are intended for interrupted cuts and heavy roughing in general.They can also produce a nice finish if enough speed and HP is availble.

I use them nearly 90% of the time at work on machines as small as a 5hp Hendey with no problem.That machine will allow .250"DOC and .018"/rev cuts in 4" OD 1018 at 450rpm all day long making nice little 6's and 9's that stream off like water carrying most of the heat with them.

There is a term,Dynamic shear I believe it is that describes the cutting action.Too slow and they make knarly looking strings and too fast they make sparks.If you hit the speed and feed right in the sweet spot(usually 350-600 sfpm) then the metal just melts away and the HP requirement drops to what a HSS tool making the same cut at a lower speed would take.