Sometimes the fancy insert ISN'T the way to go

[KC2IXE]

Production job for someting I'm hoping to sell

Here is the part that made me say "hummm"

Job is run on 3/4 6061 - and I have to turn a boss .140 long to .380 diameter. My choices of tools

1)Hss ground anyway I want
2)a CCxT insert in the 21.5x size - aka SMALL, but I have fancy grades, breakers and the like
3)A good old TPG-22x insert (or TPMR-22x) insert set square to the work

I quickly discovered that you are NOT taking much more than a .060 or so cut with the tiny 21.5 inserts - you're just NOT

Grab the TPG-221 with a nice polished insert - no breaker at ALL, except the one built into the tool holder

Two nice passes - one at .150 doc, and then the rest as a finish cut. That .150 cut was deep enough to get into the holders chip breaker (.120 or so wasn't) and that .035 was a nice cleanup pass. Quick, easy, 2 passes vs 5 or 6 passes with the little tool

(Guess I have to get myself some larger holders, eh?)

(Oh, and yes, this was being done at 1000rpm on an Atlas lathe - not exactly known for being the most rigid setup. Human screw machine - what fun and joy)

 

[gadget_lover]

That's an interesting observation. So I guess it's time for the 'why'...

The TPG-221 is thicker, with a 60 degree tip. The chip breaker is (evidently) more than .120 from the cutting edge. The releif angle is 11 degrees

The CCxT-21.5x, is only 1/32 thinner, and has a cutting edge on a narrower angle (80 degree). The relief is 5 degrees. The chip breaker is often within .025 of the cutting edge.

So which of these parameters make the TPG a better choice? The relief? Are both holders positive rake?

Just curious.

Daniel

 

[KC2IXE]

totally flat rake

What in THIS case makes it better is just SIZE. A .150 DOC is larger than the CCxT 21.5x insert. IF I had a larger CCxT tool, IT would probably work better

 

[KC2IXE]

So, based on last weeks fun, I decided that I'm going to try some of the CNMG insert tooling, lartly to get around the problem of the CCMT 21.51 tools beeing too small for that one job

Bought a TMX 3/4 shank MCLNR with 4 sized insers (aka MCLNR124) with 40 inserts - $99 (not bad with 40 inserts)

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=230463032543

 

[precisionworks]

I've run the CNMG-4xx insert for years on both big 24" machines & small (but sturdy) 10" machines. They do a nice job getting rid of lots of metal in a short time, allowing the user to switch to a sharp positive insert for taking the final few thousandths. Both neg & pos inserts have their place, and both are often used during the same job.

 

[KC2IXE]

Well see how it goes - even a cabinet base Atlas 12" (the most rigid of the Atlas lathes) is no heavyweight

 

[precisionworks]

even a cabinet base Atlas 12" (the most rigid of the Atlas lathes) is no heavyweight

That's a nice machine, and pretty capable with careful setup. You'll like the CNMG -4xx, and be surprised at how much metal it can eat per pass. The belt drive is the limiting factor - which will make you want a gear head even more

 

[KC2IXE]

Oh, I'm NOT saying it's a BAD lathe - just not all that rigid (as I've said before, I learned on a 16 LeBlond )

 

[KC2IXE]

So, I mounted up the holder for the GNMG-4xx inserts, and tried some test cuts. Believe it or not, at about .140 cut, I started to get unbelievable shuddering - Maz safe cut for 6061 on my lathe seems to be roughly .120!, so the TPG-22x still comes out the winner

I wonder if the -5 deg lead angle of both the CCMT and CNMG tools is what is killing me.

Guess I'll stick with the TPG-22x for this job - greatest DOC, fastest job cycle time, and on top of that the least expensive inserts - and I have a ton of them from back in the day that it was the only insert I used

 

[precisionworks]

Some things to check ...

Mount a 2" solid round bar in the chuck (at least 12" long) & set an indicator at the far end of the bar. Pull up hard on the bar & note the deflection. Push down hard & again note deflection. If the spindle bearings are correctly set (proper preload) you'll have no more than a few thou of upward or downward deflection. If not, adjust the preload on the bearings -

That spindle uses a pair of Timken style tapered roller bearings that are preloaded (squeezed toward each other) with a threaded thrust ring. I can't remember if there is just one ring at the back, or if there's also one at the front. I'd almost bet that the source of the shudder is inadequate bearing preload.

Also worth checking are that the tool point is dead on center & that the toolholder is set as close to the tool post as possible. Negative carbide inserts work best with very rigid setups, which usually means shorter is better. Also snug up the gibs on both the cross slide and the compound slide as these tend to get loose over time.

If you still get shudder after doing all that, a sharp positive insert may be the answer. But I wouldn't give up on the CNMG until trying all the machine adjustments first.

 

[KC2IXE]

Like I said, I take .180 cuts with the TPG-22x, can take LESS with the cnmg. Atlas lathes are NOT the most rigid things in the world. I think it's just a case of "That lathe can't do that"