Finish in climb vs. conventional

DrollTroll wrote:

DT:

Yes.

Almost always climb milling gives a better finish. It's generally thought to be the result of chip-thinning. The following is from a post I made in 2002.

================================================================== Let's do a thought experiment to attempt to isolate and simplify some of the cutting forces involved. Normal end mill, normal rotation, cut width = 1/2 cutter dia. In a climb milling situation with these parameters, the largest bite/chip load (and equivalent perpendicular force that both pushes against the material and deflects the end mill away from the cut) should result primarily from the initial contact of that part of the cutting flute contacting the stock. As a cutting flute proceeds around/through the cut, radial chip thinning occurs (the chip load and force would progressively decrease) until 90 degrees to the forward feed at which point all the material in the climb cut (at any one specific height point on the flute length) for that flute has been removed. The rear of the cutter should have very little force in trying to pull the stock around behind it (perhaps a miniscule drag by the cutter scraping on the floor of the cut. Now with the same 1/2 dia. cut width in a conventional cut the flutes will start the cut with the least amount of force and the thinnest chip load, but as the flute progresses around/through the cut the chip load and thickness increases until it's at the maximum just before the chip fractures out of the cut. Before the chip breaks free the force deflecting the end mill into the cut would be greatest. Now another force that I haven't mentioned up to this point is the rotational component of the cutting force. It would be large and the rotating cutter should attempt to impart a torque or twisting force to the material in the same direction of the cutter rotation. Of course it isn't able to twist material that is held solidly so the material itself is sheared away. This force may be the force that would be pulling the slug behind the cutter when climb milling and in front of the cutter when conventional cutting due to the slug being relatively free to move. End milling a small part that is only held down with double back tape can show that the rotational torque is strong, especially when the part lets go it often times spins around with the cutter. ==================================================================

Just a guess, but your cutter may have been dull and "burnishing" the conventional side.

My bets he had the spindle running the wrong direction......

The conventional cut was bright/smooth, like a good face mill. It would seem to me that burnishing would satinize the finish.

Also, I don't know if I grok the thought experiment.

Consider what feed does to the tangential velocity (sfpm) of the em edge. In climb, it subtracts from the sfpm, in conventional it adds. At sufficient feed, the cutter would not cut at all in climb!

So in this view, the only tangible diff between climb and conventionl is sfpm of the cutting edge.

Sorta like running on a treadmill.

Otoh, my li'l above model suggests that both climb and conventional should deflect similarly relative to a cut, just to different degrees. But apparently they deflect oppositely, which I never really understood either.

Inyway, just musing. Just glad to be done with that job.

Mebbe String Theory holds the answer... or the Big Bounce. :)

Climb milling tool steels always gives a better finish, It has nothing to do with the sfm from revolving increasing due to rotation direction. It's a geometry issue. However, Every rule has it's exceptions, and some steels and alloys are said to cut better conventionally, because they lack a grain sructure, like cast iron? (just a romour so far)

It's definetely increases the life of the tool, and the machine since power is less wasted by efficiency of cut. Instead of energy being lost in the form of sound and heat, it's used in climb milling to pull the tool along.

Also effected is deflection, and the way wear is created by deflection. Conventional cutting tends to push away, climb cutting has a tendency to pull in.

Chipping is decreased because chips aren't brought back thru cut due to the direction of the spiral on the mill.

Now.... if your cutter is dull, as in your cutting a pocket thats 8X8 by 3" thick, conventional cutting will give the best finish if no climb cutting is involved.

There's no single answer...

One thing...the more rigid the machine and toolholers, the better climb cutting will shine. Climb cut everything...left offset. that's ny motto.

One thing I can testify to, it makes a major difference when cutting graphite. Mostly in the form of wear.

OK, not argering with the mavens here, just some inneresting points:

For every 1,000 rpm of a 1/2" em, the tangential speed in inches (sipm) is

1,571 ipm. Every 15 ipm of feed will change this sipm number by about 1%: Climb will subtract from it, conventional will add to it.

Thus, the only net difference between climb and conventional cutting "should" then be material removal rate!

IOW, with the above em, to achieve the same material removal rate, a 1/2" em spun at 1,000 rpm in a 15 ipm conventional cut would have to be spun at

1,019 rpm in a 15 ipm climb cut.

Which starts to answer Cliff's "Huh?":

Imagine a feed of 2,000 ipm with the above 1/2" em at 1,000 rpm.

In a conventional cut, this would more than double the material removal rate (net 3,571 ipm), assuming chipload per tooth is not exceeded.

In a climb cut, this would calc out to *minus* 429 ipm, akin to trying to cut by spinning the em in the wrong direction!

A feed in climb of exactly 1,571 ipm is what would happen if the the em grabbed the material from loose vise jaws and flung it -- think table saw being fed in the wrong (climb) direction. Which is a net relative speed of zero between the em flutes and the material -- ie, the condition of "pure rolling" (as opposed to sliding) between two objects.

One can also think of of a merrygoround spinning clockwise on a northbound train. At the "right" rpm relative to the train's speed, one could jump off the merry go round on the east side of the train, and land standing up, unscathed; on the west side of the train, you'd hit the ground at double the train's speed.

I would then predict that deflection in an em should be in the same direction, climb or conventional, and proportional to the *net* sipm or material removal rate. Thus, at the same rpm, the deflection in conventional should be more than climb, but still in the same direction.

Or, in the above example (1,000 rpm in conventional vs 1,019 rpm in climb,

15 ipm feed), the deflection of the cutter should be in the same direction AND the same amount, as, after all, the cutter is still cutting, ergo "digging" into the material, and removing it at the same rate.

But the conventional wisdom (vs. climb wisdom? ) is that deflection is indeed opposite.

In light of the above, this is hard to reconcile.

Has anyone actually measured deflection as a function of *net sipm*, in both climb and conventional cuts? Bottle?????

I agree with you but for one thing. When climbing I always get the cutter to push away and conventional pulls it in, not the other way around. Jerry

yep...your right. my mistake.

DT:

OK, let's see how much normal feed rates affect edge SFPM. I'll take an example from a part I was doing today. 7075 Alum. 1.25 dia. insert cutter 7500 RPM @ 100 IPM. The SFPM was (.262 X 1.25 X 7500 = 2,456 SFPM). Now 12 IPM would be 1 Foot per min. 120 IPM would be 10 Feet Per min. 100 IPM is 8.3 FPM. So the two flute insert cutter I was using would drop 8.3 FPM on the climb milling side and increase 8.3 FPM on the conventional side. That's like 8.3/2,456 or .34% of the SFPM. Now to be fair, at slower speeds the effect of feedrate will be greater.

Imagine a 3/8" end mill cutting steel with a radial DOC of half it's diameter. Now when climb milling the first contact of any point along the flute will take the biggest "Bite", and of course with at that point have the greatest amount of force directed AWAY from the material. As the cut progresses LESS deflecting force is generated since the chip is thinning. So by the time the flute is 90 degrees to the end mill's travel there is relatively little force left. Now when you are conventional milling, the cut starts out with the chip being thin and as the cut progresses the chip gets thicker. And while the chip gets thicker the flute, due to it's gullet design, digs INTO the material and deflects the end mill TOWARD the material. What often results from that is a series of divots in the side of the machined material. That's my interpretation of why climb milling generally results in a poorer finish. But like anything, there will be exceptions.

Musings-R-Us.

It's funny, from a newtonian pov, the stock answer would be there is no difference -- there *cannot* be a difference -- like running on the ground vs running on a treadmill -- relative inertial frames'n'shit.

But while I'm still groggy and I focus on the micromechanics of the two cuts, I am starting to be able to visualize what you are talking about. I see the climb cut "thinning" because of the "different relative motions", even tho the sfpm could be identical!! I now seem to remember sumpn bout rotation screwing up inertial frames! :) When I fully wake up, I'll proly become resistant again. :)

All very inneresting.

Hey, iffin yer innerested in a formula for equivalent sfpm, S1 = S2 + (F1+F2)/piD where S1 is climb rpm, S2 the conventional rpm, F1,F2 the feeds, D the tool diameter. fwiw, which proly idn't much. :)

But still, my conventional cut was gleaming, my climb wasn't. And some of the ems were indeed sharp. Mebbe there was some material dependence here, I'll keep an eye out.

What about this:

Could be that in climb, in a finish cut, because of the chip thinning, the finish cut appeared a little raggedy *if the finish cut was too thin*. Mebbe in very lite climb finish cuts, the doc should be .005-.010 heavier than the conventional, for equivalent smoothness?? These finish cuts were .015 (no roughing em used).

It's just like swimming with and against the current. Jerry

----------- You have the same effect for both side and face milling.

see Moltrecht, "Machine Shop Practice - vol 2" Industrial press NY NY ISBN0-8311-1132-1 (v. 2) pages 152-153 & 169-171 for detailed discussion.

Moltrecht is an excellent reference and highly recommended. Frequently on sale from Enco.

{the one you want, but get both}

Unka' George [George McDuffee]

------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?

Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).