Chatter on climb milling but not conventional

[durableoreo]

I'm trying out some higher speed machining. I had a cut that chattered when climb milling but was quiet in a conventional cut. It was even more quiet in a slot. I'm wondering if that's a clue to a problem with the machine, length of stick out, or anything else.

From my reading/watching of the internet, climb milling may work better on less rigid machines. I'm suspicious of that conclusion but it's "out there". Recently, while commissioning a new machine, I had the opposite experience. While running a back-and-forth type facing cut, of the Fusion360 variety, I noticed sparks and noise during climb milling but quiet cool while going the other way. The cut was with in annealed 4140 using a 4-flute 1/8" solid carbide endmill (of questionable providence), about 16,000 RPM, 17 IPM, 0.063 step over.

I wonder if the conventional cut is taking a full bite (0.000,3") as a flue enters the cut, then the chip thins until the cutter is just rubbing and the little bits that are worried off the surface are hot enough that the carbon burns orange. Could the sudden engagement (as the flue enters the cut) cause the endmill to flex? I couldn't feel any vibration in the Z carriage which might indicate that the endmill is bending.

The mill is a 3030 fixed-gantry type setup but with linear rails all around. I upgraded the spindle from the stock 300-W DC motor to a 2.2 kW Chinese 24,000-RPM water-cooled monstrosity. The power is more than needed but the bearings are better than the small motors you find on most of these units.

These types of cuts are fairly unfamiliar. I used this calculator to work out some of the details. https://www.kennametal.com/us/en/re...ators/end-milling/force-torque-and-power.html

 

[Nutfarmer]

When I was taught it was on manual machines and climb milling was a big no no. If it was a loose machine, there was a good possibility of the cutter grabbing and throwing the part. I guess with tighter N C machines climb milling can be done.

 

[Huub Buis]

When conventional milling, the machine is pushing the tool. As long a you move in the same direction, play does not matter.
During climb milling, the tool will pull the machine. If your play is larger than the feed/rev, you could experience chatter.
If your setup (part, clamping, tool bit, machine, etc) is not rigid enough, you could experience chatter during climb milling and conventional milling.

For climb milling, you need "play free" spindles (ball spindles). Manual milling machines usually are equipped with lead screws that have significant play. The friction of the lead screw will, if the load isn't to high, keep the axis at position if you don't hold the handle.

When I do climb milling on my manual milling machine, I turn the spindle with one hand and pull back the axis with the other hand.

 

[Ulma Doctor]

Climb milling is best done on rigid machines at low DOC’s
Conventional milling is where the work gets done

 

[GeneT45]

Light climb milling has its place on manual machines, but it should be light (DOC, depth of cut) and if your machine has much backlash you will want to snug the gibs a bit to keep the cutter from moving the table.

GsT

 

[Martin W]

I would only do climb milling on a finish pass of a couple thousands on a manual machine. IMHO

 

[Iceberg86300]

Other comments are correct about rigidity & backlash being an issue, but by the 3030 description it sounds like your machine is "tight."

And while I'd normally say a 3030 machine is lacking rigidity, I think this is a situation where small diameter tool dynamics dominate over rigidity. (Again, I wouldn't expect a 3030 with linear guides to have a literal backlash issue)

B/c:
16k RPM seems WAY high for 1/8" diameter tool in 4140.

It's within the range of generic SFM calculations but things tend to change when you look at recommendations for specific small diameter tools.

Like recommendations from Harvey for an 1/8" end mill in 4140 are ~200-300 SFM, whereas the generic SFM recco is what, ~400-500 SFM?

Also, when talking about chip thinning, did you mean to say climb milling? B/c that's definitely an issue but with a constant depth of cut on a straight surface it occurs with climb milling, not conventional.

Sent from my SM-N975U using Tapatalk

 

[durableoreo]

I would only do climb milling on a finish pass of a couple thousands on a manual machine. IMHO

This also my experience. Just a whisker, to improve the finish.

 

[RaisedByWolves]

16000rpm and your getting sparks?

Why so fast?

 

[durableoreo]

Other comments are correct about rigidity & backlash being an issue, but by the 3030 description it sounds like your machine is "tight."

And while I'd normally say a 3030 machine is lacking rigidity, I think this is a situation where small diameter tool dynamics dominate over rigidity. (Again, I wouldn't expect a 3030 with linear guides to have a literal backlash issue)

B/c:
16k RPM seems WAY high for 1/8" diameter tool in 4140.

It's within the range of generic SFM calculations but things tend to change when you look at recommendations for specific small diameter tools.

Like recommendations from Harvey for an 1/8" end mill in 4140 are ~200-300 SFM, whereas the generic SFM recco is what, ~400-500 SFM?

Also, when talking about chip thinning, did you mean to say climb milling? B/c that's definitely an issue but with a constant depth of cut on a straight surface it occurs with climb milling, not conventional.

Sent from my SM-N975U using Tapatalk

Harvey is a reliable source but the 200 SFM figure seems wrong. I found a post on PM where people where talking 4140 finish and they liked 450-650 SFM. Here's a picture of the finish at 550 SFM. The spindle isn't quite square to the table yet, so there's some texture on the face. The 3/4 square hole was cut with 10% step over, 0.125 step down, 0.000,7 per rev on the ramp down. I used a 1/8" 4-flute carbide endmill with 1/2" of flutes and necked down so I could get down to the bottom of the pocket, 3/4" deep.



The machine has only 1 nut per axis, so there is probably some backlash. I haven't measured it yet.

Yes, I meant to write:

"I wonder if the CLIMB cut is taking a full bite (0.000,3") as a flute enters the cut, then the chip thins until the cutter is just rubbing and the little bits that are worried off the surface are hot enough that the carbon burns orange. Could the sudden engagement (as the flue enters the cut) cause the endmill to flex? I couldn't feel any vibration in the Z carriage which might indicate that the endmill is bending."

The young guys are always talking about speeds and feeds for high speed machining but I usually checked out. Now I see what the fuss is about. This is completely different from turning dials and listening to and feeling the cut.