Newbie- Why are four flute end mills better for steel?

[TXFred]

Hopefully I'm in the right forum for this. If not, Admins, can you move this thread to the right place?

I'm still a newbie. I've been a hobby machinist for several years but have less than a year's experience running real CNC machines in an industrial setting.

I want to expand on my question. I fully accept that a four flute end mill is better than a two flute when it comes to milling steel. I watched one of my programs turn a Haas VF-2 into a chattering, shuddering, tool breaking machine due to my using a two flute end mill. And then I switched to a four flute, doubled my feedrate and watched it cut steel like butter.

The superiority of the four flute for steel is an accepted fact. What I don't understand is why it does a better job. I doubled my feedrate, so the chip load stayed the same. Surface speed was unchanged. So what's the critical difference between two and four flutes?

Cheers,
Fred

[dertsap]

rigidity ! theres a lot more meat on a 4 flt

[christinandavid]

The good thing about having more flutes is you get an extra nanosecond per flute to hit the big red button when things go bad. As soon as you hear that first flute crunching....

Of course the quietest cutters are the ones which no longer have any flutes attached.

DP

[DareBee]

^what Derstap said
2 - flute cutters are made for slotting where extra chip clearance is required.

[HuFlungDung]

There is a different harmonic resonance to cutters with different numbers of flutes. And the width and depth of cut can also have an effect. A 2 flute endmill necessarily permits one flute to come completely clear of the workpiece before the other flute begins to engage (ignoring the effect of the flute helix). This shock causes the spindle to deflect like a spring. It might not be much deflection but it could significantly alter the chip thickness if the spindle is rebounding in the wrong direction when the second flute enters the cut.

More flutes helps to mitigate this deflection cycle, keeping a more constant load on the spindle so that, while it still deflects, it maintains approximately the same deflection throughout the cut until the tool exits.

[Caprirs]

The cross sectional area of a typical 4 flute endmill is much beefier than a 2 flute. This makes the tool stiffer at the expense of space for the chips.

[HuFlungDung]

Just for the heck of it, I weighed a 2 flute endmill and a 4 flute endmill the same overall length (3.56"). These were 11/16" HSS endmills.
The 2 flute had max flute length of 2.1 inches and the 4 flute had a max flute length of 1.8", so they had ground a bit extra on the 2 flute.

The weights were
2 flute: 163 grams
4 flute: 173 grams

So there is not a 'whole lot' of extra mass in the 4 flute, not as much as there appears to be. I guess you get some reinforcing effect from the extra two flutes on the 4 flute.

The definitive test would be to reduce two opposing flutes on the 4 flute by maybe .005 (so they don't cut) and see how dramatically it changes the vibration. That is, would it behave similar to the 2 flute (at the same feedrate) or would it be better?

[Geof]

Originally Posted by HuFlungDung ....So there is not a 'whole lot' of extra mass in the 4 flute, not as much as there appears to be. I guess you get some reinforcing effect from the extra two flutes on the 4 flute...

You probably get a whole lot of reinforcing effect because a four flute cutter is 'square' while a two flute is 'rectangular'.

Think of the cutters as a beam rigidly attached at one end and subject to a bending load at the other (ignore twisting which complicates thing even further). A rectangular beam is much stiffer when loaded on its large depth; the two flute cutter gets its load on the small depth of the, it really never gets loaded across the large depth, but the four flute cutter always has a large depth taking the load.

[neilw20]

I have had good success with 3 flute cutters solid carbide 10mm cutters.
When one tip on a 2fl or 4fl cutter gets damaged, the loading becomes un balanced.
With a 3 flute each tooth works on it's own, and if one chips I get more feed marks, but no deflection problems. When a tip chips it rubs, which is not good if reacting fully against a good tooth.
I am machining stainless steel, with light cuts, but require Z axis control to be really consistent. with 2 microns of set height, and no rubbing on the sides causing the SS to flow. SS is a really bad conductor of heat.
I sharpen my own. - 33 at a time. 2 hours for the lot.

[christinandavid]

I would tend to agree most heartily with post #5 describing the more uniform load cycle on a four-flute end mill as opposed to the violent oscillation of a two-flute slot drill, once you get up to the 90deg/50% engagement kind of conditions.

You only have to listen to the sound of an endmill ploughing headlong into/out of solid material - it is a lot louder on entry/exit when contact with the material becomes intermittent.

Also agree with post #9 - 3-flute is the best of both worlds, but if you don't regrind your own they are a pain to measure . . . .

Not sure if 2-fluters are any weaker than 4-fluters, they can take some severe punishment.

When you get up to the larger cutters with more flutes you start to find the workpiece flexing also, due to the amount of contact with the tool.

Some machine tools can probably handle slotmilling better than others. All the VFs I have operated have shuddered and shaked quite a lot, even when just winding the handwheel at that insane interpolation factor of 1mm a click...

DP