Milling performance using slower RPMs

[RSG]

After asking a question a while back pertaining to optimal machine settings for a 1.25" dia end mill cutter I found the results quite interesting as it performed flawless. So the other day I was working with a 3/4" ball nose end mill that was producing allot of chatter, so I reduced the RPM from 3270 to 2100. It took care of the chatter nicely. Since it's an effort to change the speed on my machine I opted to continue using that RPM and try the smaller cutters that were required on the job at that same speed. I was surprised to see that even a 3/16" two flute E/M benefited from the RPM reduction. Instead of sounding and feeling like it was hogging through the aluminum it cut smooth and had a nice soft sound.

Can anyone explain why the smaller cutters performed so well using the lower RPM? I've always been told to increased RPM for smaller dia E/Ms. The only area that didn't perform well was with a 3/16" ball nose as it wanted to stutter and made lots of noise if you pushed it's feed rate too fast.


Thanks for your time
RG

[Mr Ron]

I think the answer to your question is "FEED" rate. Regardless of the rpm, the feed rate will determine how well or smooth the cut will be. High speeds are used to speed up a machining task. If you are not in a big hurry, lower speeds will work fine. This is what I believe, but maybe Glenn can give a better explanation.

[Rick]

I believe your speeds could be too fast to start with depending on your machine. The rigidity of the machine, tool holder and tool and the feed rate has a tremendous effect on chatter. What type of mill are you using? The point I am trying to make is that spindle speed is just one of many variables that are a part of chatter.
The speeds you listed for the 3/4 ball mill seem fast for a knee mill I doubt you would have the horsepower to feed the cutter at the rate it needed to go for that spindle speed but for a CNC machining center maybe not. The speed charts you find on the internet can be all over the place, they should be used only for a reference/starting point as each setup will have its own set of peculiarities.

[Harold_V]

The use of a ball nosed cutter, doesn't really matter the size, is always going to be troublesome in that it cuts to the center, so surface speed is not a constant. In such a case, the faster, the better, assuming one remains within reasonable bounds. For cutting aluminum, speed is relatively forgiving, but you can run too fast (with larger cutters) because, as has already been suggested, feed rate may not be acceptable.

Any time you operate a cutter such that it doesn't generate a respectable chip, you can expect premature failure, as the rubbing action causes undue wear at the cutting edge, which is rapidly rounded. Once that occurs, there's a cascading effect, with accelerated edge failure. That's why one is admonished to not run slightly dull cutters, as it may take but a thou per side to restore an edge to proper performance, while prolonged used may then require far more, drastically reducing the tool's useful life. That, of course, is reliant on the idea that one sharpens cutters.

Running end mills at reduced velocity will almost always lower the incidence of chatter, and they'll generally perform perfectly well, although you may find that the sweet spot is faster than your chosen speed. Cutters that are operated too slowly often create undesirable conditions, although they don't manifest themselves unless heavy cuts are taken.

When in doubt, start slower than you might suspect is required. Too slow is safer in that you won't burn a cutter, although, with small diameters, they are subject to breakage. It would be unusual for a home shop type machine to offer enough spindle speed for small end mills (<¼") when machining aluminum.

Harold

[RSG]

I believe your speeds could be too fast to start with depending on your machine. The rigidity of the machine, tool holder and tool and the feed rate has a tremendous effect on chatter. What type of mill are you using?

I have the Canadian version of this unit http://www.grizzly.com/products/8-x-30- ... Mill/G0730

I understand feed rate vs RPM and that's why I asked the question. It appeared as though the feed rate didn't change even after the reduction in speed. In fact it was easier to move at the same speed without the hard hogging sound and feel the end mill was making when it was set at the higher speed. Also as mentioned I noted that some end mills didn't perform as well at the slower speeds but some did and that's what got my curiosity.

[Glenn Brooks]

Don't work with aluminum much, so Iam no expert here. But my old instructor in lathe class used to say: if it's making noise, your going to fast - or the tooling is dull. Quite often it's both I think... Could be the different condition of each end mill accounted for the variability you are seeing in performance of the smaller bits.