why wouldn't i use max machine rpm? SFM questions. 6061

[pld0vr]

Hey guys... I just upgraded to a HAAS mill, and I have some speeds and feeds questions.

Here is the scoop... I machine aluminum and have a 6000rpm spindle. I've been researching some aluminum specific 3 flute endmills from Swift Carb and they have some pretty nice stuff for my application.

I've been trying to learn how to best maximize the feeds/speeds on my machine, using calculations. I'd like to check my findings with you guys just to make sure I am doing this right... my previous machine only had a 3hp spindle so that was the major restriction before.

Here are the specs on the aluminum rougher I want to use... it's a 3flute aluminum hogger (serrated).

Size Chip per tooth SFM
Slotting Heavy Light Low Medium High
1/2” 0.5000 0.007 0.0095 0.0120 650 1200 3000

So lets say I am taking a heavy cut, it says CPT 0.0095. Pretty good rate... plug this into my cam software and this is what I get at 650sfm

4583rpm
130in/min

So lets say I take 50% tool width cut, 0.25", and full tool width depth

Using a calculator that I made here..
http://brassnutinc.com/tools/spindlepowercalc.xlsx

That works out to a metal removal rate of 16.25, requiring approximately 4hp at the spindle.

Now that all looks fine, but here is where I get confused. I have a 7hp spindle, and I have room for more rpm. If I set it to 6000rpm (machine max),
I get 170"/min with a MRR of 21.25 and a power requirement of 5.31hp... still well below the 7hp max on the spindle.

Is there any reason why you would not want to run at the max rpm you could so long as you are within the spindle horsepower capabilities?

Now here is the next question... I still have a little more hp available... would you consider taking a deeper cut etc, or is the theory of no more than 50% of tool width and 1x too width depth set in stone?

Now lets talk about roughing vs finishing...

They make a 3flute rougher/finisher ball nose endmill. And by the way the SFM recommendations on the right confuse me.. if they recommended 450sfm, but then also 2300sfm, surely anything in-between is fair game?

Size Chip per tooth SFM
Slotting Heavy Light Low Medium High
1/2” 0.5000 0.006 0.009 0.0120 450 920 2300

So for roughing I use heavy and get:

3437rpm
92" /min / 2.875 hp at these settings, 450sfm

Now for finishing... I use the "light" rating?

3437rpm
123in/min / hp is tiny, small cut.


Does that sound about right? And then same question applies... I could up the rpm calc on both of those... but should I? Effects on surface finish when finishing? 6061 aluminum


Now let's carry this on to harder materials like steel. I don't have the HP to make a cut at those speeds obviously... so if I am correct it is best to reduce the size of my cut and keep the feed rate high, rather than reduce the feed rate/rpm, which could still be called HSM even though I don't have a 30k rpm spindle... so even here I can use 6000rpm. So basic question... why would you not use max rpm all the time as long as you can keep up with the IPM required?

Thanks guys. This is a bit green to me but I'd like to master it.

[Geof]

So basic question... why would you not use max rpm all the time as long as you can keep up with the IPM required?

That is indeed the basic question with a MiniMill and the answer is that you can use maximum rpm provided you are not getting too much spindle load.

I don't approach it using calculations for metal removal rate I normally play with the over-rides and then adjust things to get the best removal rate.

Frequently with these low rpm, low power spindles on both the Mini and the TM machines I have found it is possible to remove metal faster by dropping the rpm as this improves motor torque.

Here are a couple of threads I started related to what you are talking about.


http://www.cnczone.com/forums/showthread.php?t=52366

http://www.cnczone.com/forums/showthread.php?t=60698

EDIT

I had skimmed over your mention of steel; again your reasoning is correct. Here is another thread.

http://www.cnczone.com/forums/showthread.php?t=73902