Newbie to CNC aluminum milling feed/speed/depth/coolant, etc.

[mrk]

Hello,

I am new to machining and trying to get my brain around cutting feeds/speed/depth calculations. My interest in CNC is mostly at a hobby level and I have no formal training.

I am working with aluminum alloys and plan to build parts for inline skate racing and later on do some stuff for bicycle racing.

I am considering using a roughing tool for my first operation. The following is an example tool from Cleveland Twist Drill. I plan to use this tool to for the initial roughing of the part from 1.5" billet.

0.75” Roughing EM (http://tinyurl.com/ve7kw)

Wikipedia lists the cutting speed for aluminum as 250-350 fpm (http://tinyurl.com/yxlsd9) this is a big range and I am hoping that someone has an idea of the speeds for the 6061, 7075, and mabey Fortal alloys. I am guessing that since these are harder the will be in the 250-275 range but that is just a guess.

Most of the work of the roughing tool will be side milling. My CNC is an Acroloc with 5hp spindle motor and up to 3500 rpm range. I have a flood coolant system and I plan to purchase some coolant soon but I have not decided what kind.

For feed/speed/depth I am using an excel spread sheet with the following calculation.

RPM = SPEED / ( (TOOLSIZE/12)*PI() )
FEED = RPM * NUM_FLUTES * CHIPLOAD
DEPTH = TOOLSIZE/2

The above calculations result in rpm/feed/depth of 1324rpm, .375 deep, and a feed of 19.86. I understand that these numbers are starting points for the operation so I am looking for some ideas from people with experience.

The J&L catalog says that the Cleveland Twist Drill roughing end mill “Doubles...triples feed rates of conventional high speed tools” does this mean that I can safely run at 2xFEED (i.e. 40ipm) while keeping the rpm and depth of cut the same? 40ipm seems like it would be way to much so I am skeptical about the claim that this tool can double feeds vs. convential HSS.

Also, since I have not yet purchased coolant for my machine and am looking for a recommendation for someone working mostly with 6061 and some 7075.

Does anyone have additional ideas or thoughts about speeds/feeds/tooling/coolant for aluminum alloys?

Thanks in advance!

[fpworks]

mrk,
It sounds like you have the basics licked, but I hope I can add a little more from our experience:

*In regards to aluminum 6061 & 7075:

Tooling: Drills: I love German HSS drills such as Guhring and Titex, even for production. For endmills, we use carbide only. They cost more, but they pay for themselves quickly since the surface speed is nearly unlimited and the tool is much more rigid. ZrN coating has proven extremely wear resistant.

Speeds: Like I said, use carbide endmills and spin it at your max rpm.

Feedrates: Most tool manufacturers severely underrate the cutting parameters for aluminum (they typically give finishing feedrates). For example, I think you should triple your feedrate in your above example (with 1324rpm and a 0.75" tool). However, I think you should use a carbide endmill and run it at [max] 3500 rpm and 150+ inches/minute. Keep an eye on your spindle load...you might run out of horsies depending on your stepover. Bear with me...I'm not familiar with your machine, so watch out that your controller doesn't try to cut corners going that fast.

Don't let me [or anyone else] talk you into something that you're uncomfortable with. I suggest that you work your way up to those feedrates. (better make sure your workholding is good!) We typically take lighter and slower cuts on secondary operations to avoid distorting/shifting/throwing the workpiece since the workholding is usually more gentle on secondary operations.

Coolant: We use Qualichem Q-cool 361. It is not cheap. ($30/gallon for concentrate) We run it at 5-7%. It smells great, it won't sour, it doesn't foam up, our machines and tooling don't rust, and we get great tool life and part finishes. We couldn't ask for more.

Justin

[GeorgeRudd]

How important is it to use a coolant for cutting aluminum on a CNC machine?

Where can I purchase a cooling system for our CNC?

A friend of mine who has a metal shop told me that it's not necessary to use a cooling system to cut aluminum. He says that aluminum cuts cooler than MDF. He has conventional lathes and mills so I am not sure that he knows what he is talking about when it comes to cutting on a CNC machine with a high-speed router.

Your advise?

We do not currently have any cooling system on our CNC. We have been cutting primarily plastic, plywood and MDF but we have a customer that wants us to cut some 1/4" aluminum sheets for them.

• Here are our table specs
• 5hp spindle that can spin at 21,000 rpm's.
• Our table can run at approximately 500 ipm

[Scanfab]

Go to CVS/wallgreens and get a large bottle of 90% iso-propel alcohol. Take a small drill, heat the back end with a lighter and melt it through the cap. Then use this peeing bottle to cool your tools when machining alu. It is clear and evaporates quickly with a blast of air-and makes the shop smell like a vodka plant. The fast evaporation is what makes it a good lube for alu, best for tappn and reaming. I do all my dry runs with it...

[Scanfab]

The rubbing alcohol will produce in alui materials better finish quality and dimension than with water based coolants. Just dont cheap out and buy the less expensive 70% alcohol, it will end up leaving rust spots on your machine table and vices-unless you hold stock in 3M scotch-brite producer.