NewsDate: 09-01-2018 by: Mr. Dũng
10 TIPS FOR CNC ROUTER ALUMINUM CUTTING SUCCESS (PART 1)
By Bob Warfield
Folks often ask whether a CNC Router can cut aluminum. They’re used to seeing the primarily cut wood and plastics. My answer to this question is always, “Yes, if you do it right.”
There are a couple of things to remember about how aluminum (and other metals) are different from wood or plastics. First, they have a much smaller “sweet spot” for optimal feeds and speeds. If you leave the sweet spot, cutters start breaking, wearing out a lot faster, and surface finish is poor at best. In fact, there are several sweet spots depending on what you want to accomplish:
The second thing is that for aluminum (and some other metals), there is a “stickiness” factor. Aluminum wants to stick to the tool. In fact, it will do so to the point that it welds itself to the tool. Once you have gummy aluminum deposits on your cutting edges, that tool is not long for this world, especially not at 20,000 rpm or more.
Despite these challenges, you can cut aluminum very successfully on almost any router. Here are 10 tips for CNC Router Aluminum Cutting Success:
1. Don’t be in a hurry
A CNC Router can cut aluminum, but it isn’t the ideal tool for hogging out big aerospace parts like wing spars. The price you’ll pay for success is slowing things down. Note that I don’t mean to literally slow down your feeds and speeds, but your overall Material Removal Rates will be less than what can be achieved with a purpose-built CNC mill. So relax and let the machine do its thing. At the very least, a good sized CNC Router can fit a lot more material on its table than most any CNC mill. Load it up, press the green button, and walk away.
2. Use a Feeds and Speeds Calculator
Look, you’re going to approaching the limits of what your machine can do in all likelihood. Cutting aluminum on a CNC Router is not a cakewalk, so let’s do it right. None of this “cutting by ear” the old timers so love to talk about. The ear can’t keep up fast enough as your machine skates around corners and through pockets. One minute things are fine, the next you’re dodging the tip of the cutter that got broken off and flung across the shop. All CNC’ers can benefit from a Feeds and Speeds Calculator, but when you’re near the edge of the performance envelope, you want to be particularly careful.
Make sure the one you get has the right features for CNC Routers. Very important features for CNC Routers that we include with G-Wizard Calculator include:
– Minimum rpm setting. The Calculator doesn’t help if it keeps telling you to go slower than you possibly can.
– CNC Router Cutter Types: V-Bits, compression bits, and downcut bits are all important for CNC Router users. Make sure your new calculator handles them like G-Wizard does.
– Deflection: Tool deflection is a fact of life and accounts for a lot of broken tools. Make sure your calculator will figure out the deflection and that it has capabilities to help find solutions that avoid excessive deflection.
– Rubbing Warning: If you slow down feedrates too much, your cutter quits slicing off nice clean chips and starts to plough along on the surface. This is called “rubbing” and really reduces tool life due to the heat it generates. Get a calculator that includes a rubbing warning.
– Chip Thinning: When you take light cuts whose width is less than half the diameter of the cutter, you get chip thinning. Your calculator needs to compensate for that or you’ll wear out the tools prematurely.
– Ability to derate horsepower for less rigid machines: See #10 below for more. It’s also nice if the calculator has multiple machine profiles so you can easily switch between full rating and derated profiles as needed.
Once you’ve got a calculator, your first problem will be dealing with the recommended rpms being too low. One of the issues for most CNC Routers is the spindle goes fast compared to a lot of CNC mills. Your average new CNC mill maxes out at 10,000 rpm and many CNC Routers can’t go that slow. Life for them begins at circa 20,000 rpm. The next couple of tips focus on solutions for this problem.
3. Use carbide coated cutters
One way to bump up the recommended rpm is to be sure you’re using cutters that are happy going that fast. The measurement that determines this is called Surface Speed (for more on this and many other feeds and speeds hints and tips, check out our Feeds and Speeds Cookbook). Carbide cutters can go much faster than HSS cutters. Forget HSS and Cobalt for the most part. A coating, such as TiAlN allows the cutter to go even faster. Shop for carbide TiAlN coated cutters. They cost a little more, but they can change your results so much it’s darned well worth it.
For example, say I need to cut a slot using a 1/4″ endmill. If I select an HSS Endmill, G-Wizard tells me it wants to run 5877 rpm and my 20,000 rpm router spindle won’t go that slow. So I switch to a TiAlN Carbide Endmill. Now the recommendation is 16897 rpm–we’re much closer. This is with a Surface Speed of 1106 SFM. You may be able to find a more aggressive SFM recommendation for your manufacturer’s tooling. With aluminum, I’d go ahead and try 20,000 rpm for this cut. It’ll probably be just fine.
4. Use smaller diameter cutters
The other way to bump up the rpms is to use smaller diameter cutters. Forget about 1/2″ endmills. Drop down to 1/4″ maximum and typically less. Because you’re going to smaller diameters, you want more rigid cutters lest tool deflection starts to be a problem–remember, you need a Feeds and Speeds Calculator that deals with tool deflection. Carbide is much more rigid than HSS, so this is one more reason to favor carbide.
Looking at our example in #3 of the carbide cutters, suppose that instead of a 1/4″ endmill, we are using a 3/16″. That seemingly small change has now kicked up the recommended rpm to 21241–very close to our 20000 rpm spindle. It’s easy for us to slow that down to 20K rpm and pick up a little extra tool life.
The moral of the story is to carefully match your tooling to the capabilities of your machine.
5. Be paranoid about clearing chips
Recutting chips breaks more cutters than most any other thing I see happening. Be paranoid about clearing the chips. Don’t count on a nearby vacuum dust collection system unless you have personally verified it sucks the chips out of even the deepest cuts. More reliable is an air blast fixed to the spindle and pointing right at where the cutter meets the material being cut. If you’re standing there, nozzle in hand (or worse a brush) thinking you can keep things clear, you’re not paranoid enough about clearing chips.
(To be continued)