NewsDate: 09-01-2018 by: Mr. Dũng
10 TIPS FOR CNC ROUTER ALUMINUM CUTTING SUCCESS (PART 2)
In the previous article, the first 5 tips for an effective aluminum parts machining was introduced. With the entire 5 tips, you will be able to manage your work with CNC Router.
6. Watch cut depths and slotting–they make it harder to clear chips
The deeper you cut and the closer to a slot the cutter travels in, the harder it is to clear the chips out of the bottom of the hole. Make more passes to cut down to required depth and to open up the shallower depths for better access.
7. Lubricate with a Mist
Assuming you’re suitably paranoid about those chips, the next issue is providing lubrication to cut down on the tendency for the chips to stick to the cutting edges. You pretty much have to use some kind of lubricant. Since you’ve presumably already rigged up a compressed air blast, you may as well run coolant mist through the same mechanism. In fact, buy a mister to provide air blast and coolant mist. It’s easy and inexpensive.
Sometimes, it just isn’t possible to use mist but you still need to cut some aluminum. If you’re cutting very thin aluminum, or taking very shallow passes, you may be able to get by without lubrication. Do some tests and see.
8. Don’t slow down the feedrate too much!
If you go too slow on your feedrate, you run the risk of making your tool rub rather than cutting. This is a much bigger risk for CNC Router users than mill users simply because the spindle is going so fast. In order to maintain recommended chiploads with rpms that high you’ll have to keep the cutting moving smartly. Our 3/16″ cutter at 21K rpm wants to feed at 91 IPM, for example. If you slow down too much, say to 1/4 of that, many will think they’re babying the machine and tool. Nothing could be further from the truth. If you wind up going slow enough that the cutter starts rubbing at 20K rpm, you’re going to heat up the whole works and drastically shorten your tool life. For more on this rubbing phenomenon, see our article on chiploads and surface speeds.
Being on top of rubbing problems is easy when you use a feeds and speeds calculator like G-Wizard that warns you about rubbing.
9. If your machine can’t feed fast enough, use fewer flutes and increase cut width
Normally, we use 3 or fewer flutes with aluminum anyway–don’t try a four or more flute cutter in aluminum! The reason is that aluminum produces especially large chips. The fewer the flutes, the more space between the cutting edges, and the more room for the big chips to escape and be blown away. With too many flutes, the chips back in too tightly, jam up the flutes, and pretty soon you have a broken cutter. Let’s suppose you are using your feeds and speeds calculator, and you come up with a situation where your machine just can’t move the cutter fast enough. For example, taking our 3/16″ example at 21K rpm, let’s say we’re cutting an 0.040″ wide cut. G-Wizard suggests feeding a 3 flute endmill at 166 inches per minute, but your CNC Router can only cut accurately and reliably at 100 IPM. What to do?
The answer is to try fewer flutes. A 2 flute cutter only needs a feedrate of 110 IPM. Slowing that down to 100 IPM is not going to run a rubbing risk–it’s only 10% slower.
BTW, we’ve been talking about cutting aluminum, but you can hit this problem even worse with wood because you can cut the softer material so much faster. Plug in these values and select Hardwood in G-Wizard and it wants to go 883 IPM at 20,000 rpm!
Here’s a tip: they make 1 flute cutters for precisely this reason.
If we take the scenario down to a single flute at 20000 rpm GW now recommends 294 IPM. If you’re burning the wood, it’s probably because you’re feeding too slowly and the cutter is rubbing. BTW, I love watching a fast moving industrial CNC Router blasting through wood and shooting up a blizzard of chips and dust. Cool beans!
The other thing to be aware of is what’s called “Radial Chip Thinning“. If your cut width is less than 1/2 the cutter diameter, you need to speed up your feedrate because your machine is producing unnaturally thin chips due to Radial Chip Thinning. Here again, you think that by taking super thin cuts and slowing the feedrate down drastically. Instead, because of radial chip thinning and rubbing, you’re drastically reducing your cutter life. The G-Wizard Feeds and Speeds Calculator automatically factors in radial chip thinning to its calculations.
10. Use a Horsepower limit to derate for rigidity
Okay, you’ve mastered the other 9 tips, and thinks are going well, but you’re now running up against the rigidity limits of your machine. If you plow in with full power, bad things happen. The machine chatters and destroys the cutter, surface finish is lousy, or the machine deflects and cuts very inaccurately.
Cutting forces for metal are likely to be much higher than for wood and CNC Routers (sometimes called Gantry Mills) are considerably less rigid than equivalent CNC Mills. This is just a fact of life. If nothing else, compare the work envelope of the mill (much lower than a router) and it’s weight (much higher than a router) against a CNC Router. Except for the biggest industrial Gantry Mills, there is no comparison. And because of that, no way that machine is as rigid as a CNC Mill. So, we have to compensate.
We don’t know the exact rigidity of a given machine. There’s not a published spec we can use to compare or calculate from. But, we can use spindle power as a proxy. It is that power “pushing” against the workpiece while cutting, that the rigidity must fight. G-Wizard has the ability to calculate a “de-rated” spindle power that matches the work envelope and weight of your machine to a spindle power that is appropriate for that level of rigidity. The results may surprise you, but they’re based on real empirical measurements.
For example, suppose you have a 4′ x 8′ router with 20″ of Z travel that weighs 1000 lbs. Note that even a fairly lightweight commercial CNC mill, like a Haas TM-1, will have travels of 30″ x 12″ x 16″ and a total weight of 3240 lbs–a much smaller envelope and a lot more weight. To perform at this kind of level of rigidity (and a TM-1 is not exactly the pinnacle of rigidity either) requires derating horsepower to 0.17 HP.
Derating will take our numbers way down–22K rpm and 79 IPM for the full slot with a 3/16″ inch and a 2 flute. But, we’ll get the job done with better surface finish, accuracy, and less tendency to deflect the machine frame or chatter.
Don’t run derated all the time, keep a machine profile that is derated and one that is not. Use the derated one for finer surface finish or for cases where the cutter keeps breaking.
Machining aluminum with a CNC Router is absolutely doable with most any router. It’s just a matter of matching your machine’s capabilities to the “sweet spot” feeds and speeds requirements of the material through wise selection of tooling and cutting parameters. A good feeds and speeds calculator like G-Wizard can help you do that. Add to that the need for lubrication and being paranoid about chips piling up and you’re ready to tackle an aluminum project.