How do you cut Aluminium with a router?

Once you understand the secrets, you will find that machining aluminium with the CNC Router is not only easy, but also very efficient. Machining aluminium parts with a CNC Router can be very successful, with just a little care and preparation.

There are a few things to remember about how aluminium (and other metals) differ from wood or plastic. Firstly, they have a much smaller 'sweet spot' for optimum feed and speed. If you leave the sweet spot, tools start to break, they wear faster and the surface finish is poor at best. In fact, there are several sweet spots depending on what you want to accomplish.

Optimal position for cutting aluminium in a cnc router

The sweet spot for metals is much smaller (narrower range of acceptable feeds and speeds) than for wood or plastic ......

The second thing is that for aluminium (and some other metals) there is a 'stickiness' factor. The aluminium has to stick to the tool. In fact, it will weld itself to the tool. Once you have a gummy aluminium deposit on the cutting edge, the tool is not fit for the world, especially at 20,000 rpm or higher.

Despite these challenges, you can cut aluminium very successfully on almost any router. Here are 10 tips for successful aluminium cutting on CNC routers.

1. Don't rush

The CNC Router can cut aluminium, but it is not the ideal tool for gouging out large aerospace parts such as wing beams. The price you pay for success is slowing down. Note that I'm not trying to literally slow down your feeds and speeds, but your overall material removal rate will be lower than what a dedicated CNC Router can achieve. So relax and let the machine do its thing. At the very least, a properly sized CNC router can hold more material on its table than most CNC milling machines. Load it up, press the green button and walk away.

2. Use the feed and speed calculator

Look, you are likely to be close to the limit of what the machine can do. Cutting aluminium on a CNC router is no easy task, so let's do it right. These "out of earshot" old-timers don't like to talk about it. When your machine is sliding around corners and in pockets, your ears can't keep up with the speed. One minute it's fine, the next you're dodging the tip of a knife that's been snapped and thrown across the shop. All CNC people can benefit from a feed and speed calculator, but you need to be especially careful when you are approaching the limits of performance. Of course, we recommend our own G-Wizard feed and speed calculator. Of course there are others, but we are the world's first feed and speed calculator designed specifically to meet the needs of CNC Router users (click on this link to see why).

Make sure you get the right product with the right features for your CNC Router. Very important features for CNC Routers that we have included in the G-Wizard Calculator include

- Minimum speed setting. If the calculator keeps telling you to go slower than you possibly can, it won't help.

- CNC cutter types: V-heads, compression heads and undercut heads are all important to CNC cutter users. Make sure your new calculator handles them as well as G-Wizard.

- Deflection: Tool deflection is a fact of life and can lead to many damaged tools. Make sure your calculator is able to calculate deflection and that it has features such as our Cutting Optimiser and CADCAM Wizard to help find solutions to avoid excessive deflection.

- Friction warning: If you slow down the feed rate too much, your tool will stop cutting nice clean chips and start ploughing through the surface. This is called "friction" and can really shorten tool life due to the heat it generates. Get a calculator with friction warnings.

- Chip thinning: When you make a light cut that is less than half the width of the tool diameter, the chip becomes thinner. Your calculator needs to compensate for this or you will wear the tool prematurely.

- Ability to reduce the horsepower of less rigid machines: For more information, see #10 below. It's also good if the calculator has multiple machine profiles so you can easily switch between full rated and derated profiles as needed.

Once you have the calculator, your first problem will be dealing with the recommended RPM being too low. One of the problems with most CNC routers is that the spindle runs fast compared to many CNC milling machines. Your new CNC milling machine has an average maximum speed of 10,000 rpm, whereas many CNC routers cannot run that slow. Their life span starts at around 20,000 rpm. The next few tips focus on the solution to this problem. 3.

Using CNC Router Bits to Cut Aluminium (Carbide End Mills)

CNC routers use a variety of special tools that should not be used with aluminium. Undercutting spirals, compression tools, etc. have no place in aluminium.

You need tools made specifically for aluminium. Most of the CNC world uses 2 or 3 flute carbide end mills.

This helps to increase the recommended speed to ensure your tool is happy at the high speeds at which the CNC milling spindle runs. The measurement to determine this is called the surface speed (for more information on this and many other feed and speed tips and tricks, see our feed and speed recipes). Cemented carbide tools can be much faster than HSS tools. Forget about HSS and cobalt tools for aluminium.

For example, let's say I need to cut a slot using a 1/4" end mill. If I choose an HSS end mill, G-Wizard will tell me it wants to run at 5877 rpm, and my 20,000 rpm router spindle won't be that slow. So I switched to a carbide end mill. The recommendation is now 17,419 rpm - we're much closer. This is with a surface speed of 1140 SFM. You can find more aggressive SFM recommendations for the manufacturer's tools. 4.

4. Use smaller diameter tools

Another way to increase the speed is to use a smaller diameter tool. Forget about 1/2" end mills. Drop down to a maximum of 1/4 inch, usually less. Because you're machining smaller diameters, you'll need stronger tools so that tool deflection doesn't become an issue - remember, you'll need a feed and speed calculator to handle tool deflection. Cemented carbide is much harder than HSS, so this is another reason to favour it.

Take a look at our example in Carbide Tool #3 and assume we are using a 3/16" rather than a 1/4" end mill. This seemingly small change has now increased the recommended rpm to 23226 rpm - mainly in the router spindle area. We could easily reduce this to 20K rpm and extend tool life a little.

The moral of the story is to carefully match your tools to the capabilities of your machine.

5. Stay paranoid about clearing chips

I can't stress this point enough, especially if the material has an affinity for bonding to the tool.

Recutting chips more than anything else I have seen. Stay paranoid about removing chips. Don't count on the nearby vacuum dust collection system unless you have personally verified that it can suck chips out of even the deepest of cuts. Much more reliable is the air jet fixed to the spindle and pointed at the point where the tool meets the material being cut. If you stand there with a nozzle (or worse a brush) in your hand and think you can keep it clear, then you are not paranoid enough about cleaning the chip.

6. Look at depth of cut and slotting - they make chip removal more difficult

The deeper the cut and the closer to the slot the tool enters, the more difficult it is to remove chips from the bottom of the hole. Make more passes to reduce to the required depth and open up shallower depths for better entry.

Here's a hint. When using material with a thin aluminium sheet bonded to it, place the sheet with the side facing upwards. This makes it easier to remove the aluminium from the tool as quickly as possible.

7. Lubricate with a thin mist

cnc router aluminium cutting requires a lubricant such as mist or flood coolant

Assuming you are quite paranoid about these chips, the next issue is to provide lubrication to reduce the tendency for chips to stick to the cutting edge.

Many CNC Router users are reluctant to use any type of coolant, but you pretty much have to use some sort of lubricant to reliably cut anything other than the thinnest aluminium. As you may already have a compressed air jet set up, you can also run a coolant mist through the same mechanism. In fact, buy a sprayer to provide the blast and coolant mist. It is simple and cheap.

8. Don't feed too slowly!

This is a greater risk for CNC Router users than for milling users because the spindle runs at such a high speed. In order to maintain the recommended chip load at such high speeds, you have to keep the tool moving intelligently.

For example, our 3/16" cutter at 21K rpm wants to feed at 91 IPM. If you slow down too much, say 1/4, many people will think they are looking after the machine and the tool. Nothing could be farther from the truth. If you slow down enough so that the tool starts to rub at 20K rpm, you will heat up the entire workpiece and shorten tool life considerably. For more information on this friction phenomenon, see our article on chip loading and surface speed.

Solving friction problems is easy when you use a feed and speed calculator such as G-Wizard to warn you of friction. 9.

9. If your machine is not feeding fast enough, use fewer chip chutes and increase the cutting width

cnc router aluminium cutting works well with single edge end mills

Single Flute End Mills

Normally we use 3 or less aluminium flutes anyway - just don't try to use 4 or more aluminium flute cutters!

The reason for this is that aluminium produces particularly large chips. The less chip evacuation there is, the more space between the cutting edges and the more room there is for large chips to escape and be blown away. With too many recesses, the chips retract too tightly, clogging the recesses, and soon your tool breaks down.

Incidentally, we've been talking about cutting aluminium, but you can solve this problem more seriously with wood, as you can cut softer materials faster.

Another thing to look out for is the so-called "radial chip thinning". If your cutting width is less than 1/2 of the tool diameter, you need to speed up the feed rate because your machine will produce unnaturally thin chips due to radial chip thinning. Again, you think by making ultra-thin cuts and reducing the feed rate considerably. The G-Wizard feed and speed calculator automatically incorporates radial chip thinning into its calculations.

10. Use horsepower limits to reduce rigidity

OK, you've mastered the other 9 tips and everything is going well, but you're now facing the rigidity limits of your machine. If you put your best foot forward, bad things can happen. The machine chatters and damages the tool, the surface finish is poor, or the machine deflects and cuts very inaccurately.

Cutting forces in metal can be much higher than in wood, and CNC routers (sometimes called gantry mills) are far less rigid than the equivalent CNC milling machines. This is just a fact of life. If nothing else, compare the working range of a milling machine (far less than a router) and its weight (far more than a router) with that of a CNC router. Apart from the largest industrial gantry mills, there is no comparison. Because of this, this machine is not as robust as a CNC router. Therefore, we have to compensate.

We do not know the exact stiffness of the given machine. There are no published specifications that can be used for comparison or calculation. However, we can use the spindle power as a proxy. It is the force that "pushes" on the workpiece when cutting that rigidity must be countered. g-Wizard can calculate a "derated" spindle power to match the working range and weight of your machine to the spindle power that is appropriate for that level of rigidity. The results may surprise you, but they are based on real empirical measurements.

For example, suppose you have a 4' x 8' router with 20" of Z-axis travel and a weight of 1000 lbs. Note that even a fairly light commercial CNC milling machine, such as the Haas TM-1, has a travel of 30" x 12" x 16" and a total weight of 3240 lbs - a smaller housing and more weight. To perform at this level of rigidity (and the TM-1 is not the pinnacle of rigidity either) would require a horsepower reduction to 0.17 HP.

Derating would bring our numbers down - 22K rpm and 79 IPM for a full slot with 3/16" inch and 2 flutes. However, we will finish the job with better surface finish, accuracy and a tendency to deflect the machine frame or chatter less.

Do not run derating all the time, keep a copy of the derated and underated machine profile. For finer surface finishes or where the tool keeps breaking, use the derated.

11. CNC Router aluminium work is different from CNC Router cutting aluminium sheets

There is a world of difference between a CNC Router cutting thin aluminium plates and a CNC Router cutting thicker aluminium plates. The thicker the material and the greater the depth of cut, the more important it is to follow these tips.

Conclusion

Most routers can be machined from aluminium using a CNC router. Simply match the capabilities of your machine to the "best position" feed and speed requirements of the material by choosing your tools and cutting parameters wisely. Excellent feed and speed calculators such as our G-Wizard can help you do this. Add to this the need for lubrication and paranoia about chip stacking and you're ready to tackle aluminium projects.