Cutting aluminium can be daunting but it does not have to be! Increase your confidence with a few tips I have gathered over the years of which I will share them here.
This guide is suitable for CNC machines ranging from hobby to industrial as the principles are the same. You will need to adjust your settings to suit your machines capability and I will try and address them accordingly. The principles apply to CNC router machines for cutting/slotting applications and basic profiling. CNC milling has its own nuances of which the concept is the same except that you will be reliant on appropriate tool-paths, coolant and 2 or 3 flute end mills.
Safety first and there should be no exceptions so wear your approved safety glasses as a minimum. A router bit that gums up could potentially start shooting aluminium chunks and/or parts of the router bit itself in a matter of milliseconds.
Before you start each cut:
- Inspect the condition of your router bit for
chips and aluminium reweld
- A chipped or gummed end mill will not cut well
- Check that your wasteboard is levelled
- You might be unintentionally cutting too deep on the first pass
- Ensure your material is secured to your
- Vibration will generate unnecessary heat and noise
Check out FSWizard for feeds and speeds. It is a handy smartphone app and online web calculator that has a lot of capability, for free! A stock X-Carve for example will not have the rigidity needed to take full advantage of a 1 flute end mill for aluminium’s recommended feed and speed but a CNC with a >1.5kw motor will take it well beyond.
Tip #1 - What aluminium are you using?
Not all aluminium should be treated equally. The first question I ask when someone calls about cutting aluminium is "What aluminium are you using?". This is usually because the grade of aluminium used causes the majority of issues they are experiencing. It is true that all aluminium grades are machinable but each grade will require a different approach.
The going rate is... the lower the thermal conductive of the grade, the lower the temperature generated when cutting. If you want to get all nerdy, read this paper. There are two types of grades that are commonly found at your local aluminium merchant for sheet between 1 and 6mm.
5005 has a thermal conductivity value of 201 W / m. °C. From experience this grade of aluminium will gum almost instantly if cut dry (without coolant) due to excessive heat. This grade is common for 1-6mm aluminium and is called aluminium sheet. Do not despair if you have bought yourself a sheet of 5005 as there are ways to cut without hassle. Refer to "the exceptions" in tip 3 regarding use of coolant to keep the heat down while cutting. A notable mention is 3003, also known as propeller plate which has the same thermal conductivity as 5005 at 192 W / m. °C.
5083 has a thermal conductivity value of 122 W / m. °C and makes for a much better cutting experience than 5005. The difference in cutting is like day and night due to the low heat generated while cutting this grade. At the local aluminium merchant, they will call this aluminium plate.
6061 has a thermal conductivity value of 166 W / m. °C and is also called aluminium plate. 6061 sits in between 5005 and 5083 from the perspecive of work-ability.
Tip #2 – Use a single flute spiral up cut CNC router bit (for aluminium)
Material gumminess is the main hurdle to overcome when cutting aluminium. Aluminium is a soft metal (HRB 50-60) with a very low melting point of 660c verses a material like stainless steel HRB 70-80 and 1400c. This makes aluminium relatively easy to work with an inherent problem of producing “gum” which is when aluminium rewelds back onto the router bit, rendering it useless.
Single or 1 flute router bit for aluminium are made from larger carbide grains with a lower cobalt content. A combination of these factors gives you a super hard router bit with a single sharp tooth shaped like a velociraptor’s nail. This has the advantage of slicing away chunks with maximum chip evacuation while generating the least amount of heat. In fact, what a 1 flute is doing is taking the heat out of the cut in the more efficient manner. You can still use a 2 or 3 flute carbide CNC router bit to profile/slot cut but you are limited to smaller diameters of 4mm and less to keep heat down.
End mill coatings are counterproductive when cutting aluminium mainly because you want the router bit as sharp as possible. Also, coatings like TiAlN which are typically classed as HRC45 has aluminium itself (Titanium Aluminium Nitrite) included which increases the chances of your material sticking to your router bit.
You should get a lot of cutting out of a good quality 1 flute up spiral for aluminium. Check your bit after every cut for chips and material reweld. I have only ever replaced my 3.175mm 1 flute for aluminium when I have hit a screw, snapped when my material came loose or another action through my stupidity.
Tip #3 – Coolants are not necessary.... but
Ok so after you have put your pitchforks away hear me out! Generally speaking, the use of coolants is not necessary when slotting if you are cutting within the router bit's parameters and a grade like 5083. Using coolants can create a lot of mess, can potentially warp your MDF wasteboard and adds cost especially for low volume work. The amount of heat that you will be generating is negligible and the way to check is using the rule of thumb is “if you can place your hand on the aluminium right after cutting and it is bearable, you are doing it right”. Of course, please do this with the spindle off and not moving.
This is only true for aluminium slotting and is not the same for engraving, 3D work and milling where coolant IS a necessity to keep heat down to provide a nice finish. Same goes for 5005 aluminium sheet where coolant is a must to stop the aluminium gumming.
If you are generating too much heat or getting a burr have a look at the following factors:
- Settings – feedrate, RPM, step over and depth per cut. Avoid excess rubbing and too aggressive cuts.
- Choice of CNC router bit – coating, tool condition and flute count. 1 flute for aluminium in a small diameter of <6mm is ideal
- Toolpath – reduce concentration of work in a close proximity. This includes drilling holes next to each other.
If you ever do want to use a coolant and do not have a mist system, a suggestion to use is Isopropyl Alcohol. Isopropyl is actually the best in my opinion to use for tapping aluminium and the same goes for cutting. It does not leave a residue, flashes at 80c, cleans your hands, can be used as an antiseptic for cuts and great for cleaning windows. I buy it by the 5ltr bottle and decant into a spray bottle.
Tip #4 – Use appropriate hold down methods and tabs
A lot of force is generated when cutting aluminium so you need to use appropriate hold down methods and tabs to secure your material to your wasteboard. Clamps and screws are some of the better options while super glue + masking tape has its time and place. Securing around the parameter might not always suffice as the internal stresses will have a tendency to warp your material over medium distances. So if you are planning on using tabs, be over cautious and make them high so that if your material does warp, it will account for the new height.
Also, aluminium likes to be cut with maximum rigidity as any vibration will increase heat and therefore degrade your finish. I have found that the use of a vacuum bed can cause issues with vibration and the tell tale sign is the loud noise generated when cutting.
Compared to cutting timbers, cutting aluminium should be surprisingly much quieter. If your cutting operation is generating unbearable noise (without a dust vacuum on), it’s a quick way to determine that something isn’t right. This is especially true if you have previously cut with the same settings before with no issues. Check your end mill condition for chips or material reweld.
Tip #5 – Choose the right direction of cutting
Last of all, the direction of cutting makes a huge difference to your finish. Conventional cutting/counter clockwise should be used for cutting out holes so as to leave a nice finish on the hole, not the material you just cut out. For example you would leave a great finish on the inside of a cut out letter “O”. Climb cutting/ clock wise should ideally be used for cutting out objects like the outside of a letter “O”
Shallow passes will give you a much nicer finish than aggressive passes. This is due to the sheer amount of force generated when cutting. If you cannot get away from using the correct cut direction, you can still get an excellent finish by using a coolant like isopropyl alcohol.
So hopefully your confidence in cutting aluminium has increased and you are ready to do some yourself. Any feedback on this article is welcomed and you can email me at email@example.com