Adam's Tips - Cutting Aluminium Sheet

Posted by Adam Malec with assistance from Garry Wilson on 29th May 2019

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 heavily reliant on coolant and 2 or 3 flute end mills.

Safety

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 wasteboard
    • 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 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.

Link to FSWizard

Tip #1 – 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 and 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 and generating the least amount of heat. You can still use a 2 or 3 flute carbide CNC router bit but you are limited to smaller diameters of 4mm and less to keep heat in down.

Here it is the link if you were wondering

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 included which increases the chances of your material sticking to your router bit.

Adam’s Tips

You should get a lot of cutting out of a good quality 1 flute up spiral for aluminium. Check your bit after every cutting operation 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 #2 – Don’t bother with coolants (with exceptions)

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 bits parameters. Using coolants creates a whole lot of mess, can potentially warp your MDF wasteboard and adds time and 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. Another way that you know your technique is not quite right is when you start to see a burr occurring at the top of holes or slots.

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.

If you are generating too much heat or getting a burr have a look at the following factors:

  • 1.Settings – feedrate, RPM, step over and depth per cut. Avoid excess rubbing and too aggressive cuts.
  • 2.Choice of CNC router bit – coating, tool condition and flute count. 1 flute for aluminium in a small diameter of <6mm is ideal
  • 3.Toolpath – reduce concentration of work in a close proximity. This includes drilling holes next to each other.

Adam’s Tips

If you ever do want to use a coolant and do not have a mist system, I suggest and use Isopropyl Alcohol. Isopropyl is actually the best 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 buy the 5ltr bottle and decant into a spray bottle.

Tip #3 – 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 give them some height 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 causes a lot of issues with vibration and therefore increases the noise generated when cutting.

Adam’s Tips

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 just fine. Check your end mill condition for chips or material reweld and your material to see if it is recurred correctly.

Tip #4 – 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”

Adam’s Tips

Shallow passes will give you a much nicer finish than aggressive passes. This is due to the sheer amount of force generated when cutting.

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 adam@endmill.com.au