A local sign shop has asked me if I could cut aluminum sign board for them on my cnc. The material is 2 sheets of thin aluminum laminated over a poly foam core, about an 1/8" thick overall. It seems to hold down well under vacuum on my spoilboard. I was thinking a 1/4" downcut spiral would work, but I'm open to suggestions from more experienced operators. Also, I'm not sure about what feeds and speeds would be recommended. Any advice would be appreciated.

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I cut this with a down shear 1/4" O flute for aluminum. Be careful of chip reweld due to the down shear. I a final pass step over of .015" and get a clean edge. As the bit dulls you will get a small burr on the lower edge.

Tom - what do you recommend for feed rate and rpm?

150 ipm at 18000 gives a chipload of .008". It works for me. That is the max spindle speed for my machine.

Chipload seem reasonable. I cut a lot of this with a single O flute tool. Onsrud 65-033 works best for me. Mine is an upcut version for better chip evacuation which is what you need to prevent rewelding.

I think I used a down shear for a job where the parts were quite small and I was worried about lifting the parts off the table. I now have the bits but otherwise I would agree an up shear would be better. I am getting only a few hours of cutting before the bit is losing performance. You might want to have a few bits on hand if a burr is going to be a problem.

What do you guys think about 1/4" vs 3/8" bits? My spindle can do 20,000, any advantage to higher rpm? This is not likely to be a high volume operation.

Nope. Why make more chips than necessary. More cutting force on the workpiece equals greater chance of lifting or movement. There is no advantage in my opinion in a larger bit. Rigidity isn't an issue. You can nest tighter and cut smaller details with a smaller bit and you eill have to empty the dust collector one less time this year!

Ok, thanks for all the feedback. I'll order an upcut O flute bit and give this a whirl.

I found a 1/4" downcut O flute bit I'd forgotten about and made a test cut with the recommended feed and speed. It seemed to cut just fine, I'll look into an upcut bit if I end up doing more of this. Thanks again for the help!

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Faster the spindle speed, the faster you can feed. Its all about chipload.

The soft nature of aluminum sheets can lead to built-up edge formation, resulting in poor surface finish. It is important to select the right cutting tool geometry, feed rate, and cutting speed to address this issue.

https://www.want.net/aluminum-grades-key-characteristics-and-cnc-machiningaluminum-grades-and-properties-gemma/