Need suggestions for pocketing aluminium

[asaraca]

Hi,

I tried for the first time to use aluminium with our Shopbot Buddy with an onsrud 65-025 bit. I was a bit nervous at the beginning not knowing what to expect but at the end I was surprised with the ease the Shopbot cut through the aluminium. But I would need your suggestions: is there a way to improve the finish of the cut ?

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Thanks

Antonio

[coryatjohn]

I've cut a lot of aluminum (mostly 6061) and I've found the secret to a good cut involves three things:

1. A stiff bit.
2. A slow RPM.
3. Good hold down.

I've found the best cut is a standard wood bit like a two flute upcut running at 6000 RPMs. The depth of cut depends on the diameter of the bit. The bigger, the better. If you hear a lot of chatter, the bit is taking too much off and the finish is going to suck. It's also going to cause the tool (the SB) to flex a lot and that's going to give you poor dimensional accuracy. Slow cutting makes for a better, cooler cut with less deflection. When everything is working perfectly, you can hear it purring like a kitten instead of screaming like a scalded dog.

I prefer the two flute bits over the single flute. The single flute is too flexible and can easily break the tip, which will cause problems with smooth cuts and dimensional accuracy. Check your bit now. I'll bet you'll see the tip has already broken off.

What alloy of aluminum are you cutting? That makes a difference too.

[asaraca]

The aluminum we have as been brought by a member of our Fab Lab, so I have no idea what kind of alloy it is ... I was thinking of purchasing a bit like the onsrud 66-315 that is bottom surfacing. Would that help ?

[coryatjohn]

I've seen good results from standard carbide bits. I doubt you need to buy a special bit. If you've got a good condition upcut two flute bit, try that with a low RPM. I think you'll find it works. You should see a nice pattern on the sidewalls and bottom that is consistent and even.

I think you may have holddown problems too. Those circles appear to be indicating a moving work piece. Aluminum is very sensitive to movement during cutting. Everything has to be rock solid or you'll see poor quality cuts and dimensional instability. Be sure everything is super tight.

Go slow, go easy and if it screams to the point where you need ear protection, you're doing it wrong.

[pro70z28]

If the part I'm doing happens to have holes I do them first and then run screws through them into the sacrifice board for the very reason coryatjohn mentioned.

[guitarwes]

Any feedrate and pass depth recommendation with that 6000rpm speed?

[tomhartnett]

Here is a doc and video we normally provide for cutting aluminum (the alloy is really important though :( some are really bad/hard to machine)

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https://www.youtube.com/watch?v=glfoB7THcqs

-Tom

[coryatjohn]

I would add a couple of things to that list at the end of the PDF:

* Be sure your hold down is perfect.
* Ramp all cuts.
* Clear chips with a vacuum (not air).
* Let your ears tell you when it's working right. Screaming is bad. Purring is good.

>> Any feedrate and pass depth recommendation with that 6000rpm speed?

Cut in half from 12,000.