I suspect that this is likely to be the limiting factor, I’ve been doing lots of tests on machine deflection and rigidity and they all support the view that cuts with low depth and lots of width work fine whilst cuts with large depth and thin width are not what the machine likes to do.
There’s plenty of other evidence from folks like Vince supporting the shallow and wide works result, which is partly why I started looking for causes.
So to get to that full 1.5" depth on the acrylic it’s quite likely that sneaking down a few mm at a time with a ramp in and lead out is the way to go.
This is the wall finish I get on 6061T6 with a single flute 6mm cutter using 3.5mm step downs, ramp down, lead out, 0.2mm stepover in each of two finishing passes after the roughing. If the machine can hold still enough for Aluminium it should be able to do acrylic. This is straight off the machine with no hand finishing at all.
Similar to the Acrylic I found that keeping the feed rate up, as well as chip evacuation was key. If I was slotting I’d be running a wider slot than the cutter to make sure I had enough room to get the chips clear of the slot.
EDIT - XXL with HDZ, you’d do better on an SO3 with HDZ
This could be key, might be worth taking a picture after each cut to see how it progresses. If you’ve got deflection on the roughing cuts that marks down to a depth below the finishing cut it won’t matter how good the finishing pass is.
I also forgot that Acrylic needs a minimum cut sorry.
Maybe, I’d try the pictures of an exposed edge after roughing, first and second finishing cut first and see what you can see.
One point of regular thought for me, at least regarding say something like aluminum vs plastic milling, is uninterestingly enough, plasticity. It is my unfounded assumption that a plastic, such as Acrylic, has a higher propensity to deform via cutter. This would assumedly interact with machine rigidity. This unscientific and baseless assumption has lead me to increase my minimum stepover in a attempt to achieve a superior finish, despite being, to me, generally counter intuitive.
Interesting, I’d only worried about it being brittle when pushed and melting if I moved too slow. If the workpiece is held down well it shouldn’t move that much.
As for the cutting dynamics that’s out of my depth and I’d be asking somebody like @spargeltarzan if they had thoughts on this?
As I’ve said before, acrylic is unforving. Think about how you’d hand cut aluminum…you keep that saw going and the aluminum will hold on until you’ve cut through. Acrylic is much different, it fractures. You score the acrylic and then it’ll snap off with a little force. In my mind, that’s what happens on a small scale each time the cutter digs in…it cuts a bit, then the acrylic snaps. Most of us have seen that slomo video of how metal deforms when being cut with an endmill. I want to see what happens with acrylic. @Julien, where can I contribute to the high speed, high definition macro video camera fund?
I’m afraid I’m still stuck at the “find a suitable camera” step, regardless of funding.
I stumbled upon this nice trick to capture frames at 660fps using a raspberry pi camera (it does not get cheaper than this), but resolution is 640x64, I doubt we would see 0.001" thick chips forming
If someone knows of a camera that
can grab high-res frames at 500+ fps
has a reasonably standard interface (USB3.0 maybe?)
has macro focusing capability
I would sure like to hear about it, just to know what the pros use. But then I would probably manage to drive an endmill into it
Sadly, I haven’t any experience with acrylic myself. However, I asked around, and a common opinion is that it helps to use extremely sharp (very large rake angle) single-flute tools with large chip gullets, preferably 1-flutes. There are vendors that sell such tools especially for acrylic (e.g. Hufschmied 110FPE series - I’m sure there are others), but they tend to be not cheap, around €35 apiece.
That same vendor has recommendations on how to achieve clear edges: with the 8 mm 1-flute tool, run a finishing pass at 24000 rpm and 3000 mm/min (120 ipm) feed with ae = 0.25 mm and ap = t + 0.1 mm, that is, cutting slightly beyond the thickness of the stock, in a single depth pass.
Its 1.250" , a massive 12x24 chunk. I was requested to make a clear warerpump housing to test impeller efficiency and cavitation. Pretty cool opportunity.
Also I have some really nice tooling on hand, this 8mm Datron SF 4-1 and Amana HSS SF Straight flute are going to get a workout.
Not exactly a great example, being much thinner material. But this is typically what I pull off of my machine. Good enough to start polishing at 320 dry.
It’s 3 layers of .5" fused together. So not bad $290 for the 4x8 sheet of .5".
On the image… it’s not crazy dissimilar… I think i might be catching the light or accomplishing the “mirroring” that the bit advertises and it just looks a little more juxtaposed then yours (like diamonds on black velvet).
I still haven’t be able to get anything close to what @Julien got in clear.
If I think I understand what Julien was spotting in his post… this looks like the shaft of the endmill is causing the marks since the marks are so long.
Are there endmills where the end is bigger than the shaft? Like the surfacing bits but cutting ones? They might be useful here.
So it’s still likely the bit oscillating ever so slightly in and out of the wall ?
I was trying to make sense of the horizontal lines, particularly the line where my red line top extremity is, but it does not seem to quite match the limit between two of the glued 0.5" sheets…
Have you tried a larger diameter endmill ? (a 8mm one should be possible on the Makita/C3D router, now finding a long LOC 8mm endmill in the US might turn to be a challenge). Can’t remember which post mentioned that larger endmills were better for wall finish (I never tested that statement though).
