Getting a better finish?

That week I just had my first experience with Acrylic while working on my Dust Shield and I almost couldn’t be more pleased how it turned out ( the cutting of the Acrylic ).

Carving the pocket in the middle produce a shit ton of pellet like pieces using a 1/4 High Speed Steel Single Flute Straight Cutter @ 1920mm/min, 16’000rpm and 1mm DoC
Everything else then was finished using a 1/8 Solid Carbide Single Flute Upcut End Mill Cutter with identical Feeds, Speeds and DoC.

For the most part I probably can’t really complain but upon closer investigation I noticed some very small kinks every now and then along the cutting surface which begs the question on how to avoid them in the future.

  • Should I have used a different Bit?
  • Should I have made use of a finishing pass? Relief Finish Pass or with Stock left to carve?
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What do you mean by kinks? Can we get a better description?

The vertical lines visible around the center of the cut surface if you zoom into the picture:

The horizontal ones could probably be removed with some sanding but the vertical ones…
Njeh… Would probably cause more harm in trying to remove them =)

One approach for the profile cut is to leave a tiny bit of material for the profile and follow up the profile with a final profile cut at full depth removing that little bit you left to remove the tool marks.

I see what your referring to. Yes a finishing pass would most likely remove all of that. When cutting did the chips you were producing look like like rice or more like granulated sugar. It should look like granulated sugar. If not your spindle speed is to high or your feed rate is too low. I always use a finishing pass on acrylics.

Mhh… From what I remember that was 11-12mm of Acrylic ( odd number, don’t ask ) to cut through.
Sounds like a lot of material even for a single finishing pass but I guess I should give it a try first =)

Not exactly sure how the chips looked like when I cut them with the 1/8 1F Bit - I guess a little like the Coconut flakes of which Coconut Macarons are made of - I do recall them sticking to the freshly cut surface every now and then.
With the 1/4 1F Bit ( same feed/speed/DoC ) they certainly looked more like small pellets which easily got flung out of the work area sticking to practically nothing - That worked really nice.

I like your 's dust collector idea! I will try make something similar :wink:

When I said do a finishing pass and remove just a bit, I am saying only a tiny amount, like 0.01" of material or less.

Yea, I assumed that =)
But as I never tried it before I was still kind of worried it might still be too much.
Do you also reduce the Feed Rate or is it only the amount the bit engages into the material?

I go normal speed.

Haven’t yet tested it with a large amount of dust generating material like MDF but when I was redoing the lazy susan turntable for my SLA 3D Laser Printer UV Oven the debris shield worked flawlessly in preventing the pellet like pieces from flying vertically into my face.

Only issue with more kinetic loaded pieces is them still being able to escape horizontally so a skirt, like shown below, is probably still an idea worth considering - I’ll most likely go with a rubber strip.

Still trying in getting the actual suction area closer to the Bit and after discovering a thin sheet of Acrylic in my cellar I came up with the idea of deepening the channel and then closing it with another piece held by 3 Countersunk Screws thus redirecting the airflow.

A full shot of the latest Variant:

If you add a Thumbscrew ( like the one that come with the Clamp Set ) as a height adjustment lock it becomes a 1-Hand operate able mechanism =)

Yes I’m thinking about brushes on perimeter and small hole for bit. But why you use big area of dust shoe? I think it’s enough to cover working area Ø 5-7 cm.

One thing led to another but to make a long story short I like the 1:1 aspect ratio of the design and the widest I could get without loosing too much travel distance was 153mm which is the width of the Z-Axis Extrusion piece.

I am though loosing like 50mm on the Y-Axis but as I’m not building parts THAT large I was more than willing to sacrifice a little bit of travel distance there in order to get the more pleasing 1:1 ratio for the Acrylic Debris Shield.

Square things make my life go round, I guess =)

To get a better finish on acrylic, use a full depth finishing pass that remove very little material and spray a mix of 50% windshield washer and 50% water. You can also use a blowtorch to melt the edges of the part you currently made.

I assume spray while cutting?

Yesterday I did a test on 11mm thick POM to see how a full depth finish would look like with 0.2mm Stock left from the previous operation using the same 1/8 1F Spiral UP Bit - Not good.

The previous operation meant to left some stock left me with a perfect finish right from the start ( this time I also used double-sided Tape to affix the part to the working surface ) but the finishing pass at 105% DoC caused the Bit to chatter like crazy resulting in a then ruined finish that looked like a block of butter over which a fine serrated knife had been dragged along - Had it’s own charm but not what I was looking for.

That being said though… This serrated surface finish might actually come in handy on sliding areas that do require friction once tension has been applied to it.

I’ll probably give it another try at full depth with even less stock left over or maybe widen the gap beforehand.

Yes, spraying while cutting will prevent chips from melting and it will be possible to reduce the feed rate for the full depth finishing pass. Hopefully this will prevent the endmill from shuttering and ruining the finish.

But as I said earlier, it might be easier to not do a finishing pass and just sand the edges. After that you can do a couple passes with a blow torch to give it a glass like finish by melting the surface of the edge. This works well for acrylic because it’s a thermoplastic.

You wouldn’t by any chance know if this Blowtorch method also works with Polycarbonate?
Going through my Amazon order history I found out that it was in fact Polycarbonate and not Acrylic xD

Yes, polycarbonate is also a thermoplastic so it should work but the melting temperature might be different. Normally, the sanded edge should become transparent pretty much as soon as the flame gets near it. Keep moving the flame along the edge and don’t stay stationary at one point, all you want to do is melt only the surface of the edge.