Shallow angle profile cut in acrylic mirror

[magazines]

Hello!
I am trying to cut shallow bevels in acrylic. That is nearly a perpendicular cut, between 2 and 10 degree angles.
Does anyone have a bit that they like for this? We thought we'd start with 3 degrees.
Thinking of trying this one:
http://www.advantagesgs.com/Gerber-R...-1-4-x-1-8-x-1
and there's this:
http://www.amazon.com/Tapered-Ball-N...B2CNP4GP91QR1B
And Amana:
http://www.amanatool.com/bits-fv/462...ated-Bits.html
and Onsrud:
http://www.onsrud.com/xdoc/TaperBits

These vary in price from $45 to $140 and it is very difficult to know what would work best.
Does anyone have any suggestions as to spindle speed or feederates for this kind of cutting?
Also, the Amana listing has 1/32" angled bits on there. We don't even have a collet for our shopbot that will hold less than 1/4". Does anyone run 1/8", 1/16", or 1/32" bits? Don't they snap all the time?
Thanks for any thoughts on this.

We posted a while ago when we were looking for a variable angle bit. This seems like a better way to go.

[Brady Watson]

Explain more what you are trying to do...

Are these profile cuts with a beveled edge, or are you adding some decorative engraving to the surface?

If the former, you can use a tapered end mill, although it will not be as clean cutting as a tool made for plastic (O-flute geometry). If the latter, you'll want some kind of v-bit and less than 10 deg is hard to find. You could do it with a variable angle chamfer bit, but the profile would have to be cut 1st and then beveled.

-B

[scottp55]

Magazines, We run 1/8 to 1/16'ths bits all the time, everything below .125" uses the 1/8" collet. Spindle or router?
We don't do plastics like you,so can't help you there.

[joe]

Who is Magazines and what do you do?

I didn't know SB would allow a person to hide behind an alien name.

Joe Crumley

[bobmoore]

We need to know what kind of machine and what you are trying to accomplish if you want constructive help.
Bob

[magazines]

Hi All,
I'm still getting used to this forum and how to enter information, please bear with me. Also I have little experience with shopbot or cnc in general. Looking to learn as much as I can.
Thanks for the responses.

Brady, yes these are to be profile beveled cuts and there are no plans to engrave the surface of the mirror. There will be hundreds of pieces of acrylic mirror cut at around 1-4" widths and 8" lengths. The bevels will be on the long sides. This is really to get around the fact that we don't have a 5 axis machine - we want to cut angles into the sheet. Generally we are cutting sheet goods, mostly baltic birch plywood and acrylic but some other stuff too.
Thanks for pointing out that we should use O flute bits. We have one and have used it with success on acrylic but it does not have an angle. I have heard conflicting things about O flutes, can someone clarify how they differ from normal flutes? I have found no O flute tapered bits, but some with 2 or 3 flutes.
A variable angle chamfer sounds perfect! Do you have any to recommend for plastics?

Scott, thanks for the info about smaller bits! We don't even have a collet for 1/8" bits but I'll move forward without fear on smaller bits now. A friend is cutting acrylic on his mastercam cnc with 1/8" LMT bits but he said he's using 24k spindle speed at 100 inches per minute. As far as I can tell our PRS standard Shopbot with HSD 4hp spindle will not spin that fast. I did not setup the machine and the person who did has moved on. I've found manuals for the shopbot and the spindle driver box but nothing about the spindle! I have been told that with plastics, it is best to run at the right speed - too fast one gets problems and the same with too slow. Is this right? It would be great to know if I can slow it down to 14k.

Hi Joe, I'm sorry about using an anonymous name. I always do for any online forums and didn't want to change for this one. I'm sensitive about online privacy and didn't want to make up a fake name (that would seem weird to me!). But my first name is Peter, so if that works I'm fine with using that. I want to learn and have found it challenging so wanted to post a question or two.

Thanks for any further advice on this project!
Peter

[magazines]

2nd part post:
Also, I wanted to add that when I called Amana to ask about their bits, I was recommended to call up Woodline.com. When I got them on the phone, they were very helpful and explained that tapered bits are pretty unusual but that a rounded ball bit such as the links I posted would probably work. He recommended that I be prepared to cut 1/8" into the sacrificial board to get past the ball part. As it turns out, they had cut some custom ones before for a job a few years back and are going to make a 3degree version for me for $80. That sounds good, so it is on it's way. Apparently 3 flutes, but not O-flutes. He said he'd include recommended feed and spindle rates, which is a huge help.
Does this sound like a good way to go?
Peter

[scottp55]

Hi Peter, As I said, No experience with ANY kind of plastics, but feed and speed are co-dependent, if you are in the "sweet spot" for the material and it's cutting the way you want---If you want to increase feed for production you have to increase speed roughly in proportion.
With smaller bits just use some scrap and shallow passes and conservative cuts until you get what you want and then tweak it up incrementally until cut quality is unsatisfactory and then back it off 5-10%.
Plastic is different than wood though.

[bleeth]

Edge cutting O flutes don't come in angles so you can stop worrying about that, but I would investigate them for straight cuts in acrylic. Which one works best is very dependant on the type of acrylic, what size parts you are cutting, thickness, hold-down etc. It varies a lot!
For any mill if you can't run as fast a spin as reccomended then you speed up the feed speed accordingly.
I don't know what kind of edge quality you need, but typically when a Shopbot cuts acrylic the edge needs at least some sanding to really look good. For your parts I would make up my toolpaths by drawing vectors to cut on, take into account the diameter of the bit, and draw a continuous squared end zig zag first in the x direction and then in the y so it is cutting continuously making long strips first and then for the squares crosscutting the parts. You need good hold-down vacuum to do this. It is difficult to stop part movement when the parts get small even with a good vacuum and I would bet that Brady would reccomend his transfer tape hold down system for this. I think he has a link to it bookmarked and will likey post it here soon! I know even though I have a pretty good vacuum I would likely use his system for these parts instead.
Since your crosscuts don't need to be bevelled I would use an o flute here. I like straight single flute cutters often as they neither push cuttings down nor try to lift the part, and when cutting at the right feed and speed the cuttings don't get hot enough to reweld. Transfer tape hold down may enable you to use an upcut o-flute, which is better for some acrylic.

Cast acrylic cuts best. Extruded is a bigger pain and I only use a straight o-flute for that.

Virtually all of my acrylic cutting is with cast that is thicker and I actually use a fairly slow speed and feed for best edge. (Like 1.5-2ips and 8-12k) I ramp in instead of straight plunge

[Brady Watson]

Peter,
You can use a regular old tapered (square) end mill if the angles are that shallow.

However...CNC as wonderful as it is, isn't always the most efficient way to cut something. I would entertain the idea of using an edge sander like the ones Grizzly sells to put those shallow bevels on the edge.

Either CNC routing or sanding will require edge polishing or flame polishing depending on your project's needs & the level of polish required.

Just a thought...

-B