Shallow angle profile cut in acrylic mirror

Thanks to everyone for their suggestions.
Well, I got a chance on Friday to try out a tapered bit. This is an onsrud 1/4 shank upcut with a ball nose. The results were not the best and I wonder if anyone has any suggestions about what to do differently.
But I fear I may have other problems. I read about problems with chatter, but the cuts that I made are wobbly has all get out. I did 2 ips at 12k Rpm and then went down to 1.5 ips and 1 ips at 14k rpm and then did .5 ips at 14k. Thanks for the suggestions for feed and rotation, Dave, can you check out these images? I'm guessing you get better quality from yours if you have it dialed in.

These are 1/8 acrylic mirror on the bottom, 1/8 sintra mounted to protect the back. Most of what you see is sintra, but it gives a good idea of variation of line. All but the 1/2 ips is pretty unusable, and that is not what I would consider great.

Unfortunately another user broke our o-flute bit, so I have to wait to try that.
I don't know whether the images will appear in line or not, but here are some pictures.
These are named with speeds.
There is also one showing how the plastic dust sticks in the cut. I had our vacuum table on with all the dust removal working.
I would still really appreciate any further guidance you all have. I can't work on this but every couple of days, but really want to get to the bottom of it!
Thanks
Peter

So in my last post, the 1st image of the O is 2 ips at 12k rpm. The second one with the 2 S's is 1 ips at 12k, the 3rd with the N is .5ips at 12k. The 4th shows the cutting debris and the 5th shows the E cut at 2 ips at 14k. These are supposed to be straight lines.

Dave, what is your recommendation about ramping?
Brady, I don't have the money right now for an edge sander and our shop doesn't have one. Im trying to get the best cut off the bat so that the hundreds of cuts have the least amount of followup work. I'd say the diamond edge thing is way out of our price range. To make the best initial cut in acrylic, I've heard table saw, router, jointer, etc. Still trying to figure out for myself though.
THanks
Peter

Oh, and is there a standard Shopbot cutting pattern to gauge how well the machine itself is working? A test pattern of some kind that people compare apple to apple to be sure they are getting what they should?
Thanks

would it help some to reverse (climb) the cuts, some of it looks like tearout

It looks to me like the first thing you need to do is power up the machine & shake down every axis with considerable force (don't overpower it) - and see if you can track down if the pinions are loose. Those cuts are horrible.

You should NOT be using a ball end tool unless you are doing 3D or decorative engraving. The tapered bit should be square. Your RPM is too low for plastic - you want to be in the 16-18k range, and slow that move speed down. Shoot for 1.2,0.7 for your XY, Z speeds and adjust as needed. No sense in running 2" per sec on a 2" tall letter, right? Do your cuts in multiple passes using a smooth ramped entry (in profile toolpath options in VCP/Aspire) - 1 pass may be too deep causing the motors to stall.

You may also want to adjust your VR settings to the numbers I lay out in the Ramping Article - but check for mechanical slop first.

Let us know how you make out. What model machine are you running...Assuming it is a Standard?

-B

It does look like machine! The cuts should look Much better.
Peter, I'm totally guessing, but you have basically 2 different materials. Maybe 2 toolpaths for the same bit? One pass at your dialed in speed and feed for the full depth (plus a hair) of the top layer, and then a second pass to your full depth(at THOSE best feeds and speeds for the bottom material).
But first figure out what is loose on machine and make sure material isn't moving at ALL.
Like I said Guessing what I would do, if I were trying it. I might even try an offset of .01" on the top layer so the second toolpath would just have to remove a skin and bit deflection would greatly lessen.
I could be ALL wrong because it's plastic.:(

I have cut acrylic with foam
you have to take in to consideration both materials plus your glue
and also your vacuum. pieces move if not glued properly.
glue builds up on bit
you may need to use 2 different bits for the 2 different materials or at least multiple passes
2 materials could cause vibration
if you would like additional info PM me

Frank

here is a pic of acrylic and foam I did

Your issues are starting before your cut. There is something or a bunch of things not dialed in on the mill. Brady got here first with the reply and he is completely correct. Check pinions for wear and tightness. Check every bolt. Make sure your motors are engaging properly. It could also be time for a new collett. Your vacuum may not be holding well enough. No matter what bit you use in what material you can't get professional results unless your tools are tuned up.

Sandblasting should be left to the pro's. If you have a CNC go for that.