Shallow angle profile cut in acrylic mirror

[bobmoore]

When I look at the witness marks in your spoil board on that last picture I see uneven vectors. You may need to do some serious node editing. Bob

[Brady Watson]

There are a number of things that stand out. The original post implied that there were straight cuts in the 8-10" range in acrylic mirror that had to be made. Your latest post shows a laminated material, with materials of different densities and curvy letters - not the same animal at all. There are several concessions that have to be made or you might as well just use a jig saw.

Unfortunately another user broke our o-flute bit, so I have to wait to try that.

You absolutely need to have o-flute tools to cut this layup with any degree of quality. An end mill isn't going to give you the edge quality you want & you'll spend a TON of time on the back end cleaning up. You need to get single and double o-flute tools to make your best effort. A triple flute may be in order if you need to sneak up on final edge quality. Find your Onsrud dealer & let him tell you what tools you need. This is what they do for a living.

If you still require a beveled edge, then you do the majority of your cutting (say 90% through cut) with the o-flute tool, then swap out for a SQUARE end mill with taper to just kiss the edge and go the final depth. This will result in a cleaner edge than using the taper itself, but still not as clean as a square cut o-flute bit will.

This entire job - laminated dissimilar materials with beveled edge - is a lesson in compromise. You need to experiment with tooling and technique and carefully select the best overall compromise in edge quality, time, expense and hold that up against what the customer wants & is willing to pay.

-B

[magazines]

Wow, thanks to everyone for the replies. I can't work on that machine today but will get back to it tomorrow. We are in an academic setting and the technician who set up and maintained our machine moved across the country a couple of months ago. We are trying to get a handle on maintenance and running this particular job at the same time.
Brady, thanks for the suggestion of feed and rotation with ramping. I've never run ramps before and will be looking at your article which seems quite helpful. I tried the speeds that I did based upon other recommendations and slowed it down as it became clear that we were not getting what we should.

From the bill of purchase and looking at the machine itself, we are running a PRS Standard shopbot with a 4x8 bed. The spindle says that it is HSD brand and 4 hp. As far as I can tell it maxes out at 18k rpm. I know someone who cuts acrylic with 24k at 100 inches per minute and gets excellent results, but that totally different setup with a mastercam etc.

Yes, the goal is to cut acrylic in the manner that the original post says. I had a few minutes at the end of the work week to test out this tapered bit so decided to cut out a couple of words - it seemed like a good test.
Does anyone know of a standard cut test?
The vendor of the acrylic mirror recommended the mounting to sintra to protect the mirror backing. Due to it's softer material makeup, the sintra has been fine to cut on the tablesaw ,but it appears this may be a drawback with the cnc. I can say that it appears to not be causing a problem. We may consider mounting to another plain sheet of acrylic to standardize moving forward.


It's unfortunate to find out that the ball nose is not a good tapered bit for this purpose. We've got another one coming at a 3deg angle from woodline that they custom made.
Is this the kind of endmill bit that you were thinking of:
http://preview.tinyurl.com/o2hthan

For tapered bits, Amana send us to Woodline and Onsrud recommended a similar ball nose.
Thanks,
Peter

[magazines]

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

Thanks for the recommendation Tim. We are going to design a test pattern and will try climb cuts in multiple passes to get a better edge.

[magazines]

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's good to hear that we should expect more from the machine. It is a standard shopbot with HSD 4 hp spindle. It appears that 18k is max rpm.
I was not involved in setup or construction of the machine, so this is quite helpful but also unfamiliar.
So, to check pinions I should turn it on and run the spindle warm up routine, zero it, then I push it physically - by hand? Is there a way to look for what area to tighten? On the gantry or the sides? I'll certainly give it a once over and tighten anything that seems loose, just trying to figure out what to look for.
The collet is new.
Thanks

[Brady Watson]

If the only purpose of the Sintra is to protect the mirror, then I would just get a 12 or 24" roll of transfer tape, which is like masking tape, available through sign supply houses & cover it with that. It's cheaper in the long run. I don't know how you are attaching the Sintra to the mirror - ???

Sintra cuts beautifully on these machines with a single or double o-flute. So it isn't an issue with the right geometry. You can get by with an end mill, but the finish is not as nice.

FYI - ShopBot has techs in your area that can give you training and tune up the machine in your shop. Call them to inquire. 2 days of training would pay for itself in no time considering you'd be 'getting all the answers' to your questions right away.

Your 300Hz spindle is fine. It's nice to have a few extra RPM, but there is no impediment to you cutting what you want.

You don't want to use a BALL end mill - if for no other reason than you will be left with a cusp on the parts that looks like this---> л - which is probably NOT what you want. You could get around this by burying the bit into the table until it no longer imparts a cusp on the edge, but this is frowned upon for a number of reasons.

-B

[magazines]

If the only purpose of the Sintra is to protect the mirror, then I would just get a 12 or 24" roll of transfer tape, which is like masking tape, available through sign supply houses & cover it with that. It's cheaper in the long run. I don't know how you are attaching the Sintra to the mirror - ???

Sintra cuts beautifully on these machines with a single or double o-flute. So it isn't an issue with the right geometry. You can get by with an end mill, but the finish is not as nice.

FYI - ShopBot has techs in your area that can give you training and tune up the machine in your shop. Call them to inquire. 2 days of training would pay for itself in no time considering you'd be 'getting all the answers' to your questions right away.

Your 300Hz spindle is fine. It's nice to have a few extra RPM, but there is no impediment to you cutting what you want.

You don't want to use a BALL end mill - if for no other reason than you will be left with a cusp on the parts that looks like this---> л - which is probably NOT what you want. You could get around this by burying the bit into the table until it no longer imparts a cusp on the edge, but this is frowned upon for a number of reasons.

-B

We have applied 3m pre-mask to the front surface of the mirror prior to cutting. The sintra is applied to the back surface to protect the mylar mirror part during cutting but also when these individual pieces get glued to the final support - wood columns. So there has to be some kind of backing for gluing and sintra was recommended due to its ease of cutting and porousness - the recommended adhesive is silicone for flexibility. The mounting material is also silicone based for flexibility between the acrylic mirror and the sintra. It is a pressure sensitive film called opti-clear.

Also, I realize now that one of the tests that I posted pictures of (the 2ips letters, the most wobbly ones) were solid sintra, not this laminated acrylic sintra thing.
These tests were cut .05" into the sacrificial LDF board to avoid the ball part. The tabs that I used definitely show the curvature and it is not great for the part. But since I have now two ball nose tapered bits in shallow angles, can you elaborate more on what the drawbacks of cutting into the LDF are?

I am now planning on several approaches for testing.
1.2 pass profiling : rough cut with Oflute unbeveled, followed by tapered end mill.
2.Ball nose bits we have purchased, in two passes first to clear the sintra, adhesive and mylar then 2nd pass to cut the acrylic itself.

I will be tugging and pushing on the head tomorrow and tightening anything that I can find. That said, I am certainly going to push for us to have a shopbot person come by to check out our setup and give direct suggestions.
I'm assuming that I ought to contact Shopbot directly for this, correct? You don't know anyone in the general Southern California area do you?
Thanks

[magazines]

When I look at the witness marks in your spoil board on that last picture I see uneven vectors. You may need to do some serious node editing. Bob

Thanks for the heads up. I'm going to make another test file and be sure the vectors are right on with that one. This was text made in sketchup and imported, then cut with an "on" vector path. This may have led to the characteristic you describe.

[magazines]

here is a pic of acrylic and foam I did

Frank, those look like excellent multi-material cuts. I'm going to see what I can do with the machine and then may very well send you a message with a question once I get everything in order.
Thanks for the offer!

[Brady Watson]

Yes - contact SB directly for services.

You don't want to eat into your LDF because it now becomes a channel for vacuum to escape...and it fuzzes up the board and causes other problems that need attention. If you are only running 1 sheet - not a huge deal...but if you are running several, it will give you problems.

-B