Cutting Acrylic

[Mark Kruszka]

I plan on getting a laser and starting my own business. My employer has expressed interest as having me for a vendor as I have identified several parts that could be cut to print faster and cheaper with a laser.

The main job that I can foresee doing on a regular basis would be cutting acrylic. This particular part would entail cutting 6 1/4" diameter rings 1/8" wide out of 3/8" thick cell cast black acrylic.

I have two primary questions regarding this particular part.

1 - What kind of finish should I expect?

2 - Is there a chart or table somewhere that plots laser power/wattage vs speed of cut?

As it stands, I'm currently favoring the Pinnacle M series with at least a 60W laser.

Your thoughts appreciated.

Thanks!

[Rodne Gold]

You will get a polished edge with some striations.

Issues you will face cutting 1/8th inch wide circles , 6 1/4" in diameter out of 3/8th cast pex are:

1) the thickness of cell cast pex will vary up to +-10-15%

2) Perpendicularity of the sides of the cut might not spot on

3) circularity of the item will have to be adjusted using scaling factors

4) possible warpage of the ring due to heat and its very small thickness

5) You will have to compensate for spot size and kerf to get an exact 1/8" inch thichness of ring.

6) Air assist will have to be real precise to maintain repeatability

7) bed leveling and laser alignment will have to be perfect to utilise the full bed for this

I dont think a laser is the best tool for this , you would get better results dimensionally with a cnc router if measurements are critical.
Its also an incredibly expensive method of doing these rings in terms of material wastage and cost especially if there is to be significant production. If big quantities are required , I would imagine extruding a tube with a 1/8th wall thickness would be a LOT cheaper.
60W or more seems the right sort of power if you do want to do this with a laser.
Speed and power are proportional in a laser , so an equivalent 30 w vs a 60w would take 2 x longer to cut
However to to extrapolation , acclerations , ramping , beam quality , power density etc , its not really possible to have an accurate chart as a rule of thumb guide.

[Joe Pelonio]

I agree with everything Rodne says. I have cut similar items with 1/4" acrylic
and anything that thin is very fragile. A better material might be ABS which laser cuts pretty well too but is more stinky. I've actually gotten smoother edges with ABS than acrylic but others have had problems cutting it.

At the very least, see a demo of the machine and bring some 3/8" acrylic to have them cut for you as a sample before buying.

[Mark Kruszka]

Thanks Rodne & Joe, very useful!

I'm glad I asked, cleared up a lot of questions for me. BTW, I was planning on taking some 3/8" acrylic to distributors to see first hand what to expect.

Regarding the CNC Router, think I'll start looking around and see what the capabilities & prices are. Sounds like I have the right idea, just the wrong tool.

Maybe I'll start a shop with a CNC router and a laser, that would be fun.

Things that make you go hmm....

[Larry Bratton]

Mark:
Start with the CNC router unless you got piles of money. You can buy a reasonably good new machine for 40k or so and you have a huge amount of capabilty that you simply don't have with the laser. Laser is great for engraving and cutting certain things,but to get table size and good power for cutting your going to spend 30k and you still can't cut metal and your limited to thickness in wood and other materials. Table size will be limited to say 24x36 where a router can give you at least 48x96 in that range. Software however is extra by a good bit for a router, contingent on what you want to do. Good luck with your new business!

[Martin Reynolds]

Plus, with that thin a ring I would worry about residual stresses in the material. I like to see cuts leave material wider than the thickness, but there again I am new at this.

[Rodne Gold]

Stress would be a MAJOR problem with a laser . Perspex that is laser processes is almost guaranteed to develop stress cracks or crazing if any solvent is applied or is indeed nearbye.
It would also be a problem if your CnC machining involved any heat put into the material.
You need specific tools to cut perspex or other plastic in your CnC machine to avoid heat build up. The secret with plastics is to cool em and allow massive chip removal when machining.
A laser and an overhead cnc router go hand in hand , using both machines you can do just about anything anyone can throw at you and can combine various materials exceptionally well.

[Steve knight]

cnc routers can cost far less then 40,000 a shobot can be hand for far less depending on how big you get and if you shop around for accesories.

[Dave Jones]

Have you considered having these injection molded? The amount of waste material in doing this on a laser or CNC is going to be expensive. Injection molding has an initial tooling cost, but after that is pretty cheap.

[Jim A. Walters]

My brother builds injection molding tooling. The part will have flat round ejector pin marks on one side, and a few gate mark s(where the plastic flows into the cavity).

If it is a simple ring, and the tool shop is CNC capable, they should be able to build an aluminum mold for $3000 to $4000 dollars. It will be "soft", so you should make sure the shop that runs the tool won't pry out parts with a screwdriver .

He has a customer that only uses aluminum tooling. They mold the big sand/water tables for schools and daycares.

I have never figured out how tool shops can quote the same job and one will quote $12,000 and another $4,000 for a simple tool. Must be the "nuisance" factor!

Shop around and get 4 or 5 quotes.

[Bill Cunningham]

I think for a job like this, Rodne has mentioned the simplest route, and thats a 6" o.d. 5.75 i.d tube. The pieces can be parted on a bandsaw slightly oversize, and then edge finished/sanded , if you need a smooth edge surface..
The plastic tube may even be a stock item!

[Mark Kruszka]

Well, I guess I should clear up some things.

I'm a machinist for a Vet & Dental equipment manufacturer and am responsible for machining this acrylic ring.
The ring comes in a solid 6 1/4" circular blank. We have a fixture that we use on a conversational CNC lathe. The first tool rough cuts out the inside, then we use an insert ID Profile tool and a custom ground carbide grooving tool to finish it up.

What I was originally thinking of doing was to cut the blanks myself from acrylic sheet (as a vendor) and have it as close to net dimension as possible to make for an attractive selling point to my employer. If I could sell it with the OD & ID to size it would be eliminating 2 machining steps within the process.

Those inside 'cut-outs' that my employer has been throwing out (an approx. 5 1/2" diameter blank)...I've been saving. I have over 1000 of these and that's what got me thinking about lasers, that I could do something neat engraving-wise and sell it.

So the saga continues... lol.

Thanks for all the help and comments everybody.

[Mark Kruszka]

Oh, one more thing. I've already talked to the engineers about getting extruded rings from a vendor and they agree with me. The main problem atm is that we were acquired by a company a few years back and they are swamped with Engineering Change Requests, product revisions and integrations.
We are an ISO 9001 compliant company and some other standards I can't recall right now. Anybody familiar with these standards knows how much paperwork is needed for any little change to anything.
So although everybody agrees about the extruded rings, it's not going to happen for a while still.

[Larry Bratton]

Steve:
I know you can and I have a machine that is in the $25-30k range with the vacuum pump and all. However, I am speaking of a machine with automatic tool changing, 10hp router and a bigger vacuum table. Actually a machine like that would probably press $60k. I am associated with a shop in another city that just bought a medium sized Multicam-60" x 12' table, 10 or 12hp router and a 30hp vacuum pump -$75K and that didn't include software. Shopbots are considered to be more of, dare I say, hobbyist machines. However, looks like their making a lot of strides with their newer machines.
I bought the router I have without a lot of knowledge or practical experience. When I buy another one, I will get the auto tool changer for sure. That's about a 10K ticket alone with most I've seen. If your planning to make sizable production runs it will pay for itself, but if your just going to cut signs and short runs the Shopbot would be fine. I just installed a $2000 dust collection system on mine. We cut a lot of MDF and those small 2hp dust collectors just were not doing the job. All of this stuff is expensive and it's good to have a good plan in place before buying it. I'm not too good at that sometimes.

[art baylor]

Mark:

Bill is right. 6" od x 5 3/4 id acrylic is available in both cast and extruded tube. The cast will look better and finish better. Parting off rings on a lath would be very accurate and minimize finishing vs a bandsaw cut.

Art