Cutting PVC Letters

[srwtlc]

Hi,

I would like to know if anyone out there is cutting letters from 1/2" thick PVC (I think thats what it is). I have a sample piece that I have cut some 6" letters from and it cuts very nice, but I'm concerned with edge quality (rippling).

I'm using some high quality upcut spirial bits (3/8" and 1/4").

What speeds, ramps, or other settings are you using? What kind of edge quality are you getting?

For instance on a "M" where the angled cuts are I get noticable step ripples approx. 3/32 apart consistantly. I've always been concerned with this type movement (jiggly) on any low angle moves and have never found a combination that smooths it out. Is this common to all, or are some of you getting better step resolution? I have a PRT96 with the direct drive steppers.

This issue could make or break this job. Approx. 5000 letters!

I also would like to know of any ideas on how to hold all these letters in place while cutting out a 4'x 8' sheet with no tabs or huge amounts of vacumn. Perhaps some adhesive paper on the back of the sheet, but it would need to be somewhat thicker in case of a slight through. What ever I do it also needs to be inexpensive. Which brings me to my last question, how much on average would you charge to just cut out letters that range any where from 6" down to 1-3/4". I come up with approx. $0.75/letter +/- a little. Out of line, or close for the northwoods (WI)?

Thanks for any replies,

Scott

[rgbrown@itexas.net]

Scott,

You might look through the "Onsrud" site and see their recommendations. As I have been told, they are the true 'experts' on plastic cutting. Heck, they even have a complete website dedicated to cutter selection:

http://www.plasticrouting.com/

Ron Brown - rgbrown@itexas.net (mailto:rgbrown@itexas.net)

"In general the art of government consists in taking as much money as possible from one class of citizens to give to the other."
--Voltaire

[jkforney]

Ron
I have a couple of Onsrud bits that we will cut plastic with at the midwest camp shopbot. The bits were donated by Onsrud as well as brochures. I will report on the bits after the camp.

Don't want to try them before since they sent them specifically for the camp.

Scott
I know this may be too late for you and it may be a good idea to call them directly at 800 234-1560. I know that they have reps that cover different areas.

John Forney

[rookie432@aol.com]

Scott,
My 2002 official signwriters price guide states that the national average for 3/16 " acrylic o 6 mil pvc letters cut on a cad cam system goes for:
6" = $12.00
5" = $9.00
4" = $8.00

now keep in mind these are national averages and are meant for retail sign buyers. I can't get that price in my small town nor would I attemt to get that for 5000 letters but it is a good rule of thumb to go by. It really all depends on where you need to be pricewise and still get the job without losing your butt. Profit is not a four letter word.
The sighnwriters price guide is a great book to have and fits in my back pocket and goes with me everywhere for that current question "how much would a _____ cost me?" you fill in the blank

The guide is about 25.00 and has well paid for itself already. they also have computer graphics pricing guide.
signwriters publishing co 406-252-2214

Bill

[joecrumley]

Scott,

I have been using 1/2' PVC (Centra) for years and have had no trouble with the white. I use it for outside lettes. They need no paint.

PVC does not require special bits. We use a double fluted, carbide tip, cheap $6.00 bit. Your problem sounds like something other than a cutter.

If you continue to have ripples edges, just put in a bridge and double cut. The second pass could be a little closer in. Sound like a bit that is not chucked up tight enough.

Watch out for the colored PVC's. They faid fast out in the sun. If you need colors on letters let me know.

Joe

[threadfx]

I've cut PVC in the past also with the excact same results. I never did figure out how to get rid of the ripples.

I did find that using a .125 spiral O bit and running the Porter Cable router on it's slowest speed, the cut material mushrooms out of the cut kind of like a Chia Pet and holds the part in position. It didn't appear to effect the cut quality or heat up the bit in the process.

If you find a solution, I'm all ears. I finally attirbuted it to the jerkiness of the Y-carriage.

Aaron

[Mayo]

Jerkiness is sometimes from setting your RAMP speeds improperly. On my PRT96, everything goes good if I make the ramp speeds around 60% (or less)of the cut speeds.

[grant@shopbottools.com]

It is true that on very shallow angles the step increment can be seen. However, the step resolution is less than .002" even on our older tools, so I find it hard to believe you are getting serious cosmetic issues from this alone. Is it possible there is some problem with the toolpathing or with bit chatter? If you let me know what software you are doing your toolpathing in and I can have a look at the file I may be able to prove helpful here.

As far as hold down goes, I do have an idea. If you leave a very thin skin of material on the back, and do not cut all the way through, you can remove the letters after. This can be accomplished by putting the whole sheet through a wide belt sander and freeing them all at once. Alternatively, you can pop a trim router through and cut it free, or you can actually do a final pass on the ShopBot that cuts the skin. While the skin holds the letters in place during the heavy passes, it puts very little load on the cutter and material to cut it by itself, and it is uncommon for the letter to move much as it is cut. John Forney has had good luck with that last technique.

[srwtlc]

Grant,

Toolpathing shouldn't be a problem. The sample path was generated from Vector v9.4. No tool chatter because this stuff cuts very easy. I've been testing some new Vortex finisher spirals and they cut really nice.

As far as holding I might try the skin at the bottom, but I'd like to save the time needed to free them. I'm thinking of trying some paper mask and seeing if I can cut through to the paper and then just peel it off. It's worth a try.

To everyone else, Thanks for your input!

Scott

[donchapman]

Masking or single-side taping the back won't work unless your depth of cut is so precise that you cut through the material but not the very thin mask. But spray adhesive or double sided tape on the back of the work sheet will hold the otherwise loose letter parts in place.
How about tabbing? That's why it exists, although it does slow down the effective cutting speed by inserting a lot more z-axis cycles.

[srwtlc]

Cleaning up the tabs on 5000+ letters by a second pass or sanding is what I'm trying to avoid.

[gerald_d]

You only need one tiny tab in each part - right at the end of the cut. (ie. when the tool has nearly gone right around). Program so that the tool lifts say 0.1" before it cuts the last 0.5" of the circumference. Then you can trim the single little 0.25" X 0.1" with a knife as fast as you can peel off any adhesive.


But remember that your 4x8' board is going to loose it's shape and strength if the parts are very close to each other. You have to plan the sequence and starting positions of your cuts very carefully. But it is more than worth the trouble for 5000 letters.

Come to think of it - how about the good folks at ShopBot editing their Tabbing function so that we can have a single tab right at the end? Bill Young, have you considered this? Maybe this has happened already - time to pay a visit to Bill's corner . . . . .

[swims@mindspring.com]

IMO, your pricing is way too cheap.


If you are a small time operator like I am, you may find that the logitics of large quantity jobs may not accrue the kind of time and cost savings that you may expect. But anyway, my shop isn't your shop... if you do the job at this rate and it works out well for you, let me know, I'd be interested to see how you accomplish it.

[srwtlc]

I haven't heard anything back from the customer yet after doing the samples.

Note that all I'm doing is cutting them out, all materials are provided by the customer, and I'm the third person down the food chain. ;^)

[larry]

I cut acylic alot and got a great bit form onsrud bit 61-082 cut great! with this bit the cut direction is climp. It leves only dust that holds the part in place even if the masking is cut of the bottom and has no melting back on the part. there is a down side , the bit has a tip or hook on one side. when i use the zero plate some times it dings of the tip. Hope this helps,Larry

[gerald_d]

Larry, for our zero plate we use aluminium with a 1/8" foam cushion underneath, so that the plate can deflect. (The foam is just thick double-sided tape). No problems with bit damage.