Type 1 PVC

[Keith Larrett]

I'm looking for some advice on how to improve the cut quality on Type 1 PVC. I cut some prototypes for a client yesterday. The results were ok, but the cut quality was hardly a "mirror finish" and I would like to improve it if possible. The PVC is 1" thick.

I was using the Onsrud 1/4" O upcut bit that comes with the ShopBot starter kit. Spindle speed was 14 000 rpm, feed rate was 120 ipm and plunge rate was 60 ipm with a pass depth of 0.25". I was using a climb cut with an allowance of 0.02" and a final pass in the opposite direction.

The cut edge looked like it has chatter marks on it.

I think a big part of the problem was the CEL of the bit is 7/8" and so there was no chance of the chips being ejected. I want to order another bit with a longer CEL and do some test cuts.

Any advice on feed speeds, rpm, cut direction and whether a last pass is necessary would be appreciated and help me with my testing. Thanks in advance!

[Keith Larrett]

Hopefully this will help someone in the future.

After a lot of experimentation I found the following bits, feed rates etc gave me an acceptable cut. I was cutting 1" and 3/4" Type 1 Grey PVC.

On the through cuts I used a 1/4" Spiral 'O' Flute Upcut Plastic Bit with a cutting edge length of 1 1/2". I ran the bit at 18 000 rpm and a feed rate of 180 ipm. I took off slightly less than 1/4" per pass with a climb cut and a separate last pass with an allowance of 0.01". On the last pass I set the last pass thickness to 0.0625". I did not reverse direction on the last pass.

The cuts around the gear teeth were at a depth of 1/2" and I used a 1/8" Spiral 'O' Flute Upcut Plastic Bit with a cutting edge length of 3/4". I ran the bit at 18 000 rpm and a feed rate of 90 ipm. I took off about 0.1" per pass with a climb cut and a separate last pass with an allowance of 0.01". On the last pass I set the last pass thickness to 0.0625". I did not reverse direction on the last pass. Because the material is so hard, I also took the added precaution of "roughing out" the teeth first with the bigger 1/4" bit before running the same profile with the 1/8" bit.

I also tried Carbide Spiral Upcut Bits at similar feeds and speeds. The results were similar although it sounded like the bits were laboring a lot more. They also left some swath in the gear teeth that was difficult to clean out.

I also tried some HSS Straight Plastic Cutting Bits but they did not work at all. They chipped the material badly and the bit lasted a couple minutes before br

I certainly didn't achieve the "mirror finish" I've got on softer plastics like Starboard, but I'm not sure that is possible with this material. I'm also sure that someone with more experience could tweak my cut strategy and improve my results. If anyone has anything to add I'd be very interested in learning more about cutting this material.

[paul_z]

I am curious why you chose PVC for gears. Acetal (Delrin) might be a much better choice for gears especially if they are going to see heavy use, high temperatures or chemical exposure.

Paul Z

PS Acetal machines nicely.

[joe]

Keith that some impressive work.

Thanks for the tests and report. Like you I'm working with PVC on a regular basis. I'm not sure your Type 1 is what we have.

At this time we are finishing up on a dozen 28"X79" panels on a contract with the city of Norman. Over the past year I've used several manufacturers. The edge smoothness does vary depending on the manufacturer. My work is on .75" and 1". I'm not sure it's possible to get a slick edge no matter what cutting tool used. This is always a problem when I'm wanting to do a 23K gild on the interior of Vcarv letters.

Another consideration is how tough this material is to sand. On the edges of my large panels I've tried a carpenters furniture scraper. Even then it has a texture. Perhaps the Type 1 is better.

[joe]

Paul,

Is the density of the acetal increased over PVC. I guess it is but must be more expensive.

[paul_z]

Joe,

Acetal may be double the density of solid PVC. It would be very bad for sign work because almost nothing sticks to it. In a way, it's like polypropylene.

I am amazed how abrasion resistant it is and yet how nicely it machines.

Don't use it until you read the MSDS. Although it is food safe, there is a slight chance of contact dermatitis while machining it. It's never bothered me but that is a sample size of one.

It is not cheap but neither is the machining (nor replacing a failed part).

Paul Z

[Keith Larrett]

I am curious why you chose PVC for gears. Acetal (Delrin) might be a much better choice for gears especially if they are going to see heavy use, high temperatures or chemical exposure.

Paul Z

PS Acetal machines nicely.

Paul

The PVC was chosen and supplied by my client. I'm primarily a cabinet maker. I offer CNC routing services as a means to keep my machine busy when I am not using it. My knowledge on plastics is pretty slim. I appreciate your advice and I will talk to my client and give him your suggestion as this stands to be a repeat order. Thanks.