Cutting .472 Foamed PVC

[Mark Farris]

I have been cutting quite a bit of foamed PVC letters lately with very good success. I recently got a job cutting 50 sheets of .472" foamed PVC into hundreds of knock down parts. I have been trying various 1/4" bits (O single flute upcut, Super O single flute upcut, O 2 flute upcut) following manufacturers chipload ranges with various RPM's and feed rates, and cutting in 1 pass. I have a vac table with 4 motor Black Box continuously pulling 7-8 inches. All cuts with every combination are good. My only aggravation is the chips pack into the 1/4" kerf and not up into my dust collection. I have a 3HP Super Dust Gorilla cyclone and have very good suction. The chips are not re-welding but just packing. I assume the hold down vac is helping keep them there. It is just a lot of clean-up between sheets. My question is does anyone have experience with this? Typically does more or less RPM vs more or less feed speed help with the chip evacuation or is this just typical of cutting this material. I have not gone out of the recommended chip loads. Any insight would be appreciated.

Thanks,
Mark

[Brady Watson]

There is no practical way around this. The chips in the kerf have more to do with static cling than anything else. Static Guard doesn't last long enough to work.

Get good at clearing the parts and scrap off of the table. Then use compressed air or a wide vac attachment and the shopvac to clean the table off of chips. Then load the next sheet...It is what it is.

-B

[Mark Farris]

Thanks Brady, I figured that was the response I was going to get. I hadn't really considered the static electricity. I guess it is time to build my high powered, large volume custom vac I have been dreaming up. I have had it with weanie shop vacs.

[knight_toolworks]

this can be a good thing on small parts. but you can control how it works. if you cut a climb cut they stick to the scrap. thats what I do then the parts come out clean and the shavings stick to the scrap or fall off.

[Brady Watson]

Thanks Brady, I figured that was the response I was going to get. I hadn't really considered the static electricity. I guess it is time to build my high powered, large volume custom vac I have been dreaming up. I have had it with weanie shop vacs.

Rather than just throwing horsepower at it, you might consider adding a compressed air nozzle or two to the underside of the dust foot. This would stir up the chips in the kerf and get them into the DC...

-B

[Mayo]

When cutting 3/4 inch and 1 inch Azek PVC I have found that using an upcut spiral takes more material out of the kerf than a straight bit, and it doesn't pack in as hard. The other thing that helps is to try increasing your feed rate. Just don't increase it so much that you get bit deflection.

I rely on screws to hold material in place and so I typically make 3 passes for 3/4 inch, 4 passes for 1 inch.
Because of the material left in the kerf, the 2nd and 3rd pass show a visible step on the edge of the cut.
My feed rate on my PRT w/4G is 2 inches per second for this material. If I slowed down the feedrate I may eliminate the stepped edge, but these are corbels which go about 25 to 30 feet above the sidewalk, so it's never noticed. I think my RPM is 13K - it's the second setting on the variable speed Porter Cable router.

[ckurak]

Mayo,

Regarding the visible step issue...

I cut 3/4" (18.5mm) veneer core plywood with a 5mm compression bit at 58mm/sec (or 2.28"/sec). So, there is some bit deflection. I also use screws for hold-down.

I make three passes. Passes #1 and #2 are in the CLIMB direction and go down to 17.0 mm. Then I make a final pass in the CONVENTIONAL direction that is 3.5mm deep (starting at 17.0mm). The total depth of cut is 20.5, but the plywood is only 18.5. The last 2.0 mm are cut in the MDF spoilboard. 2.0mm may sound a bit deep, but if there is any bowing in the plywood (remember I am not using a vacuum) this usually avoids partial cuts.

The bit will deflect outwards somewhat on the first two cuts making the part a little bit oversized. The final pass will make a clean, step-free cut at the exact dimension. There is no apparent bit deflection on the final pass since so little material is being removed.

Using this method gives me a nice, clean, step-free edge that is ready for the edgebander (or final sanding if appropriate).

I know you are discussing PVC, not plywood. But, maybe this cutting strategy will help eliminate the visible steps. It is true that I need to create two cutting toolpaths for each profile, but the exact, smooth edge is worth it for my projects.