# issues with pvc trim board and cnc



## flyforever (Jan 4, 2013)

I am experimenting with Azek pvc trim wood. I am using a 3/8" upcut carbide and cutting at 50ipm and 20000 rpm router. I am also cutting the .750 stock at .125 per pass. I've got pretty good suction, but at the end of the cut the chips literally remain glued to the workpiece. So I know it's heat related, but have no idea how to eliminate it.
I've tried all different router speeds; I've even elevated the board so that there's space under it during the last cut.

I like the material because it creates little dust and cuts cleanly. However, if stuff gets glued on it, it's tough to remove. 

Perhaps there's a cutting technique other than going vertically on each pass. Perhaps I can use a multipass approach, where the tool starts farther from the actual line and works its way in. It's expensive wood, and hate to waste it trying to experiment without any clear ideas.

any ideas?
tc


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## Phil P (Jul 25, 2010)

Hi "Fly" and welcome

What you are experiencing sounds like "weld back" which occurs with many plastics materials like PVC, acrylic, etc and is caused by friction heating the cutter. There are several approaches to dealing with this; first use the biggest cutter you can carry because the mass of a larger cutter will help dissipate friction build-up; consider going to an O-flute design of cutter (take a look on the Onsrud Cutter website for details) rather than a conventional wood cutting cutter; try to reduce the spindle speed whilst increasing the feed rate - the idea is to create a true chip rather than dust off the cutter; if you haven't installed it already get some good quality (high volume) dust extraction on the cutterhead; consider cooling the cutter with compressed air if necessary. Finally, don't remove the protective film that the plastic sheets should come wrapped in from the manufacturer. One or more of those combined should help alleviate your problem. The aforementioned Onsrud Cutter website has a lot of good gen on machining plastics and is well worth a browse

Regards

Phil

PS Is that really your name? Oh, well, better than calling you "n/a". Please be polite and fill in your profile


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## MEBCWD (Jan 14, 2012)

I agree with Phil, 
If you increase you feed rate and slow down your spindle speed you should be able to decrease the heat build up on your bit. Also a larger bit will build up less heat if you can use one on your project. 

Another thing to consider is the direction of you cut. If you are climb cutting it can contribute to some of the heat build up you are experiencing.


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## flyforever (Jan 4, 2013)

MEBCWD said:


> I agree with Phil,
> If you increase you feed rate and slow down your spindle speed you should be able to decrease the heat build up on your bit. Also a larger bit will build up less heat if you can use one on your project.
> 
> Another thing to consider is the direction of you cut. If you are climb cutting it can contribute to some of the heat build up you are experiencing.


thanks for very good observations. I will try them out and report. In the meantime,
I did try making a rough cut that removes all but .010 from the item, and then with a finish cut I removed the last .010. This seemed to open the gap a bit more and leave practically no molten debris.

One reason why I am trying to stick to smaller endmills is that this material costs a lot, so I am trying to use every inch of it. I will also try a 0 flute bit instead of the 2 that I am currently using. 

Tony


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## Phil P (Jul 25, 2010)

flyforever said:


> I did try making a rough cut that removes all but .010 from the item, and then with a finish cut I removed the last .010. This seemed to open the gap a bit more and leave practically no molten debris.


Hi Tony

I'd sort of assumed that you'd be doing a hogging cut first then doing a finishing cut on plastics in order to overcome the polymer memory of the material which can leave really riugh edges. As you are doing that then the hogging cuts can be done at much higher feed speeds than the finishing cuts as the surface finish is immaterial. Should have mentuioned that because larger diameter cutters generally have relatively deeper flutes they can carry a greater chip load - that translates into higher potential feed speeds

Regards

Phil


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## fixtureman (Jul 5, 2012)

I cut pvc all the time if I use an .125 bit I cut it at 270 IPM and 10000 rpm .125 deep each path and If I use a .25 I nstill use those settings and sometimes bump the seed up to 300IPM


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## roofner (Aug 1, 2010)

I used that last summer on my deck what I did was cut a 3/16 by 1/2 rabbit and wide trim board at the 6 in piece on top and 4 in on thebottom and slide my latisse and use the rabbit as a track .
Worked great easy to get under deck. I cut these with my table saw with a high axillary fence came out nice and clean.


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## flyforever (Jan 4, 2013)

thanks for the great suggestions. I found that by adding a slight offset before the finish cut, all the melting goes away.
tony


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