I'm cutting parts for another shop and I've run across something I've never seen before. It's the first time I've cut Rangerboard, which I understand has a pretty good rep as far as MDF goes. It's been mostly Plum Creek in the past, but I don't cut MDF often. The Rangerboard sheets are about 1/8 out of square coming from the plant, but I don't know if that's significant. Cabinet parts are cut and seem fine initially, but over the next few days, I get a callback that some appear to grow or shrink or parallelogram by up to 1/16 over 48". These parts are good sized, some 24" x 60", far too big to be shifting on the table during processing. I skin cut down to .040, then a final pass to help clean the kerf. Cut quality is good, tool is new, and parts check ok coming off the router. When talking to the other shop's sawyer, he mentioned that when cutting this material on their slider, he'd have to oversize parts in order to deal with the apparent tension within the sheet, with long rips banana-ing on him. So my thought is, if this tension is prebuilt into these sheets, where does it go when cutting on a router? Anyone else experienced this?

Are you cutting the first pass oversize?

Not yet but I've considered it as an experiment to see if it behaves differently than the non-oversized parts.

We always cut the first pass 0.020 to 0.030 oversize.

On all parts or just MDF? And do you do this to solve a particular problem?

That and less dust in the grooves.

Melamine and MDF.

Here are some destructive testing results for posterity. I routed 2 rectangles 24 x 96 out of a 49 x 97 sheet, burning 1/8 off the factory edges. After a thorough cleaning and pushing the parts together, the rangerboard touched at the outside corners and left a .060 gap in the middle. Flipping both parts and pushing the previously outside edges together, they contacted in the middle and left a gap at both corners. I now have 2 bananas.

I ran the same test with a leftover sheet of Weyerhauser MDF, formerly Plum Creek, and got 2 proper rectangles, with no measurable gaps. This appears to be a cut and dried demonstration that 1) my machine is healthy, and 2) CNCs can make bad parts if the material is under stress. Dropout, I'll be using your strategy to oversize parts by a bit for the first pass from here on out.

Sounds like you need to get your customer to change their sheet supplier. We have had a good bit of issue with tension in MDF but mainly with regards to heavily carved on one face. It would seem to create havoc for MDF door shops but... To think there would be that much lateral tension is insane.

Cutting oversize is fine but in my mind your porked on small parts in that your having to take a pretty hefty final cut which somewhat negates the skin on the final pass. But I guess you buy more vac.

Id be telling them you cant meet tolerance with their supplied product but you can supply a better product that can.

Stress Cut on Slider or CNC We get this on both over 42"

On these larger ones we are precutting over size let them sit a day, then finish cut on slider.

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