The Trouble with Heat

One moment you are working away quite happily, the CNC is humming along and everything is just so.

The next, things are NQR – items are breaking loose, engravings are either too shallow or too deep, and the vacuum table is not doing its job.

It is the afternoon of a warm day, and the MDF spoilboard had suddenly started to warp and flex, pulling away from the vacuum table. It wasn’t something I had experienced with very thick MDF, but once the thickness had come down to around 8-12mm or so, it was a real problem.

I first thought the solution was easy – buy more 32mm thick MDF, after all, the first lot came from Bunnings.  But there was a problem.  That was apparently intro stock for the new store, but not something they were going to carry in the long run.

I got to thinking, and one thing that I had tried unsuccessfully, was to place a piece of 3mm MDF on top of the spoilboard (on top of the vacuum table). As a trial it was unsuccessful – to many losses in the system.  But what if I ditched the spoilboard altogether, what then?

So I milled the base really flat, (still a sacrificial piece), then instead of placing a spoilboard on top, I placed a simple sheet of 3mm MDF on top.  It is thin enough that air is drawn straight through it without having to mill off the heavily compressed portions top and bottom, and it is a very uniform thickness.  Also, rather than having to mill, and re-mill it flat as it gets chewed up, it can be flipped over and the other side used, or simply discarded and another $2 sheet bought in.

The other main drawcard of the 3mm spoilboard, is it is thin enough that the vacuum base pulls it flat whether it wants to tend to curve or not.

The first few runs really proved how effective it is.  Not only was the sheet being cut held down well, and very flat, there was significant vacuum that kept the pieces in place as they were cut free, and drove the dust deep into the cut groove.  This was packed rather tight, so the bits did not move even though no tabs were included, even when the entire board was picked up and turned over to sand the other side.

Small refinements to technique, as a result of an adverse situation.  Happy days.

6 Responses

  1. Been hot and humid, has it? Raw mdf is a bit of a sponge, I’ve had my share of surprises with its behaviour.

    You might try scrounging building suppliers for 32mm mdf. It’s sometimes used as cover sheet on packs of doors etc, last time I bought some I picked it up for ten bucks a sheet with a minor bit of fork damage. Not bad for 2400×1200 and 2700×900!

    • Just hot- warps the boards mid afternoon that then become flat again in the evening. My latest attempt with 3mm MDF is working well. The base (which I have routed all the air channels in for the vacuum table) is 32mm MDF, and I did have that as the top as well, but it got sacrificed away! When I went to buy more, it turned out it was only a new store opening deal (Bunnings), and not a stock item.

      Thanks for the suggestion on the door suppliers – will look into that.

      Cheers

      • I’d be surprised if it was just heat on its own. MDF is pretty dimensionally stable within normal temp ranges, and doesn’t have grain stresses like solid wood to be as affected. Humidity does have a noticeable effect. I encountered it when using a dado head to cut housing joints, some I had previously made and checked no longer fitted. So I went looking for an explanation and found this PDF, which indicates a linear hygro expansion of 6% in thickness. I thought it unlikely, but leaving the pieces on the bench for a couple of days and sure enough they fitted together.

        http://www.ewp.asn.au/library/downloads/ewpaa_facts_about_pb_and_mdf.pdf

        I haven’t gone as far as putting a thermometer and hygrometer in the shed, but maybe I should.

        • Bought home a stack of 3mm & 6mm MDF yesterday (1220×2440)

          Not a humid day, but definitely hot & sunny. Had the boards out in the sun as I was ripping them down to size, and by the end of the session, there was a good 25mm or so warp (if not more).

          Didn’t worry me, as I knew once I had them in the cooler shed, lying flat on the concrete floor, the warp would drop straight out over the remainder of the day.

          Sure enough, by today they were all as flat and straight as you’d want.

  2. you could try Tile Importers in Oakleigh they often stock the 32mm plus theyre worth a look around for the odds and sods they stock.

    • Will have a look. 🙂

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