How thick is 16mm particle board?

[Glider]

I have just tried to dry assemble a set of pantry shelves made from 16mm melamine particle board bought from Bunnies. To add to the strength of the shelves I rabbeted the back and sides of the frame 8 mm deep to a thickness dimension taken from one of the sheets. Big mistake. Half of the shelves didn't fit the rabbeted slots. So I got out the trusty calipers and went measuring. The thinnest sheet was 15.45mm and the thickest 16.6.


On a manufactured item like this I would have thought the variation in thickness would have been 3/5ths of nothing. And maybe it was before it was exposed to atmospheric moisture. I mostly work with solid timber so this has been a good lesson.


Does anyone know if melamine coated MDF is more uniform in thickness?


Mick

[malb]

There will be some variation in both MDF and chipboard substrates, but 1 in 16 seems a lot. I am guessing that you bought the material in different styles/sizes of precut rather than as a couple of full size sheets to cut to requirements and edge.

I deal with packs (30 to 35 sheets) and we don't find a lot of variation within a pack, but some variation pack to pack. For our work (kitchens) absolute thickness is not critical and the place where we notice it most is edgebanding, as we have to accurately set the bander for trimming.

We use Australian made HMR (highly moisture resistant) chipboard for carcase work. I suspect that the material purchased would be imported and most likely not HMR, in which case the production tolerances might be somewhat broader, and prior moisture exposure would also have a significant effect. Once chipboard swellls form moisture exposure, it will not return to original thickness even if the moisture is removed. Exposed solid timber is more dynamic, it will expand and contract with variation in moisture content.

With composite materials like MDF and chipboard, the outer skin is denser than the inner core and carries must of the stress as the material is loaded. It is not normal to groove the sides for shelves etc as this leaves the material significantly unbalanced and prone to failure. This is why material thickness is not critical to our application, we don't cut a groove that has to accurately match the shelf thickness for cosmetic reasons.

[andrewr79]

I read the title of the post and thought "Surely he can't have just asked that" but on reading I can sympathize. You'll also find that besides the variation in thickness, it won't be square. I bought a piece to use as an under-desk tray, took it home ready to cut, and thankfully checked for square first because it didn't look right. Ended up having to take half from each end to fix the issue.

I think it's sad that we almost have to take a try square and calipers when we go shopping!

[Avery]

Did you measure your slots?

I bet they vary just as much.

I would have thought +/- .5mm or there abouts was OK for particle board.

[Ian Smith]

I think it's sad that we almost have to take a try square and calipers when we go shopping!

Andrew,
I am presuming you got your piece from the off-cut piles stacked in the walkways.
I learned very early on they are rarely, if ever, square. the best you can hope for is a straight edge or two.

They are, after all, off-cuts bought, no doubt, at cut price rates and in bulk from big maunfacturers and sold at a premium. Handy to fit in the family car but not a manufactured sheet.

Never checked them for thickness but it's anyone's guess where they came from, and how long they've been exposed to the elements

Ian

[Glider]

Did you measure your slots?

I bet they vary just as much.

I would have thought +/- .5mm or there abouts was OK for particle board.

Avery, the slot widths were almost spot on varying +/- 0.025mm. Any planar wobble is easily offset by the angular velocity of the blades and supplemented by a reasonably slow feed and a decent quality fence.

I concede that the acceptability of a thickness variation exceeding 6% is a matter of of opinion, but it sure surprised me. It's not something I would have expected to see through my own thicknesser working with solid timber; but I imagine the particle board makers use steel rollers.

Malb, how do you fix a shelf against a side, the outside surface of which is in plain view? One reason I slotted the sides was to avoid a visible row of screws or other fasteners.

Mick

[andrewr79]

@Ian

Correct - never knew that was offcuts. Will avoid in future though!

[thompy]

Not speaking for malb, but given he works in cabinets and "packs" probably makes his shelving adjustable such as seen in the 2nd and 3rd pic on this link White Melamine Cabinets Painted Red (Caromal Colours) | Fabulously Finished

They're not seen from the outside of the carcass.

Neal.

[malb]

Hi Glider and thompy,

When planning a kitchen, we use fixed shelves screwed through the sides and back, and adjustable shelves mounted on shelf pins fitted to predrilled holes in sides and back. To hide the screws in visible sections, we use panels that match the doors etc. These are screwed from inside the cabinet, so have no externally visible screws. The fixing screws near the front of the cabinet can be mounted under the hinge mounting plates to hide them. The rear screws are normally set as far back as possible and positioned just below the top and fixed shelves, or just above the base to reduce their visual impact. The panels are set so that their edge is flush with the doors etc.

[ian]

Malb, how do you fix a shelf against a side, the outside surface of which is in plain view? One reason I slotted the sides was to avoid a visible row of screws or other fasteners.

Mick

Mick
there's a range of knock down fittings designed for just that purpose

try googling "hettich cam fitting" and VB 35, VB 36, or VB 135 for examples

[Glider]

Thanks Ian. The cam fittings will certainly do the job. Hettich have a pretty impressive range although I guess I should call them first before trying to place my small order. Their catalogues are so extensive, I am also wondering if they have a showroom where I could get a closer look at how their various products are fitted.

Mick

[specialist]

I worked as a cabinet maker for 9 years and found that the processed boards didn't vary at all. Mind you I was working with packs. Most of the time we used adjustable shelving But, when you have 10 uniit in a row, any discrepancy will show in the overall length, often the variance was only 1-2 mm given that there were up to 20 thicknesses and shelf errors to contend with.

My guess is that the board is imported and from different batches which could account for the variation.

Robert

[Glider]

[QUOTE=specialist;1315048 My guess is that the board is imported and from different batches which could account for the variation.[/QUOTE]

Spot on, Robert! My mistake was assuming that the material had an identical thickness because it was the same brand,

Mick