Biscuit and slot sizes

This post is about the thickness of biscuits and biscuit slots and the related sundry issues I found while looking in to the topic. Some of these seem to be traps to catch the unwary and unfortunate.
If the internet is to be believed there are different standard thicknesses of biscuit, as follows.
* 3.5mm. According to the following link the Draper blade is only 3.5mm thick and the reviewer even commented that he had to file normal biscuits to get them to fit.
(Amazon.com product link shortened)
* 3.8mm. The following ad is for a SIP blade that is 3.8mm thick. Is this the real standard blade thickness and their specification is more precise than manufacturers who describe their products as 4mm?
http://www.sipuk.co.uk/sip-64529-blade-for-07904-biscuit-jointer.html
* 4mm seems to be the standard size but it could be a nominal one. The following video shows biscuits being too small for the slots and falling out whereas in other videos I have seen the biscuits are a snug fit even when dry. Could the presenter in this video have the wrong type of biscuit? The relevant section is 12:30 to 14:30.

https://www.youtube.com/watch?v=xZLi_L43aUM

* Biscuit slots in chipboard.
Chipboard is more compressible than other timber. Would biscuits be as effective in chipboard as they would in other types of board? I recall seeing something about using them to join kitchen worktop but cannot imagine they would be much good in that case if the worktop is chipboard based.
* Biscuit material
I saw a reference to biscuits being made of comressed beech so that they expand once glued and such biscuits have hatching patterns on them where they have been through compression rollers. By contrast I also saw reference to uncompressed birch plywood biscuits that did not have hatching marks. Are the latter designed for MDF or other weaker material? According to the following link, normal biscuits can cause a bulge in MDF. You can see in one comment that it refers to birch plywood biscuits being uncompressed and used in such cases. So it's beech vs birch....
http://forums.finewoodworking.com/fine-woodworking-knots/general-discussion/biscuits-causing-budges-mdf%E2%80%A6
James
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wrote:

If you ignore everything about biscuits made or designed by Merkins and just take the word of Lamello who invented the concept in the first place then the biscuits are nominally 4mm, the blade kerf is exactly 4mm, the biscuits fit snugly, always. They swell the right amount, always.
Piss about with things made to how the Merkins think they should be made, redesigned as they are in a semi-random manner to imperial standards etc and you will have problems.
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On 04/09/2014 09:43, James Harris wrote:

Tthat's not a standard biscuit size. (although I expect you could stick a different blade on the machine)

Try this - gives a fairly good demo of biscuit size:

https://www.youtube.com/watch?v=_YIGavdFp0U


IME they should make a nice light interference fit - if you turn the wood upside down with no glue on the biscuit, they will still stay put in the slots.
If you use undersized biscuits / oversized blades then you get the problem Colin highlights in the video.

They were originally developed for chipboard edge jointing, since once glued into place they don't weaken the edge of the particle board, but simply become a part of it. Since they are quite wide they also spread the load out over a wider area than say dowels.

Not sure if the latter are specifically designed to be non swelling so much as just cheaper! (even original lamello biscuits are actually quite cheap if you buy a big bag).
I have some birch ones that I bought from TS - they are ok, but as you say, smooth in finish. (they feel more like plywood cutouts than traditional biscuits)

Yup they are designed to swell very slightly when they are glued in.
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On Thursday, September 4, 2014 12:08:56 PM UTC+1, John Rumm wrote:

and they will swell slightly if kept in the wrong environment. Running them through a microwave is reputed to restore them to correct size but it never made much difference when I tried it. YMMV
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On 04/09/2014 15:46, fred wrote:

Bit like welding rods - better to keep them dry in the first place...
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