Thread: Using MDF inserts as biscuits?

Biscuits are made from timber to give that lateral strength. MDF would not be a good alternative. Plywood would be better. But hey, it wouldn't take long to duck down to your local hardware and buy the real thing! You know they work!!!

Biscuits are only used for alignment in the assembly stage. As a joint strengthening device, they have the same consistency as Weetbix,as you will find on threads on this Forum from those who have disassembled biscuit joints.

Suggest you use some ply or Masonite biscuits that you can make yourself

Bammo,
It would be a mistake to believe that biscuits contribute anything to the strength of a joint.
Their sole purpose is to assit in aligning the pieces to be joined. Thereafter the integrity of the joint relies solely on the quality of the glue.

Biscuits have next to nothing tensile strength.

Bearing in mind that biscuits are a compressed material which expands to fit the slot I think that, if the MDF is a snug fit, it will perform a similar function.
Ply wood may be a little better and even make some contribution to the joint strength as a floating tenon but, again, it would need to be a snug fit.

Ian

edit - too quick Jill

All of the above, if you you wish to strengthen, make feathered joint, use a strip of ply the length of the joint stopped short at he ends this is called a feather. Personally, for a bookshelf I would just use glue and cramp method

Originally Posted by bammo

Hi All,

I need to join a heap of Jarrah boards as part of a bookcase project. Ordinarily I would biscuit join them but I'm out of biscuits (Triton type) and the bloke at Aus Timbers suggested I just use MDF instead.

My immediate thought was that MDF hasn't got the same lateral strength as a hardwood biscuit but thought I'd check with the group first...?

My immediate thoughts are
1) I don't particularly like cutting MDF
2) shaping ply, MDF or real wood into biscuits takes far too long -- ones from the hardware are much easier

Cool - that's kinda what I thought. Just means a drive out to Carbatec to get a bucket'o'bikkies....

Originally Posted by Ian Smith

edit - too quick Jill

No problem, Ian, wasn't sure if you would be reading this thread, so used your Weetbix analogy. You certainly convinced me

In the old days when gluing up boards for shelving and or the tops for bank tellers ( which were up to 1200 wide) we would groove the edges of the boards full length to approx 14mm deep and use ply strips of suitable thickness and width. If the bords were seen on the ends the grooves would be stopped short of the full length. Prior to gluing up the edges would always be shot with a no 7 for straightness.
In those days we always used animal glue. I can not remember ever seeing a joint coming apart.
Mac

I wouldn't bother using MDF as a form of joining material, it has no strength in tension. Malcome's suggestion is basically a long loose tenon and when made from quality ply (like marine ply) this is tried and tested method of joining planks into boards.

Triton biscuits are a bit too small, I use them myself but only for narrow rails and muntins where a size 10 bicky won't fit and the end result doesn't warrant proper M&T joinery.

I'm afraid I can't agree with the weetbix anology; maybe because I only use quality (expensive and not made in oz) biscuits. Real good biscuits are made from compressed beech, some are made from beech ply. These are bloody hard and strong as hell. I have hundreds of "Bix" biscuits that I only use where alignment is more important than strength; these are Australian made from....hoop pine????? Throw a handfull on the ground and some of them will actually break on impact; I suspect that many of the detractors of biscuit joinery may be using similar rubbish. Try Lamello biscuits for a real comparison; at a pinch and when I'm in a hurry I'll use Haron but only because they're available at the Big Green Shed. The only time I've ever had joint failure is on a jarrah outdoor storage box/seat where I didn't allow sufficient expansion room for panels and then moved from dry WA to humid QLD. In this case it wouldn't have mattered what kind of joinery I used, it would have still broken due to the massive internal stresses.

The other important point is the slot and the mating faces; these need to be ACCURATELY cut and flat. You need to use a glue spreader that puts the glue into the slot and coats the two sides. Coat the biscuit too. PVA glue works well but you need to use one with a long open time because getting the glue into where it needs to be takes time unless you can get a good quality gluing system that speeds the process up. If the glue surface skins over before you join the wood together then you've already lost some strength as it can't penetrate the wood as deeply.

Where biscuit joinery really shines though is in alignment. I've made plenty of bookcases and have always biscuited the planks together, the resulting boards need very little if any planing afterwards; usually just a lick with a smoother over the joints. Some bookcases I make solid with the shelves biscuited into place, these are as strong as hell and I've never had one break.

Originally Posted by Chief Tiff

I suspect that many of the detractors of biscuit joinery may be using similar rubbish. .

Cheif T,
I gather from your post that you would regard Triton biscuits as quality.

I am the originator of the Weetbix analogy and I'm here to tell you that those Triton biscuits (the only type I have ever used) failed comprehensively under the stress of a poorly assembled post and rail panel door in humid conditions.
I was able to disassemble the doors with one of those "break off the blunt bit and push out a sharp bit" type of hobby knife by simply slicing through the biscuits which were exposed at the failed joints. Several of these biscuits exhibited fracturing.

I would agree that biscuits assist in alignment but disagree that they contribute any additional strength to a joint.

It'll be a cold day in hell before I use another one.

If your experience is to the contrary then so be it

Ian

Triton biscuits are of a somewhat less than average quality; they are an Australian product (like Bix) so are unlikely to be manufactured from decent timber. I can't identify it and it's nowhere near as strong as beech. Yes, I do use them but "only for narrow rails and muntins where a size 10 bicky won't fit and the end result doesn't warrant proper M&T joinery". The box I mentioned in my post that self-destructed due to inadequate allowance for expansion was constructed using Triton biscuits; it was the only big thing I ever made with them and never again. I don't use them where structural integrity is important. When my bucket runs out they will be replaced by bickies of a decent manufacturer but will still only be used for simple non-stressed joinery.

I was under the impression that your dislike of biscuit joinery stemmed from experience with the proper tooling, to denigrate a whole system of joinery on the results of a somewhat unique local manufacturer who uses crap material is a bit...well...unfair. You have put your argument so forceably that there are now forumites who are quoting you with probably even less experience to their names. If you were specifically bagging the Triton system for outdoor use I would understand and probably agree with you, but real biscuit joinery is quite different. I don't claim to be well experienced in biscuit joinery but I've never had a failure with the proper gear; and as for strength there is a set of oversized kitchen cabinets that have been hanging off a wall in Victoria for the last 9 years holding a shedload of crockery. Every joint used biscuits aligned so they are under shear load.

Lastly Bammo is building a bookcase, even Triton biscuits are more than adequate for joining planks into boards for this task. I wouldn't use Triton biscuits for supporting the shelves unless the bookcase is really small but he hasn't indicated how he's going to fit them yet.

Chief T,

Firstly, lets not confuse the terms "joinery" and "alignment".

I have many items of furniture in my house which I have built over the years, and the vast majority were constructed using biscuits....Triton, as it turns out, because that was the system I had, and Harron for the cheap as chips Ryobi biscuit cutter for the jobs the Triton could not do, such as faces. Now, in spite of my inexperience, I was in IT not woodwork, so just a week-end hacker trying to save a bit of cash, they all seem to be holding together.

Until recently I had presumed that the biscuit was a version of the floating tenon and therefore was a useful addition to any joint and made it much stronger by it's presence. However, the recent near disaster in my bathroom reno has altered my opinion. It appears to have been a combination of poor practice and bad glue, but whatever the cause, the result was that 6 panelled doors literally fell apart at the joins, exposing the biscuits I had inserted in the belief that I was making the joint stronger. As I said, in doing the repairs, the doors were very easily de-constructed. Most of the biscuits showed evidence of fracturing, so even with poor quality glue the biscuits were failing, and all were easily sliced through with a knife.

I don't doubt that your Victorian cupboards are still on the wall, but in my view it would be a mistake to assume they are there because you used biscuits.
They are still there, and my furniture is still holding together, because the glued surfaces are holding up to the forces exerted on them, be they shear or tension, and they look nice because biscuits were used to make sure they all the bits lined up.

I really should qualify my statement in my previous post to read that " It would be a cold day in hell before I rely on a biscuit to reinforce a joint". They still have their place as an alignment device.

There are members of this forum with experience ranging from vast to half-vast and all are entitled to their views and to express their opinions. It is up to each and every one of them to either take what is said as the gospel truth or do more research before coming to any conclusion.

My opinion, based on my own experience, and from reading the many contributions to this forum, is that "biscuit joinery" is an oxymoron.

Ian