This woodworker desires to know if he can use biscuit joints as a substitute of mortise and tenon joints. The quick reply is, in fact, it relies upon. But the bigger query is when ought to woodworkers use biscuits and when does a challenge name for mortise and tenon? Are there tips for making this choice?
Rick White: It all comes all the way down to energy. The biscuit joint doesn’t provide the energy you’ll get with a mortise and tenon. One subject to think about is how the piece is designed and the place the stress factors can be. As a normal rule, nonetheless, if I’m constructing furnishings, I exploit the mortise and tenon. Furniture will get loads of stress and put on and wants to carry collectively. For cabinetry, I’ll use a biscuit joint. Most cupboards simply cling on the wall and don’t see loads of stress, so biscuits are normally sturdy sufficient.
Ian Kirby: The bigger query asks about tips for making selections in regards to the energy of joints.
What we do is make our judgment primarily based on empirical values; we depend on expertise and statement. The consequence is that we normally make the joint overly sturdy. We don’t know by how a lot it’s overbuilt, however so long as it doesn’t break throughout its helpful life, that’s OK. Mortise and tenon sort joints are those we use to hitch elements collectively to make buildings which have to face up to stress ? chairs, beds, tables and the like. Maximizing joint energy appears to me to be a good suggestion – realizing extra about it was what brought about me to place ahead a proposal for a dissertation while finding out for my Wood Science and Technology diploma. It’s now extra years in the past than I care to consider, however loosely mentioned, it was a comparability of the energy of mortise and tenon joints and dowel joints.
Well, the proposal obtained nixed as a result of it was decided to be “insufficiently academic.” Instead I did analysis on some microscopic comparability between two uncommon species of softwood – a topic of curiosity to me and two different folks on the earth. A few years later, FIRA (The Furniture Industry Research Association), a British group, determined to fund analysis into what amounted to the identical examine I had proposed for my first dissertation.
The analysis was funded for one yr – no report. Then it was funded for a second yr – no report. Then a 3rd yr; the consequence was “there are altogether too many variables for the data to be of any practical value to a woodworker.” Here is a sampling of the variables:
– How tough or easy are the partitions of the tenon, the mortise and the dowel gap?
– How spherical is the outlet?
– How spherical is the dowel?
– How tight or free are the interfaces of the elements?
– What glue was used?
– How totally had been the elements wetted with the glue?
– What species of wooden was used?
– What was its development charge?
– Were the joint elements tangential or radial tissue?
– What was the moisture content material of the wooden?
– And so it stays.
In a mortise and tenon sort joint, there are two issues to take into consideration on the subject of energy: the mechanical energy and the glue line energy. To get a gauge of the mechanical energy, put the items collectively dry and take a look at the form of stress it could take to trigger them to return aside. In different phrases, how a lot does the joint depend on the glue with a view to keep collectively?
To illustrate the purpose, I’ll briefly describe 4 of the choices you typically have with this form of joint scenario: a standard MandT, a free tenon, a dowel joint and a biscuit joint.
Of the 4, the normal M and T is the one two-half joint. The others all deliver into play a 3rd factor. Since the sizes and sections of the elements differ, every mortise and tenon is designed to be as energy-balanced as doable. The tenon half is not any stronger than the mortise half. It’s additionally designed to have the utmost glue space doable.
A free tenon has a lot the identical energy as a mortise and tenon. What you start to understand now’s that type doesn’t comply with perform ? type follows financial system. You solely want one machine to make a free tenon: a slot mortiser. The query is: are you able to afford one?
In spite of all of the theoretical disadvantages and failings of a dowel joint, it largely works.
The biscuit joint has fairly a bit much less mechanical energy than the opposite three and its glue space isn’t nice. But it scores large on the accuracy of its glue line. The partitions of the joint are easy, the plate swells to type a good interface and, in lots of instances, it’s doable to double up the biscuits so the gluing space is doubled.
At the tip of the day, no matter you resolve, it’s essential to ask: Is the joint sufficiently sturdy to outlast the helpful lifetime of the piece?
Rob Johnstone: Mortise and tenon joints might be ornamental, however the principle motive to make use of them is to hitch two items of wooden in a powerful environment friendly method. My opinion, and I do know it’s within the minority, is that biscuit joints are useful for precisely aligning joints and nothing else. I don’t imagine an edge that’s joined for glue-up is any stronger for having biscuits in it, however it’s simpler to line the sides of the glue joint up in a rush. And if you’re edge gluing 5 or six boards, biscuits are an actual time-saver. So if the mortise and tenon joint is inteded to help in alignment of a chunk fairly than creating a powerful joint, I say go forward and biscuit away. If you’re asking if the biscuit joint can be stronger or superior in every other manner, I say no.
Ellis Walentine: Yes and no, relying on the sort and severity of the stresses you anticipate. Technically, a biscuit joint is a sort of mortise-and-tenon joint ? the place one piece of wooden penetrates and is glued into one other. Both kinds of joints hold elements aligned and supply resistance to twisting, shearing, levering and pullout forces on the meeting. The energy of each sort of mortise-and-tenon joint depends upon a number of issues.
1. The sort of wooden determines the mechanical energy of the joint: tougher, stiffer woods are extra proof against breaking.
2. The design of the joint can be essential. It includes the proportions of the mortise, the tenon and the “shoulders” the place the 2 elements really meet.
3. But, for sensible functions, the glue space is what actually holds the joint collectively. Biscuits don’t supply as a lot glue space as conventional tenons, so that you want extra biscuits to equal the pullout and levering energy of a tenon. On the opposite hand, biscuits typically present sufficient glue floor to maintain the shoulders tight, which makes the joint simply as stable. Biscuits are greatest suited to plywood case building, for which they had been initially designed. They are also high quality for lengthy edge joints, some cupboard doorways and different kinds of frames, together with these with mitered corners. Properly designed biscuit joints ? typically involving a number of, ganged or stacked biscuits – have additionally been used for demanding duties comparable to full-measurement doorways.
Personally, I choose the safety of deep, precisely machined tenons for heavier-obligation purposes the place higher mechanical energy could also be wanted.
