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I am wondering if there is any guideline for using fasteners (screws, dado and biscuit) for joinery. Specifically, I am wondering about how to determine where those fasteners are located in the joint and how many are needed in total. I was trying to find the info from the internet but could not find any relevant info and was wondering if there was any principled guideline.

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    Dados are not fasteners. A dado is a slot cut into the face of a board, which another board is then inserted into. Biscuits are used in joinery, but they are not considered a fastener. They are used for alignment, not structural strength. That leaves screws, which are fasteners. I do not know the answer to your question, but I thought maybe this comment could help clarify some things nonetheless. – Charlie Kilian Apr 28 '16 at 22:41
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    Are you talking about spacing and edge distance, for instance? – grfrazee Apr 29 '16 at 14:30
  • @grfrazee That is right. Is there any rule-of-thumb or guideline for spacing and edge/end distance? – DSKim Apr 29 '16 at 17:44
  • I usually do it so it looks right to me. Give me a couple hours and I'll add an answer about fastener spacing based on the NDS code for wood structures. It's not required for furniture, but good guidance nonetheless. – grfrazee Apr 29 '16 at 17:49
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Specifically, I am wondering about how to determine where those fasteners are located in the joint and how many are needed in total.

As far as I'm aware, there are no real guidelines or rules of thumb for this sort of thing besides "make it look right to your eye." I understand that this is possibly the vaguest of ways to put it, but that's really how it is.

Furniture is generally designed to be aesthetically pleasing. As such, the proportions are more fitting with what looks right, not so much with what's structurally-sound. This means that most furniture is way overbuilt from a strength perspective, and this usually includes fasteners.

For example, a #10 wood screw can probably be relied upon to resist at least 300 lbs of shear or tension. You will likely not see these magnitude of forces in a properly-designed piece of furniture.

WARNING: ENGINEERING CONTENT TO FOLLOW


That being said, I'm going to attempt to outline some recommendations based on the National Design Specification (NDS) for Wood Construction, 2015, which is an American standard for the design of wood-framed buildings. Consider the following two images, which depict loading parallel to and perpendicular to the grain of specific members (NDS 2015, Fig. 12G):

12G-1

12G-2

In these, you can see the spacing between fasteners and the edge distances for the fasteners called out. For wood building design, the NDS specifies minimum spacing and edge distances. Table 12.5.1A below shows the minimum edge distances.

Table 12.5.1A

The value D is the diameter of the fastener. For example, for loading perpendicular to grain, an edge distance of 4D is required for full fastener strength.

Table 12.5.1B below shows the minimum spacing between fasteners.

12.5.1B

Again, these tables are meant for use with wood buildings and don't really apply to the design of furniture, which has loading several orders of magnitude lower. However, the spacing requirements given above can be applied to furniture design, just on a smaller scale.

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I know this question was asked some time ago but if anyone else is searching for the answer here is a link to an easy guide for determining screw placement on wood projects.

https://www.woodworkersjournal.com/easy-math-formula-spacing-fasteners/

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    The problem is that single-link answers age poorly. This link will probably be dead in an internet moment which makes this answer not very useful for future visitors. Why not give a brief synopsis of the relevant parts of that link here? – jdv May 17 at 16:04

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