Friday, February 20, 2015

Gateway (56)

Post 56 in an ongoing series describing the design and construction of a kabukimon, a type of Japanese gate. This is a project for the Museum of Fine Arts in Boston. Post 1 in this series can be found here if you'd like to start at the beginning. Each post links to the next at the bottom of the page. Recent installments also to be found in the 'Blog archive' index to the right of the page.

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Time to get something done around here. Can't let a little head cold slow you down. Actually, it was good to have rested a day I think as I feel half decent. Looking forward to the arctic weather coming to end, let me tell you.

Setting up for tenoning on the shaper takes most of the time in terms of the entire task:


The tenoning stop is in place and things are getting almost to the point where wood could be introduced to the situation:


After a few test cuts on an extra piece to calibrate the cut, tenoning can proceed:



I look a bit like the Michelin Man during his 'homeless migrant period' - wearing 4 jackets is an inexpensive way to look like I've bulked up I guess.

The cutter head is so large and the tenon so deep that the tenoning hood only covers about half-way, which means a certain amount of chip spray.

The tenoning doesn't take much time at all - these are the ends which connect to the hanging stiles on the main doors:


Another view:


I've got a little crack in one of them, but as these are wedged joints I'm not too concerned about it. I'll glue up the crack in the next day or so.

After doing one end of each rail, the cutter head is swapped out on the shaper and then following some calibration cuts, the opposite ends of the rails were processed:


Another view:


Tenons are 123mm (4.84") long. Very pleased with the way these came out.

The twin tenons are supposed to come in at 15mm in thickness, however they were about 0.1mm under:


Hence the reason to leave off cleaning up the corresponding mortises until after the tenons are done. The heads could be disassembled and the shims played around with - it wouldn't take much to dial it right in - but it will not be necessary since I haven't made the mortises to the nominal values, and can easily adjust them to suit these dimensions. The other ends of the rails have 20mm tenons and those came in exactly at 20.0mm

While there is a bit more work left to do on these tenons, the rails are now most of the way to completion:


Tomorrow I should be able to complete the tenoning on the side door rails, along with the tenons on the battens for all doors. Once the shaper is set up, production goes pretty fast.

All for now - thanks for visiting the Carpentry Way. On to post 57

2 comments:

  1. What tolerance do you aim for with the thickness of tenons and widths of the mortices? Should they be the same, or, if not, how many thou less on the tenons?

    ReplyDelete
    Replies
    1. Hi Tico,

      thanks for the question.

      Tolerance of fit is an interesting topic. It depends partly upon:

      - which species of wood you are using, certain woods being more compressible than others
      - whether the joint is glue-less or glued. If glued, you need to leave at least a couple of thou for the glue
      - how much friction is part of the equation. A long tenon passing through a post and into another beam, for example, would be harder to draw together than a stub tenon.
      - whether the joint is exposed or not. While a slight looseness in fit might be acceptable in a blind tenon joint, it is less acceptable in a through-tenoned joint. Put it another way: blind joints are more forgiving of errors than exposed joints.
      - the qualities of the wood which affect how well it slides. Some woods are oilier or more slippery than others and can be shoved together more readily.
      -the size of the joints. Consider fitting, in the same species, a 1/2" tenon with say 5 thou of interference versus a 2.5" tenon with the same 5 thou of interference. The larger joinery can tolerate a greater amount of interference as the percentages of compression, are the same. If the percentage the material compresses is, say, 1%, then 1% or 2.5" is a larger amount than 1% of 0.5"

      I think only with the very hardest woods, without glue, would you look for a fit which the mortise and tenon were exactly the same size or the joint was a thou or three on the loose side. Whether the loseness falls on an undersize tenon or an oversize mortise hardly matters. A wood like ebony or lignum vitae for instance has next to no 'crush' so it you have interference and try to put them together you risk splitting the mortise.

      Softer woods, like the cedar i am using, in the sizes i am using, can surely tolerate 1/100" interference and probably more. I'm generally working with 5 thou interference as a target, and in certain connections am allowing more or less room as the situation dictates.

      Add into the mix the practice of pre-compressing the grain on an oversize tenon so as to achieve a fit....

      Of course there are dimensional targets you desire to hit and then thee are the targets you actually hit, and only on a good day do those lists jibe to a fuller extent.

      ~C

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