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I'm a beginner. I'm concentrating on hand-tool joinery for now. I've watched a lot of YouTube videos, trying to learn joinery. For the purposes of this question I'm most interested in mortice & tenon joints and dovetail joints.

When fitting a joint it's clearly necessary to remove a little wood where the joint doesn't fit yet. But, best not to remove wood that doesn't need to be removed since (e.g.) this can make the joint fit more loosely than necessary.

So the basic idea is only to remove the wood that's preventing the joint from fitting. Often, videos show people making a trial fit and then figuring out where the wood is "bruised" in order to deduce what to remove. But I can't see this bruising on the videos, myself.

At the moment I find tenon fitting seems easier, since I can do a trial fit and then wiggle the part-way-inserted tenon in the mortise to figure out where the tenon pivots; this is the high point and I can remove material from either the mortise or the tenon to make it fit (and the shoulders of the tenon will hide some mistakes in this). Sometimes, the layout marks may provide a hint as to what to remove, but that's not the case that's interesting to me here (since I already know how to do that). Anyway, in this way I can more or less fit a mortise and tenon joint without being able to spot the bruising.

However, with a dovetail joint there are multiple places where the joint could be binding during a test fit, so spotting bruising is, I suspect, more important for dovetails.

So my question really boils down to these specifics:

  1. How do I distinguish the wood that must be removed to fit a joint from the wood that should not?
  2. How do I identify where wood is "bruised"?

Pointers to learning materials would be very welcome (i.e. you don't have to explain it all directly in your answer if there are e.g. videos showing how to work on these skills).

  • Just a quick point: remember that if you overshot you can often glue a shim to the surface and try again, and if you come at all close to matching grain folks will have to look pretty darned closely to see the patch... certainly much closer than spotting a gap in the joint. – keshlam Jan 6 '17 at 18:38
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When fitting a joint it's clearly necessary to remove a little wood where the joint doesn't fit yet. But, best not to remove wood that doesn't need to be removed since (e.g.) this can make the joint fit more loosely than necessary.

Yes this is very important with most glues used today. Some glues have good gap-filling properties but the three main ones in use with leisure woodworkers today — PVA (both white and yellow types), foaming polyurethane and room-temperature hide glue — all have poor to very poor gap filling, so the strength of loose joints is significantly compromised.

So the basic idea is only to remove the wood that's preventing the joint from fitting. Often, videos show people making a trial fit and then figuring out where the wood is "bruised" in order to deduce what to remove. But I can't see this bruising on the videos, myself.

Bruising may not be the best word for it. It's sometimes referred to as burnishing, which I think is the better term since it gives an immediate idea of what to look for, which is slightly glossy areas.

Wood type (and colour) play a big part here, in general it's extremely easy to see the burnishing on darker, denser woods and much harder on lighter and/or low-density woods. So you'll find it much easier usually to see what you need to see on ABW than on poplar :-)

With softwoods in particular the burnishing may only show up properly on the dark grain lines, since the pale areas in between (the soft earlywood) can be so yielding that it doesn't show the rubbing at all.

At the moment I find tenon fitting seems easier, since I can do a trial fit and then wiggle the part-way-inserted tenon in the mortise to figure out where the tenon pivots; this is the high point and I can remove material from either the mortise or the tenon to make it fit (and the shoulders of the tenon will hide some mistakes in this).

This may indicate that you're over-thinking the fit of joints a little bit. With a M&T the tenon doesn't need to slide in evenly, it's great if it does and by all means aim for this, but basically all it needs to be able to do is slide into the mortise, maybe with the help of a couple of mallet taps (an easy sliding fit is too loose, although you can get away with it).

If it's fairly tight in one spot that's OK, as long as you can still get the tenon into the mortise once glue is applied you're good to go. Shaving off that high spot, even removing just a few thousandths of an inch (hundredths of a mm) will actually make the joint slightly weaker.

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  1. for all things like this, it helps to have the right kind of lighting. A low raking light is useful for highlighting subtle differences in wood surfaces - like this burnishing or in preparing a surface for finishing.

  2. it can also help to rub some soft pencil lead on one of the mating faces. When you try to push the joint closed, the lead will come off on the other face at the binding locations.

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