Why set mortise gauge pins wider than chisel??
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19 July 2018 at 3:42 am #549500
At around 1:40 in video 3 of the workbench build series, Paul states that he sets the mortise gauge pins just a bit wider than his chisel. He goes on to say not to worry because the tenon will be marked out at the same width.
I get that the mortise and tenon will have the same dimension, however I don’t understand why he doesn’t just set it exactly to the width of his chisel. By not doing so you would always be a little thin while chopping the mortise. In other words it seems like you would have to pare the sides of the mortise to get it to fit properly.
Also, do you just pick one of the mortise lines and put the chisel against it and stick with that side as you move along chopping?? Or do you try to split the difference and chop in the middle of the mortise lines knowing that the chisel won’t touch either of the lines exactly?
I can see how having a thin mortise could aid in getting a tight fitting joint. However another videos I swear he to make sure to set the points of the pins to the outer corners of the chisel.
19 July 2018 at 10:37 am #549506For a through tenon, you don’t want to see any gaps where the tenon protrudes through the mortise, so it’s common practice to set the mortise gauge just a little wider than the chisel you intend to use. Remember, you’re going to use that same gauge setting to mark the tenons. Just stay between the two gauge lines when you’re chopping. After you finish chopping, paring the sides of the mortise out to the gauge lines will form perfectly straight mortise sidewalls and a tight fitting joint. Some types of wood cut easily and you end up with a pretty neat hole right from the chisel but most likely the sides won’t be perfectly straight until you pare the opening. Other stuff, like oak, will tend to want to leave very ragged edges but that all disappears when you pare the sidewalls.
Appearance usually isn’t so critical for stopped tenons. The tenon shoulders are going to hide the mortise opening so you can be tighter with the gauge setting and not worry about paring the sidewalls of the mortise. Tenons are cut the same way regardless of whether it’s a though tenon or a stopped tenon. In theory at least, if you set the gauge pins to the exact width of the chisel and you use that same gauge setting for both the mortise hole and the tenon and if you cut both the mortise and the tenon exactly to those gauge lines, it should be a perfect fit. In practice, that’s nearly impossible to do so we cut the tenons a little fat (1/32″ or so outside the gauge lines) and then fit them to the mortise hole either by very carefully paring them or by planing them like Paul does using a router plane.
20 July 2018 at 12:33 am #549515Thanks Harry for a very thorough answer. I have heard others say that you should never pare the mortise, rather only the tenon should be pared to fit the mortise.
“Never” is rarely an accurate word to use it seems. Thanks again!
20 July 2018 at 2:17 am #549518You’re welcome Seth. And true enough, you don’t normally pare the mortise – just the tenon. You should always fit the tenon to the mortise no matter what kind it is. In other words, don’t pare the mortise if the tenon is still too fat. Pare the tenon to make the joint fit. Once in a while, if you forget to stay plumb or some “fuzzies” got left in the hole or something like that, you might have to go back and pare a mortise a little to straighten things up, but with a little practice you won’t need to do much of that.
A through tenon needs a little extra care. If you set your gauge to the exact width of the chisel all it takes is one slight misplacement of the chisel or side-to-side movement when you lever and the side of the mortise hole will have a little divot that will show. Worse still, after the mortise is chopped, it’s just about impossible to go back and run any new gauge lines to pare to. I think on the video Paul did on just a mortise and tenon joint, he made a through tenon without paring the mortise at all and in oak to boot. Perfect fit, but that’s Paul! Mine don’t ever seem to turn out like that – go figure?
Thanks from me too Harry. This clarifies things. I cut a few practice through mortise and tenon joints in pine with little or no paring of the mortises and they were not bad. But oak? I just cut four stopped mortises in oak for a small table. Though my tenons fit beautifully, the mortises were ragged looking around the edges. All that raggedness is covered up by the tenon shoulders, but if I had wanted a through tenon it would have looked terrible. So I now see that paring the mortise to the line is the way to go for through tenons in oak. Thanks again.
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