I am using a powermatic table top mortiser. I am drilling some 1 1/2″ deep mortises in table legs. the mortise is 3/8 x 1 1/2″. The mortising chisel is a Clic. Following Roland Johnson’s advice in his article in FW #185 “Get more from your mortiser” I am drilling straight mortises in each end of the slot so as to keep the mortise straight at the ends, then I drill out the center of the slot. The procedure worked fine at first but after about 8 completed mortices ( I need sixteen because I am using double tenons) the mortising chisel began binding on removal as the mortise depth exceeds about 1/2″. I deal with this by drilling the mortise in several passes but there is more hand chisel clean-up now because the ends of the slots tend to slant inward. Is this a common problem with deep mortices or is perhaps my technique in drilling the deep mortices faulty? Is it possible the friction/heat on the chisel is actual changing it’s dimensions or perhaps bending it (I haven’t put a straight edge on it but eyeballng tells me it remains straight). I have experienced this problem in the past using other mortising chisels. I have polished the chisel faces down using down to #600 wet dry sandpaper. Thanks in advance for any advice.
Mike
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Replies
Mike,
I forwarded your question to Roland Johnson. I'll let you know what he says.
David Heim
FineWoodworking.Com
David
Thanks for contacting Roland. In the meantime I have a thread working on knots/ joinery and I think I have worked out most of my problems and am having great fun doing it. I have come to the conclusion that I misinterpreted Roland's guideline to drill each end of the mortise slot before cutting out the web. Roland doesn't say that the end mortises should be cut to full depth in one push but that's what I assumed. If I cut the mortises in two or three layers the problem disappears.
MikeSawdust & Sunsets
Mike,
Glad you're on the way to solving the problem. Here's Roland's reply, which has plenty of useful advice for using a mortiser without problems:
Without being able to look over your shoulder or try the machine myself I would have to say that the most likely cause of your slanted mortises is techinque. You are starting the mortise correctly by cutting the ends first. The rest of the mortise should be removed by making plunges with the mortise bit fully supported on all four sides. The web or bridge between each plunge can be almost as wide as the mortise bit. Fully supporting the mortise bit eliminates the typical tendancy for a a bit to angle toward the unsupported, open mortise, especially as the bit dulls slightly. The webs between the holes are easily removed and won't cause the bit to flex and drift.
Keep the bit sharp. Some bits won't stay sharp through more than few full mortises.With each plunge make several extractions, leaving the tip of the chisel in the mortise, to assure complete chip ejection. Chips binding in the mortise can also cause a wandering bit and also unacceptable heat levels in the bit. Clean out the mortise frequently. Sawdust and chips can bind between the tapered bit and the straight sides of the mortise. I use a quick blast of compressed air to clear away the detritus. (make sure you have your safety glasses on when you do this) Heat generated during the mortise plunge will slightly change the shape of the bit; simple physics. Most bits are slightly tapered so the body of the bit is clear of the sides of the mortise and only the chisel end is tight in the mortise, eliminating any problems caused by a minute increase in the size of the bit from heat expansion. Problems can also arise if the wood is not sufficiently dry; the damp wood swells slightly binding the bit on extraction. This problem would only happen with wood with a moisture content quite a bit higher than is acceptable for furniture contstruction.
My recommendation: Keep the bit fully supported during the plunge, make sure the bit is sharp and that the chips are cleared often and use a feed rate that doesn't overheat the bit but is fast enough to keep the chips moving up the auger.
Roland
ref "...Keep the bit sharp. Some bits won't stay sharp through more than few full mortises.With each plunge make several extractions, leaving the tip of the chisel in the mortise, to assure complete chip ejection."
It had never occurred to me that when I remove the tip of the chisel from that mortice I am leaving chips in the mortise that might increase friction on the next plunge! Moreover, I get lazy about sharpening, assuming that the chisel will stay sharp for the entire project. My Bad.
BTW, I never invest in the most expensive hollow chisel so perhaps my cheap chisels lose their edge more quickly. Which brand do you prefer?
Thanks for the help
MikeSawdust & Sunsets
Mike,
You mentioned that you use Clic mortise bits. Do you mean Clico?
If so, those are good bits, I use them, and they should hold an edge as well as any. Keep in mind that a mortise bit gets quite a workout and that even the best quality mortise bits can require frequent sharpening. I doubt that anyone could chop that many mortises by hand without sharpening their mortise chisel a few times. I'm glad that you are finding solutions to your problem on FineWoodworking.com; it's such a tremendous resource.Good luck,
Roland
Clico, right.
Thanks for the help.
MikeSawdust & Sunsets
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