Hollow chisel mortising question - Woodworking Talk - Woodworkers Forum
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post #1 of 8 Old 07-26-2009, 03:01 AM Thread Starter
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Hollow chisel mortising question

Greetings from Colorado,

My name is Bill and I am an amateur woodworker/hobbyist - at it for about 15 years (I am almost 50 now). This is my first post and I need some advice. I am building a set of six Harvey Ellis style dining room chairs out of quarter-sawn white oak. I have some very nice black ebony that I plan on using for a detail on each chair. Up at the top end on the front of the rear chair legs I want to put four 3/8" square holes (about 1/4" deep) and glue 3/8" ebony squares in the holes. I have a Delta hollow chisel mortising machine but when I drill out the squares, there is a small but noticeable round cut on a couple of the faces of each square that is slightly outside of the square. I could use some advice on how to deal with this problem - I could go in afterwards with a chisel and enlarge the hole a little bit but I want consistency; the cost of ebony is quite expensive (as well as all the time invested already in shaping the legs) so I really don't want to screw these up. I am also trying to do as much as I can by using machines and jigs to keep this project moving ahead in a timely manner. I would enjoy hearing any ideas on the topic if anybody has had a similar experience. Thanks in advance for your help - Bill
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post #2 of 8 Old 07-26-2009, 03:35 AM
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Hi Bill. I'm sure you'll get plenty responses to this from more experienced members than me but I would do the job by hand with a registered mortise chisel. I'd mark out the square with a mortising gauge, cut the hole perimeter with a sharp blade then maybe use a forstner bit just smaller than the square to hog out the bulk and give me a flat bottom to work to while I cut the rest with the chisel. Sometimes power tools just won't do it. It sounds like the back of your chisel is not flat.

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post #3 of 8 Old 07-26-2009, 07:28 AM
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From your description, sounds like your chisel isn't true. You might check it out for square. Also check for play in the stroke. You might be feeding a bit too fast. A square sharp chisel should give a nice square hole. Try dressing the bevel only on the inside, and if need be check the outside for flat.

As it was mentioned, you could just clean up the holes with a slight oversize with a sharp chisel. Worse case scenario would be is having to fit each Ebony insert.

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post #4 of 8 Old 07-26-2009, 10:42 AM
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For small square plugs try just using the square chisel and lightly hit it with a mallet to get the perfectly square hole started, then switch to a regular chisel and finish the job.
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post #5 of 8 Old 07-26-2009, 12:02 PM
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Hello Bill,
The bit is set too far below the chisel. As the bit starts to cut it is wobbling and cutting outside the chisel leaving the circle that you describe.. The fix is easy, just raise the bit. The bit should not project more than 1/32" below the chisel.
Also check the bit for sharpness. A couple of strokes with a sharp file will make a big difference.
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post #6 of 8 Old 07-26-2009, 01:10 PM
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Bill, I had the same problem on a recent project. My fix was to cut the square plug about 1/64 oversize. I then took a chisel and beveled the bottom of the plug slightly so it would go in straight. When I drove the plug in place the round cut went away. Try it on a scrap piece first and see if it works.

I also agree with KJuly. I raised my bit in the chisel and the problem went away.
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post #7 of 8 Old 07-26-2009, 08:15 PM Thread Starter
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square mortise solutions

Hi guys,

Thanks for the suggestions - I'll try them out to see what works best for my project. I appreciate the help - I'll take some photos of the finished work.

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post #8 of 8 Old 08-04-2009, 09:32 AM
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Welcome to the forum.


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