Hollow Chisel Mortising in the Horizontal Mode

[forrestb]

As I said on another thread I am using slip tenons in a hall table. The aprons are 4 ft long and I don't have a tall enough bench for router mortising, don't have a mortising machine, and don't trust my self with cutting tenons on the end of a 4 ft. board.

So I thought to do the hollow chisel mortising on my Shopsmith in the horizontal boring mode.

Getting the chisel sharp enough was a chore and so was getting the main table stable enough to provide the needed support. Early work showed me that even with horizontal support, the leading edge was actually flexing. To eliminate that I installed three 1 x 1 x ⅛ angle irons by drilling into the main table:

Support.jpg (364.9 KiB) Viewed 2399 times

The fixture runs in the miter slots and is made of BB plywood with clamps to support the articles and tie the fixture to the table. Note the 2 x 4 running up against a shelf support to provide firm backup. It is loosely installed and jammed tight with a shim and I will use that arrangement running the aprons up against the post. The shimming will be at the post. The L-piece will provide support for the legs and will be removed for mortising the aprons.

Fixture.jpg (512.38 KiB) Viewed 2399 times

I set the main table as close to 90 degrees as I could and then used reality to check that. I drilled a ½ inch hole through a 2 inch square piece:

Level Hole.jpg (394.53 KiB) Viewed 2399 times

A ½ inch dowel installed in the hole (it was a tight fit) gave the dimensions shown from the reference surface to the dowel on each side. At the working site, the table/fixture is level within 1/64 over the 2 inch span. My mortises will be 4 inches long by 1 inch deep.

How Level.jpg (471.95 KiB) Viewed 2399 times

I will be able to cut the ⅜ mortises in cherry:

Mortising.jpg (435.51 KiB) Viewed 2399 times

I have cut a complete test mortise and it is fairly good but across the grain the cherry tends to tear rather than be severed so the mortises in the legs will be assembled with a gap-filling glue. We will see how the end grain aprons go - I expect that they will be clean.

I have more work to do to eliminate a minor main table flex that is causing more effort than needed.

Even though I am having success and will cut the hall table mortises this way, I think that if I have a similar occasion (hmm, one comes to mind) I think I will explore stopping all vertical movement and use a router bit instead of the hollow chisel.

Should I have a LOT of such occasions, I am buying a mortising machine that I can turn on its side!

I will re-sharpen before next use on the real thing and will take Ben's advice to sharpen every night.


Forrest

[forrestb]

I added some stops cut from Schedule 40 PVC split in half and held in place by tape. Turns out a 1" Cap is just the right inner diameter. The table height lock could use the assistance over the long haul.

Carraige.jpg (218.42 KiB) Viewed 2297 times

To resolve the minor table flexing I referred to in the last post, I added the leftover 1x1 angle iron to the front edge of the table. Two 7/16 threaded rods were bolted to that and then bolted to a scrap piece of bar aluminum. The nuts on the bottom were then tightened to bring the aluminum rod into firm contact with the lower way tubes. Turns out to be working to get rid of that minor flexing.

Table Support.jpg (335.4 KiB) Viewed 2297 times

I believe that I (probably not the first!) have found one task that the Shopsmith is just not rigid enough to be an excellent solution.

Forrest

[algale]

I am utterly confused. The left edge (viewed from operator's position)of the table was flexing? I can't see how the drilling/mortising operation in horizontal mode puts force against the table edge.

[forrestb]

I am utterly confused. The left edge (viewed from operator's position)of the table was flexing? I can't see how the drilling/mortising operation in horizontal mode puts force against the table edge.

Yes it was lifting.

The mortising chisel contacts the wood about 2 inches above the level of the table top. That kind of chisels relies on a lot of pressure being applied to cut the wood left after the drill bit. That's why mortising machines are very sturdy and have a long handle.

Remember that the fixture is clamped to the table at the left hand edge or that force would just lift the fixture off the table. So a lot of the force from the chisel is transmitted right to the left edge of the table through that 2" lever.

The addition of the angle iron helped to stabilize the left edge and the aluminum bar would not allow much, if any lifting of the table.

I should have taken a video before and after. When I get all my mortises cut I will try to do so.

Forrest

[JPG]

I am utterly confused. The left edge (viewed from operator's position)of the table was flexing? I can't see how the drilling/mortising operation in horizontal mode puts force against the table edge.

I do not think flexing is descriptive enough. I think 'raising' due to the table/trunion/carriage rotating under that lateral force. I do think flexing of the table by itself was also occurring. I reach that conclusion due to the last 'fix'.