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Cutting Mortises with a Hollow-Chisel Mortiser

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Hollow-Chisel Mortiser

Dedicated Hollow-Chisel Mortiser

Photo Courtesy of Jet Tools
Mortise and Tenon joints are elegant, strong and relatively simple. A single mortise can easily be cut using a power drill and sharp chisel. However, if your woodworking plans call for a considerable number of mortises, you'll easily tire of cutting them by hand.

In this case, a dedicated, hollow-chisel mortiser can be a godsend. A mortiser is little more than a modified drill press that holds a square, hollow chisel with a drill bit in the center.

Features to Consider:

When looking for a dedicated mortiser, there are a few features to consider. First, most quality mortisers keep their bit speed on the low end (1500-1800 RPM) to reduce excessively heating the chisel. Most are table-top models with a small table and hold-down that keeps the workpiece square to the chisel. Some larger, floor-standing models offer the ability to tilt the table for cutting mortises at an angle.

Look for a model with a travel of at least 4-1/2". Any less travel and you may have a difficult time cutting through-mortises on some boards.

The mortiser should use a smooth but strong steel rack-and-pinion system with a single arm to drop the chisel and bit into the stock. Pushing the chisel into hardwood can take a bit of muscle, and the action should be strong enough to handle considerable torque from the operator. Additionally, the chisel should complete the entire length of the travel in less than a 90-degree rotation of the arm.

Chisels:

Most mortisers are equipped to handle 1/4", 3/8" and 1/2" square chisels. Some are even capable of handling a 3/4" chisel, but keep in mind that this is a pretty healthy chunk to cut at one time and may require a lot of physical exertion. Remember that even if you're cutting a mortise that is 3/4" wide, you don't need to cut the entire width in one pass. A few cuts down one side of the mortise and then back up the other with a 1/2" chisel will be just as effective and not nearly as taxing on the operator.

Using the Mortiser:

The first thing to remember when using a mortiser is to always use the hold-down. The downward movement of cutting is rarely problematic, but often the upward movement of releasing the chisel from the stock will cause the wood to rise off of the table rather than the chisel releasing as desired. The hold-down will solve this problem.

Also, remember that the bottom of your mortises will not be nearly as clean as the sides, since the drill bit isn't square on the bottom. This is rarely an issue, but something to keep in mind if the bottom of the hole is ever going to be visible.

Before using your mortiser, always check to see that the chisels are square to the fence, and that your stock is securely against the fence. Otherwise, your mortises will not be square to the face of the board held against the fence.

Also, before cutting your mortises in your stock, make test cuts in scrap boards of the same thickness. This will allow you to set the depth of cut accurately and make any mistakes on scrap so you don't damage your project.

Caring for Your Mortiser:

To keep your mortiser running smoothly for a long time, there are three tips to keep in mind. First, keep your chisels and bits sharp. If the chisels become dull, have them professionally sharpened or simply replace them.

Second, keep your unit as clean as possible. Sawdust can wreak havoc on the rack-and-pinion and drill chuck.

Finally, over time, the teeth on the rack-and-pinion can wear down a bit. Check your tool's manual for instructions on adjusting for normal wear and tear on the mortiser's travel mechanism.
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