As your woodworking interests evolve, it isn't long before you want to make mortise-and-tenon joinery. But how to cut those mortises?
Perhaps you’ve read about the hand chiseling method, and considered trying it. Or, you may have considered making mortises with your router, although doing so would limit you to mortises with rounded corners. A mortising attachment for your drill press would work, but you may not want to set it up for each session.
There are a variety of ways to make a mortise, each with their advantages and disadvantages. However, if you want to make great mortises that fit and look like it was done by a professional, you will want to consider a dedicated mortising machine.
Dedicated mortising machines, often referred to as ‘hollow chisel mortisers’, deliver dependable results with minimal set-up time. They are quiet, safe and don’t produce a lot of dust. When you get one, here is bit of advice: don’t rely on the low end mortising bits often included for free with your purchase. Buy higher quality bits and learn how to sharpen them.
A solid, reliable machine is most important. Let’s look at some of the key features you need to look for when shopping for a benchtop model.
Most benchtop models offer a ½ hp induction motor, which is more than sufficient. As I have stated in previous articles, compare amperages instead of stated horsepower if you want to compare apples to apples. The truth is that these machines don’t require large motors to work well. Essentially, the motor just drives an auger bit. The force required for the chisel is provided when the operator pulls down on a long lever, so motor amperage really isn’t a large factor.
Motor RPMs is a more important concern. The faster the bit rotates, the more chance of burning the walls of your mortises with heat build-up, especially when using larger diameter bits. Most mortisers run from 1700 to 1750 rpms, and the speed is not adjustable. I would prefer a speed around 1200 rpms or less. 1700 rpms is almost too fast for the ½” bits, but you’ll manage if you do not hesitate too long in the mortise. Work extra quickly with burn-prone species, like cherry.
It’s important to determine the head travel or ‘stroke’ of the machine. This indicates how far down the bit will travel when you pull on the lever. Typical stroke capacity would be from about 4 ¾” to 6″. You aren’t likely to cut a mortise that deep, but the extra travel gets you down to your work piece even before you start cutting. If the head travel is too small, and the distance to the work piece is large, you will have to shim under the work piece to get it closer to the bit. You might also take a look at the distance between the head and the table. The larger this distance, the thicker the work piece that can be mortised.
For me, table size is extremely important. You don’t want a work surface so small that you have to support even small work pieces at all times. I would go for the machine with the largest table size, all other things being equal. You will probably still need additional support on both sides (for larger work pieces), so you might want to design/build a workstation that provides additional support.
Most benchtop models will take up to ½” x ½” mortising chisels. Some claim to take ⅝” or even ¾” chisels. Unless it’s a much larger, heavy duty machine, I don’t pay much attention to such claims. Benchtop models are light-duty machines that, in my opinion, just aren’t designed to force the larger chisel sizes into hardwood. Remember that you supply the power to force the chisel into the wood. Try a ½” x ½” chisel in hard maple and you’ll see what I mean.
Concentrate on finding a machine that offers quick and easy bit changes, preferably with few tools required. A tool and chisel holder at the back of the machine is convenient, although perhaps a luxury. All benchtop models will accept ¼”, 5⁄16″, ⅜”, 7⁄16″ and ½” chisel sizes, which is more than enough for most needs.
A mortiser needs a depth stop to control the depth of the mortises, just as a drill press needs such a system. Choose a machine with an easy to adjust depth stop that is accurate, and doesn’t flex much under load. The bottom of your mortise needs to be at least 1⁄16″ deeper than the length of your tenons. Going a little more may result in going through the other side of the work piece, but is otherwise fine.
The fence system is vital on a mortiser. You want a fence that is as large as possible while also being flat. Ideally, go for a fence that can be moved forward and back with a rack and pinion gear system and stays exactly parallel throughout its movement. If your mortiser’s fence isn’t dead flat across its face, or 90º to the table, I’d suggest adding an auxiliary fence made of wood and shimming in between the two fences to create the accuracy you need.
The hold-down system is also important. When you drive the chisel into hardwood, a lot of force is required to retract the bit from the wood. Unless you have a solid hold-down, you’ll lift the work piece off the table. Make sure that the hold down is flat, to ensure full contact with the wood. Also, make sure that they are not so thin as to cut into the top surface of the wood, creating a dent as you retract the bit. Most woodworkers use the hold-downs that the manufacturer provides, and either shim (or otherwise modify) them to improve their performance.
Mortisers have a long feed lever to give you extra leverage for those larger chisel sizes. This is one huge advantage of a mortising machine over a mortising attachment on a drill press. The drill press usually has fairly short handles, requiring more muscle with mortising chisels. Look for a feed lever that is long, sturdy and can be repositioned for different depths of cut. That will give you more options for leverage and comfort.
There are some other interesting features you might find on some machines. I use a model that comes with a riser block. This allows me to raise the head almost 2″ higher from the table, for when I need to cut mortises in unusually thick work pieces.
Another interesting feature on my mortiser is that the head can be removed from the base and repositioned 180º. This allows me to mortise off the table when I need to cut a mortise in the middle of a larger object. It also allows me to cut a mortise in something that has already been assembled, which would be impossible otherwise. By turning the head such that it hangs over your workbench, you can rig up just about anything to put mortises in. You might not need this feature very often, but it could come in handy for some antique restoration projects, where the piece is not fully disassembled.
One of the biggest drawbacks to achieving good results with hand tools is the inability to tune up and sharpen them. This also holds true for the mortising chisel and bit sets used with benchtop mortisers.
Don’t base your purchasing decision on how many free bits the manufacturer offers with their machine. Typically they aren’t very good quality bits. Just like router bits, find a top-end product and you’ll make your woodworking safer and more enjoyable. Even better quality bits require a certain amount of tuning and sharpening straight out of the package. Learn to do it right and your mortises will look better than ever.
The router is an effective tool for occasional mortising. However, when you have a lot of mortises to cut, then the mortise is the way to go. It’s not as noisy as a router, and when used for extended periods of time, it is not as tiring. You will also find it quicker and easier to move stock into the mortise than to maneuver a router over clamped stock. The longer chisels in a mortiser can cut much deeper than router bits – and the mortiser can cut mortises with a single pass. Additionally, with the router, tenons will have to be rounded or mortises squared off in order to work properly.
For the dedicated hobbyist or the small one-person shop, a benchtop mortise is a good choice. However, for the production shop, or where you cut a lot of large mortises, such as for doors, a stationary mortise might be a better choice. These machines feature heavy duty hold downs, larger moving tables, longer handles for better leverage, tilting heads, and they can take larger bits (up to 1″ chisel shanks). Both Canwood and General offer stationary models.