Monday 9 July 2018
The somewhat tedious job of cutting mortices is much easier and quicker if you have some form of powered help, particularly as small morticing machines are relatively cheap these days.
They range in size from the adapted drill stand to the full-blown workshop machine, but although the principle is the same on them all the cutting capacity is very different.
You do not need a lot of power on a morticer, it only has to drill a hole, but you do need plenty of leverage to pull the chisel into the work. This is what really differentiates the various classes of machine and explains why the drill attachments are rather limited. They just do not have enough leverage or accompanying casting weight to withstand the forces needed to drive in a big square chisel. Even bench-mounted morticers are restricted to a chisel size of half an inch, whereas a proper morticer will handle three-quarters of an inch or even 1in chisels.
How it works
The seemingly impossible task of drilling a square hole is achieved with specialised morticing tooling consisting of two parts. A hollow square chisel is fixed securely in the head of the morticer. An auger bit is then set up to revolve inside the chisel and remove the bulk of the waste.
The chisel and bit are accurately matched, and if you look at them closely the end of the bit is actually slightly bigger in diameter than the chisel, but although there is very little wood left after the drill has done its work, the chisel still requires some considerable force to penetrate the wood and remove the remaining corners.
It is this action of forcing the chisel into the timber that really determines the capabilities of the various machines. At the lower end of the scale there are the drill-powered models, which are just modifications of the basic drill stand, but these are not able to withstand the forces needed for large chisels, so you are usually restricted to a maximum capacity of a half inch chisel, and realistically most will struggle at this.
The necessary leverage to plunge the chisel into the work is produced by the morticing arm. Obviously the longer this is the more force you can generate, but it must be in balance with the overall construction of the machine or something will break. The length of this arm is a good indicator of the capacity and exemplifies why attachments on drill presses are rarely any good.
The tooling must also be held very securely, so consider how this is achieved if you are looking to buy a morticer. The chisel has to be locked firmly, not only to withstand the pressure of being forced into the wood, but also then being withdrawn again.
On most machines this may be as simple as a grubscrew tightening onto the shoulder of the chisel, but continual usage may wear burrs on it, making accurate setting more difficult.
The bit has to rotate within the chisel, and you must be able to set the amount of clearance really accurately, so clamping it in a drill-type chuck is the easiest answer. Once again, however, you must be able to get the bit really secure, or it tends to press back into the chisel during use, losing the vital clearance which then causes burning and severe damage to both the chisel and bit.
Clamping the work firmly is another major consideration. It has to be held in place for each cut, but then be free enough to move along quickly for each subsequent cut. On the simplest machines the clamping system consists of a hold-down plate and a back fence, but you then have to pull the wood through by hand to make each cut.
This works OK, but it takes some time to set the fences square to the chisel, and it is often tricky getting the chisel lined up precisely on the line of the mortice. There are no fine adjustments at this level.
Having forced your chisel into the timber it is essential that it returns back out easily, or the job becomes hard work. The wood tends to grab the chisel so look at the return mechanism on the arm. At the very least this should be a very strong spring or a gas strut, but bigger machines often incorporate counterbalance weights.
Some form of depth stop is also vital as you need to control how deep the mortice is, particularly when they do not go right through the work.
All things considered
Small dedicated benchtop machines are excellent if you have the occasional need for mortices up to half an inch in width. Admittedly you can always double cut if you need one wider, but the necessary setting up for this is quite time consuming with the lack of fine adjustments.
You do not require much in the way of motor power though with a morticer as it only has to drill a small hole, but look for a machine with a motor of about 370W.
The next step up is an industrial-grade morticing machine. These may be bench mounted but more often than not are freestanding. Because they are heavier and more robust the castings will withstand more leverage, so the arm can be bigger, allowing much bigger mortices to be cut in one pass.
A three-quarter inch capacity is now possible, which is actually a big mortice when you see it cut out. The biggest machines will even cut a 1in mortice, but it does take some effort, particularly in hardwood.
The table needs to be a heavy casting, with an equally robust fence. On the cheaper machines they are separate and the fence has to be adjusted by hand to line up the mortice, so check how easily this operates. You need to be able to make fine adjustments that stay in place when you tighten up the clamps.
Top of the range machines have really heavy duty cast iron tables that incorporate the fence all as one piece.
These morticers incorporate a screw adjustment for the fore and aft movement of the table, which makes exact positioning of the timber extremely simple, and is worth the extra cost if you anticipate doing a lot of morticing.
On some machines this adjustment is on a separate handwheel, on others it is incorporated into the transverse handwheel. The former option is better because there is no confusion about which setting you are altering.
The material clamp needs to hold the work securely as the bit is being withdrawn from the work, but beware of big cast knobs that are often uncomfortable to tighten properly.
You should also have plenty of adjustment on the clamp to accommodate different sizes of work. As well as the actual thread adjustment for tightening, the whole clamp assembly should be relocatable to different positions on the table.
The biggest refinement you get with a dedicated morticer is the ability to move the wood from side to side with a handwheel control for the table. This speeds up the cutting process enormously, particularly when you have to work from side to side across a long mortice. It is also far more accurate as the work is held absolutely rock solid.
Motor size is probably now about 550W to cope with the extra capacity, anything less than this being underpowered for a three-quarter inch mortice in hardwood.
Adjustable gib strips
To maintain the necessary accuracy you must also be able to take up any wear on the moving parts of the machine. The head can soon get sloppy in its slides if you are doing any serious amounts of work, so ensure that there are adjustable gib strips to eliminate any play.
Mortice chisels above half an inch usually have larger-diameter shanks. To accommodate these the holder is drilled out much bigger and a variety of bushes is provided.
These lock into the head in the same way as the smaller machines. This allows you to fit any type or make of chisel, but these bushes are usually specific to a particular machine, so make sure they are present if you are looking at something second hand.