Monday 9 July 2018
Last month I looked in some detail at morticers, but as with most machines you will only get the best out of them if the tooling is correctly fitted and, more importantly, sharp. This is particularly important on a morticer and no matter what type of machine you have, whether it is drill powered or fully industrial, it is vital that the chisel and bit are set up properly, both in the machine and relative to each other.
If you get the setting wrong the chisel will not cut properly and you will probably ruin it into the bargain. Using them set with inadequate clearances always leads to premature tool burning or even complete failure.
The traditional 2-wing mortice bit now seems to have been largely superseded by the Japanese-style bit with a single cutting lip and spur and a longer flute. This latter type of bit seems to be much freer cutting, and clears the chips better, but is inherently weaker so correct setting is more critical.
Chisel and bit combos
Chisel and bit combinations vary from quarter inch width up to one inch, but the end of the chisel that fits into the machine is traditionally one of two standard sizes, either thirteen sixteenths or one and one-eighth of an inch, though metric sizes are now becoming more common.
The larger shank sizes are gripped directly in a fixed holder that is part of the machine, but the smaller ones have to be fitted with a bush first. I prefer chisels to have a large chip clearance window, particularly if they have a hole at the top as this minimises the risk of shavings travelling back up into the body of the bit.
Incidentally, if you buy a new chisel and bit, the bit will probably be far too long as they are supplied to fit a huge range of machines. If so just hacksaw off a section of the plain shank to reduce it to a suitable length.
Until recently chisels and bits were hugely expensive items and the bit often wore out before the chisel. Consequently you could buy replacement bits only, but these are less common these days. If, however, you do find a source of bits make sure they match the original make of chisel as they are rarely interchangeable.
Start the setting-up procedure by pushing in the chisel, but for a single flute cutter, sandwich a washer about 3mm thick between the shoulder of the shank and the machine holder. I find a Â£1 coin ideal. If you are using a 2-wing cutter you will get away with slightly less clearance so use a 2p piece.
Tighten up, leaving the coin in place. Now slide in the bit and push it right into the chisel as far as it will go and lock it firmly in the chuck, then remove the coin and push the chisel right home in the holder and retighten, making sure the chisel is parallel to the fence and most importantly that the chip clearance window is aligned with the length of the mortice. I wind out the back fence until it just touches the bit to get the necessary parallel alignment.
This procedure should give the essential spacing between the end of the bit and the chisel. If there is not enough clearance the bit will rub and overheat the chisel which will eventually lead to the chisel losing its strength and edge-holding properties. It may even split.
Once the chisel and bit are set up correctly, adjust the position of the timber to line up with the required mortice and clamp it firmly in place. There is a tendency for the chisel to pull the timber off the table as it withdraws if it is not held firmly enough, so do get it tight.
Adjust the depth stop so that the mortice will be flat along the bottom and to stop you cutting right through the timber into the machine table. If you need a through mortice put a piece of scrap underneath and cut through onto this.
The procedure for cutting the mortice is equally important. It is virtually impossible and certainly undesirable, to try and plunge the chisel into full depth in one go. Instead start at one end of the mortice and just pull the chisel in for about 5mm. This often requires some considerable force to get started.
Then move the timber along and make another slightly deeper cut. Continue in this fashion until you reach the other end of the mortice by which time you should be at full depth.
If the timber is relatively soft you can make each cut the full width of the chisel. On harder materials this may need too much leverage, so overlap successive cuts, taking only half the width each time.
Once you have reached the end of the mortice, start working back along it, now making each cut to full depth. Note how the chips are ejected through the window into the mortice rather than jamming up the inside of the chisel.
As you work do stop occasionally and check that the chisel and bit clearance is still correct. On harder woods the bit sometimes gets pushed back into the chuck, but with experience you will both see and hear the result. Watch out for tell-tale signs of blueing on the chisel tip or excessive noise, both of which indicate potential clearance problems.
You may also find that some timbers jam in the bit flutes even with the correct clearance set. In this case, increase the gap between the end of the bit and the chisel slightly until they do clear.
Some users recommend waxing the outer faces of the chisel to help it slide into the timber but this always worries me if you are going to glue the joint together as there is bound to be some contamination. Providing the chisel is set up correctly you should not need any sort of lubrication.
The finished mortice always needs cleaning out by hand as the extreme corners are not removed. This may not be so important with softwood where the tenon will force in, but on hardwood they are often enough to stop the joint closing together properly.
Sharpening chisels and bits
Even with sharp cutters morticing can be quite hard work, and if the chisel and bit are blunt it becomes extremely difficult to get them to cut at all. Many users are put off sharpening them in the mistaken belief that the process is complicated, mainly because of the complex shapes of the cutting edges.
In fact, both chisel and bit are quite simple to sharpen and a few minutes spent touching up the edges will save hours of arm-aching drudgery trying to force blunt tooling.
To sharpen the chisel there are two options, depending how much you use the chisel and how much investment you are prepared to make in sharpening it.
The ideal answer is a dedicated countersink-style sharpening device, but these are a bit pricey. They come with a range of interchangeable pilots to fit virtually any size and make of chisel. Simply select a suitable pilot that is a snug fit in the chisel. In years gone by these devices were operated in a brace, but nowadays you can use them in an electric drill running at slow speed. Just feed the pilot down the chisel gently until the wings of the countersink bit start to rub, then apply light pressure to hone the four cutting edges simultaneously.
If, however, you only use the mortice chisel occasionally, it may well not be worth investing in an expensive sharpening device, in which case you can lightly hone the cutting edges with a very fine needle file, maintaining the existing bevel and taking care not to sharpen one edge more than another, so ending up with them uneven. Never file the outside faces.
The bits are less complicated and again just require light filing of the main cutting edge, working through the flute, and then a quick touch-up of the outer wings, sharpening from the inside only and not from the outside or you will reduce the diameter of the bit.
You will be amazed at what a difference this will make to the cutting performance, particularly as many of the cheaper chisels and bits are often not properly sharpened when new.
The chisels in particular are very prone to damage. Even a slight knock can seriously deform one of the sharp corners enough to stop it plunging in, so do consider some form of safe storage for them, preferably fixing them on a board where they are both safe and accessible.