Hollow chisel mortisers: use and maintenance

Words and photos: Neil Erasmus

Machined mortises, or deep slots, can be made square or round-ended. The latter are made with rotating cutters fitted either
to hand-held routers or dedicated, oscillating slot-mortise machines. These slots are created very quickly through multiple, shallow plunges while simultaneously traversing, with depth and end-to-end stops adjusted to the dimensions required.

Properly set up and with sharp tools, the HCM is a joy to use.

Traditional, square-ended mortises are either cut by hand or by machine, using either a chain or hollow chisel mortiser. The plunging head of the HCM consists of a hollow, square chisel into which a special drill bit called an auger is fitted. It holds the fixed chisel in place and also a motor- driven chuck that houses the auger.

The length of the auger is smaller in diameter than its nominal size so it fits into the hollow in the chisel – only its end flares out to match the chisel size. Its end has either one or two cutter lips and spurs. Most HCM auger bits are directly-driven from above, some with a blower tube to clear swarf and to cool the chisel.

The plunging head has a hollow, square chisel which holds an auger.

Essentially, these devices ‘drill’ square holes – the chisel squares the round hole drilled by the auger. A clearing slot on one of the four faces of the chisel allows chips to exit. The wood is placed down on the cast table and clamped to a fence with the built-in clamp. The table travels left to right and can be finely adjusted and fixed in the appropriate fore/aft position.

Most of the larger woodworking machinery manufacturers built these, notably the older British ones such as Wadkin, Dominion and Robinson. Some older machines doubled as chain mortisers too and were primarily used for heavy joinery work, thus the sheer size and heft of these behemoths.

Tool sizes

The chisels and their matching bits come in mostly imperial sizes starting at 1/4” (6.35mm) and going up in 1/8” (3.17mm) increments up to 1” (25.4mm).

The larger sizes are rarely used in domestic or commercial furniture applications, and require huge leverage on the plunge arm to bite into hardwoods, even with the sharpest chisels and bits. Furniture making applications typically require four chisel sizes: 6.35mm, 7.9mm, 9.52mm and 12.7mm, the 9.5mm being the most common size for chair construction, while the smaller sizes are nicely proportioned for side-by- side, twin mortises.

Setting up the HCM

Setting these machines up to perform at their best is what it takes to realise their usefulness. The first and most important thing is to ensure the chisel and bit are sharp. I’ll cover the processes of fettling and sharpening later. Here’s how to set up the HCM.

Get the chisel square

1. Carefully slide the auger out from the end of the chisel and place it down on something soft, then slip the chisel up into the chisel holder, clearing slot facing back, and temporarily lock in place.

2. Next, take a five cent coin or similar spacer, undo the chisel, then place the coin between the chisel and the holder, then again re-tighten temporarily, see above.

3. Again taking care not to knock the end against the metal table, slide the auger all the way into the end of the chisel and fully tighten the chuck around its shank. Note: some augers are too long to fit properly and all it takes is to cut some off the shank end.

4. Loosen off the chisel, remove the coin, and slide the chisel all the way and fix it in place ready for squaring up. The coin thickness provides enough space between the end of the auger and the chisel to effectively eject swarf and help keep everything cool.

Getting the chisel square with the back fence is one of the most important ways to ensure clean mortise walls, a feature that is so important when cutting decorative through-mortises. Rotate the auger by turning the upper shank area by hand so neither the auger’s spur, nor its cutter at the end of the device face backward. We want them out of harm’s way for what comes next.

5. Drop the chisel and bit down by pulling down on the lever until the chisel is at the same height as the back fence and leave it there. Now, loosen the chisel from its holder (it can’t fall out) and bring the left/ right sliding table forward until its fence just nudges the chisel – don’t force it as the chisel can easily bend. Pinch the chisel and fence together by hand while tightening the chisel to its holder, then slide the table back out of the way (see above). The chisel should now be perfectly square and parallel to the fence, and ready for cutting.

Set the stops

The only other settings required are the stops – end-to-end, and depth. Set the cutting depth by clamping the work in place and eyeing a depth mark on the end of the piece. The chisel head typically has a bar and stop like many older drill presses, and a pair of adjustable stops behind the fence. On many machines the head adjusts up and down to allow for different timber sizes. On some machines the range of movement on the plunge arm may be adjusted by simply pulling it out against a spring and re-positioning on its spline.

Using the HCM

Bear in mind these machines are strictly for solid wood only, and should never be used on abrasive and corrosive man-made boards. Wood to be mortised must be clearly marked out with a mortise or plain marking gauge, square and marking knife, and the appropriate bit and chisel placed in the machine.

HCMs are designed to clamp fairly large pieces of wood, and dressed wooden props may be required to raise smaller pieces a little so the built-in clamp can engage them properly. An inadequately clamped workpiece may pull free as the head is retracted, so be sure to get this right.

The depth stop on the plunging head may now be set. If through mortises are called for, be sure to place some sacrificial wood underneath and clamp firmly down to prevent tear-out, or mortise from both faces if this can be done. After the wood is clamped in place, the fore/aft position of the table is adjusted to the set-out as shown above.

Next, set the end stops and a positioning block to reference your work relative to your mortise width marks. Now you’re ready to turn the machine on and begin cutting. Remember that the sliding table must be in position and stationary before entering the wood. It can only end-cut, not side-cut, so start the cut at one end of the layout and slowly plunge the chisel into the wood – too slow and it will burn, too fast and it will resist. A happy medium will soon become apparent.

When plunging I always find that several shallow cuts and quick withdrawals work better, as it gives the chisel a little time to cool down and clear the swarf out the ejection slot in the back, out of view. Never force the chisel into the wood – it will cut at its own pace, and don’t allow the chisel to linger inside the mortise for any longer than you have to, otherwise it will quickly overheat and possibly ruin the chisel and bit.

Once the two outer squares have been cut, the rest of the mortise can be done by working from one end to the other, overlapping the mortise already cut. It is important to finish the outer squares first to maintain a perpendicular cut – cutting a mortise with a void to one side allows for chisel wander; leaving a slanted cut is a problem requiring handwork to rectify.

Sharpen the chisel

The single most important discipline with the HCM is to know when and how to sharpen the chisels and bits. If this is understood, it is a joy to use – relatively quiet and creating gentle dust – the kind of machine I like when pulled away from handwork. When you are engulfed in wood-smoke, you know you have missed the mark. Sensible storage of spare chisels and bits is a must as they rarely survive a fall on a concrete floor.

Starting with the chisel, I use a diamond impregnated conical bit held in a battery drill to grind the inside reverse cone in the end, see above. Select a speed that doesn’t cause the conical bit to vibrate or jump around in the chisel end – generally quite slow.
Grind away until a neat little burr is evident all the way around and right to the corners of the chisel.

Sometimes I find I need to gently dance the drill around in a neat, but smallish circle to get to the very corners – that’s fine too. Go through the grits until nice and smooth. If left rough, the waste can’t quite slip through this choke-point properly, causing choking and burning, which results in accelerated dulling.

Next, the outside faces must be honed on a perfectly flat waterstone. I say ‘perfectly flat’ because doing it on a hollowed one will likely result in a chisel that binds tightly in the wood due to the creation of a bigger ‘waist’ than the cutting tip. In my teaching career I’ve seen too many hollowed waterstones and convex-backed bench chisels for one life. You’ve got two, maybe three minutes of continuous honing before you need to flatten that stone again – believe me!

When sharpening a new chisel, I always apply extra finger pressure mid-point to actually achieve the reverse – creating a slimmer waist to help avoid the chisel binding in the wood. In fact some chisels, such as the excellent Clico brand, are purposely made like that.

Finishing off this process may require several back and forth actions on the stone and the finest conical grinding bit (held between the fingers) to achieve a burr-less transition at the very edge. Occasionally I have taken a small square file to the inside corners of the chisel to create extra space for shavings to travel freely to the twist of the auger.

Sharpen the auger

The auger requires a little more care and skill, but anyone can do it. The areas that need fettling are at the bottom tip of the tool and quite small, so a jeweller’s loop and good light may be required if your eyes are old like mine. First, clean the entire length
of the bit with a pitch solvent as the intense heat generated by these bits leaves a tough wood tar on the surfaces.

Next, I ‘paint’ the cutter/s and the spur/s at the tip with a permanent black marker so I can visually check and adjust the sharpening process. Before you begin, a word of warning: these augers have a limited number of sharpens in them, so take off only what you must to get it sharp again.

The auger is firmly held in a padded vice, tip facing up, while a small, fine triangular file is used to sharpen both spur and cutter lip. Forward passes only. The cutter lip is filed both at the bottom and on top while the spur only gets done on its inside edge. After that a little fettle on the waterstone to remove the burr on the outer edge of the spur is all it takes to complete the job.

Sometimes I have had to fettle the outside of the cutting tip of a new auger to reduce its diameter, when the small lateral movement within the chisel causes the auger to overshoot the chisel’s square parameters, leaving a messy-looking mortise. I achieve this by chucking the chisel and auger setup in the machine, but leaving about 20mm of the auger protruding. With the machine running, I offer a small diamond plate up to the edge of the auger tip, at a slight angle to maintain a leading edge. I do this several times until no part of the augur protrudes beyond the chisel.

Properly set up and with sharp tools, the HCM may well become your favourite machine.

Neil Erasmus is a furniture designer maker based in Perth.