How a hollow-chisel mortiser cuts
The hollow-chisel mortiser is the machine most workshops mean by a mortiser. A square, hollow chisel is clamped in the head; an auger bit runs up through it into a chuck and turns at motor speed. Pulling the handle drives both into the wood: the auger bores out most of the waste and the chisel's four sharpened edges pare the round hole square. The chips leave through a slot in the side of the chisel. A long mortise is cut as a row of square holes, the head lifted and the work moved between plunges.
- The auger leads the chisel. Its tip sits a little below the chisel's points, by a gap the maker sets: Grizzly gives 1/16 to 3/16 in for its G0645 and G0448, depending on the wood and the work, and 1/16 in for its T33127. Set it from the machine's own manual.
- The chip slot faces sideways. Grizzly's T33127 manual has the slot facing the side; facing front or back, it says, can leave a rounding pattern in the cut.
- Deep holes are cut in stages. For the G0645 Grizzly cuts about 1 in deep, backs off to let chips clear, then goes deeper.
- A set sequence. For long mortises Grizzly's G0645 manual gives a numbered sequence of cuts, and has the chisel centred over the last cuts, which use only part of the chisel.
Kinds of mortiser
| Type | How it cuts | Example, as its maker lists it | Suits |
|---|---|---|---|
| Benchtop hollow-chisel | Square chisel and auger, hand lever | Grizzly G0645: 1/2 HP, 70 lb, chisels 1/4 to 1/2 in, 3 in maximum depth | Furniture-size mortises in a small shop |
| Floor hollow-chisel | The same, on a heavier head and a moving table | Grizzly G0448: 1-1/2 HP, 332 lb, 6-1/4 in stroke, head tilts 30° each way | Larger chisels and parts; angled mortises |
| Chain | A cutting chain plunged into the work | Makita 7104L: 1,140 W, 130 mm wide and 155 mm deep as listed, clamps onto logs | Heavy timber and construction work, not furniture-size joints |
| Slot (horizontal) | A rotating bit moved side to side and in | An optional mortiser on Felder's HAMMER C3 31 combination machine | Round-ended mortises for rounded or loose tenons |
Figures are each maker's for its own machine. The kinds cut different mortises and are not substitutes for one another. Not a ranking.
Each kind leaves a different hole. A hollow chisel leaves square corners that take a square-shouldered tenon; a slot mortiser leaves round ends, so the tenon is rounded to fit or a loose tenon is used; a chain mortiser is built for timber far larger than furniture parts. What the joint needs — size, proportions, fit — is explained in mortise and tenon joints, and the tenon side is cut with the jigs in the tenon jig guide.
Reading a data sheet
| Specification | What it decides | Grizzly's figures (G0645 · T33127 · G0448) |
|---|---|---|
| Chisel size range or maximum | The widest mortise in one pass | 1/4 to 1/2 in · 1/2 in · — |
| Collar or shank size | Which chisels fit the head | — · 3/4 in collar · — |
| Chisel travel or stroke | How far the head moves | 4-5/8 in · 4-7/8 in · 6-1/4 in |
| Maximum mortising depth | How deep one hole can go | 3 in · 3 in · — |
| Fence to chisel centre | How far from the face a mortise can be | 4 in · — · 3 in |
| Stock width and thickness | The largest part under the head | 8-1/2 × 6 in · 13-3/8 × 4 in · 9 in wide |
| Motor and weight | Power and stability | 1/2 HP, 70 lb · 1/2 HP, 60 lb · 1-1/2 HP, 332 lb |
Grizzly's figures for three of its own machines, from their manuals; a dash means the manual reviewed gives no figure under that name. Other makers measure some figures differently. Not a ranking.
Chisels and bits
A mortising chisel and its auger are a pair: the auger must be long enough to reach the chuck and narrow enough to turn freely inside the chisel. Grizzly's T33127 manual tabulates the pairs it takes — a 1/4 in chisel with an auger of no more than 3/16 in, a 3/8 in chisel with 1/4 in, a 1/2 in chisel with 3/8 in, every auger at least 6-1/2 in long — and asks for chisels from elsewhere to match those dimensions. The chisel's shank must also fit the head's collar or bushing. Keep both sharp; a dull auger or a chisel packed with chips burns the wood, as Grizzly's troubleshooting notes.
Mortising by hand, with a mortise chisel and mallet, is the alternative for a few joints; see mortise chisels. Hollow-chisel attachments are also sold for some drill presses, but the drill press manuals BenchRoot has reviewed do not describe them, so mortising on a drill press is not covered here; see the drill press guide for what those machines are documented to do.
Safe use
Safety— Power off to fit the chisel; floor machines bolted down
Grizzly's manuals have the machine disconnected from power before a chisel or bit is fitted, and leather gloves or a wrapped towel when handling the chisel. Set the hold-down just above the work — 1/16 in on the G0645 — so the work cannot lift with the chisel, never lengthen the handle with a pipe, and wear safety glasses and a respirator. The G0448 is top-heavy and can tip with its head tilted, so Grizzly has it bolted to the floor. See woodworking safety rules.
Choosing a machine is covered in how to choose a mortiser and, for the smaller machines, how to choose a benchtop mortiser. A mortiser fitted to a combination machine is described in combination woodworking machines. The other joinery tools are mapped in the pocket hole guide.
Before cutting a mortise
- Machine disconnected while the chisel and auger are fitted
- Auger-to-chisel gap set to the machine's own figure
- Chip slot facing the side; chisel turns without binding
- Fence set so the mortise sits where the layout puts it
- Hold-down set just above the work; depth stop set
- Test mortise cut in scrap of the same stock
Sources
Every capacity, clearance and setting is the maker's for its own machine, from its manual or product sheet; nothing here is a figure for mortisers in general. Grizzly's G0645 and G0448 give a range for the auger-to-chisel gap and its T33127 a single figure, which shows why the gap is set from each machine's manual. Makita's sheet lists the 7104L's 'maximum hole width' without saying which way it is measured. Mortising attachments for drill presses are not covered: the drill press manuals BenchRoot has reviewed do not describe one.
- 1.
Model G0645 Benchtop Mortising Machine, Owner's Manual. Grizzly Industrial, 2010.
Used for: A 1/2 HP, 1725 rpm benchtop mortiser: 70 lb; maximum stock 8-1/2 in wide and 6 in thick with the column extension; 3 in maximum mortising depth; 4-5/8 in chisel travel; 5 in column to chisel; 4 in fence to chisel centre; 1/4, 5/16, 3/8 and 1/2 in chisels; the drill bit set 1/16 to 3/16 in beyond the chisel depending on wood and operation; chisel rotated 90° if it binds; deep mortises cut 1 in at a time with the chisel backed off to clear chips; disconnect power and wear leather gloves or wrap the chisel to fit it; hold-down 1/16 in above the work; never a cheater pipe on the handle; respirator and safety glasses; mortising in wood only.
- 2.
Model T33127 Benchtop Hollow-Chisel Mortiser, Owner's Manual. Grizzly Industrial, 2024.
Used for: A 1/2 HP, 1720 rpm benchtop hollow-chisel mortiser: 60 lb; 3/4 in collar; 1/2 in maximum chisel; 3 in maximum mortising depth; 4-7/8 in chisel travel; 5-1/8 in column to chisel; its chisel and auger dimensions by size; the chisel's chip-ejection slot facing the side, not front or back, and let down about 1/64 in from the bushing to prevent binding; the auger 1/16 in beyond the chisel.
- 3.
Model G0448 Mortising Machine, Owner's Manual. Grizzly Industrial.
Used for: A 1-1/2 HP, 1725 rpm floor mortiser on a cabinet stand: 332 lb assembled; 6-1/4 in maximum chisel stroke; 3 in fence to chisel centre; 9 in maximum workpiece width; a 19 × 12-1/2 in table with longitudinal and cross traverse; a head that tilts 30° each way and makes the machine top-heavy, so it is bolted to the floor; the bit 1/16 to 3/16 in beyond the chisel.
- 4.
Chain Mortiser 7104L — product sheet. Makita (Makita Power Tools India).
Used for: A chain mortiser: 1,140 W, 3,200 rpm, maximum hole width 130 mm (5-1/8 in) and depth 155 mm (6-1/8 in), 17.8 to 18.2 kg, with an adjustable vise for clamping onto logs.
- 5.
Combination Machine C3 31 (HAMMER), product and technical data page. Felder Group.
Used for: An optional horizontal mortiser on the planer side of a combination machine.
About this guide
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