How to Use a Drill Press Mortising Attachment for Square Mortises Without Tearout
What's inside
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Contents
How an Attachment Actually Cuts
A drill press mortising attachment turns your drill press into a low-speed square chisel press. It clamps to the quill or column and holds a square hollow chisel stationary while an auger bit spins inside it. The auger removes most of the waste, the four-sided chisel shears the walls square. It is not a high-speed operation. Unlike drilling, you are forcing a square edge through wood, so you need low RPM, high downforce, and zero play in the setup.
Most attachments require a drill press with at least 1/2 HP, 3-1/4 inches of quill travel, and a 5/8-inch collar or a flat on the quill for the yoke. Light benchtop presses with tubular columns and plastic depth stops will flex and wander. If your quill has noticeable side play when extended, fix that first or skip the attachment entirely. A dedicated benchtop mortiser will outperform a sloppy drill press every time.
Chisel and Auger Setup That Prevents 90 Percent of Problems
Tearout and burning almost always trace back to setup, not technique. Get three things right before you cut wood.
First, auger-to-chisel clearance. Install the chisel in the yoke, then slide the auger through the chisel until its cutting spurs are visible. You want the auger tip to protrude 1/16 to 1/8 inch below the chisel points. More than that and the auger pulls the chisel into the work, causing grab and tearout on entry. Less than that and the chisel hits wood before the auger clears waste, which burns and requires excessive force. Lock the auger in the chuck, then tap the chisel up slightly to set that gap with a dime or feeler gauge as a spacer.
Second, orientation. The open slot in the hollow chisel must face to the side or front where chips can eject, not trapped against the fence. Most fences block the rear slot, so turn the slot to the side. You will see chips firing sideways when it is right.
Third, sharpness. Factory mortise chisels are rarely sharp enough. The outside bevels need to be flat-honed and the inside polished. A budget hollow chisel mortising set can cut cleanly if you spend 20 minutes per chisel on 1000 and 4000 grit stones. If you can shave end grain with the chisel point, it is ready. If you cannot, you will crush fibers instead of cutting them, which looks like tearout even when your technique is fine.
Speed, Feed, and Hold-Down
Run the drill press much slower than you would for twist drilling. For 1/4-inch to 1/2-inch chisels in hardwoods like white oak or hard maple, 1200 to 1700 RPM is the range. In softwoods or poplar, you can go up to 2000 RPM, but there is no advantage to spinning faster. Heat is your enemy. Wax the chisel and auger with paste wax or paraffin before each mortise.
Feed pressure should be steady and firm, not a sharp slam. Sink the chisel about 1/4 to 3/8 inch per plunge, then retract slightly to clear chips, then continue. For mortises deeper than 1-1/2 inches, clear chips completely every 1/2 inch of depth by raising the chisel out of the cut. If chips pack inside the chisel, friction spikes and you will scorch the walls and potentially seize the auger.
The fence and hold-down are not optional. An attachment kit without a proper hold-down is almost unusable. The chisel acts like a wedge on withdrawal and will lift the workpiece with 50+ pounds of force. If the board lifts even 1/32 inch and then slams back down, you get elongated, ragged mortises and bad tearout on the top face. Use the included hold-down bar and add a wood-faced fence so the chisel has a reference. Clamp the fence itself to the table. Any fence movement during the cut ruins wall straightness.
Technique for Clean Walls Without Tearout
Square mortises tear out in three places: the entry corners, the exit side if you go through, and the side walls from levering the chisel.
Start with scoring cuts. Plunge 1/8 inch deep at both ends of the mortise first, defining the outer walls. These light scoring plunges sever the surface fibers cleanly. Then come back and hog out the middle, overlapping each square by about 25 percent. For a 1-inch long mortise with a 1/4-inch chisel, you need 5 to 6 overlapping plunges rather than 4 edge-to-edge. Overlapping improves wall straightness and reduces corner chips.
Use a sacrificial backer. Put a piece of scrap hardwood behind the workpiece against the fence if you are mortising within 1/2 inch of the edge, and put a scrap board underneath if the mortise goes through. Without a backer, the chisel blows out fibers on exit. With a tight backer clamped with the work, you get a crisp edge.
Do not wiggle the chisel to widen the mortise while it is buried. That flexes a small drill press column and bells the walls. If the mortise needs to be 1/64 inch wider for fit, pare it with a hand mortise chisel after, or make a second overlapping pass offset by 1/32 inch, fully retracted and re-plunged. Keep grain direction in mind: mortising across the grain tears more than along it. When you must cut near an end, leave an extra 1/8 inch of waste length and trim to final length after mortising.
Keep chips clear and keep the chisel cool. If the chisel is too hot to touch, you are feeding too slowly, not clearing chips, or the auger is dull. Stop and re-hone.
Attachment vs. Dedicated Mortiser vs. Router
All three make a rectangular mortise, but they compromise differently. This is where cost and shop space matter.
| Method | Best For | Limitations | Typical Cost |
|---|---|---|---|
| Drill Press Attachment | Occasional mortises, 1/4 to 1/2 inch width, 1 to 2 inch depth | Needs strong press, limited chip ejection, slower, more setup | $35 – $90 plus chisels |
| Benchtop Hollow-Chisel Mortiser | Frequent joinery, repeated production | Single-purpose machine, takes floor/bench space | $250 – $500 |
| Plunge Router + Jig | Through mortises, long mortises, loose tenons | Rounds ends, needs squaring by hand, noise and dust | $150 – $300 plus jig |
If you cut fewer than 20 mortises a year, a drill press mortising attachment on a solid drill press is the economical choice. It is slower than a dedicated mortiser but accurate enough for tables and doors if you take time with setup. If you are building chairs or doing a kitchen’s worth of mortise-and-tenon joints, buy a dedicated mortiser. It has a longer, gas-assisted lever, better holddowns, and proper chisel alignment with gibs. The router is king when you want speed and use loose tenons or Domino-style joinery, but you will still need to square the ends with a chisel if you use traditional tenons.
Common Failures and Fixes
Chisel will not enter wood even with heavy pressure: chisel dull, auger too deep inside chisel, or you are feeding too fast without clearing chips. Re-hone, reset the 1/16 inch tip projection, and use peck drilling.
Burning and smoking: speed too high, auger dull, or chips packed. Drop to 1200 RPM in hard maple and white oak, wax the chisel, and fully retract every 1/2 inch.
Mortise walls tapered or out of square: fence moved, column flex, or you levered the chisel while buried. Clamp everything solid and never side-load the chisel under full depth. Use multiple light plunges.
Workpiece lifts on retraction: hold-down too high or too weak. Bring the hold-down bar down until it almost touches the board before the cut, not 1/4 inch above it. If your attachment came with a flimsy sheet-metal hold-down, replace it with a shop-made clamp bar or add a separate drill press hold down clamp to the table.
Ragged top shoulders: dull corners or no scoring pass. Hone the outside faces of the chisel and always make shallow end cuts first. A strip of blue tape on the layout line helps but is not a substitute for a sharp edge and a backer.
When to Buy Better Chisels
The attachment yoke is usually fine even in cheap kits, but the included chisels are hit or miss. If you have honed correctly and still get crushed, fuzzy corners in cherry or red oak, upgrade the chisel and auger pair for your most-used size, typically 1/4 inch and 3/8 inch, and keep the cheap ones for softwoods. Good chisels are thicker-walled and hold an edge longer, which directly reduces tearout on the final walls. Keep them in a dedicated rack, not loose in a drawer where the corners get nicked.
