Drill Press Adapter for Mortising Bits: Compatibility and Setup Guide
Most woodworkers have a drill press gathering dust in the corner. A mortising attachment can turn that machine into a surprisingly capable hollow-chisel mortiser - but only if your quill collar actually fits the kit. Getting that fit right is the part nobody explains well. This guide covers the adapter question directly: which kits work on which machines, how bushings function, and what to do when your quill diameter falls outside the standard range.
What a Mortising Adapter Actually Does
A mortising bit is a hollow square chisel with a twist bit running through its center. The whole assembly attaches to the drill press quill via a collar and interchangeable bushings. If the kit’s collar doesn’t fit snugly around your quill housing, the chisel holder wobbles, the fence goes out of parallel, and your mortises look rough.
The Two Kits Worth Buying: H7789 vs. D4031
Grizzly H7789 Mortising Attachment Kit
Grizzly Industrial H7789 - Mortising Attachment Kit
Six bushings covering quill flange diameters from 1.57” (40mm) to 2.945” (75mm). Four chisels: 1/4”, 5/16”, 3/8”, and 1/2”. Cast-iron chisel holder, fence, hold-down, alignment pin, carrying case.
Pros: Six bushing combinations cover a wide quill range. Includes alignment pin. Cast-iron construction.
Cons: Not compatible with small 8-inch bench-top or radial arm drill presses. Chisels arrive functional but not razor-sharp - sharpen before first use.
Bottom line: If your drill press quill is in the 40-75mm range, the H7789 fits and performs well.
Shop Fox D4031 Mortising Attachment Kit
Woodstock D4031 Mortising Attachment Kit
Nearly identical to the H7789. Same six bushing sizes, same four chisels, same cast-iron holder. Both from Woodstock International. Chisels from one kit are interchangeable with the other.
Pros: Identical bushing coverage. Complete kit. Sometimes available at a lower price.
Cons: Same limitations as H7789.
How to Match Bushing to Quill
Measure your quill flange outer diameter with calipers before ordering.
| Bushing ID | Quill Diameter It Fits |
|---|---|
| 2.600” | 2.6” quill |
| 2.366” | 60mm quill |
| 2.170” | 55mm quill |
| 2.000” | 50mm quill |
| 1.880” | 48mm quill |
| 1.570” | 40mm quill |
If your quill is between sizes, use the next smaller ID.
The G9716 Adapter: When Your Quill Is Non-Standard
The Grizzly M1039 and Shop Fox W1680 drill presses have a quill diameter of 2.825” - just under the largest bushing ID in both kits. The collar sits loose. Grizzly sells the G9716 Collar specifically to bridge this gap (66mm ID, 92mm OD). If you own a G7947, G7948, M1039, or W1680, you need this collar before the standard kits will work. Find it at Grizzly.com.
Annular Cutter Adapters: The 3/4” to 1/2” Question
Annular cutters (hollow-core hole saws for structural steel) have a 3/4” Weldon shank. Most drill press chucks are 1/2” keyed. You need a 3/4”-to-1/2” adapter.
JESTUOUS Weldon Shank Adapter 3/4 Inch to 1/2 Inch
Converts a 3/4” Weldon socket to a 1/2”-20UNF threaded shank. Reasonable for occasional use. For heavy or frequent annular cutting in steel, a magnetic drill press with a dedicated Weldon chuck is the right answer.
Setup: Getting the Chisel Aligned
- Install the bushing matched to your quill OD.
- Slide the column collar onto the quill housing.
- Insert the alignment pin through the chisel holder into the chuck.
- Tighten the collar, then remove the alignment pin.
- Chuck the auger bit.
- Set the fence distance from the chisel’s outer face.
- Set the depth stop so the chisel plunges fully through the material.
Leave about 1/16” clearance between the chisel’s bottom edge and the bit’s cutting tip. If the chisel rides the bit, chips can’t evacuate and you’ll burn wood.
Comparison: H7789 vs. D4031
| Feature | Grizzly H7789 | Shop Fox D4031 |
|---|---|---|
| Bushing range | 1.57”-2.95” ID | 1.57”-2.95” ID |
| Chisels | 4 (1/4”-1/2”) | 4 (1/4”-1/2”) |
| Holder | Cast iron | Cast iron |
| Carrying case | Yes | Yes |
| Non-standard quill fix | G9716 collar | G9716 collar |
FAQ
Can I use a mortising attachment on any drill press? Not on 8-inch machines or radial arm drill presses. You need a full-size bench-top or floor-standing machine with quill diameter in the 40-75mm range.
My quill diameter is 2.825”. Which bushing do I use? Neither standard kit fits. You need the Grizzly G9716 collar first, then either the H7789 or D4031 clamps onto the collar.
Why does my chisel burn the wood? Either the chisel is too close to the twist bit (less than 1/16” clearance), the chisel is dull, or you’re plunging too fast.
References
- Grizzly H7789 manual: cdn0.grizzly.com/manuals/h7789_m.pdf
- Shop Fox D4031 manual: cdn0.grizzly.com/manuals/d4031_m.pdf
- Grizzly G9716 Collar: grizzly.com
