Delta Mortising Attachment for Drill Press: A Practical Guide

Delta built its reputation on stationary woodworking machines, so it makes sense that woodworkers go looking for a Delta mortising attachment when they want square holes from a Delta drill press. The search gets confusing fast, because Delta’s own branded attachment has come and gone from shelves over the years, while Delta chisels and bits are still very much around. Here is the practical picture, so you can cut clean mortises on your Delta press without chasing a part that may be hard to find new.

A mortising attachment for a drill press is a kit that clamps a hollow square chisel to the quill and runs an auger bit inside it, so each plunge bores a round hole and squares its walls into a socket. Chain those sockets and you have a mortise. A “Delta” mortising attachment refers either to Delta’s own kit or to Delta’s chisel and bit sets used in any compatible attachment.

The direct answer: you do not need a Delta-branded attachment body to mortise on a Delta drill press. Any mortising attachment whose bushings match your quill will work, and Delta’s real strength today is its chisel and bit sets, which fit most attachments thanks to a common 5/8 inch shank. Match the attachment to your quill, fit good chisels, and the brand on the casting matters far less than the setup.

What a mortising attachment does

The hollow chisel is a square steel tube, sharp at the tip, with an auger bit spinning inside it. The bit bores out the bulk of the waste and lifts chips through a side window, while the chisel’s four corners pare the round hole into a crisp square. A cast holder clamps to your quill, a fence sets the distance from the edge, and a hold-down stops the workpiece from riding up when you pull the chisel back out.

Fitting it to a Delta press

Fit comes down to the quill, not the badge. Attachment kits include bushings, and you match a bushing’s inside diameter to your drill press quill diameter. Delta presses are well documented, so finding your quill size is easy. You also want enough quill travel for the mortise depth you want and enough downforce to drive a chisel through hardwood. If your Delta is a floor model with a long quill stroke, you are in good shape; a small benchtop Delta will prefer softer woods and shallower cuts.

The smart way to buy today

If you can find a genuine Delta attachment body, it is a fine casting. If you cannot, do not stall the project. A current all-in-one mortising attachment kit gives you the holder, fence, hold-down, and a starter set of chisels that fit standard quills, and it gets you mortising now. Then lean on Delta where the brand truly earns its money: the chisels. A Delta 17-003 professional mortising chisel and bit set uses a 5/8 inch shank that fits most attachments and cuts cleaner, cooler mortises than the bargain steel that ships with budget kits. That pairing, a working attachment plus quality Delta chisels, beats holding out for a discontinued branded body.

Setup that gives clean square holes

Get three things right. Set a small clearance gap, roughly a coin’s thickness, between the auger bit and the chisel tip so chips clear without the bit rubbing and burning. Square the chisel to the fence so a row of holes chains in a straight line without stepping. And cut in stages, letting the bit auger the waste out instead of forcing a single deep plunge that packs chips and overheats. Keep the chisel honed, and back thin stock with scrap so the bottom does not blow out.

The verdict

Do not let the hunt for a Delta-branded attachment body stop you from cutting mortises. The body is just a clamp and a fence; the parts that decide quality are the quill match and the chisel. Buy a current attachment kit that fits your quill, fit a quality Delta chisel and bit set, and set the gap and alignment with care. Your Delta drill press will cut square holes as clean as the joint deserves, and you will have spent your money on the parts that actually touch the wood.

FAQ

Do I need a Delta attachment to mortise on a Delta drill press? No. Any attachment whose bushings match your quill works. The brand on the casting does not change the result.

Will Delta chisels fit other attachments? Usually yes. Delta chisel and bit sets use a 5/8 inch shank that fits most mortising attachments and machines.

How do I know which bushing I need? Measure your drill press quill diameter and match it to the bushing’s inside diameter included in the kit.

Why are my mortises burning or packing with chips? The bit-to-chisel gap is likely too tight, or you are plunging too deep at once. Set a small gap and cut in stages.

Can a benchtop Delta press mortise? Yes, best in softer woods with shallower cuts. Squaring hardwood needs downforce that larger presses provide more easily.

What chisel sizes should I start with? 1/4 and 3/8 inch handle most furniture joinery. Add 1/2 inch for heavier tenons.

References

• Delta Machinery, 17-003 Professional Mortising Chisel and Bit Set specifications: https://deltamachinery.com/product/17-003-3-8-in-professional-mortising-chisel-and-bit-set/
• Grizzly Industrial, H7789 Mortising Attachment Kit (bushing sizing and universal fit): https://www.grizzly.com/products/grizzly-mortising-attachment-kit-for-drill-presses/h7789