Drill press mortising attachment: chisel sizes, fit, and setup

A mortising attachment converts a drill press into a hollow-chisel mortiser: a square, four-sided chisel mounts in a holder clamped around the quill, a spinning auger bit runs up its center, and the pair cuts square-bottomed holes for mortise and tenon joints ^{[4] [8]}. The concept has been sold for decades, under labels from the vintage Rockwell 976 to today's Grizzly H7789, Delta 17-924/17-935, Rikon 29-202, Shop Fox D3582, and import kits from SKYSHALO and Klutch ^{[10] [1] [11] [6]}. Buying one is mostly a fit question, and using one is mostly a setup question. This page covers both: which chisel sizes the kits carry, which drill presses the major brands fit and how their bushing systems differ, the setup steps the manuals agree on, and the speed question, where the manuals genuinely disagree.

Every row of every table below is backed by at least two independent sources, and the disagreements between manufacturers, including two outright misprints we found in current manuals, are disclosed on this page rather than smoothed over.

The short answers

Chisels: every mainstream kit ships the same four sizes, 1/4, 5/16, 3/8, and 1/2 in ^{[1] [2] [3] [7]}. Fit: the attachment's chisel holder clamps around your press's quill or quill collar, and bushings adapt it to that diameter, so the number to measure before buying is the quill, not the chuck ^{[1] [6]}. Setup: the one step every manual repeats is setting the auger about 1/32 in proud of the chisel points before locking everything down ^{[4] [5] [6] [9]}. Speed: the manuals publish three different ranges, 650 to 1000, 650 to 1800, and 1000 to 1500 RPM, all saying the same thing in different words: run slow ^{[5] [4] [6] [13]}. Harbor Freight: no attachment, only the WARRIOR chisel and bit consumables ^[12].

Hollow chisel and bit sizes

The four-size set is an industry standard. Grizzly's H7789 page lists 1/4, 5/16, 3/8, and 1/2 in chisels ^[1]; Delta's 17-924 kit contents are the same four ^[2]; the Rikon 29-202 retailer listing repeats them ^[7]; and Lee Valley's replacement chisels for attachments and dedicated mortisers confirm the same four as the common core, in both economy and premium lines ^[11]. Two manuals deserve a warning, disclosed here rather than laundered. The SKYSHALO DPA65 manual lists its included chisels as 1/4, 5/8, 3/8, and 1/2 in in two places, a near-certain misprint for the standard 5/16, since no other kit in this class ships a 5/8 as its second size and the rest of the manual's text is copied from the standard format ^[4]. The Klutch 57860 kit genuinely does ship a 5/8 in chisel as its fourth size, but omits the 5/16, listing 1/4, 3/8, 1/2, and 5/8 ^[6]. So: 5/16 is standard, 5/8 exists, and one manual swapped them.

Every chisel needs its matching auger bit; the four-sided chisel squares the corners while the auger inside removes the waste ^{[4] [8]}. Mixing a chisel with the wrong bit is the classic way to burn both. The chisel itself is held by its shank in the holder or yoke, 5/8 in diameter on the mainstream kits, and the auger shank goes in the chuck ^{[7] [4]}. Sizes above 1/2 in are sold separately: Lee Valley catalogs replacements from 1/4 up to 1 in ^[11], and Harbor Freight sells a 1/2 in chisel and bit that works with any standard attachment ^[12]. This page deliberately does not restate the general RPM-by-diameter tables; those live in our drill press speed chart.

Quill collar and bushing fit

The chisel holder clamps around the quill, the sliding steel tube that carries the spindle, or around a quill collar at its base. None of them fit every press out of the box; each brand adapts with bushings, and the bushing lists are where the brands genuinely differ ^{[1] [6]}. Grizzly machines it in inches: the H7789's six bushings have inside diameters from 1.570 to 2.600 in, and you match your quill to the nearest ID ^[1]. Its support article translates the same kit to metric quills of 40, 48, 50, 55, 60, 66, and 75 mm ^[3], which matches the Rikon 29-202's published collar list almost exactly ^[7]. Delta's 17-935 uses split-ring bushings chosen by quill diameter, 50.8, 52, 55, 60, and 66 mm, with published bushing thicknesses (7.5, 6.9, 5.4, 2.9, and none) so a 66 mm Delta 17-900 quill needs no bushing at all ^{[9] [6]}. The SKYSHALO DPA65 lists four collar sizes, 52.5, 55, 60, and 65 mm ^[4]. The Klutch 57860 parts list names its split-ring bushing IDs as 52.5, 55, 58, and 60 mm ^[6]. The overlap is real but not identical: 58 and 65 mm each appear in only one brand's list, so measure the quill and read the specific kit's list before ordering.

Which drill presses each kit fits

With the bushing logic in hand, the fits lists make sense: they are just presses whose quills fall inside the bushing range. Two lists below, one for each of the widely documented kits. Divergences between them are real and disclosed: Grizzly's H7789 explicitly excludes radial drill presses, while Delta's older 17-905 manual supported Delta's own 11-090 32 in radial drill, so the exclusion is a brand-by-brand decision, not physics ^{[1] [10]}.

A note on the Jet rows: the same three-press list, JDP-14MF, JDP-17M, JDP-17MF, appears both in Jet's own JMA-582 attachment copy and in Grizzly's H7789 fits list ^{[14] [1]}, and a replacement Jet quill part is cataloged for the JDP-17M/MF ^[16]. The three presses share quill diameters inside both kits' bushing ranges, which is why the lists agree.

Setup: the steps the manuals agree on

Strip the brand names and the manuals converge on the same sequence. The critical adjustment, repeated in four independent manuals, is the bit-to-chisel relationship ^{[4] [5] [6] [9]}.

1. Unplug the press. Mount the chisel holder on or just above the base of the quill, with the correct bushing in place and its slot aligned with the gap in the holder, then tighten the holder's bolt ^{[4] [6]}.
2. Mount the fence to the table with its bolts through a sacrificial layer (next section) and square it to the chisel faces; if fence and chisel are not square the cuts come out staggered ^{[6] [10]}.
3. Insert the bit through the chisel and into the chuck, hand tight only. Clamp the chisel in the holder with its shoulder about 1/32 in below the holder's bottom ^[4].
4. Push the bit up in the chuck until its end seats against the chisel end, and tighten the chuck with the key ^{[4] [6]}.
5. Loosen the chisel and raise it to close that 1/32 in gap, then tighten. This is what sets the auger's cutting lips about 1/32 in proud of the chisel points, the running clearance that lets the auger lift shavings up through the chisel ^{[4] [6]}. Grizzly's G1083 sheet states the same 1/32 in proud rule directly ^[5].
6. Set the hold-down so it presses the work flat without pinching it, and set the curved guide rods to hold the work against the fence while still letting it slide ^{[4] [6]}.
7. Set table height so the chisel starts about 1/2 in above the work, set the depth stop, and cut the mortise ends first, then overlap cuts between them, raising the chisel often to clear chips ^{[4] [6]}.

Two technique notes the manuals insist on. The chisel's bit slot should face the previously cut hole on overlapping cuts, so chips eject into the open cavity instead of packing ^[6]. And the chisel and bit arrive factory ground but the SKYSHALO manual states they must be sharpened before use, with a slip stone from the inside for the chisel and a flat stone for the auger ^{[4] [5]}. For keeping every other bit in the shop sharp, see our drill bit sharpening guide.

Backing board and hold-down: through mortises

For through mortises, put a plywood panel between the fence and the table before bolting anything down. Delta's and SKYSHALO's manuals specify 1/2 in plywood; Klutch says a minimum of 1 to 2 in thick, a difference in wording we disclose rather than average, since any of those thicknesses does the same job: it stops the chisel point from striking the cast-iron table when it breaks through, and it enlarges the working surface ^{[9] [4] [6]}. The hold-down exists because the chisel pulls the workpiece up with it on every retract; the guide rods and hold-down together are what keep the stock on the table and against the fence through a full mortise ^{[4] [8]}.

The speed question: three camps, one direction

The manuals disagree on the exact range, and this page shows all three positions rather than picking one. Camp one, Grizzly's G1083 instruction sheet: for best results, select a drill press speed of 650 to 1000 RPM ^[5]. Camp two, the SKYSHALO DPA65 and Klutch manuals: best speeds are 650 to 1800 RPM, use lower speeds on hardwoods and larger chisels, and test on scrap ^{[4] [6]}. Camp three, Popular Woodworking's technique piece: 1000 to 1500 RPM depending on mortise size and wood density, found by trial ^[13]. The reconciliation is that all three start slow: the overlap zone of every camp is roughly 650 to 1500 RPM, and within it the rule the sources share is directional, slower for larger chisels and harder woods, faster for small chisels in softwood ^{[6] [13]}. Dedicated benchtop mortisers run fixed 1725 to 1750 RPM motors, which bounds the top of the plausible range for the same tooling ^{[13] [15]}. For the general RPM-by-bit-diameter logic on your specific press, use the drill press speed chart; this page covers only the mortising-attachment-specific numbers.

The Harbor Freight question, answered plainly

Harbor Freight does not sell a drill press mortising attachment. As of September 2026 the Mortise and Hinge Bits category on harborfreight.com contains three items, and none is an attachment: the WARRIOR 1/2 in. mortising bit and chisel, item 68679, listed at 14.99 with a 3/4 in shank and explicitly stated to work with any standard drill press mortising attachment or mortising machine; a 3 piece WARRIOR spiral mortising bit set in 1/4, 3/8, and 1/2 in; and a 3 piece hinge drill bit set ^[12]. So the honest answer to the search question is: buy the attachment from Grizzly, Delta, Rikon, or Shop Fox, and Harbor Freight will supply the consumable chisel and bit that goes in it ^{[12] [1]}.

Attachment versus dedicated mortiser

Whether to convert a press or buy a benchtop mortiser is a capacity and rigidity question. The attachment route costs a fraction of a dedicated machine and stores in a case, but setup and removal are time consuming, and drill press handles can bend under the torque of plunging a chisel into hardwood, a limitation Woodsmith names directly ^{[8] [15]}. Popular Woodworking's counterpoint: on a press you can slow the speed well down, and the results can be first-class ^[13]. Depth capacity is the other limit: the Klutch manual warns against cutting deeper than the listed capacity for each chisel, since beyond it the ejection slots clog and the tool damages ^[6]. For the woodboring bits themselves, spade versus Forstner versus hole saw, our wood boring bits compared page owns that comparison.

Common problems, sourced

Chips jamming between chisel and bit is the failure mode the troubleshooting sections return to, with the same three fixes everywhere: lift the chisel periodically on end cuts, keep the chisel's open side facing an already drilled area, and increase the overlap between successive holes ^[6]. Burning and smoke trace to speed and feed: too fast a spindle speed for the chisel size, or feeding too slowly, and the remedy is dropping RPM and matching feed to the chip flow ^{[6] [13]}. If the noise level climbs mid-session, Delta's remedy is re-centering the chisel holder with the alignment pin ^[9]. Excessive friction and heat from a dull chisel or bit makes the drilling difficult and damages the tooling, which is why the sharpen-before-first-use rule above is not optional ^[4].

Related

For RPM by diameter and pulley mapping on your press: drill press speed chart. For the spade versus Forstner versus hole saw comparison in wood: wood boring bits compared. For Forstner size charts specifically: Forstner bit sizes. For the adjustable expansion bit that covers 5/8 to 3 in holes: adjustable wood bit sizes and models. For bit choice across all materials: drill bits by material, and for keeping chisels and bits cutting: drill bit sharpening. For hinge and hardware boring with a press: hole saw sizes.

Sources

1. Grizzly Industrial, H7789 Mortising Attachment Kit product page: 1/4, 5/16, 3/8, 1/2 in chisels; bushing IDs 2.945 x 2.600, 2.600 x 2.366, 2.366 x 2.170, 2.600 x 2.000, 2.600 x 1.880, 2.600 x 1.570 in; FITS list incl Delta 11-990/14-070/17-900/11-090 and Jet JDP-14MF/JDP-17M/JDP-17MF; not recommended for baby, 8 in, or radial presses; M1039 2.825 in quill needs G9716 (grizzly.com/products/grizzly-mortising-attachment-kit-for-drill-presses/h7789)
2. The Home Depot, Delta Mortising Kit 17-924 listing: four chisel bit sizes 1/4, 5/16, 3/8, 1/2 in; fits 38, 48, 50.8, and 66 mm quill diameters including Delta 11-090, 11-980, 11-990, 14-040, 14-070, 17-900, 17-925, 17-965; 4 in capacity under hold-down, 2-3/4 in clamp rod to fence (homedepot.com/p/Delta-Mortising-Kit-17-924/203026212)
3. Grizzly Industrial support article 4412910275479, How to turn your drill press into a mortising machine: kit components, four chisels 1/4 to 1/2 in, six bushings, fits metric quill sizes 40, 48, 50, 55, 60, 66, and 75 mm (support.grizzly.com/hc/en-us/articles/4412910275479)
4. SKYSHALO DPA65 Mortising Attachment instruction manual (manuals.plus): collar sizes 52.5, 55, 60, 65 mm; 1/2 in plywood backing; chisel shoulder 1/32 in below holder then raised to seat bit; bit end against chisel end; chisels listed 1/4, 5/8, 3/8, 1/2 in in the parts list (5/16 misprint, disclosed on page); 650 to 1800 RPM, lower on hardwoods; ends first then overlapping cuts; sharpen before use (manuals.plus/skyshalo/dpa65-mortising-attachment-for-drill-press-manual)
5. Grizzly G1083, G1370, G1514, G1515 Mortising Attachment Instructions: bit tip at least 1/32 in below chisel edge; 650 to 1000 RPM for best results; ends-first overlapping sequence with spacing less than chisel width; sharpen chisel inside with round slipstone, bit with flat stone (manualslib.com/manual/62748/Grizzly-G1083.html)
6. Klutch Mortising Attachment 57860 owner's manual (Northern Tool): chisels 1/4, 3/8, 1/2, 5/8 in with dimensions; split-ring bushings 52.5, 55, 58, 60 mm; 1/32 in gap-then-close setup; 1 to 2 in plywood minimum; 650 to 1800 RPM; ends first, bit slot faces previous hole; chip-jam troubleshooting (assets.northerntool.com/products/578/documents/manuals/57860.pdf)
7. Acme Tools, Rikon 29-202 Morticing Attachment listing: collar adapters for 40, 48, 50.8, 55, 60, 66, 75 mm quills; chisel held in yoke by 5/8 in shank; 4 chisels and bits 1/4, 5/16, 3/8, 1/2 in; fits 13, 17, 20, 34 in presses (acmetools.com/rikon-morticing-attachment-with-chisels-fits-13-in-17-in-20-in-34-in-drill-presses-29-202)
8. Woodsmith, Using Hollow-Chisel Mortisers: yoke over the quill locked with a collar; 1700 versus 3400 RPM fixed-speed motors on dedicated machines; attachment setup/removal time consuming and drill press handles can bend under hardwood torque (woodsmith.com/article/using-hollow-chisel-mortisers)
9. Delta 17-935 (17-924 kit) Mortising Attachment manual: quill diameters 50.8, 52, 55, 60, 66 mm; bushing thicknesses 7.5, 6.9, 5.4, 2.9 mm, no bushing at 66 mm; 1/2 in plywood piece under through mortises; alignment-pin re-centering for noise; do not exceed chisel depth capacity (delta.manymanuals.com/power-drills/17-925/user-manual-18698 and manualslib.com/manual/2132732/Delta-17-935.html)
10. Delta 17-905 Mortising Attachment manual (archive.org mirror, revised 4-24-96): for use with 11-980 10 in, 11-990 12 in, 14-040 14 in, 11-090 32 in radial, Delta 15 in, and 17-900 16-1/2 in drill presses; split-ring bushing assembly; fence square to chisel or cuts stagger (archive.org, manualsonline id 3cf7d91f)
11. Lee Valley, Economy and Premium Chisels and Bits for Mortisers and Drill-Press Mortising Attachments: sizes 1/4 through 1 in, common core 1/4, 5/16, 3/8, 1/2 in; chisel shanks 5/8 in; economy hardened Rc 48-52 (leevalley.com, item 04J1020 family); also Timbertoolshed comparison naming Shop Fox D3582 for 55 to 66 mm quills and the Delta 17-905 as the cast-iron benchmark (timbertoolshed.com/best-mortising-attachments-for-your-drill-press)
12. Harbor Freight, Mortise and Hinge Bits category and WARRIOR 1/2 in. Mortising Bit and Chisel item 68679 (also 99963): works with any standard drill press mortising attachment or mortising machine, 3/4 in shank, 14.99; plus 3 pc spiral mortising bit set 1/4, 3/8, 1/2 in; category holds 3 items, no attachment sold (harborfreight.com/12-in-mortising-bit-and-chisel-68679.html)
13. Popular Woodworking, Mortising on the Drill Press: 1000 to 1500 RPM slow-speed rule depending on mortise size and wood density; ends first then clear the middle; engage all four sides or two opposite sides; auger about 1/16 in gap inside chisel for chip ejection; watch chip flow to set feed (popularwoodworking.com/techniques/mortising-drill-press/)
14. Accura Machine Tools, D4031 Drill Press Mortising Kit listing (H7789-pattern copy): 1/4, 5/16, 3/8, 1/2 in chisels, same bushing set, FITS Delta 11-990, 14-070, 17-900, 11-090 and Jet JDP-14MF, JDP-17M, JDP-17MF; and Jet JMA-582 (708582) Mortising Attachment product copy: fits JDP-14MF, JDP-17M, JDP-17MF, 5/8 in shank chisels (accuramachinetools.com/d4drprmokit.html; drillpresstools.blogspot.com 708582 review citing Jet copy)
15. MISec Best Mortising Machines guide and BoundByFlame hollow-chisel mortiser reviews: dedicated benchtop mortisers ship fixed 1725 to 1750 RPM motors (Powermatic PM701, Jet JBM-5, Delta 14-651), slow-speed machines easier to control, fast motors scorch (misec.net/best-mortising-machines; boundbyflame.com/best-hollow-chisel-mortisers)
16. Global Industrial, Jet Quill JT9-10905612 replacement part for JDP-17M/MF and JDP-12: confirms shared quill part across the 17M/17MF presses the fits lists name (globalindustrial.com/p/quill-jdp-17mmf-10905612)