3/4-Inch or 20 mm Dog Holes? How Bench Hole Size Decides Your Holdfasts, Dogs and Clamps
3/4-Inch or 20 mm Dog Holes? How Bench Hole Size Decides Your Holdfasts, Dogs and Clamps - A step into the workshop.
Stop Wasting Wood
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The first hard frost came to the valley this week, and the shop floor held the cold the way it does in late autumn, so that the maple slab on the sawhorses was chilled through by the time I laid a hand on it. It is a bench top, or will be. It has been flattened, squared, and left to settle, and all that remains is to bore the holes. I have the 3/4-inch auger that came down to me from my grandfather and a new 20 mm Forstner still in its sleeve, and for the better part of a morning I stood there with the layout square and did nothing at all.
The holes are the cheapest thing you will ever do to a bench, and the only thing you cannot take back. Rockler sells a high-carbon Forstner in 20 mm for $14.99 and in 3/4 inch for $15.99, a dollar's difference. But the moment the chips fly you have chosen the holdfasts you can own, the dogs and clamps you can buy, and the catalogues that will speak your size. For a decade or more, the bench will hold what those holes allow.
A Millimetre, Give or Take
Three-quarters of an inch is 19.05 mm. Twenty millimetres is 0.787 inch. The gap between them is 0.95 mm, a shade over 37 thousandths, which is nothing to the eye and everything to a shaft that is meant to wedge.
Fine Tools in Germany organizes its whole workholding catalogue by hole "system" and lists four: 19 mm, 20 mm, 25.4 mm and 30 mm. Its Bessey page explains why: one bench had 3/4-inch holes, another 20 mm, and outliers had 1-inch holes, so Bessey sells the clamp head separately from shafts made for 19, 20 and 30 mm. A serious clamp maker had to split one product in three.
And the 1-inch hole is not a museum piece. Sjöbergs' Elite 2000, listed at $2,800 by Woodcraft, bores 1-inch holes in a 3-3/8-inch beech top, while the same maker's Elite Clamping Platform, $635 at Woodcraft, uses 19 mm holes in a 1-1/2-inch top. Lie-Nielsen's 7-foot bench, $3,750 in 4-inch maple, runs rectangular dogs along the front and separate 3/4-inch holdfast holes. Dog holes and holdfast holes were never obliged to match. It is only the gridded, everything-through-one-hole bench that asks them to.
What the Holdfast Knows
Of everything you will push into a bench top, the holdfast cares most about the number you choose, because it works by nothing but friction and a little bend. Set the pad on the work, strike the top, and the shaft cants in the hole and bites. Tap, and it holds. Tap from the side, and it lets go. The holdfasts I learned on taught me early that the hole and the shaft are a pair.
Christopher Schwarz has written at Lost Art Press that most vintage holdfasts he has encountered in his research are an inch or larger, and that the early French examples in Roubo run nearly an inch and a half. He suspects the 3/4-inch standard is modern and guesses it grew out of Lee Valley's bench-dog specification, though he is careful to call that an inference rather than history. Gramercy's holdfast, $24.95 each or $39.95 the pair at Tools for Working Wood, wants a 3/4-inch hole in a top 1-3/4 inches or thicker. Lie-Nielsen's, at $95, wants the same hole in a top between 1-1/2 and 4 inches, with the bottom third relieved to an inch if your top runs thicker. Crucible's 1-inch pattern, five pounds of ductile iron, claims tops from 2 to 8 inches.
The fit that makes any of this work is measured in thousandths. In a Lost Art Press open-wire thread from May 2023, a reader with 3/4-inch holes in a 3-5/8-inch pine top could not get holdfasts to cinch; his calipers put the Gramercy shafts at 0.716 to 0.720 inch against bores of 0.742 to 0.750. Schwarz's diagnosis was that the shaft was not a close fit, and he noted that Crucible makes its shafts a few thousandths under an inch for precisely that reason.
Now set the 20 mm hole against those numbers. A 3/4-inch holdfast in a 20 mm bore begins its life 37 thousandths undersize, before a single season of wear. Fine Tools, asked whether its 3/4-inch hold-downs can go into 20 mm holes, answers with what it calls "a clear no": the play is excessive, the pressure lands unevenly, and the hole edges wear fast. I have never found a reason to argue with them.
The Same Hole in Two Materials
A hole is not just a diameter. It is a diameter in a material, and the material decides whether a wedging shaft is a grip or a wound.
The Forest Products Laboratory's Wood Handbook puts numbers to what a bench-builder's hands already know. Compression perpendicular to grain at 12 percent moisture: sugar maple 1,470 psi, white oak 1,070, Douglas-fir 800, eastern white pine 440. Hard maple pushes back more than three times as hard as white pine, which is why a maple top takes a holdfast's cant season after season and a soft one bruises. The Handbook classifies MDF under ANSI A208.2 but offers no comparable figure, and I will not invent one. The people who sell it say enough.
Rob Cosman offers his 3-inch laminated bench with either an MDF or Baltic birch top at the same $1,999 to $2,099, and prints on both pages that if you plan to use holdfasts, buy the birch. Richard Maguire at The English Woodworker is blunter: MDF and hollow-core doors won't work effectively, because you need strength in the material to resist the force pushing against it. Festool designs around this. Rockler's listing for the MFT/3 notes the MDF top is meant to be cut into and flips over to double its lifespan, and the replacement top is a stocked part, $190.38 at Acme Tools. A top you expect to replace is a different philosophy from a top you expect to hand down.
Twenty Millimetres and the Grid
None of that is a case against 20 mm. It is a case for understanding what the 20 mm hole was designed to do, which is not to hold a holdfast at all.
The Festool MFT/3 top, per measurements gathered on the Festool Owners Group, is 1102 by 718 mm carrying 11 by 7 holes on 96 mm centres. That grid is the point. Two dogs in a row make a straightedge; two in a column make a square, repeatable to the accuracy of the drilling. The accessory world built around it is deep: Festool's MFT-SP clamping elements at $139 the pair, Benchdogs UK's fence dogs at £34, Precision Dogs at $26.99 machined to a thousandth.
But the "20 mm" hole is itself a bit of a fiction. Peter Millard at 10minuteworkshop reports that Festool's own 20 mm router bit cuts between 20.16 and 20.2 mm, and that Benchdogs UK makes its dogs to 19.98 mm to sit snug in a true 20. A Festool forum administrator conceded the holes are "meant to be 20mm" but drift with humidity even in MDF; one user measured a new table at 20.12 mm, an old one at 19.95, and his dogs at 19.84. The ecosystem is precise and the holes are not, and the makers resolve the tension by aiming under.
The 3/4-inch side buys a different shelf. Benchcrafted's Split-Top Roubo notes call for 3/4-inch holes to take Veritas and Gramercy accessories and recommend the Gramercy holdfast for top, leg and deadman. Kreg's nylon bench dogs run $13.99 for four at Woodcraft, Rockler's are $12.99, and the Veritas planing stop on Lee Valley's Canadian site rides on 3/4-inch posts. A few products straddle the line. Kreg's Bench Clamp Base, $24.99, fits either hole in tops to 1.5 inches, but it bolts through rather than wedging, so it dodges the question. Tool Theory's Convert-a-Dogs, $17.99, set a 19.01 mm base in a 3/4-inch hole with a 20 mm shaft above, so a solid top can borrow from the grid. A dog can live with an adapter. A holdfast, which lives on the bite of iron against wood, cannot. The 3/4-inch hole grips; the 20 mm hole locates.
Drilling Is the Cheap Part
Enlargement is a one-way road. Schwarz has published a method for taking 3/4-inch holes to an inch, a kerfed dowel carrying a spade bit as a stepped pilot, and he calls it what it is: a permanent modification. You can go bigger, once. You can never go smaller. Wood keeps moving besides. Schwarz's own bench dried over a few years until its 1-inch holes tightened on the shafts, and his fix was a 1-1/16-inch bit and a caution not to over-do it, because a holdfast wants a close fit above all.
If the top is solid hardwood, thick, and meant to outlast me, I bore 3/4 inch, because that is where the holdfasts live and where the makers I trust have built their fittings. If I were building a light, flat, sacrificial top for a track saw and a square, I would bore 20 mm and buy the dogs without apology; the grid does that job better than a dozen holdfast holes ever will. Schwarz, asked whether the MFT grid could be brought to a heavy bench, allowed that it could, then said he had never lacked for a better solution than a planing stop and about a dozen holdfast holes. That is my experience too.
Whichever size you choose, choose the places as carefully as the diameter. Schwarz bores eight holes on his bench and can name the job of every one, and his advice is to have a reason for each rather than a vague feeling about its location. The auger will be warm in the hand by the third hole, the shavings spiralling up and dropping to the cold floor. Bore the ones you can justify, leave the rest of the top whole, and let the bench tell you over the winter where the next one belongs.