Mar 23 2008
The big thing about using steel cross dowels for knock down construction is that your layout must be absolutely meticulous. I have, and continue to use these a lot in jig construction, but there are a lot of other great applications.
While a lot can be done with these, a common application is for use in workbench base construction. Real life happens. People move, circumstances change. Sometimes the dream shop in the basement relocates to a garage or an outbuilding. Many of us cannot build a bench with the certainty of knowing it will never need to be easily transported to elsewhere at some future point. This makes the use of steel or brass cross dowels a wonderful option.
While this article is about what it takes to install cross dowels, due to popular demand I have researched and included links to what I feel are the best sources for this hardware in it’s various sizes out there. The links are included at the end of this article.
Ok, if you already have your hardware in hand, then layout for cross dowels in knock down bench legs works like this. If you have not obtained the parts you need, please consider waiting until you have it in hand.
Lay your two legs and cross piece on the bench, Inside face facing up, already milled, mortised and tenoned, and dry fit them. Clamp them top and bottom to hold the unit together with enough force so that things will not move while you perform the layout, and check them for square. Make sure you have them square.
If you have a Veritas saddle square, or equivalent, that would be handy, but if you don’t, no matter, here is a work around:
Position the leg assembly by allowing it to hang over the edge of your bench and support it with a something like a telescopic roller stand. Take a six-foot length of string line and tie a nut or a heavy washer on each end for weight, and drape it over the leg assembly, along the centerline of the crosspiece. This string line forms somewhat of a double-ended plumb bob, which will indicate the centerline of your layout work and will project these lines around corners. You can eyeball centerline of your work piece or measure. Once the string is located, go ahead and put a piece of blue painters tape on each end near the corners where it drapes over the edge.
The best position for the cross dowel on the bolt is at the end of the bolt with all the threads in the nut fully engaged. The formula for figuring cross dowel hole location from any combination of length and diameter hardware is to take the working length of the bolt (which differs if the cap screw is hex head, allen head, button head or flat head) and subtract half the diameter of the cross dowel from that length to find hole centerline for the cross dowel. The length of the bolt, when you are choosing the length optionally, is best determined by the thickness of what it has to pass through on the bolt head side, plus 1-1/4 inches minimum into the adjoining board for cross dowel centerline.
If you are planning to recess the bolt heads in a counterbore, it is best practice to drill all these counterbores all at once, with the stop on your drill press (as a for instance) set so the depth of all holes will be uniform. Then factor the depth you drilled these counterbores into the layout equation for the cross dowel. Doing this can allow you to shorten the length of the bolt in some applications, or compensate for the length of the threaded part of the bolt, so the nut is properly positioned on the bolt threads and surrounded by ample timber on the nut end.
Now for specific example, if you are using Veritas bench bolt hardware, it measures 1/2 x 6 and the cross dowel is 1 inch in diameter. Take a 4 or 6-inch Double Square if you have one, or perhaps a steel hook ruler, and measure center of your legs on the string line and make a light pencil mark where the bolt heads will be. Consider the exact depth you plan to recess the bolt into the leg, and add that amount to 5-1/2 inches. (remember the diameter of the cross dowels for this application are approximately 1 inch) Measure that same distance in from the edge of the leg, along the cross piece and make a light pencil mark. Repeat this for the other side. Next to your tick marks mark your left and right orientations, remembering you are working on the inside face. Now disassemble.
Center punch all your tick marks and begin drilling. For your legs, begin with a forstner and compensate the hole depth for the thickness of the washer, and the diameter for the washer by 1/8th inch. Stop drill this exactly to the intended depth. Be careful, because you are exactly locating the cross dowel to work with this depth. Then back the drills exit location with waste board and drill perfectly at a right angle. A Drill Press is the best tool for the job, drilling from the center divot the forstner bit left, through the leg. Repeat this for all the legs until finished.
On the cross pieces, drill the bolt hole first. This is a stop-drilled hole. Stop drilling will ensure the outer look of your bench base will not reveal the knock down construction so conspicuously. To find the depth of the hole on the drill bit, measure from the end of the Tenon in to the location of the center punch mark for the cross dowel. That is the depth you need to drill. I like to mark this on the bit with a sharpie marker if I am drilling freehand. Acetone will clean it off.
You can do this in your Drill Press if you have the capacity, that would be best. You can set the drill press depth stop for this depth if you like. If not, Drill a hole through some 2-inch stock as a guide piece on the DP, and use it to help you drill square while starting the hole. Fixture the work, upright would be best, and start your hole. Remove the drill guide when you are deep enough and finish the hole. Squares set at right angles to the drill are wonderful for visual references to keep the hole on course as well.
To drill for the cross dowels, I like a hole to be slightly larger, as this allows clearance for easy adjustment, so I would select a 1-1/8th Forstner. I again recommend stop drilling this hole. To find the required depth, I would go from edge of the top edge of the stretcher to the centerline of cross bolt hole, plus the half width of the cross dowel plus 1/8 inch, and stop drill to that depth.
From there the assembly should fully line up and assemble with ease. Use a screwdriver in the slot to align the cross dowel in the hole for the bolt.
Just remember this string trick as it has a lot of application elsewhere. A plumb line and a square will always make a makeshift level. You can plumb up any vertical board with a plum bob of some sort, and a bob can be made from a bolt or washer, so it needn’t be expensive or high tech. The trick is in the string hang, not the weight, so the bob doesn’t need to have a point. Where the string hangs is where the alignment is.
Also consider acquiring a saddle square. The saddle square is a great way to transfer layout around corners, and is very useful for properly locating cross dowels when the scale is smaller and the placement is random. They are wonderful to have. The layout tool arsenal is your friend.
There are a couple of different sources available for Cross Dowels, though not all places carry the same things and pricing may vary. For convenience sake, here are a few. There may be more, but please understand that retailers reevaluate inventories and update links, so please forgive if these links have expired or the source has dried up over time.
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