Step 7: The Base, step four - Trestle rod holes
Matching up the parts
Not every part and not every cut will be perfect. Match up your parts so that the less-than-perfect parts are in less-than-critical locations.
The top is supported by the top ends of the legs and the top sides of the top stretchers. Stand your legs on end on a flat surface (like my door) and see if they wobble. If you have an end that isn't quite stable, use it as a foot, where the leveler will make it's flaws unimportant. Check the top edge of each stretcher for straightness. If one has a bit of a bow, use it for a lower stretcher. It's less critical that the shelf be well supported along its length.
Do a trial layout to see how the parts fit together. Label each part to indicate which part joins with which.
Mark the holes
The holes we want to mark are the holes through which the threaded rod connecting the two legs will run. This threaded rod will run through the 3/8" groove along the bottom of the short stretchers,. The hole for the upper stretcher has to be positioned so that when the rod is running through this groove, the top of the short stretcher is even with the top of the legs. The most precise way I've found for marking the position of this hole is to use a dowel center. Fit the dowel center into the bottom groove, line up the stretcher, and bang on the end with a rubber mallet. The dowel center will leave a mark indicating the center of the hole.
The precise position of the lower stretcher is less critical. I marked out a position 8" from the end of the legs.
Drill the holes
In the "Getting Started with Woodworking" video, the holes through the 4x4's were drilled from the back. That is, they start on the side opposite the precisely-positioned mark, and drill through to hit it. If they can do this, more power to them, but I can't drill through 3-1/2" of wood to emerge at a precise mark without a drill press - and not always then.
I drilled from the mark. That way I could ensure that the hole was where it was supposed to be, on the side where the position was critical. I have two 3/8" bits -- a brad point bit that came with my doweling set, and a perfectly ordinary 3/8" twist bit. Brad-point bits are far more precise than twist bits -- they're more likely to start where you want them to, and they're more likely to stay straight. My problem is that my brad-point bit wasn't long enough to go through 3-1/2" of wood. So I started each hole with the brad-point bit, then finished it off with the twist bit. I clamped a piece of ply on the back, to reduce tear-out.
When the holes were complete, I flipped the legs and drilled the countersinks with a 1" Forstner bit. Trying to drill a countersink when the center was already drilled would be impossible with a spade bit or an auger, but Forstner bits are guided by their edges, not their center, so they can handle this job. On thing about Forstners, though -- they have a tendency to skitter around a bit when starting, before they bite. An easy fix for this is to drill a hole through a piece of ply, and to clamp that to your work, creating a jig that will prevent the bit from drilling in the wrong spot.
The countersinks should be deep enough to hold a nut and washer, plus a little bit.