Monday, February 28, 2011

Beyond Relative Dimensioning

Once upon a time, I thought you could draw up a plan for a woodworking project, make all the parts according to the plan, and then have them magically fit together.  After all, that's how they make cars and washing machines and ink jet printers.  The result was, of course, that things almost fit, but not always, and not always very well.

Relative Dimensioning

Eventually I learned the trick of relative dimensioning.  When you're making a cabinet, for example, you should make the carcass according to the plan, then toss the plan aside and make the doors and drawers to fit.  In fact, you should toss your tape measure aside, too, and where possible transfer dimensions directly from the already-built parts to the not-yet-built parts.

The same idea applies to all kinds of projects where the parts have to fit together.  Here's a really simple example.  This is a drill press table.  Minus the T-rack, it's just a piece of melamine with some solid wood trim around the edges.

Just for grins, let's say the melamine itself is 18" x 24", and the trim is 2" wide.  In theory, then, the shorter trim pieces on the ends should be 18" long, and the longer trim pieces at the front and rear should be 28" long.  But rather than try to cut out all five pieces ahead of time and then assemble them, it's much better to do it like this:
  1. Cut the melamine to size and rip all the trim stock to width.
  2. For the end trim pieces, transfer the actual width of the melamine to two pieces of the trim stock and cut them to length.
  3. Attach the end pieces of trim to the melamine.
  4. For the front and rear pieces, transfer the actual length of the resulting assembly to two more pieces of the trim stock and cut them to length.
  5. Attach the front and rear pieces to the melamine.
If your cuts are accurate, this should give good results.  But they still might not be perfect, for any of a number of reasons.  Such as:
  1. Your weren't able to cut the trim pieces to exactly the right length.
  2. One or more of your cuts was not perfectly square.
  3. When attaching the trim pieces to the melamine, you weren't able get everything lined up perfectly flush.
  4. Etc.
So what to do?  In many situations, the only answer is to try to work more accurately.  And that of course requires practice, diligence, patience, care, and all manner of similar good stuff that is not always in abundant supply.  In other situations (and the drill press table just happens to be one of them), a different approach can give better results with less skill.

Cut It Big and Trim to Fit

The trick is to cut your parts a little bit larger than you really want them, and them trim them to their exact size. What I'm getting at here is different than "sneaking up on the fit" by carefully removing small amounts of material until the fit is correct.

Here's how it would work with the drill press table. Start by cutting the melamine piece to the desired length, but a little bit wider than you want. Then attach the end trim pieces as shown at A. Make them a little bit long, and also a little bit wide. Now trim the assembly to its final width as shown by the dashed lines. The result, at B, will have the end trim pieces exactly flush with the melamine.

Now add the front and back trim pieces and trim again as shown by the dashed lines at C to get the perfect result shown at D.

As I mentioned, this trick doesn't apply always, but when it does, it will save you some time and a few headaches.  Maybe you can find ways to use it in your projects.

Monday, February 21, 2011

Bandsaw Dovetail Jig - Free Plan

Something got into me a couple of weeks ago and I decided for no particular reason to try to make some hand-cut dovetail joints. The results were not good at all, partly because I don't have a proper dovetail saw, but mostly because my saw kerfs were not even in the same county as my cut lines.

So rather than upgrading my saw and simply repeating the failure with a different tool, I decided to try a method I had read about in separate articles by Lonnie Bird and David Marks. These guys both cut their pins first on the bandsaw, using a system of spacers along with the bandsaw's fence to guide the cuts and to establish the layout of the joint. After chiseling out the waste between the pins by hand, they then mark the tails and cut them freehand on the bandsaw. Finally, they chisel the waste from between the tails to complete the joint.

Besides the spacers, Bird and Marks both describe a sloping auxiliary table that you need if your bandsaw doesn't tilt to the left.

This technique appeals to me for a number of reasons:
  1. You can cut dovetails even if you can't saw to a line. Yay!
  2. You don't need any fancy and expensive equipment.
  3. You can size and locate your pins any way you want.
  4. It eliminates most of the tedious measuring and layout required for hand-cut joints.
The one thing I don't like is the spacer idea. Making the spacers seems simple enough, but figuring out how wide they should be looks to be fairly confusing. In The Bandsaw Book, Lonnie Bird tells how to make the spacers for evenly spaced pins, but doesn't explain what to do if you want variable pin spacing. Most of the other descriptions I've seen either ignore the problem, or else cop out completely with nonsense like, "some experimentation will be needed to determine the width of the spacers." Great.

Rather than messing with the spacers, to me it seems simpler to just mark the desired pin positions, then saw them using the fence to help keep the cuts straight and square to the end of the board. If you're making multiple joints, you still only have to mark one part, set the fence, and then use that one setting for all the corresponding cuts on the other boards.

The Jig

My bandsaw doesn't tilt very far to the left, so I needed to build the sloping auxiliary table. As I was thinking about how to make it, I realized that it would be easier to use if the fence was part of the auxiliary table than it would be to adjust the bandsaw's fence and move the auxiliary table for every cut. With that in mind, here's what my jig looks like:

The small cleat on the right side squares the jig with respect to the bandsaw table, while the C clamp just keeps it from moving around. The slope of the platform determines the slope of the pins and tails.  The fence assembly slides in a dovetailed (!) slot in the platform, and is held in place with a second clamp once set.

The following images link to PDF files containing detailed drawings of the jig as I made it.  It's sized to fit my generic 14" bandsaw. There's nothing especially critical about any of the dimensions, however, so the plans should be easy to modify for use with other bandsaws. I used 5/8" particle board and 1/4" tempered hardboard for my jig. There's nothing critical about that, either.


How to Use It

Cutting dovetails using this jig is much like cutting them by hand using the pins-first method, except 1) you use the bandsaw instead of a hand saw to cut the pins and tails, 2) the layout of the joint is much simpler, and 3) if you're doing multiple joints, you only have to do the layout once.  Given that similarity, there's no point in rehashing all the gory details of hand cutting dovetails when they've already been expertly discussed many times before.  Instead, I'll just give a quick (?) step-by-step run through, with some pictures to show how the jig works.

In case you're new to the subject, there are a couple of entries (Part 1 / Part 2) in Glen Huey's blog that explain the pins-first method in great detail.  There's also a very nice video in Keri Hultman's blog that's definitely worth a look, if only to see how she uses a big fat plane blade to position a block to guide her chisel.

Stock Preparation and Layout

After you've milled your stock to be flat and square in every direction, use a marking gauge to mark baselines on both the pin board and the tail board, just as if you were going to cut the joint by hand.

Then lay out the locations of the pins. I like to do this with a knife just for the sake of precision. I also like to make rough pencil marks on the end of the board to indicate the ends of the pins. These don't have to be accurate; they're there simply to reduce confusion when actually cutting the pins. The layout shown has a half pin on each side of the board, with two full pins spaced relatively close to the half pins. This will result in two small tails next to the half pins, and a single, wider tail in the middle.

Cutting the Pins

To cut the pins, set up the jig as shown, with the small cleat on the right side and the platform sloping up and to the left. Register the cleat against the edge of the bandsaw table so the jig is square, then secure the jig with a small C clamp.

Now hold the workpiece against the fence on the jig, and move the fence so that the bandsaw blade is lined up on the waste side of the mark that defines the half pin farthest from the fence. (This will be the right side of the leftmost half pin as viewed from the normal operator's position.) Apply a small clamp to keep the fence from moving. Verify the setup by comparing the blade position with the rough pencil marks on the end of the board. If everything looks good, use the jig's fence as a guide to make the cut from the end of the board to as close to the baseline as you dare. If you are making multiple joints, leave the fence set and make all the corresponding cuts on the other pin boards.

Now move the fence so the blade is lined up with the right-hand side of the next pin. As before, clamp the fence in position and check the blade position against the pencil lines on the end of the board. When everything is ready, make the cut. Repeat this process until you have a cut on the right-hand side of all the pins on all of your pin boards.

To finish cutting the pins, turn the jig around 180 degrees so the cleat is registered against the left side of the table, and secure it with a clamp. Now the board will be angled the other way and you can make the cuts on the left sides of all the pins.

With all the pins defined, you can use the bandsaw to remove much of the waste between the pins. Do this by making repeated freehand cuts in the spaces between the pins. Stop these cuts 1/16" or so from the baseline, and make sure that you do not cut into any of the pins. Finally, break out the thin slivers left by the bandsaw, then chop the remainder of the waste from between the pins using a chisel in the traditional way.

Cutting the Tails

Mark the tails from the pins just as you would if you were cutting the joint by hand. Again, I like to use a knife for greater precision, followed by a tap with a chisel if needed to make the marks deeper and easier to see.

With the jig removed from the bandsaw, carefully saw out the tails freehand. Make sure to keep the blade on the waste side of the cut line, and don't cut past the baseline. As you did with the pins, make repeated freehand cuts between the tails to remove most of the waste in preparation for chopping.

Finally, chop the remaining waste from between the tails using a chisel. Adjust the fit, assemble the joint, and throw it on the heap with all of your previous attempts.

Saturday, February 19, 2011

Bandsaw Table Clamping Pads

If your Taiwanese copy of the Delta 14" bandsaw is anything like my Taiwanese copy of the Delta 14" bandsaw, you've surely discovered how difficult it is to clamp things to the table. The lip around the edge makes it awkward to get a clamp positioned in the first place, and when you do, chances are good that one of the funny webs in the casting is right where you want to put the clamp. So you jury rig something, finish what you were doing, and mumble to yourself, "I've got to fix that someday."

Well, someday for me came a week or so ago, only five or so mumble-filled years after I first got the saw. I grabbed some 1-1/2" x 1-1/2" poplar and made the pieces shown in the picture by hacking away at each one as needed to provide clearance for all the obstacles on the underside of the table. The resulting pieces might have passed for some exotic Japanese joinery had they actually fit together in some exotic Japanese way.

I attached them to the bottom of the bandsaw table using hot melt glue, then reinstalled the table on the saw. A day or two into the "and everyone lived happily ever after" phase, one of the pieces fell off onto the floor. After more hot melt glue and a couple more days, two of the other pieces fell off. Time for Plan B, which was some double-sided foam tape that 3M calls "Mounting Tape".

The coolest thing about the 3M tape (besides the fact that it works) is a little note on the package that says, "Not recommended for use on hard-to-stick surfaces." That can only mean one thing, and that is: "This stuff won't stick to anything that it won't stick to."

I guess we can't say they didn't warn us.

Thursday, February 17, 2011

Trapped in the Laundry Room

If ...

you need a rack to hang up your laundry as it's coming out of the dryer,

And If ...

you make some brackets even half as awesome as this one:

And If ...

you decide to mount your rack above a pocket door like I did,

Make Sure ...

that you don't use really long screws because one of them will go into the pocket door and you will be stuck in your laundry room forever.


Tuesday, February 15, 2011

Grinder Tool Rest - Free Plan

After watching me burn up a few tools on a high speed grinding wheel that looks like it was made from recycled earthquake rubble, my folks took pity and got me one of those $100, no-name 8" slow speed grinders with the soft white wheels for sharpening my chisels and plane irons. It works great, but like every other grinder in the world, the tool rests that came with it were not the best. I had the usual build vs. buy debate with myself, and (as usual) my inner tightwad prevailed and I decided to make my own. This is what I came up with:

It has three main parts. I have mine set up so the base mounts (more or less) permanently to the same table with the grinder.
If you wanted, you could also simply clamp it down for easy removal. The second part is a platform that rotates about a pivot so you can adjust the grinding angle. Once you get the angle set, you can lock it in place with a bolt that runs through the base and a curved slot in the right-hand platform support. Finally, a secondary platform slides in and out in another set of slots so that you can move it close to the grinding wheel after you have set the grinding angle.

The Details

If you want to make one of these for yourself, the following pictures link to PDF files containing detailed drawings of all three parts.  The dimensions given work with the common grinder shown in the first picture above.  If your grinder is different, you might have to adjust one thing or another to suit.

Secondary Platform
I used Baltic birch plywood for all the 1/2" thick parts, and some 3/4" oak for the two vertical uprights on the base.  The bolts are all 1/4-20 flat head machine screws.  I used a piece of 1/4" threaded rod for the pivot pin.  An unthreaded shaft would work there as well.

Construction Hints

There are two or three things to watch out for if you decide to build one of these.  First, the holes for the pin between the base and the rotating platform all have to line up. For this reason, it's a good idea to wait until after you have assembled the base to drill the holes.

The second thing to be careful about is the location of the vertical pieces of the rotating platform. These need to be spaced so that the platform rotates freely on the pin, but without excessive side-to-side play.

Finally, it helps to make the bolt holes (but not the slots) just slightly undersize so that the bolt threads actually bite into the wood a little bit in order to keep the bolts from turning. If you do this, you can use wing nuts and/or star knobs for tool-free adjustments. If you find that the bolts still turn in their holes, a bit of epoxy under the heads will get them under control.


When you're ready to mount the tool rest, temporarily remove the secondary platform and position the tool rest in front of the grinding wheel, centered from side to side. Then rotate the platform through its entire range of motion, and move the tool rest so that the platform is close to the wheel, but does not interfere with it at any position of the rotating platform.

Wednesday, February 9, 2011

A New Road to Flatness

Sooner or later, after tearing themselves away from the internet and maybe making an end grain cutting board or two, most woodworkers wind up building a workbench. There are a zillion variations on the theme, but one thing that every workbench needs is a top that's flat and likely to stay that way.

The Old Ways

To make flat work surfaces, the traditionalists glue up mondo slabs of wood, grab their hand planes and go to town. Other folks glue up the same ginormous slabs, then flatten them with routers and sleds. Both of these methods involve quite a bit of work, and the router method in particular makes a mess and a half. Worse, because wood is wood, it will move over time and it's a pretty good bet that a thick wooden top that's flat today won't be so in a year or two or five.

A completely different approach is to abandon the thick, heavy slab, and build a torsion box instead. The result is a stiff, lightweight structure that will remain stable over a long period of time. That stability is great if the torsion box comes out flat to begin with. But if something goes wrong during the build and it ends up with a bend or a twist, about the only fix is to start over from the beginning.

A New Way

"If you can't make it perfect, make it adjustable." I don't know who first said that, but it works for a lot of things, including yet another way to make a flat work surface.

Suppose you have a square piece of 3/4" MDF, say one foot on a side. It's going to be flat, and it's going to be stiff and stable enough to remain flat over that small area. But a panel of the same material as big as a workbench top will sag under its own weight, and will not be acceptably flat over its entire area unless it is supported by, um, something flat?

Sounds like a Catch-22. It is, almost, but not quite. Here's the trick. If the panel can sit on a series of regularly-spaced supports, and if those supports can move up and down, then it's possible to adjust the elevation of the panel by a small amount at each support position. That in turn makes it possible to tweak the panel until it's almost perfectly flat. I've done this with both a workbench top and a router table, with very good results.

The Support Mechanism

To make the adjustments easy, I used two 1/4-20 bolts at each position, as shown in the figure. Both bolts are threaded into T-nuts that are embedded into the rigid frame of the supporting cabinet. The bolt on the left is the one that actually supports the top. It bears against a metal disk (a penny, actually) embedded in the top so that the bolt doesn't dig into the MDF. The bolt on the right simply holds the top down and in position against the first bolt.

Both my workbench and router table have pairs of bolts like this arranged in a grid with the pairs spaced about twelve inches apart. Here's what one of the pairs looks like in real life, viewed from the bottom:

Adjusting the Supports

It's easiest to adjust the bolts in two stages. Start by installing all of the bolts that come up through the frame to support the top. Then select a bolt somewhere in the middle as a reference, and use a carpenter's level to adjust all of the other bolts so that they are at the same height as the reference. If you have a good level and do this carefully, the tips of all the bolts will define a surface that is not only flat, but also level.

Next, set the top on the support bolts and install the hold-down bolts. Tighten them until they are snug, but not really tight. Then, using a straightedge for reference, adjust the bolts so that there are no humps or dips in the surface. To raise the surface, loosen a hold-down bolt slightly and then snug up the corresponding support bolt. To lower the surface, loosen the support bolt first, then snug up the hold-down bolt.

This sounds easy enough, but it can be frustrating if done in a haphazard fashion. A methodical approach works much better. Here's one procedure that works well. Assume that your bolt pairs are arranged in three rows of three, as shown in the picture.

Start by placing a straightedge along the diagonal from corner A to corner I. Then adjust the bolts at A, E, and I so that there are no gaps between the straightedge and the top surface. Next, place the straightedge along the diagonal from corner G to corner C. Now, without changing the bolts at E, adjust the bolts at G and/or C so that there are no gaps. Finally, without changing any of the previously set bolts, adjust the bolts at B so there are no gaps when the straightedge is placed along the A-B-C edge, adjust the bolts at D so there are no gaps when the straightedge is placed along the A-D-G edge, and so on for the bolts at and F and H.

The procedure for other arrangements of bolts is similar: first set the diagonals, then without changing any of the previously-set bolts, adjust all the bolts in the intermediate positions between the bolts that have been set.