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AW Extra 7/11/13 - Adjustable Height Assembly Table

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Adjustable Height Assembly Table

Check out these clever legs: they ratchet up and down with ease.

By Alan Schaffter


“An adjustable assembly table, huh? Well, you’re wasting your time unless it’s a simple design that’s easy to operate.”

That’s what a fellow woodworker said when I told him about the latest scheme to improve my shop. I don’t know how many times I’ve wished for a worktable that was shorter, or taller, than what I had.

I took my friend’s advice as a challenge and went to work. I designed an inexpensive mechanism that allows the legs to ratchet up and down, plus a lightweight but very strong tabletop. He was just as pleased as I was with the result: a huge table that goes up and down with ease.

These two-part legs employ a system of notches and locking arms (ratchets and pawls in engineering terminology) that allow you to change the table’s height in 1” increments. The weight of the table and the geometry of the mechanism firmly force the leg sections together.

Operation of the legs is simple. To increase the table’s height, just lift each end. As you lift, the ratchet arms move freely, “clicking” from notch to notch. They lock automatically once you stop lifting. To decrease height, you step on a bungee cord, lift the table slightly to unlock the pawls, then lower the table. When you remove your foot from the bungee the pawls engage the ratchet and lock the legs and table at that height.

 

Make the ratchets

1. I used white oak to make the legs, but any strong hardwood will do. It’s OK to glue them up from thinner pieces. Mill the upper legs (A) and lower legs (B) to size (see Cutting List, below). These legs will make a table that adjusts from 22” to 35” high. If you’d like a different range of heights, make the legs shorter or longer (see Leg Sizing Chart, below).

2. Begin making the ratchets in the upper legs (Fig. B) by cutting dadoes. Make an indexing jig for your miter gauge to ensure that all the dadoes are accurately spaced (Photo 1). My jig is 3/4” x 3” x 24”, but the exact size isn’t important. Use a dado set to cut a 3/8” wide by 15/32” deep notch in the jig’s center. Glue a 4” long alignment pin in the notch–it should fit fairly tight.

3. Position the jig on the miter gauge so that the distance from the dado set to the indexing pin is exactly 5/8”. Make some test cuts to verify this spacing, then fasten the jig to the miter gauge. Raise the dado set 1/32” to cut a notch 1/2” deep.

4. Cut 14 dadoes down the length of each upper leg (Photos 2 and 3). Cut 6 similar dadoes in a scrap piece to help set up the next operation.

5. Cut the angled sides of the ratchets. First, remove the dado set and install a crosscut or general purpose blade. Tilt the blade to the 38° mark on the saw's bevel scale. Raise the blade to make a cut 1/2" high–the same as the notches. Reposition the indexing jig so that the blade cuts to the corner of the notch (Photo 4). (On a right-tilt saw, like mine, position the alignment pin on the left side of the blade. On a left-tilt saw, position it on the right side.) Temporarily clamp the jig to the miter gauge and make test cuts in your scrap piece. Check your accuracy by placing a 1/2” bolt in the ratchet– it should nest in the bottom. When everything looks good, fasten the jig to the miter gauge. Begin at the top of the leg (Photo 5) and finish cutting the ratchets.

 

Cut sliding dovetails

6. Large sliding dovetails keep the leg sections together when you adjust the table’s height (Fig. C). To help guide your cuts, draw tails on both ends of one of the upper legs. Draw sockets on both ends of one of the lower legs. My dovetails have an 8° angle because I have a large 8° router bit in my collection, but a different angle would work OK, too. Whatever bit you use, it must be large enough to cut a 3/4" deep dovetail. Begin by removing most of the waste on all of the legs using the tablesaw. Install a 3/8" wide dado set in your saw and set its height to 11/16". Plow out the waste from the center of the lower legs (Photo 6), staying 1/16” away from the dovetail layout lines. For the upper legs, reset the fence and remove most of the waste in the same manner.

7. Finish the dovetails on a router table, beginning with the sockets in the lower legs. Raise the bit 3/4” high. Position the fence to make a light cut, then make two passes, one from each side of the leg. Reset the fence to make another light cut, and continue the sequence until you reach the layout lines.

8. Next, rout tails to fit the sockets. Lower the bit 1/32” so there will be clearance between the tail and socket. Before you begin, mark all the upper and lower legs in pairs: two A’s, two B’s, etc. Rout tails on each piece, again taking small cuts from alternate sides (Photo 7). Test the pieces as you go–some tail pieces may require a bit more taken off than others in order to fit their mates. The finished dovetails should slide easily but have minimal freeplay. When you’re done, apply paste wax to all the sliding surfaces.

 

Fit the legs

9. My table has sockets to receive the legs, but you can attach the legs to a table by a variety of methods. I removed part of the tail on the upper legs so they would fit in the sockets (Photo 8). This cut is 2-1/2” long. For a permanent installation, you can glue the legs into the sockets. I made the legs removable, attaching them with machine bolts and T-nuts (Photo 9).

10. Use a drill press to make a 1/2” dia. hole in the top end of each lower leg (Fig. D). This hole will receive a 1/2” bolt, part of the ratchet arm mechanism. A tight fit is OK–the bolt doesn’t have to rotate.

11. Add stops (J) on the bottom of each upper leg to prevent the ratchet arm pawls from sliding off the legs if you lift the table too high (Fig. F). (If you add braces to the legs, an optional step below, these stops aren’t necessary.) Make the stops from 3/8” x 3” x 3” metal corner braces. Use a hacksaw to cut one side of the brace. Use an existing hole or drill a new one on the other side of the brace and fasten the brace to the leg.

 

Make the ratchet arms

12. You’ll need to make four ratchet arm assemblies, one for each leg (Fig. A). Each assembly is composed of two metal plates, which are available at any hardware store. One plate has a 90° angle-it's made from a flat corner brace. The other plate is straight-it's made from a mending plate or flat bar stock. You'll have to drill holes in the plates, and it's very important that all the holes line up with each other. Most mending plates and corner braces are pre-drilled, though, which could be a problem. If you attempt to drill a new hole that overlaps an existing hole, your drill will wander off center. You may need to be creative in locating the bolt holes. For one of the holes in the corner braces, I enlarged an existing hole. I couldn't find mending plates with suitable hole locations, so I used flat bar stock instead.

13. Make a jig to hold the plates while drilling the holes (Photo 10). You can stack the plates or drill them one at a time. Use a center punch to mark the holes first, then begin with a 1/16” pilot hole running at 1,000 rpm. Gradually increase the hole’s size using larger diameter bits. Reduce the drill speed with each bit and lubricate the bit with machine oil. Drill all similar holes in both types of plates before repositioning the jig.

 

Assemble the legs

14. Assemble and attach the ratchet arm assemblies to the lower legs (Fig. A) using bolts (E and F), washers (G), and stop nuts (H). Tighten the stop nuts until the arms are parallel and there is minimal slop (Photo 11). The arms must rotate freely.

15. Lift the ratchet arms and slide the lower leg onto the upper leg. Temporarily clamp the legs sections together and attach the legs to the table.

16. Thread a 6” piece of vinyl tubing (L) over a 48” long piece of bungee cord (K). (The vinyl tubing protects the bungee from premature wear.) Thread one end of the bungee through the hole in the end of the ratchet arm and knot the end. Thread the other end through the other ratchet arm. Tie a knot so the cord hangs 6” above the floor, but is not tight. Leave the excess cord for now. If there is interference between the knots and the legs, slightly bend the ends of the arms away from the legs.

17. Raise the table by lifting at each end. The ratchet arms should “click” freely from notch to notch. If they do not, you may need to loosen the ratchet arm stop nuts or enlarge the bolt holes in the ratchet arms.

18. Lower the table by stepping on the release cord and lifting the table slightly. The pawls should disengage immediately and allow you lower the table. If the ratchet arms swing, but the pawls don’t fully disengage, shorten the bungee. As soon as you remove your foot from the bungee the pawls should immediately fall under their own weight, re-engage the ratchet, and lock the table.

 

Optional braces

19. You can easily add two metal braces to each leg for additional stability (Photo 12). Fabricate the leg braces from 1/2” thin-wall EMT electrical conduit. On each brace, flatten both ends with a vise and/or a hammer. Make sure the flat sections have the same orientation.

20. On my table, the short braces are attached to the top with lag bolts, which go into the reinforcing blocks around the legs. The long braces are connected to the top with machine bolts, which go into T-nuts in additional reinforcing blocks. To install the braces, start by removing the ratchet stops, if you’ve installed them. Drill a 3/16” by 1” deep pilot hole centered in the bottom of each upper leg. Drill the holes in the short braces, bend the ends to fit the table, and install the braces. For the long braces, drill holes on the ends that attach to the legs, and attach the braces to the legs. Mark the locations of the T-nuts on the other ends, remove the braces, and drill holes in the braces for the machine bolts that go in the T-nuts.


Cutting List


Leg Sizing


Fig. A: Exploded View


Fig. B: Notch Layout


Fig. C: Dovetail Layout


Fig. D: Pivot Bolt Hole in Lower Leg


Fig. E: Layout of Ratchet Arms


Fig. F: Pawl Stop

Click any image to view a larger version.

1. Make an indexing jig for cutting ratchets in the legs. Cut a notch in the jig using a dado set. Glue an alignment peg in the notch and attach the jig to your miter gauge.


2. Use the jig to cut evenly-spaced notches in the upper sections of the legs. Place the bottom end of the leg against the side of the alignment peg to make the first cut.


3. Reposition the leg after the each cut. Place the new notch over the indexing peg and cut the next notch. Repeat the procedure to cut all the notches.


4. Tilt the sawblade to cut a relief angle in each notch, making a ratchet. Reposition the indexing jig and make test cuts on scrap– the spacing is correct when a 1/2” bolt bottoms out in the notch.


5. Saw all the relief angles. Start with the notch nearest the top of the leg and work your way down. Each ratchet is identical and evenly spaced.


6. Begin making the legs’ sliding dovetails by removing most of the waste. The lower half of each leg has a socket, shown here. The upper half has a mating tail.


7. Finish the dovetails on the router table. First, rout all the socket pieces. Then cut the tail pieces to fit.


8. Saw off the tails at the top of the upper legs. This leaves a square section.


9. The bottom of the assembly table has a socket for each leg. Bolts and t-nuts allow you to remove the legs so you can store the table out of the way.


10. Make four ratchet arms from hardware store flat corner braces. Drill identically-spaced holes in the braces by stacking the pieces in a jig.


11. Mount the ratchet arms to the lower legs. Add a bolt between the arms as a pawl to engage the ratchets.


12. To make the legs extra-sturdy, add braces made from electrical conduit.




This story originally appeared in American Woodworker December/January 2010, issue #145.

December/January 2010, issue #145

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