When neither a 4-leg nor a 3-leg table will do, a simple suspension allows us to combine some of the best attributes of both: the wobble-free performance of the three-leg table and the high tipping energy of the standard four-leg.
We couple this suspension with a table built for stiffness, a table-top mini-pallet to make it easy to disassemble the thing into parts which can be carried, and a set of cheap, strong, drawers which will stay put while you wheel the whole thing around.
Overall, I'm pretty happy with the way the design has worked out. There's a few compromises (see Design) which you might want to reconsider, depending upon your facilities and needs.
The tables take about 10-15 hours to construct, including one drawer, using a circular saw, a table saw, a radial arm saw, a drill, a pneumatic staple gun, screws, glue, and staples. This time could be reduced substantially with a nice table to do the construction on, and a panel saw or big table saw.
Cost is about 65$, due to about 15$ in 2x4s, 15$ in wheels, 10$ in 3/4" plywood, 2$ of ¼" plywood, 4$ in screws, 5$ in bolts, a 10$ hinge, and 1$ in glue.
For those who just want to get right to it, here's a link to a Pro/E parametric sketch of the lathe table, here's one for the mill table... and just for kicks, here's one of a (considerably more spindly) work-bench-tool-chest-table that I'll eventually make (design is still a work in progress).
For further reading, see my upload of the (unedited) design log, which includes rejected ideas, numerical figuring, a derivation of some drawer physics, and shopping/cut lists.
Step 1: Design
Some background on the suspension:
With uneven floor surfaces, like most of the concrete floors around here, not all the legs of a typical (four leg) table will be in contact with the ground. Instead, if the weight of the table is centered, the table will essentially be supported by two opposing corners, leaving the other two corners floating. This presents a few problems:
- Because only two of the legs are weight bearing, instead of all four, the weight capacity of each of the wheels must double. These stronger wheels are generally taller, reducing drawer space in the completed table, and more expensive.
- The table can wobble, which is a pain in the butt and decreases the quality of the work you can perform (on any type of table, not just a lathe table)
On the other hand, going with a three leg table makes it much easier to tip the table - for long tables like this, about a factor of two in required force and a factor of four in required energy. With a grand of tool on top of the table, this made me a tad bit worried. (A little physics analysis spreadsheet here, for those who are interested)
However, if you could make it so that the first little bit of tipping was done from a three-legged position, and then the table somehow transitioned to tipping in the four-legged manner, these issues would be avoided. Here, we accomplish this by using a range limited whippletree on one set of wheels: at first, it shares the load between the two wheels in the set into a single point in the center, emulating 3-legged performance; but if tipped somewhat more, the limited range of movement in the mechanism causes the table to "puts its foot down", putting all of the load through the two wheels best suited to prevent the tipping.
While several ways to accomplish this were sketched up (see the design docs) and/or investigated, this design decision was made in the context that the stuff on the table is heavy compared to the weight capacity of the wheels, that this weight is centered over the chassis, and that all the wheels are the same. If this is not the case, there's no real need to have four wheels; three (with the lonely one at the end taking 50% of the load) will be fine. You'd still need a pair of "kickstands" set at a height a bit above the floor, so that the weight shifts to them in the event of a tip, but making this substitution should save you about 15$.
Outside of that, there are several other compromises here:
- There is no built-in provision for leveling the thing. This is less than ideal for a lathe, and could be solved with considerable complexity... but that would be a pain to do every time you moved the thing, and if I ever end up in a situation where I'm not moving the things, I'm just going to stick shims under the feet, like a normal person.
- While the diagonal supports in the design of the side panels, meeting at the hinge point in the center, does a good job of making that structure stiff, and thus mitigating vibration by producing a structure with a high natural frequency, it's a bit of a pain to get the side panels square. It'd be a lot easier, and about the same cost, to just use a ¼" plywood sheet.
- There's no provision for changing the work surface height - it has to be built at the height you want it.
(for lathes and mills, incidentally, the rule of thumb is put the crossfeed / y-axis crank (respectively) at the height of your handshake.)
- The mini-pallet which holds the machine tool is not glued to the thin sheet of plywood under it. This means that we haven't quite built a big ol' box beam on the top... so the top isn't as insanely rigid (esp. in torsion) as it could be. This is mitigated by the clamping force of the mini-pallet hold-downs and a couple small screws, but it is still somewhat true.
- The simple style of the drawers means they have a limited range-of-motion (or limited strength - but we're putting heavy stuff in these, so we went with limited ROM here), which limits their capacity.
- The table shown, as a whole, is built really heavily. For most applications, this is overkill and just serves to drive price up. Feel free to pare things down if you don't need so much stiffness/strength.
That out of the way, we can actually design the thing. Essentially, just:
Find the depth-axis center-of-mass of the machine tool (or whatever heavy stuff is going on the table),
- decide on a width for the table,
- decide on a depth for the table,
- decide on a height for the table (see point 3, above, in "compromises", as well as this doc),
- decide on the wheel spacing depth (determines how hard it is to tip the thing),
- decide where on the table the tool will sit (determines the location of the table with regards to the wheelbase),
- and decide exactly how you'd like the little shelf - the one formed by the ¼" plywood under the mini-pallet - arranged (determines some dimensions in the mini-pallet).
Most of these are dimensions in the Pro/E sketches I link to on the first page; plug 'em in and you should get a valid design which achieves them.
With the sketch done, it is most useful to create a "shopping list" of the sizes of boards you need, for two reasons:
- You can sum the lengths and add a bit to account for waste, and then you know how much material to buy.
- Its easier to work from a list like this than from a sketch - you can just go down the list, checking things off as you cut them.
With your shopping list in hand, we can go to work...
Step 2: A Note on Precision
This design has a lot of dimensions determined by the length that the pieces of wood are cut to, a lot of butt-end connections, and assumes that 2x4s are exactly the same width, so we need angular precision and repeatable length and width. This means three things:
- You really should have an adjustable stop on whatever you will be using to cut things to length, so that you can set it and then cut all of a set of members. There's two main reasons:
* For most of our cuts, repeatability is more important than accuracy. IE, if the width-wise members of the mini-pallet (or the vertical supports in the side panels) aren't the same, these things won't be square.
* It saves an enormous amount of time.
- The ends really should be square. Check.
For the angle cuts on the 2x4s in the side panel, this is less important (and harder to do), but if you have the means, do it.
- We frequently use the width of the 2x4, assuming that they are all the same. Turns out... they aren't. It's probably best to rip all the boards to some width just under 3.5" - or at least the boards used to make the mini-pallet.
Once all the pieces are cut to length, get out your screws, your glue, and your stapler, and continue onto...
Step 3: Assembly of Mini-Pallet and Sides
The assembly of these two substructures starts off essentially the same:
- Start the screws which will hold it together. Two things are important:
* first, if they are near the ends of the board, you should drill a pilot hole, so that you don't split the board;
* second, they should be screwed at least 1" into the boards, so that they won't tip later, but not more than about 1.6" (if they stick out very far, they interfere with your ability to line everything up).
- Make marks indicating where boards go. Some of them are flush with the end of another, but some of them are just located in the center... and those need to be marked, so that you put them in the right location.
- Apply glue to the joining areas.
- While holding each pair of frame members together, in the right position, roughly square, and down against your work surface, assemble the perimeter of the subassembly you are working on.
At this point, the assembly diverges:
- For the Mini-Pallet, square up the frame and make it stay there by attaching the plywood surface.
Staples are excellent for this, with screws added later, once you have enough staples that you can be confident that nothing is going to move.
- For the Sides, square up the frame while fitting the diagonals into it (you may need a mallet), then hold it there with screws.
Step 4: Assemble the Base
With the Sides complete, we can move onto assembling the entirety of the Base.
Change over the staple gun short staples suitable for holding down ¼" plywood and follow this sequence in order:
- Space the Sides about the right distance apart on the floor and apply glue to their top surfaces. Make sure they are facing the same way.
- Find the squarest corner of the piece of ¼" plywood, and align this with the front outside corner of the right Side.
- Keeping the corners coincident, line up the edge of the piece of plywood with the right edge of the right Side.
- Staple the plywood to the right Side.
- Line up the left-front corner of the plywood and the front-outside corner of the left Side.
- Line up the left edge of the plywood and the outside edge of the left Side.
- Staple the plywood to the left Side.
- Apply glue to the contact areas, line up the front 2x4 flush at both ends, and screw it in place.
- Again, applying glue and lining everything up, screw the back 2x4 in place.
- Screw the short 2x4 "bolster" to the back face of the back 2x4
- Make sure the top surface of the structure is square to the Sides, and glue/staple/screw the furring strips to the Sides and the bolster to make it stay square. (You may want to drill pilot holes so the screws don't split the furring strip.)
Step 5: Assemble the Mini-Pallet to the Base
This part is pretty simple, though it does involve drilling a long hole:
- Line up the Mini-Pallet and the Base, and arrange for them to stay together with a weight or clamps.
- Drill holes through the Mini-Pallet and into the Base. Make sure you drill them in a location such that the bolts which will go in them don't interfere with the diagonal or vertical elements of the Sides or whatever you'll be putting on the Mini-Pallet.
If your drill doesn't go all the way through everything, and it, make sure it goes at least deep enough to make marks in the Base. Then, disassemble the Mini-Pallet from the Base, and use those marks to locate and complete the holes.
- Switch to slightly larger drill bits and enlarge the holes until you can successfully bolt the two parts together.
(I used 3/8" bolts and enlarged the ones through the mini-Pallet to 25/64", and the holes in the Base to 13/32")
While you have the Base and Mini-Pallet assembled, you might as well mark and drill the mounting holes for whatever you'll be putting on the table. Here, I was able to use the chip tray to layout the holes... you may or may not be able to do this.
Step 6: Suspension and Wheels
The Suspension is just a beam attached to the rest of the Base via a hinge.
Because the lathe makes a lot of vertical-axis vibration, the beam for it is quite heavily built. This is to make the natural frequency of the base high, so as to avoid amplifying the magnitude of that vibration. See the design docs for an example of this sort of calculation, and why we DID NOT want to use a simple 2x8 for the Suspension beam. (hint: fn was pretty near 2500 rpm)
The mill, on the other hand, doesn't make (nearly) any vertical vibration, so it doesn't matter near as much. So I was a bit lazy, and it (while not a simple 2x8) isn't nearly as beefy.
I used a 10" T-Hinge, like you might use on a barn door, because the strength of the hinge is important, and because the larger size allows the bolts holding it to the Base and beam to be spaced further apart, reducing the forces on the wood (which is probably the weak part of the whole arrangement). A large strap hinge would be a fine substitution, letting you use a board smaller and more elegant than a 2x8.
That design issue out of the way, we can build the Suspension:
- If using a built-up beam, make that first.
- Carefully, turn the base up-side-down.
- Place the hinge in the right location on the Base.
(Note that while, in the picture, I show a square... I ended up not using one. It doesn't work very well and would be excessively precise if it did... the demand for accuracy in mounting the hinge is pretty weak; don't sweat it.)
- Clamp the hinge in position and drill through the mounting holes in the hinge into the base.
(You may want to drill an extra hole in the hinge, because you don't like where they put them. Go ahead!)
- Bolt the hinge to the Base.
- Position the beam on the hinge so that it lines up with the base, and clamp it there.
- Drill one hole through the hinge and into the beam.
- Bolt the beam to the hinge through that hole.
- Confirm that the beam lines up with the Base.
- Drill the rest of the holes through the hinge and into the beam.
- Finish bolting the beam to the Base.
On the other side of the Base, you'll need to build some risers so that the bottom of the beam and the bottom of the risers is level (or approximately so), and then you can mount the wheels and flip the thing back over.
Step 7: Drawers
The drawers are composed of three parts:
- The guides, which are 2x4s ripped in half, with notches (3/4" deep) cut in them.
- The drawers, with rails 3/4" wide made from a strip of 2x.
- A bar at the back which keeps the drawers from being pulled too far out and off the rails.
I decided upon a horizontal clearance of 3/16" between the rails and the drawers, and a vertical clearance of about 1/16".
To make the guides:
- Rip some 2x4s in half.
- Cut some notches in them. I don't have a dado blade, so it saves me time to mount them all together first.
- Clean up the bottom and sides of the notch with a file and sandpaper
- Lightly crown the bottom of the notches. This makes a big difference.
- Screw the guides onto the insides of the Sides of the Base, recessing them 3/4" and spacing them 10" apart.
To make the drawers:
- Cut the materials to size.
- Cut otherwise produce including angles on the top and bottom of the face panel, to keep chips out, and a groove on the sides to afford pulling it out.
- Glue/staple the rails to the side panels.
- Glue/staple the bottom to the side panels.
- Glue/staple the front panel in place.
- Glue/staple the back panel in place.
Step 8: Finishing and Use
Finish by cleaning everything of sawdust, and applying a coat of your wood finish of choice (I used polyurethane).
Once that's dry, you can support the Mini-Pallet on jackstands, or similar, and install the tool, then (locking the wheels on the Base) transfer it to the Base and bolt it in place.
Drawers can be installed by putting them on the guides, sliding them to the back, and installing the retention bar with a pair of screws; for transport, the drawers can be retained by tying the retention bar to the furring strip.