Cast Iron Legs and Live Egde/Slab Tabletop -- best way to attach?

[adam breuer]

I want to build a slab/live edge table using a set of cast iron legs from an old (early 20th century) lathe that was left outside then dropped (legs are fine, but the rest is toast). I would like your input on how to attach the legs to a tabletop. I have not yet bought the slab, but it will be finished between 10/4 and 12/4 thick. If the slab sits directly atop the legs, it will already be about the maximum height I would want it, so I'm looking for a system that adds little or no height. Also, the tabletop/legs will have to be able to be disassembled for moving.

I'm currently thinking of epoxying in three threaded inserts in each side of the slab side to attach each leg pair. I would use bolts that are smaller than the holes in the legs to allow for movement. However, I'm very concerned about movement of the wood compared to the cast iron. How would you guys attach them?

Lathe showing similar (but not identical) legs


Threaded inserts I have in mind

[Rod Sheridan]

Lag bolts.......................Rod

[adam breuer]

Lag bolts.......................Rod

Thanks for the reply. I thought about lag bolts, but I would prefer to be able to disassemble and reassemble a bunch of times (nature of my job means I move a lot...). While this is possible with lag bolts, I've found it to be a pain. Also, what are your thoughts on wood movement and the likelihood of splitting my slab if it's just bolted tight to the legs?

[Sam Murdoch]

Adam, here is my idea. Make up 2 mending plates - flat steel plates that you can often buy at any hardware store - that are long enough to span the top of each leg x 3/16" to 5/16" thick. I would mortise the mending plate flush into the underside of the slab making the mortise a 1/4" longer than the plate at each end for wood movement (this is more than you need). The side to side mortise should be tight. 2 threaded holes in each plate to make a hard connection to the cast iron legs (I assume that you would attach a few short bolts to the mending plate from the underside of the legs). To allow for wood movement I would create 2 elongated slots in each plate through which you would attach up into the slab with wood screws or with bolts into those threaded inserts. If you have room for screw or bolt heads between the mending plate and the iron legs you could use pan heads so that they can easily slide along the mending plate slot with seasonal wood movement. If you need them to be flush I would just use a countersink head and make the elongated slot angled on each edge to receive the countersunk screw or bolt head.

Is that clear as mud

Now, using this method or the threaded inserts will require that you use caution when moving such a heavy base with its attached top. Unless you trough bolt the entire assembly you will never (well more than once anyway) be able to lift on that top to move this table without pulling the screws right out.

[Mark Kornell]

Adam,

The total width of the lathe's legs at the top is somewhat less than 12", correct? If so, you don't have a huge amount of wood movement to worry about over that range. You only have to worry about wood moving over the distance from the center of the leg to a side hole, which is maybe 5". If you allow for 1/8" wood movement either way, you are good.

Threaded inserts would be fine, but use the longest ones you can find. Use three holes on each side and hex cap screws of an appropriate length. The hole in the middle should be sized to just fit the bolt, while the holes on the sides should be elongated to allow the 1/8" movement either way. Tighten the middle bolt well, but leave the sides just shy of tight so that they can move. Put a bit of oil or wax between the washer and the leg to try to inhibit rusting at that spot, even if you use a stainless cap screw.

[Jerry Miner]

I was going to reply, but Mark said everything. Great advice, Mark

[Sam Murdoch]

I agree that Mark is suggesting a good and relatively easy solution. Mark's comments about dividing the wood movement in 2 and therefore minimizing the effect is spot on.
I proposed the mending plate idea however to address Adam's specific comment that "but I would prefer to be able to disassemble and reassemble a bunch of times (nature of my job means I move a lot...). " my thinking was that the mending plate idea, though more work, would allow for many disassemblies with no issues with threaded inserts ever coming loose. The mending plate would in effect be permanently attached to the underside of the slab. Removing the top would be by removing the 4 short bolts through the cast iron base and into the threaded holes of the mending plates. You, or others with no experience, could take this apart and reassemble hundreds of times with no issues. My experience with using threaded inserts for repeated ability to take things apart has not been as successful as I expected. The epoxy will help immensely but eventually they will likely loosen over time, also long threaded inserts are not always easy to get in perfectly straight without boring the holes on a drill press. I would be more inclined to use T-nuts, rather than threaded inserts in this case, as they have always seemed more secure for the long haul and install more predictably.

[Jamie Buxton]

There's no need to use metal inserts. You can drill and tap hardwood for machine screw threads, and they hold perfectly well. Use something like a 3/8 bolt, and you're good to go.

[Chris E Smith]

Adam
I would put an angle iron frame under your slab that spans the legs and has a piece of plate across either end for the legs to attach to. Then attach the slab to the plate as others have mentioned. This will make your life a lot easier when you have to disassemble the table, provide support for the slab, and you won't have to worry about moving the table and pulling the legs off of it.
And it will look fabulous !

[Mark Kornell]

Sam has a good point - using an embedded plate is a probably a good idea if there is a fair amount of overhang to the sides. Unless the slab is already completely dry, it will want to warp or cup as it continues to dry. Having a couple pieces of plate or (even better) angle iron span most of the width would help to counteract the wood movement.

[Jim Matthews]

I'm still trying to wrap my head around moving a lot and cast iron.

If the legs are separate, I gather the live edge slab will be all that keeps them together?
I would mortise the bottom of the table as suggested above, to allow the legs to slide in from each end.

I would route a channel wide enough to accept a full-length run of all thread rod from end to end.
Cut an access hole in each casting to allow a paired nut/locknut and washer to pull the legs together.

(If this is unacceptable, or if the legs aren't tall enough - fabricate a block to fit atop each leg to slide into the mortise, preferably keyed.)

Have a look at this clever attachment rail for leaves on a trestle table - this could make the same connection as with a threaded rod,
with the long spars across the top of the legs, and the hard points fixed to the table top.

This would be easy to remove, although you might need a deadblow mallet to remove the fully seated legs.
It might be worthwhile to line the opposing leg mortises with a particularly hard wood or delrin (not metal to avoid galling).

I'm not convinced this would be sufficient to prevent splay in the legs.