I have been dabbling in “woodworking” for several decades so I have some rudimentary skills, but having finally secured the money and space for a cabinet saw and a router table, I am about to embark on my first serious furniture project, an oak pedestal table. The anticipated design is basically a 6×6 column with 5/4 oak legs and arms to support a 38″ diameter granite top.
For the pedestal, my thinking was I’d construct a 4″x4″ core by gluing up plywood to make it dimensionally stable, and then clad it with 3/4″ thick oak boards using lock miter joints. Then I would use blind mortise and tenon joints on each face of the pedestal for the legs and arms, which would be made of 5/4 white oak and extend out 12″ to give a total supporting width of approx 30″.
However, I’ve just been reading up on the guide lines for dimensioning the mortises and tenons, and now I’m beginning to doubt I will be able to achieve the strength and stability I need to support the granite top, which I calculate will weigh 140lb!!
Is the design even feasible, and if so how massive do the mortise and tenon joints need to be?
Is an alternative “simple” pedestal design possible, or am I just crazy to think I can support this lump of granite with a wooden pedestal table?
Replies
Yes, you are a bit crazy - that's a lot of ambition for first use of those tools, but if you take it easy you can achieve amazing things and (usually) still remain attached to the relevant parts of your anatomy. Go slow and have fun.
Some principles to remember -
The table will topple if at any point the centre of gravity goes outside a line drawn between its feet.
The toppling will be more likely to occur with a heavier top as the centre of gravity is much higher, probably within the top or near as in your case, so a smaller angle of tilt will cause toppling as the higher centre of gravity will move further for a given tilt angle..
You can help prevent tipping with wider, heavier feet, and a heavy column support.
Heavier tops will also impart more significant torsional stresses if knocked, and this will place side-loading on your legs, and longer legs mean greater side-loading. Also, if the table begins to lean, or the leg is kicked, you have a long lever (the leg) acting to multiply the force to be applied to the tenon again, side-loading it.
Were this my piece, I would make a few changes:
1. I would NOT use lock mitres - they are really tough to pull off, require very precisely milled stock and add no significant strength for the effort. Given the amount of glue surface, I'd just use plain mitres - reinforce with slip tenons (you can rout slots) if you feel paranoid, but the old dudes did it without, and with hide glue so I'd not be stressing. Personally I throw in a couple of biscuits on long mitres but they are for alignment only.
2. Square column and round top??? It's your table but if you lose the lock mitres you can do octagonal which will look way better.
3. I would build the column out of 3/4" boards without any plywood core - you don't need it and it won't keep your wood straight - having properly dry timber, milling it properly and using it as soon as milled is the secret. Don't leave it lying around in a pile for a week. I'd expect you could very easily support well over a tonne on a square or octagonal 18mm column, especially if it's only as long as a table support.
4. You need heavier legs. 5/4 is not enough - you need at least 1 1/2", preferably 2". Thinner will be strong enough, but will look silly and will tend to flex. You need something with presence to support a heavy top.
5. Whilst you can mortice the feet into the column, that is inherently weaker than having the feet half-lapped together and passing through the entire column as the loading can be shared over two joints rather than one.
6. Have a look at similar table designs that you like online and see if you can copy them.
On the thickness point- you really can use remarkably thin pieces of wood to achieve the look you want - it's phenomenally strong in tension and compression. Try pulling both ends of a wooden skewer (about 1/8" or less) - it'll never break until you twist it or bend it.
The key is understanding where, when and how the wood will get subjected to the sorts of forces that will tear and snap fibres.
Can you upload a sketch?
Will all due respect, your query/description is in good part meaningless without a drawing or sketch. E.g., exactly what kind of leg? There have been thousands of thousands of legs designed over the centuries - but all that is given here is a thickness and length? There is an old proverb...a picture paints a thousands words.
Thank you for the feedback and advice. Much appreciated. Although I feel somewhat encouraged, thoughts of tomfoolery remain!
Apologies for the lack of sketch in the original post. Please find attached a jpg.
The plywood core will not buy you much unless you count on it for the joinery for the table supports and legs. To hold the legs and table supports, tenons will not work in this application, those are cantilever beams and applying force at one unsupported end will act as a lever to pull the tenon out, the legs/supports should be continuous across the center post with a notch where they meet, see this plan of what you are attempting to do : https://www.finewoodworking.com/membership/pdf/19783/011249038.pdf?fww_sid=VDAxRlJFRmZTVVE5TVRFeE9URTVPVFF4T0NaWFVGOUpSRDB4T0RJMU1URW1WRmxRUlQxdFpXMWlaWEltZEdsdFpYTjBZVzF3UFRFMk56YzFPRGMxTWprPXwxNjc4OTg3NzI0&fww_token=4e846dbb4d42cb271bb72877732671d5&_gl=1*sgn9nv*_up*MQ..*_ga*MTMxNDU5MTk2LjE2Nzg5ODc0NzQ.*_ga_XZ40MPD7CK*MTY3ODk4NzQ3My4xLjAuMTY3ODk4NzcyNC4wLjAuMA..
The above is a very good solution and drawing is very helpful. It looks like your column is narrower than 6" looking at leg height dim?
Thanks again guys. I had actually looked at the pedestal dining table build in my search for information, and have certainly learned from it.
The drawing is not to scale, though the column will in fact be 5 1/2" assuming I can get the locking miters to work!
I'll go with notched/overlapped legs and arms to make things stronger, though now I might need a thickness planer!! The wife won't be happy!!
I'd consider building a scrap, lightweight round top as your assembly test piece. Once you get it built; are satisfied with it's stability; and like the way it 'looks', replace the sacrificial top with the 140lb granite one.
Best tip here.
Are you open to a design change, to an arrangement that would probably be a less precarious and stronger support for a heavy granite top? If so, consider this Gimson round pedestal table design, which has four vertical legs rather than one.
https://www.finewoodworking.com/readerproject/2021/03/30/gimson-round-oak-table
There are five pics in the gallery.
Lataxe
Nice design, Lataxe. I've always enjoyed your input/perspective in this forum. Hope you are doing well!
Harvey
Not sure where to start...
First, 140 pounds is not really that much. At 35 pounds per leg, it is pretty easy to engineer for that load. However, that is just the dead load (in architecture terms.) Much more significant in use is the possible live load, of someone heavy pushing down on one edge of the table. That could easily add 100 pounds to just one leg.
I believe that the length of the legs you plan is too short. The legs functionally make a square support base, with the ends of the legs at the corners. Tho the legs end 15" from the center, the distance from the center to the middle of one side of the square is only about 10.5". This leaves the table quite tippy from live loads applied to the edge of the table midway between the legs. And being top heavy won't help. I always end the legs about 1" short of the table edge, so they are not a trip hazard but extend out as far as possible.
I agree that an octagonal base looks good under a circular top.
Don't use plywood under solid wood for the pedestal; the solid wood will behave totally differently across its width, either buckling or cracking open with humidity changes. Make the pedestal out of 8/4 lumber; you need the strength. The standard way to attach the legs is with 2 carriage bolts (typically 3/8"), from inside the pedestal. They thread into the ends of the legs. Threading into end grain is known to be weak. To counter that, I drill a blind hole into the bottom of the leg near the pedestal end, and put in a dowel that the carriage bolts thread thru, giving much more strength to the connection. You will need a thicker leg than 5/4; I would go with a minimum of 1&3/8". 1&1/2" would be better, with a 3/4" dowel. Of course, you will predrill holes for the shank (thru the pedestal) and for the root of the threaded section (into the leg ends) before inserting. You will need a simple jig to get the holes the same distance apart and in the same locations for each leg and the pedestal. I would do a test with scrap pieces to make sure you have the holes the right size.
At the top, you can do the same thing. It is typical to attach stone to wood bases with adhesive, so you may want larger contact surfaces between the upper braces and the granite top for more glue surface.
I have built several round tables with this leg attachment system, from 42" cherry to 60" walnut (1.5" thick) with no problems from dead or live loads. I'll try to attach a picture or two.
Thank you SO much for taking the time to pen such a comprehensive reply. This keeps the pedestal design I need for the space where the table is and gives me the information and confidence I need to start making sawdust!
Very much appreciated!
I clearly have a huge amount to learn and I value all the insight. Thanks to all for the feedback.
Hi all. I am newbie. Awesome))