screw hole to allow for expansion in oak breakfast bar

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Hi - have built a steel pedestal for a high freestanding solid oak breakfast bar - kind of like a chunky poseur table. I am going to bolt the base to the floor, but the query is regarding fixing the 5mm steel plate to the underside of the oak.
I understand that I need to allow for expansion - as the kiln dried oak is made of two 500mm wide sections biscuited and clamped together. The 400mm square steel plate will be fixed at a diagonal to the woodgrain (so covering more of the crossgrain) at each of the four corners by a single woodscrew - is it enough to have an oversized hole to allow for movement? How much bigger do you reckon it should be - and can I use a washer so I don't lose the screwhead through it - or will this washer just grip to the powder coating on the steel - defeating the point of allowing movement?
Phew! Sorry for the essay!
Any help appreciated! thank you! :)
 
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Fix to underside and don't bolt to floor, will allow all movement required. ;)
 
Oversized holes and washers or slotted holes would be OK,500mm are wide sectiones and you could get alot of movement,stability will depend on size of tree and growth ring direction
 
do not use steel near oak the tannin in the oak will severely corrode the metal and cause black streaks
 
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Thanks all! Big-all it is powdercoated steel so i hope should be ok. Harbourwoodwork - how much movement should i allow approx, i read somewhere 1/8" fot every 12" - what do you reckon? Thanks again for your help!
 
Thanks all! Big-all it is powdercoated steel so i hope should be ok. Harbourwoodwork - how much movement should i allow approx, i read somewhere 1/8" fot every 12" - what do you reckon? Thanks again for your help!

I got a good 1/8"shrinkage on a 6"plank of kiln dried oak ,the other day ,If in doubt over estimate youve got a couple of big lumps there what is the grain configuratioin 1/4 sawn or through and through
 
Thanks for your reply, through and through i think ( one whole slice of the tree but grain configuration looks close to centre of original tree). Also it's 50mm thick (hence the need for a steel pedestal!), which i presume affects rate of movement?
 
Over size holes, and don't over tighten the screws....all will be fine, especially if the timber has been standing inside for a while.
John :)
 
sorry if this seems patronising but how refreshing to read of someone understanding the need for allowing for this. Most people dont have a clue ;)
 
sorry if this seems patronising but how refreshing to read of someone understanding the need for allowing for this. Most people dont have a clue ;)

I just remember my dad labouring over a beautiful piranha pine kitchen table, he didnt think about this and it split completely apart twice down it's length :(
 

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