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Thread: Why are slabs so popular?
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30th July 2013, 09:53 PM #1
Why are slabs so popular?
I was at the wood show in Sydney on the weekend and a thought occurred to me.
Why slabs?
Yes they are big shiny bits of wood and everyone loves bigger and better. How many table and bar tops can you make though.
Do people who buy these cut them up into boards more suitable projects?
Just a "dumb question" that went through my head on Sunday and I have nobody else to ask except you fine gentlemen
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30th July 2013, 09:56 PM #2
Well I guess you can always make them smaller more easily than making them bigger. More options? Cheaper? More challenging for transport ?
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30th July 2013, 10:32 PM #3Banned
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why and why not
In softwoods where you have 2 grain directions because of the presence of medullary rays, generally the timber is more stable and cutting slabs makes more sense if you like the absence of visible joints!
In hardwood however, if you view this diagram - showing radial (Red) and tangential (Blue) grain;-
You can see that the yellow depicted slab must contain both radial and tangential grain in the one board/slab.
Stats for radial and tangential grain shrinkage in hardwoods typically show a variation in rates.
Published shrinkage & expansion rates figures for Jarrah lets say,
Forest Products Commission - developing the sustainable use of the States plantation and native forest resources in Western Australia
Tangential and radial shrinkage before reconditioning are 7.5 and 5.0 per cent respectively, and after reconditioning 6.7 per cent and 4.6 per cent respectively.
What this means, is that when that one slab drys (and moves between summer and winter seasons, once dry) it MUST either cup badly or split to take up the difference in shrinkage and expansion rates inherent within the one slab containing 2 different grain directions (Radial & Tangential)
Example if the slab is a meter wide the dry difference in expansion and shrinkage in say Jarrah is 21mm! (2.1% of 1000mm)
How does a flat table top fixed to a sub frame not cup or split if its to shrink and expand that much width wise every season?
MY way of overcoming this impossible to avoid problem - was this
1. Make the table from 2 or 3 slabs cut one under the next from the same log.
2. Re-saw the slabs to get the wayne edge (and thus tangential grain boards) from the nicest slab!
3. Use the Middle boards (Radial Grain) from the slabs for the undercarriage and support framework,
4. Resaw the second (and or 3rd) slab so that the wayne edge is removed BUT you still have a matching radial grain board from the outside of the log and join this to the wayne edge radial grain board from the slab above or below.
5. keep doing this until you have a top either all out of radial grain (or all out of tangential grain - whichever you prefer) so you have ONE uniform shrinkage and expansion rate across the entire table top.
6. Ensure you separate radial from tangential grain boards in your table top and use one in the top and the rest in the frame and undercarriage.
7. Slabs like the trees they are cut from invariably taper from butt to crown so if you used the one slab your table might be 1200 at the base end but only 1ooo (a meter) wide at the other end. In furniture - this looks CRAP!
8. By resawing if your happy to not use all parallel boards, (i.e. tapered) it's possible to make both ends of the table the same width (1200) while retaining the wayne edge.
In essence this is how they come out when you do it this way.
Others mileage may vary.
That's how I do MINE for what its worth.
Cheers!
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30th July 2013, 10:32 PM #4
Yes you can cut them up as a cheap source of boards, but slab furniture is very popular and relatively easy to do.
Neil____________________________________________Every day presents an opportunity to learn something new
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30th July 2013, 10:49 PM #5GOLD MEMBER
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Most cabinet makers I know, if given a slab and the choice of how to handle it ( as opposed to what a customer wants) are going to rip it into wide planks then join them back together. As Timeless Timber says, this alleviates all the issues with differential shrinkage and if done carefully the few mm lost in sawcuts is going to be nearly invisible.
I've been trying to explain this to people who request slabs from me for years. Just take them as planks and number them 1A, 1B, 1C, 2A, 2B etc etc for later joining. Alas slabs are popular.
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30th July 2013, 11:16 PM #6
Wow, thanks for the responses guys!
That has helped answer the question quite well.
Especially thanks to Timeless, the diagram and pictures really helped
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31st July 2013, 12:08 AM #7Banned
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You are
You are very welcome.
It's a question that few really understand the real answers too - and thus the popularity of slabs is inversely related to the ignorance of the masses, who like and buy them!.
Then they ask someone with the skills and machinery etc to make a slab table out of the slab - should be simple - right, just smooth it, add legs & shine her up! - viola a slab table (cheap)?
Anyone knows what they are doing them slaps their head and runs a mile... or thinks of a figure and triples it...
I well remember the umbilical hernia operation I had at Pt Hedland Hospital to repair the herniated umbilicus I got working on Jarrah slabs - humping them off the saw ans intro the kiln and back out again and all over the workshop etc.
You couldn't PAY me to make another slab table out of one slab.
Different story, if I could cut it up however and do it as it should be done.
Theres no getting away from how good a single slab table looks initially - and I've made some that way in the early days.
They all split in from the ends after the event tho - and then you start with buckets of epoxy resins and butterfly stitches to try and pull them back together, and next season they split alongside the butterfly's...because simple physics says they must!
If you don't mind repairing your table every year with the change of the seasons then make it from one hardwood slab.
Funny the lessons you learn in life - the hard way usually!
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31st July 2013, 12:38 AM #8
Been an interesting read. Timeless timber thanks for sharing your experiences. I'll point a friend to read this that I just brought home 2 large slabs home from the SWWS last week i dropped them off and ran!
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31st July 2013, 01:39 AM #9Banned
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NO worries
No worries.
If you have 2 slabs - you should be OK.
This is what I was getting at.
You take the two wane edge boards from slab 1 (red)
You take 2 boards from just inside the wane edge of slab 2 (Blue)
You join up a slab top, of the 4 boards all with similar radial grain to make one slab top.
If you allow a nominal say 300mm plus dressing straightening of say 15mm on both ends of the boards and do the same with the lower blue boards - when they all go together your outside wane edges should be roughly parallel to each other at nominal 1200mm.
Also with the ends - don't cut them square across, it looks crap - there's few straight edges in nature - instead use a length of say 8mm timber beading and cramp it 2 inches in from the end at each edge & then pull the bead say 50 mm outwards and hold it while you scribe a natural looking arc.
Use the other wane edges to make the legs and splay them also... and slant them out to the corners so you can get chairs around the table...
Its a pretty simple design actually and not hard to construct. I used tusk tenon-ed joints with the timber peg for the foot rail. I also used a wane edge piece for the foot rail...(and the cross rails on the legs)...
Anything to keep the natural edge effect, contiguous throughout the design...
By all means copy it if you like - its not patented or anything.
Happy to help out.
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31st July 2013, 08:33 AM #10Skwair2rownd
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Be honest you blokes!! We all love ####!!
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31st July 2013, 08:51 AM #11Senior Member
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Dai, a question that I've been wondering about, in Aus at the moment are slabs generally enough cheaper per m3 than rough sawn to make them a cheaper source of boards?
After a quick glance of some timber places over here in NZ, I got the impression that slabs would actually work out as an expensive source of boards because the current market seems to pay an in-fashion premium for them? (I'm thinking of $/m3 RS vs $/m3 of RS yielded from a slab after ripping the edges off and discarding the waste).
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31st July 2013, 10:37 AM #12
My slab tables float on the sub frame, unglued dowels in slots/oversized holes so not fixed, so the slab can move as required and haven't had a problem. You mention radial vs tangential, but the killer is usually longitudinal vs tangential, so if you fix a slab or boarded slab to a rigid frame it will crack.
Or am I missing something in your explanation?Neil____________________________________________Every day presents an opportunity to learn something new
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31st July 2013, 12:55 PM #13Banned
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I also
Dai - I don't think your missing anything - as I too hold my tops down with slots that allow for the sideways expansion.
I've posted in some detail just recently in "big stuff" on a chest thread about longitudinal and lateral expansion (Use of bread board ends on hardwood tables & box lids etc).
Yes theres an even larger variation in expansions rates because longitudinal is quite a low figure around only 1% from memory in Jarrah (versus 2.1% differential between radial and tangential grain in sawn boards). (Must fix my 3% math error in my post above).
The thing is I've seen slab tops held down with tek screws, coach bolts, heck even railway sleeper dog spikes, in the past and thru a hole (not a slot) that allows zero movement. They typically get screwed down from underneath, often thru holes not slots thru the sub frame cross member - which as you correctly point out will most likely be longitudinal grain (with a shrinkage and expansion rate of around 1% while the top could be either before reconditioning 7.5 and 5.0 per cent respectively, and after reconditioning 6.7 per cent and 4.6 per cent respectively for radial and tangential grain boards depending of the slab was well seasoned i.e. dried down to an EMC of around 12%) before being made into a table. In My experience many slabs, air dried, never get much below 18% moisture content unless the area has extremely dry relative humidity (i.e. well inland or snow country).
I've seen slab tables made from what I consider to be raw green slabs even (Typically on outdoor tables - but some were intended for indoor dining use).
Things could alter if using different timbers.... softwoods for example with medullary rays (2 grain directions) - tends to be a lot more stable in my experience (I'm thinking of our local coastal sheoke A Fraseriana which I milled a fair bit of over the years and has a shrinkage rate around only 1% green to dry - which is why its so favored here in our boat building industry/marine environments).
Other eastern states timbers I am less familiar with - maybe the shrinkage rates are a lot more forgiving? - I've never bothered (had occasion) to look!.
Qualifier.
Maybe I miss understood you Dai.
Are you making your slab tops from one piece?
In that case the slots / sliding dowell's, certainly help stop the top from being torn by the longitudinal cross member due to it's difference in rate of expansion contraction (~1%) compared to the much larger rates for the radial and tangential grain rates quoted above.
The trouble with single slabs is that one contiguous piece of timber, has both radial and tangential grains in the one board. When you dry them (I had a EBAC evaporative kiln) over 3 months from green to 12% they split in from the ends...usualy in a 4 meter slab about 900 - 1200 long and a gap (split) of approx 12 - 18 mm...tapering to zero away from the end!
This is the variation in sideways expansion rates between the radial grain and the tangential grain posted above.
It's not the sub frame fixing causing this (coz the slabs are strip stacked they aren't yet fixed to anything) its purely the variation between shrinkage and expansion rates between two different grain directions in the one contiguous piece of wood!.
Theres almost the same rates differential between seasoned slab from summer to winter (the reconditioned figures above) - so even if you cut the split ends off the slab after its dry and make a slab table out of it - the summer winter reconditioned rates differential means the slab will tear itself a new split in from each end... not as severe as from green to dry in the kiln 0 maybe half as bad coz the rates differential is reduced in the reconditioned dry slab.
So this isn't just a matter of how you fix the slab down (with or without slots). Maybe I didn't read your question correctly above - sorry if that's the case.
Cheers
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31st July 2013, 04:06 PM #14GOLD MEMBER
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Although I'm not a fan of natural edges, have a look in here Dining & Boardroom Tables | Boranup Gallery theres some stunning stuff with stunning prices.
Experienced in removing the tree from the furniture
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31st July 2013, 07:50 PM #15
Yeh, some people just have no idea
Not always, but yes that is what I often do
I don't necessarily agree. The radial and tangential grain differential shrinkage is in the same direction, ie lateral, so wouldn't cause the cracking. The cracking you mention is usually due to the difference between the end grain drying rate compared to the other areas. Some timbers are just cranky and crack no matter what, but most if you seal the slab properly at the ends plus another 6" top and bottom can avoid this, except the slabs that contain pith from the centre of the log.
The differential radial and tangential grains are in the same direction (both lateral to the slab), so shouldn't cause any issues, provided the slab is not constrainedNeil____________________________________________Every day presents an opportunity to learn something new
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