It is currently 28 Mar 2024, 11:47
Malc2098 wrote:Nice tip.
x = chord mid-ordinate
RogerM wrote:Malc2098 wrote:Nice tip.
x = chord mid-ordinate
Don't go getting all technical on me Malcolm!
Looks like the same formula. In my case, the minor springback is not anything more than of academic interest, but I've allowed for it anyway, and it's good to see that it works in practice. I think I will be drawing a circle on the underside of the table, attaching blocks to the inside of the line, and then screwing the apron to it through these blocks. But it's aways nice to understand the issues as there may be times when absolute precision is required.Andyp wrote:I think your springback calculator is the same as one that Scrit (is he still around) gave me many years ago when I cold formed some curved backs for tripp trapp like chairs
s = r / p**2
Where:
s = spring back at edge of curve
r = rise of form in centre, e.g. the distance from the centre of the form to the highest point of the curve
p = number of plies
Must you have zero springback before you attach the apron on the table? Surely the glue would hold such a small gap.
9fingers wrote:
I'd stay away from the glues that go off glass hard such as cascamite family...
Bob
RogerM wrote:Forgive the "brain fade" but are we talking about a solid sheet, 3.6m x 1.5m and 4mm thick after the components, which are 40mm wide, have been glued edge to edge? So will this be attached to some sort of framework? What will hold it in position?
I wouldn't be frightened of vacuum bagging if you have the pump. It's "easy peasy" to make a large bag from a sheet of heavy guage polythene. The difficult bit would be building the former around which it will be bent.
Thought of the day. "It's easy to be wise. Just think of something stupid to say, and then don't say it"
RogerM wrote:Forgive the "brain fade" but are we talking about a solid sheet, 3.6m x 1.5m and 4mm thick after the components, which are 40mm wide, have been glued edge to edge? So will this be attached to some sort of framework? What will hold it in position?
I wouldn't be frightened of vacuum bagging if you have the pump...............
9fingers wrote:Are we talking all the same timber?
What timber is it?
I assume you are bending the 3.6m axis and leaving the 1.5m straight.
On a 600mm radius bend of 4mm thick stock, the outer will be stretched by 2/600 or ~0.3% and the inner will be compressed by a similar %. The neutral axis will stay put and won't be far off from the centre for the wood
So the creep on the glue will have to accommodate that stretching and compression.
Pinch wrote:Roger, my experience of lams and forms is 0% spring back using 2.4mm lams and Titebond 3 adhesive.
I used to steam the lams first and then clamp them up in the form; leave until dry and then straight on to glue assembly. My biggest lot was 16 staggered lams creating an overall width of 600mm and thickness of approximately 19mm over a half-circle dia of 800mm - no spring back at all.
RogerM wrote:Pinch wrote:Roger, my experience of lams and forms is 0% spring back using 2.4mm lams and Titebond 3 adhesive.
I used to steam the lams first and then clamp them up in the form; leave until dry and then straight on to glue assembly. My biggest lot was 16 staggered lams creating an overall width of 600mm and thickness of approximately 19mm over a half-circle dia of 800mm - no spring back at all.
Interesting. Based on a chord of 600mm of a circle with a diameter of 800mm, the distance X from the chord to the circumference will be about 135mm. With 16 laminations, 135/16(squared) = approx 0.5mm. That's as near to nil as makes no difference at my level. I'm sure a bit of steaming would reduce that to nil. I didn't steam my laminations at all.
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