composite block warping

[Farrell]

> Bob Hull wrote: 
> I've had a couple of problems with making and
> drilling composite blocks.    Just one to bring up
> with this post. 
> 
> On the three composite blocks I have made thus far,
> (5/16 cap of delignit on multi-lam)   I am getting a
> warping after we expoxy the two together.  The plate
> web screws are able to straighten this out but, this
> isn't what I want.  

Warping after laminating the cap on? Is that to suggest that both the Multilam base and the Delignit cap were flat prior to bonding? Or do you mean the base was warped after resawing and planing, and remained warped after capping?

> Does this have something to do with how we're resawing
> or planing the block down to thickness?  We're not
> paying any special attention to how many laminations
> are remaining.  I have heard that this isn't important
> and also the contrary.  

I can only assume (until I hear differently) you planed down the Multilam (but prior to capping) and found that it developed a good curve to it. I have had that happen also. I just capped a Mulitlam pinblock with 8 mm Delignit and had the most bowing occur I ever have seen. After resawing and then planing the Multilam from 37 mm to about 24 mm thick it developed about a 15 M radius curve (or bow or warp). 

I figured I needed to bend the Multilam backwards and bond my cap to it - and then maybe it will end up straight - because I know there will be significant springback from applying only one thin lamination. Well, I bent it back to something smaller than a 15 M radius (curve was reversed), glued the cap on and ended up with a permanent reverse curve - less than 15 M maybe, but still too much for me to accept. So clearly, this was bending the base into too small a reverse radius curve (or arc actually).

So I resawed the cap off, planed it down to 4 mm, resawed another thin cap and planed it down to 4 mm also and planed my Multilam base back down to 24 mm. BTW, after cutting and planing the base down to 24 mm, I was right back to the 15 M radius curve in it.

Boy, am I glad I upgraded from my two-blade 12" Dewalt hobbyist planer to my new 20" Grizzly planer. That little guy woulda' been toast running all these pinblock hunks through it!

So I bent the Multilam base backwards again - but only a little bit past flat this time (a 15 M radius over the length of a 60-inch cord has a height of about 20 mm - the reverse curve I put on the Multilam base this time had a height in the middle of 3.5 mm). Then I bonded the two 4 mm Delignit cap pieces to it while in that shallow reverse curve. I used a two-part urea-formaldehyde resin adhesive that sets up to hard-rubber hardness in three hours. I unclamped the block after three hours and clamped it to a flat table overnight. I just checked it today and it came out perfectly flat.

So this effort ended up a success, but it sure was a round-about way of getting there.

I can only guess that the proper reverse curve to put in the base with a single 8 mm thick cap would be maybe double my 3.5 mm reverse curve - or about 7 mm or so (three laminations will have less springback than two). But that's just a guess. So if I were to try and state some kind of rule to use when this happens, with the very little bit of data I have - or have estimated, it might be to bond the cap to a reversed curve on the base that is 1/3 the height of the unstressed warp/bow/curve. That's probably what I'll try next time I make another capped block.

Oh, and, of course, cross my fingers!  ;-)

Hope this helps.

Terry Farrell
Farrell Piano
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