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Glue block failure = lessons learned (Read 885 times)
 
Louie Powell
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Glue block failure = lessons learned
Mar 13th, 2016 at 1:48pm
 
I was turning a jar lid from a piece of ash held on a glue block with double-sided tape.  The glue block was made a couple of years ago from a scrap of plywood that I had drilled and tapped to screw onto the headstock of my lathe, and with a layer of hardwood glued to the end.  I opted for this design because I concluded that threads cut into plywood should be tough and long lasting because of the face-grain laminations.  Due to its age, it has seen catches in the past.

I finished the bottom of the lid with its recess, and turned the blank around to do the top.  The double-sided tape looked OK, so I opted to reuse it rather than replace it with fresh tape.  After all, it's expensive!

As I was finishing the lid using a scraper for the infamous final cut, I got a minor catch, and the piece flew off the lathe and bounced around the shop.  Bad words were said.  When I picked it up I was amazed to find that the problem wasn't that the recycled double-sided tape had failed, but instead the plywood glue block had come apart - a failure in one of the laminations.

The good news was that the lid itself wasn't damaged, and I was able to remount it and finish it fairly quickly.

Two lessons from this experience:
1.  Double-sided tape is really tough and is a great way to mount small pieces for turning.  Note that I used the tailstock to provide additional support throughout most of the turning, and that reduced the stress on the tape.  Also, the tape was under pressure (from the tailstock) for 10-15 minutes before I removed the tailstock pressure, and that helped secure the bond.  I use ShurTape double-sided tape (with the blue film) from Lowes).
2.  Plywood may be a great product for flat work, but the laminations can be weak and make it unsuitable for repeated use in turning.  In the future, my glue blocks will be made from solid wood.
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Ron Sardo
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Re: Glue block failure = lessons learned
Reply #1 - Mar 13th, 2016 at 2:41pm
 
Louie Powell wrote on Mar 13th, 2016 at 1:48pm:
Plywood may be a great product for flat work, but the laminations can be weak and make it unsuitable for repeated use in turning.


Smiley

I think we all have to learn this at least once.
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Ed Weber
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Re: Glue block failure = lessons learned
Reply #2 - Mar 13th, 2016 at 3:02pm
 
Louie Powell wrote on Mar 13th, 2016 at 1:48pm:
The good news was that the lid itself wasn't damaged


The good news is, you weren't damaged.
You can use plywood in turning applications but you can't rely solely on it's adhesive for support or work holding.
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Alan Hollar
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Re: Glue block failure = lessons learned
Reply #3 - Mar 14th, 2016 at 12:41pm
 
I have had good luck with the old poplar core interior plywood and with baltic birch plywood as glue blocks, with screws that reach a for through the plywood as possible.  I used them a lot for platter mounts when the stock was thinner than I liked.  I would never again use fir core like BC, BCX, CD, or CDX plywood.  One launch of a 14" platter was enough.
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Glenn Jacobs
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Re: Glue block failure = lessons learned
Reply #4 - Mar 14th, 2016 at 1:19pm
 
And you thought flying saucers were a myth!!  Grin Grin Grin
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Louie Powell
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Re: Glue block failure = lessons learned
Reply #5 - Mar 14th, 2016 at 1:37pm
 
Alan Hollar wrote on Mar 14th, 2016 at 12:41pm:
I have had good luck with the old poplar core interior plywood and with baltic birch plywood as glue blocks, with screws that reach a for through the plywood as possible. - - - I would never again use fir core like BC, BCX, CD, or CDX plywood..


I think that the plywood i used in this instance was a scrap of some fir core material that I had left over from a project many (>30) years ago.  In looking at it after the fact, it was obvious that a lamination had failed and not a glue line. 

I made a replacement this morning using ash for the threaded body (ash threads better than maple), and maple for the face (which is easier to smooth between uses. I drilled into face grain on the ash, and the resulting threads were clean and very sharp.  After reinforcing them with thin CA and letting it cure, I ran the tap through it again and then smeared some paste wax on the threads. 

I thought about drilling some holes and gluing some dowels in to reinforce the joint between the body and the face.  Instead, I cut a recess in the body, and a matching tenon in the face, so that the two pieces would mate mechanically to reduce stress on the glue joint between them. 

As in all my glue blocks, I drilled a hole on axis through the glue block.  The hole is very slightly larger than ordinary coat hanger wire.  I can then insert a piece of coat hanger through the hole to line up the glue block with the marked center of the blank to assure that it is centered properly.
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Louie
 
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Charlie Zapalac
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Re: Glue block failure = lessons learned
Reply #6 - Mar 16th, 2016 at 6:14am
 
it's an amazing thing to watch, depending on your point of view at the time.  I've had things spin so much on the floor I though I heard rubber burning out as it took off.
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