For several years we have been making laminated wood panels using a glue line ripsaw. We would like now to use our moulding machine to prepare the wood to be glued. Before starting full production we conducted some joint solidity tests which so far have not been conclusive.
We made some test blocks using 8/4 sugar maple (acer saccharum), the moisture content was 6 to 8%. For the test we compared the glued joint created by the glue line ripsaw to the joint made with two types of cutter heads, either, straight knives Tersa style , or a multi knives spiral style. Both cutters had new knives.
In order to test the solidity of the joint, we hammered a wood chisel into the glued joint. Before gluing, we did a dry assembly to evaluate the quality of the joint, which were
The results are very disappointing. As expected, the glued joint in the test block made with the ripsaw never failed. However, the glued joint in the test block made from the moulding machine failed most of the time.
We suspect that the glue is either evacuated from joint when it is pressed or the wood is "glazed" and does not absorb the glue.
We decided to redo the test using slightly dulled knives. The glue line was not nice but the joint did not fail.
Would you have any suggestions to solve this challenge?
I never glue machine planed wood without sanding first. The surface after machine planing is scalloped, not truly flat, and the fibers are compressed and won't absorb glue well enough to bond properly.
A couple of years ago this question or a similar question came up and I suggested glue starving of the joint by over-clamping. I decided to do a test of how much is too much clamping. It can't be considered definitive in any way because I only used two short samples of cherry for each of rub joint no clamps,moderate clamps typical of a regular glue up and extreme clamping with heavyduty F clamps at 4" intervals with maximum force.
The conclusion was that there was glue line failure on the rub joint sample but the moderately clamped and extremely clamped samples had mostly wood failures
I think glue starving for PVA is not a factor - remember the ripsawn glue up is using the same clamping.
It's been decades since I read Bruce Hoadley's Understanding Wood. I recall there being a chapter on gluing that was extensive.
The suggestion that the scallops left by the moulder would interfere with the glue bond is intriguing but I suspect unless the moulder is cranked up to maximum feed the scallops would be in the range of .001" or less.
I would like to see if the OP tried passing a sanding block over some moulded staves then gluing and seeing if there is a benefit.
One last thing. Are you gluing the parts immediately after machining in both methods? A delay would allow distortion of the surface however small due exposure of the fresh face to drier conditions.
It could be a couple different issues. My first guess is your moulder cannot hold the tolerances for a glue line edge. This could be how well the knives were cut or tolerances on the head and spindle. It could also be your feed rate/speed. Too slow and the wood can glaze over, too fast the knife marks per inch will be too spread not allowing for good contact and not producing a glue line edge. And as someone already mentioned, sharp knives will also help your case. Clamp pressure is also important. Too much pressure will force too much glue into the wood, not leaving enough to fill the bond line. But you also want to make sure you have enough pressure. Another possibility is you are using too much or too little glue, but I doubt this is the case because you are already having success with the rip sawn edges. I put a blog together not too long ago on this exact topic. It mentions glue line edge tolerances and other basics. It's a really quick read, so its worth the read.
2/24 #9: Ripsaw versus moulding machine for ...
RE: Starving a joint from over clamping with PVA....
This topic seems to get beaten to death and Tom also contributed to the post linked. Still wish I could find the article from FWW but every test I have ever seen shows that there is almost no way possible for most shops (even those with air/hydraulic/actuated clamp carriers) to ever starve a joint by over clamping. You could space I-bars as close as they can be operated and put every ounce they will allow on the glue up and likely still technically be under-clamping.
Link to post
2/24 #10: Ripsaw versus moulding machine for ...
Thank you all for your input. In previous test we did try light sanding and different clamp pressure. The end result was joint failure. In our test we glued immediately after machining. I had a second look at our failed test block, please see pictures, some glue seems to have dry on the surface. Instead of clamping immediately after application of the glue, I wonder if letting the glue "soak" longer into the wood before clamping may rectify the problem?
How long did you wait before performing your test? Did you give the glue enough time to fully cure? Just because it is dry on the surface doesn't mean the glue is cured all the way through the joint. For most PVA glues with normal working conditions, the glue should cure for at least 24 hours before testing. As for the idea of letting the glue "soak" in before applying pressure, I think you might cause some more problems for yourself. This opens up the possibility for the glue to precure before pressure is applied, resulting in a weaker bond. I've heard of people doing that with end grain, but with the sole concept of end grain soaks up all the glue and doesn't leave enough glue to fill the bond line. Just out of curiously, what is the temperature of your shop and the temp of the wood when the glue is applied? This could also be a factor of some joint failures, especially with colder temperatures during winter months. Also, if you could, look at the failed joints and inspect to see if the glue or the wood is failing. The cured glue should be stronger than the wood itself. So if the wood is failing, you will see wood fibers adhered to the bond (goal being 100% wood failure, with this you should only see wood fibers on the bond line). If the glue is failing, you would see little to no wood fibers adhered to the bond.
If you would like to pursue the glue soaking in, glazed surfaces, etc. look up the link. It discusses a water drop test that helps you know if the surface is good to glue or not.
Drop a single drop of water on the surface to be glued. Watch to see how long it takes before it soaks into the wood. Surface tension holds it as a drop, but eventually it gives way to the drop soaking in. This is also called 'wetting out' and is critical when gluing large surfaces like in veneering.
2/25 #15: Ripsaw versus moulding machine for ...
Thank you again for providing additional insight. The glueing environment is the following, the average temperature is 18 Celsius (minimum acceptable following manufacturers recommendation is 15C. ), the RH when dust collector is running is 20% and 40% without, the wood is at room temperature. The joint solidity test where performed 6 hours and 24 hours after gluing. We didn’t use any lubricant during the machining.
We have decided to conduct more test next week. This time, we will test three wood species, run the moulding machine at different speed and do the water test as suggested by Mr. Sochar. Any suggestions for our test will be appreciated.
Hey I just looked at your web address now, I'm just down the river from you in Ottawa.
One thought I had is that the major difference between sawing and milling is the sawn edge will have machine marks cutting across the wood cells. This may allow for better glue absorption. So what may be necessary when sanding after the moulder is to sand across the grain to mimic the saw.
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