New on here with problem # 1

[Kirkll]

Glue starved joints do not typically result from squeeze out. This is most commonly seen from only applying glue to one side, or one of the boards you are glueing together. It’s always best to apply a thin layer of glue to each piece prior to lay up.

Another cause for glue starved joints is type of application. Some use foam rollers or paint brushes. Some use a bondo spreader, or putty knife. And some may use a thin piece of wood or fiberglass.

The type of applicator successfully used may depend on the type of adhesive you are spreading  too. EA-40 commonly used in bow building is easier to spread with a putty knife , a thin piece of fiberglass, or bondo spreader. Where a roller works better for low viscosity, or thinner epoxy like the Huntsman product, or other thin material. Binghams used to have a thinner epoxy called Epon Versa Maid.(spelling)

Some of the high end boat building epoxy made by west systems has lower viscosity, and is great stuff….. but it’s working time is very short. Hot stuff! 

.02   Cents…. Kirk

[Roy from Pa]

Glue starved joints do not typically result from squeeze out. This is most commonly seen from only applying glue to one side, or one of the boards you are gluing together. It’s always best to apply a thin layer of glue to each piece prior to lay up.

Good point, Kirk. Yes always apply glue to both surfaces.

I spread unibond800 with a 1 inch paint brush and cover it good. Another thing is applying excessive pressure can spread the glue out too thin.

[simk]

Good point 👍
I'm now using for years a simple universal laminating resin (resin-L). Its super thin and creeps into any micro crack for repairs. For my glueups i thickening it with cotton fibres. At least 50bows, no delamination indestructable (see pic). But right now I'm testing again white pva woodglue, wich I like for the ease of use and because its non toxic. Looks like its working fine but needs further testing.

[Roy from Pa]

OMG, what kind of bow you building?

Here is my tillering tree.

[simk]

It was just a test - but wasn't able to break this boo yew - took a little set after pulling it to 103# @ 44" but its still shootable, tiller still allright.
(This site always flips my pics 180degrees and while my tillering rig is inverted it looks very odd 😅)
Nice tillering Roy

[Roy from Pa]

I flipped your picture.

Ok On test.

[Buggs]

Reflecting back on the original question, your sandpaper might not be the optimum one for gummy woods.
You did not mention what you are using? The best no load paper is going to be a stearated silicon carbide. And as far as grit is concerned, lots of opinions. Here is some factual information;

https://appliedadhesionscience.springeropen.com/counter/pdf/10.1186/s40563-015-0050-4.pdf

[Kirkll]

That was an interesting read, but had to skim over some of the math. I found it interesting that they stated that wood in general has a stronger adhesion with smoother sanding prep, but didn't get into different types of  glue used.

I believe using a rougher paper with deeper toothing works well on higher pressure applications like an air hose in a form. The smoother surface you have with wood, the less clamping pressure you want to use. I believe this to be true with epoxy as well as white glue, and Urac  (uni-bond 800) Same stuff. Too much pressure on porous woods sanded smooth could easily starve a joint.

I've had pretty good luck with Zirc  paper for longevity. it cleans up well with a rubber paper cleaner.... But...some of these oily woods can load up and burn easily, and gum up the paper real bad if you try and hog too much at a time on a drum sander. I have best luck going real easy on speed and depth, and use lots of light passes, use different locations on the drum, and keep the paper clean.....

Bocote, and cocobolo are tough on sand paper, as well as many varieties of teak.    Kirk

[Buemaker]

Taken from the book Encyclopedia of wood.

[Buemaker]

A planer with sharp blades must of course be used, with dull blades you will more compress the surface.

[simk]

Thanks Buggs for the interesting read! I'm a little astonished on the results. I always thought, a rougher surface would enlarge glueing surface leading to a better bond. That maybe wrong but the other factor is that a rough surface probably decreases the risk of a starved gluejoint and maybe this factor is more important in practical use and everyday work. This maybe also the reason why practical experience generally favors the rougher surface...but that's only guessing.

The quality of the sanding paper maybe another factor; however sanding now works pretty fine after switching from 80 grit to 60 grit already and that's what counts for me now. Could be that the colder temperatures in the shop also help some to sand the oily wood. I'm happy when things work well and at this point I'm not longer questioning the "why".

Problem solved, thanks everybody for contributions. Cheers

[mmattockx]

I always thought, a rougher surface would enlarge glueing surface leading to a better bond. That maybe wrong but the other factor is that a rough surface probably decreases the risk of a starved gluejoint and maybe this factor is more important in practical use and everyday work.

Glues do bond to a rough surface, we just have to remember that the glue sees the roughness at a microscopic level and what is rough enough for the glue bond may feel pretty smooth to our senses. I wouldn't be surprised at all if your theory about a rough surface preventing glue starvation is accurate.

Thanks to Buggs for posting that link, it is a good read with some quality info. I find it interesting that both the aluminum and wood bonding strength peaked at very similar surface roughness values even though their surface and material properties are very different. I also note that all of the wood samples appear to have failed in the wood and not the glue joint itself. If the glue joint is good enough to pull the base wood apart then I would say higher surface roughness is not a problem for our purposes and should help with glue starvation, which is a much more likely cause of failure for us.


Mark

[Kirkll]

You should try some 36-40 grit in your drum sander next.

Grinding laminations in a drum sander efficiently , and getting a nice consistent thickness is what we all  strive for. The more aggressive sand paper cuts the wood easier with less drag on the motor and less friction and heat. It’s real easy to take too much material on one pass as you tighten down the drum. With finer grit paper it doesn’t take much at all to burn the wood and trash out your sand paper.
The finer grit paper will also flex the drum more if too much pressure is applied.

But with the heavier 36-40 grit,  it’s more user friendly. The paper lasts much longer, your thickness consistency is better and it’s easier on your drum sander too. I believe with bow laminations, and  all that toothing running the same direction, it provides an interlocking shear strength. There is mention of this in the info Buggs provided. I’ve been using heavy grit on my drum sander for 15 years with very few issues.

When we have a limb failure, it’s typically always the belly side of the limbs, close to the fades, and shears the bond between the glass and wood due to compression. The wood compresses and the glass buckles.  With glass bows it’s not as common as using carbon backing on the belly because glass does have some compression latitude where carbon does not….. using rock hard maple on the belly side will help a lot…..

All wood bows are a whole different ball game requiring more skill in reading the wood grain and run out , and a bit of luck too. But having laminations separate of glue joints failing would be the least likely reason for a failure….   Kirk

[Buggs]

Taken from the book Encyclopedia of wood.

That pic tells it all. Look at the cell walls in the bonding surfaces.
 In every other endeavor involving glue joints, other than the one advocated by some bowyers, tight, smooth, clean joints are the norm. Just look at 500yr old stringed instruments or furniture made with hide and fish bladder glues, that are still in service. Not going to see any fat glue lines on a Strat or a Chippendale.

[simk]

🍿very interesting thoughts everybody! Maybe a properly planed surface finished with toothing plane would be the best of both worlds?

Edit:
Kirk also is perfectly right; with wooden bows the gluelines seldom are the main problem. I only had one failure of many on which I could possibly blame the gluejoint. I did glue with white pva carpenters glue, fishglue, and multiple 2k. All work with a more or less proper handling.

[Kirkll]

Taken from the book Encyclopedia of wood.

That pic tells it all. Look at the cell walls in the bonding surfaces.
 In every other endeavor involving glue joints, other than the one advocated by some bowyers, tight, smooth, clean joints are the norm. Just look at 500yr old stringed instruments or furniture made with hide and fish bladder glues, that are still in service. Not going to see any fat glue lines on a Strat or a Chippendale.

These limbs were laid up with 40 grit sanding on everything but the glass.  There are no glue lines.... These were laid up with an air hose clamping method at about 55# of pressure.

I've worked with Luthiers in the past. The skill level of some of these guys with hand tools alone was incredible. If you could see a line in a dry fit joint with only hand pressure, it wasn't close enough. Nothing is forced together with clamps on high end musical instruments. Clamps are just used to hold things together until the glue cures.

 At the time i worked with these guys we were building high end Motor Yachts. Half of the guys in the cabinet shop had grey beards and experience building musical instruments. You would not believe some of the projects we did in that shop. Everything was done by hand from rough cut exotic woods.   That was a wood shop to die for....This is where i recieved my doctorate in finish wood working. Those kind of craftsmen are few are far between now days. I was very fortunate to have the experience working with these guys..... I spent two years in that shop working for lower wages than i was comfortable with just to get the experience. It was a great job.....

There were some guys that ran the milling machines in that shop that were incredibly talented too. I worked with a few of these guys milling radius crown moldings a couple times.  The cutters looked like curved lawn mower blades they were so big, and that material was fed into those blades by hand.  

These were BIG boats....   check it out...    https://www.christensenyachts.com/

[simk]

The interesting pics buemaker provided here....

Taken from the book Encyclopedia of wood.

...made me think about my childerns microscope. So let me share the results of a deeper look into one of my bows (-;

These are taken from a failed experimental maple/ash/yew bow (swapped the usual maple midlam and ash backing and quickly found out my maple clearly does not work as a backing). Lams here were sanded to 80grit before glueup with white carpenters glue and a lot of pressure. For taking the pics the surface of this sample was sanded to 320grit and polished. Doesen't the earlywood of the white ash just look frightening 

Have a nice day  everybody!

[simk]

This series was taken from the other side of the same sample - this side was just cut with the japan saw, no sanding, no polishing. The resolution is worse but the interesting thing is, the gluelines are visible better.

I'm curious how my epoxy gluelines made with less pressure would look like....? To be continued.

cheers

[Buemaker]

Interesting pictures and yes, that Ash earlywood looks scary.

[Buggs]

Very cool! Thanks for posting those pics. Interesting that the Ash looks pocketed, it's very strong wood.