Q: I'm looking for some help on resolving an issue I'm having while gluing up large panels of maple. When laying up a panel in our clamp carrier, the staves move up slightly and create an uneven panel surface. Getting a smooth panel surface after gluing is next to impossible without planing the panel excessively. We have the clamp pressure at the exact manufacturer's specification, yet the staves slide past each other. We end up loosening the clamps, pushing the pieces down flat again and tightening them up to five times per panel before the panels stay flat.

This only happens with our maple panels and not with our oak panels. Our adhesive shouldn't be the problem, as it happens with fresh glue and with glue that's been in the pan for an hour or so. We're applying the adhesive at the recommended rate or slightly higher and are getting adequate squeeze out. The panels are flat when the clamps are tightened, but as soon as the hold-down bars are retracted, the staves start to slide past each other.


A: The problem you're having is caused by having one or more of the following five factors out of control. The factors are: 1. the pressure applied by the clamps, 2. the amount of adhesive used, 3. the absorptivity of the wood surfaces, 4. the tack of the adhesive and the speed of assembly after spreading and 5. the squareness of the edges to the faces of the staves.

I'm suggesting a few things for you to try; if one of my test ideas works, then we can put our heads together and determine how we can come up with a production solution. I encourage all of the readers who are gluing panels in a clamp carrier to check my answers as there may be some helpful pointers for them as well.

Pressure setting

First, I'd question the so-called "manufacturer's specification" for pressure. Certainly, this pressure setting varies with species of wood (for example, less pressure for lower density species) and with the area of the joint (height by length). I hope you're making such adjustments, because if you have too much pressure, you can expect to see some movement. So, my first suggestion is to reduce the pressure and see if the problem improves.

From a technical point of view, the role of pressure is fourfold: to move the two pieces being glued very close to each other (0.002 to 0.006 inches apart is best in most cases), to spread the glue evenly throughout the joint surfaces, to squeeze the glue into the nooks and crannies (microscopic in size) on the joint surfaces (glue is not squeezed into the wood itself) and to squeeze out excess glue.

Second, if you've applied too much adhesive (and possibly if the adhesive is too thick to properly squeeze out when the pressure is applied), you'll develop pressures within the joint that try to move the two staves relative to each other. In other words, as the excess adhesive moves out of the joint, the movement of the adhesive causes movement of the staves. My suggestion is to make sure that you have the proper viscosity adhesive and that you aren't applying too much. You can tell if you have excessive adhesive in that you will see excessive squeeze out (especially on the bottom side). A little squeeze out is good in that it shows that you have enough adhesive. Excessive squeeze out is bad. (Your comment about taking five attempts to get a good panel is certainly a strong indicator of too much adhesive or an excessively thick adhesive.)

Spreading adhesive

Often with a glue spreader, you'll spread adhesive on just one edge of every stave. As you lay up the panel, you'll join an edge with adhesive on one stave to an edge without adhesive on the adjacent stave. However, when you get to the last stave, this stave will be turned so that its face with adhesive mates with the adjacent stave's face with adhesive, giving you double the amount of glue in this one joint. Is this where you see the movement? If so, don't spread glue on this one last stave in a panel. In fact, this is a good idea in almost all cases, even if you're not having problems. Why waste glue? Why run a risk of movement?

Special note: There's a relationship between pressure and glue spread rates. For example, if you lower the pressure when you have excessive adhesive, your glue-joint quality will likely deteriorate.

Third, perhaps the maple surface isn't very absorptive. Perhaps the machines that prepared the edges to be glued actually heated, burnished or polished the wood edges slightly. (This can be due to over-drying the lumber in the dry kilns or due to poor saw blade sharpening. Remember that it is the sides of the tooth, and not the top, which prepare the wood surfaces that we'll be gluing later.) The adhesive then will act as a lubricant between the two staves, and will not be adsorbed at all; any pressure that's even slightly off center will cause the staves to slip. I suggest you check this possibility by very lightly sanding the stave edges (just one pass with a piece of 100-grit sandpaper) before you spread the adhesive.

Fourth, it would also be helpful to have a stickier adhesive. Although we seldom suggest this procedure in full production, as a test you should wait for 60 seconds or longer between spreading of the adhesive and assembly of the panel. This open assembly time will allow the moisture in the adhesive to partially "soak into" the wood and the adhesive will become less slippery. Technically speaking, this stickiness is called tackiness. Of course, if the problem is that the wood surface cannot adsorb the moisture, then this delay will not be helpful.

Note that the instant tack of an adhesive varies from formulation to formulation. You may wish to explore using a little tackier glue. But watch the spread rate and squeeze out, as mentioned previously.

Fifth, although it may not pertain to your situation, as you only have problems with one species, it's critical to make sure that the edges that are being glued are both 90 degrees to the surface. In other words, they're parallel to each other and perpendicular to the face. One situation that will cause them to be non-parallel will happen when you glue off of a ripped edge and the lumber that you ripped was slightly cupped. We normally will plane rough lumber enough to get a flat surface (hit or miss, but not a lot of "miss") and thereby avoid this problem. Cupped lumber can be accentuated by incorrect kiln operation, so maybe we have to work with your drying people to avoid as much cup as possible.


Q: Your previous comments about rough mill yield have been helpful. Can you be more specific on how you suggest we analyze our waste? And do you have any other ideas?


A: A very useful technique for evaluating a rough mill cut-up operation is to examine the wood waste pieces. You should identify the reason why a particular piece of wood is going into the waste hog rather than into the pile of acceptable pieces. The more separations or reasons that you have, the more useful the data will be.

  • Some of the important factors are:
  • Knot or knot hole or associated cross grain (Make sure that you include the cross grain area around the knot, as such wood is hard to machine and glue.)
  • End check or split (Almost all end checks, which are splits smaller than 1/4 inch wide, are avoidable with proper end coating in drying.)
  • Surface check or internal check, also called honeycomb (These again are almost 100 percent avoidable in drying and should not be tolerated.)
  • Stain (Mineral stain is stain that was in the tree; sticker stain and other discolorations are often drying related and can be controlled.)
  • Color contrast (Trashing wood because of heartwood to sapwood color contrast is wasteful. Avoid purchasing such lumber.)
  • Wrong size
  • Planer skip or thin pieces (Should not happen. Detective work needed to figure out if it's thin lumber or if it's due to warp.)
  • After the rough mill, pieces may be culled because they warped, and if this is warp that developed after the rough mill, then they were at the wrong moisture content. Culling may also occur because of fuzzy grain (usually a growth defect and not controllable) or chipped grain (usually an indication of over-dried lumber or poor knives).
  • Another avoidable problem that results in waste is due to an open glue joint, which is almost always related to incorrect moisture content.

Asking the employees

Another approach to improving the rough mill operations is to ask the chop saw operators (or strip markers if you have an automatic saw) and rip saw operators to spend about three minutes every hour and make note of why they're discarding every piece of wood during this time period. For this to be effective, they do need some prior training, but once trained, they can be an important quality improvement force. After all, who else really sees every piece of wood in the plant? For each piece during the time period, have them note if the problem is due to a "natural" wood factor, is due to poor sawmilling, is due to poor drying, is due to poor moisture control, is due to poor machining or is related to operator and management issues. At the end of the day, you'll have a pretty good idea of what the employees feel is the basic problem with wood quality. It's always surprising to me how often these people know what's really going on, but management has never asked them to share.

Managers should ask

Managers might ask themselves, in addition to the above items, if poor yield is related to poor operator judgment due to poor vision, poor lighting, over-working, uncomfortable working conditions (cold, for example) or poor instructions.

I think I've said it before, but a very good training tool is to have a large display board in the lunch room that shows acceptable and unacceptable defects and also gives the reason (such as drying defect) for such defects occurring. Along with this defect board, have another board with dollar values attached to piece of scrap and to acceptable pieces. It's surprising how many people don't know that a small piece of wood 2 inches wide and 24 inches long is often worth 60 cents or more. If they knew, maybe they'd think twice about throwing it away or wasting so much wood.

Have something to say? Share your thoughts with us in the comments below.