Knife checks in veneer – FineWoodworking

When contemplating veneer and its high quality, we normally suppose solely of traits equivalent to species, thickness and determine, or defects equivalent to knots, stain or pitch streaks. Past that, veneer is veneer. However of most severe concern needs to be knife checks. These are parallel-to-grain fracture planes produced within the veneer on the time of its manufacture and which can go unnoticed, solely to trigger agonizing issues later. As a result of they’re in all probability the commonest explanation for checks within the completed surfaces of veneered work, the woodworker ought to perceive what these knife checks are, how they’re fashioned, how one can detect them and the way to deal with them.

Sawn veneer doesn’t have these checks, however immediately most veneer is knife-cut by peeling (rotary slicing) or slicing. In both technique, the fundamental slicing motion is analogous. A knife sharpened to an angle approaching 0º would distort the wooden construction the least, however would, after all, break too simply. Veneer-cutting knives are due to this fact sharpened to an angle of about 21º—a compromise between a small angle that will reduce distortion of the wooden construction and a blunt angle that will reduce knife breakage. Which means because the knife separates the veneer from the flitch, the separated layer of wooden is severely bent, and stresses construct up within the area close to the knife edge. When the power of the wooden is exceeded, the stress is relieved by failure, and the aircraft of fai·lure thus fashioned is known as a knife verify, or lathe verify. This bending and breaking cycle is repeated as slicing continues, so every layer of veneer has checks at pretty common intervals.

The facet of the veneer that was in opposition to the knife and has knife checks penetrating into its floor is known as the unfastened facet, or open face. The opposite facet is known as the tight facet, or closed face.

To stop knife checks, lathes and slicers are outfitted with a strain bar or nosebar, a strong bar or curler that bears in opposition to the veneer as it’s being minimize. Its strain holds the cell construction collectively within the area the place checks normally develop. An excessive amount of strain crushes the cell construction of the veneer, so there’s a theoretical optimum opening between the knife and the strain bar that produces the highest-quality veneer. Expertise has proven that checks may be minimized or eradicated when the space between the strain bar and the knife is 80% to 90% of the thickness of the minimize.

The phrases tightness and looseness confer with the relative depth of knife checks. In producing veneer with out nosebar strain, tightness is improved by slicing lower-density species of wooden, by heating the wooden, and by skinny cuts.

Anatomical options of the species being minimize are additionally associated to checking. If structural planes of weakness-such as the big rays of oak or the earlywood layer in ring-porous hardwoods- coincide with the possible aircraft of verify formation, the checks can be worse. Diffuse-porous hardwoods with superb, well-distributed rays usually tend to yield tight, uniform veneer.

The tightness of veneer may be assessed in a lot of methods. Floor roughness or corrugation (particularly of the unfastened facet) is usually related to checking. Veneer having any suggestion of a washboard floor might be loosely minimize.

Manually flexing the veneer will enable you see the checks. As well as, the veneer will really feel stiffer when flexed to shut the checks, however will really feel extra limp when the checks are flexed open. Tightly minimize veneer will flex about as simply each methods, so in case you can’t inform, it’s in all probability minimize properly.

In some woodworking functions, it’s vital to know the precise depth of the checks. This may be decided by staining with an alcohol or spirit answer of dye equivalent to machinists’ format dye, then beveling the veneer. Minimize pattern strips of veneer from the ends of sheets and stain them liberally on each side, preserving the stain away from finish grain. Enable to dry totally, then glue or cement the veneer onto blocks of scrap wooden. When the glue is ready, bevel the veneer with a superb sander disc, or with a pointy chisel or knife. The relative depth of the checks can be obvious throughout the bevel .

The results of knife checks needs to be fairly apparent. The commonest downside is parallel-to-grain cracks within the end on veneered surfaces—almost at all times traceable to knife checks.

This downside is very aggravating as a result of it’s normally a delayed response, showing months or years after the piece is completed. A floor could also be flawless at completion, however the regular shrinking and swelling of the wooden in response to seasonal humidity fluctuation trigger hidden knife checks emigrate to the floor and thru the end itself. This downside is second solely to delamination as a explanation for the dangerous repute veneered merchandise have undeservedly acquired. One often sees it when softwood structural plywood equivalent to Douglas fir is used for completed or painted surfaces. Structural plywood is designed to hold stresses parallel to the grain path of its plies, and this functionality is little affected by knife checks. Apparently, little severe effort is made to manage tightness of veneer in manufacturing industrial softwood plywood. Plywood manufactured with surfaces of medium or high-density impregnated paper overlay (designated MDO and HDO ) is greatest the place easy painted surfaces are wanted. Giant lumber yards normally inventory MDO plywood, which is routinely used for outside indicators and comparable merchandise.

Finish-grain plywood surfaces might also reveal end defects attributable to knife checks. That is particularly frequent when moisture loss leads to extreme shrinkage stress .

One other visible impact of knife checks is bleed-through of glue, which reveals up as a sequence of evenly spaced traces on the veneered floor. That is particularly obvious in light-colored woods equivalent to maple or birch sapwood.

In woodworking, veneer ought to routinely be inspected for tightness. Checks penetrating not more than 2 5 % of the thickness of the veneer may be tolerated underneath most circumstances. When laying up veneer, unfold the unfastened facet with glue. With luck, the glue will penetrate the checks and maybe glue them closed. It could assist to put the veneer over a barely convex floor so the checks can be open to the glue. (This can even be sure that the tight facet is the uncovered face on floor plies. ) Care should then be taken to not sand by means of the tight facet of the veneer. I’ve seen numerous conditions the place veneer surfaces have been sanded proper down to show the knife checks and the glue in them.

Bookmatched surfaces are a predicament, as a result of the veneers have to be positioned with alternate open and closed faces up. In such instances, it is very important have comparatively tight veneer to make sure uniformity. You’ll have seen bookmatched patterns through which the end high quality alternated with each bit of veneer, a consequence of knife checks.

Lathe-check troubles aren’t restricted to visible floor results. Vital mechanical issues might also outcome. Most common is a few type of rolling shear developed when plywood is careworn within the type of a beam over a brief span, so that prime ranges of horizontal shear are developed. If the shear coincides with the path of stress that opens the checks, rolling shear failure might outcome. The sides of plywood panels ” broomed” over on this method are sometimes misinterpreted as “delamination, ” which erroneously implies glue failure.

I would not have any particular suggestions for ending veneer that has knife checks. I believe a end that would offer one of the best moisture barrier and thus scale back dimensional variation could be greatest. Additionally, if the checks had been on the floor, any end that will seal them shut would assist. A end like linseed oil would have little to supply. A low-viscosity lacquer or varnish in a number of coats would possibly work greatest.

On the entire, most hardwood cupboard veneer produced by respected mills is minimize with sufficient manufacturing high quality management to make sure cheap tightness. However watch out for ” clearance” gross sales or “closeouts, ” as a result of unfastened veneer is hardly a discount at any value. One of the best guideline is to purchase veneer from respected sellers and know how one can detect, and deal with, the occasional unfastened veneer.

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