There are several factors that allow wood on wood to slide more easily compared to other materials. The main reasons are the smoothness of the wood surface, using lubricants, and the friction properties of wood. Understanding these key elements provides insight into how to create wood joints and surfaces that glide effortlessly.
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Smoothness of the Wood Surface
One of the biggest contributors to reducing friction between two pieces of wood is ensuring the surfaces are as smooth as possible. Any roughness, bumps, or irregularities on the wood can catch as the pieces slide over each other. Sanding the wood creates a very uniform surface to allow excellent sliding.
The finer the sandpaper grit, the more smooth the final surface will be. Starting with a coarse 60-80 grit paper to remove bigger flaws, then progressing to 120 or 220 grit will generate an ultra-smooth finish. In some cases, certain hardwoods can even be sanded up to 2000 grit or more for a perfectly polished effect. The smoother the surface, the less friction occurs when sliding another piece of wood over it.
Moisture content of the wood is another factor for smoothness. Wood that is very dry and porous will not slide as well as wood with a bit more moisture. The ideal moisture content is around 6-8% for most furniture woods. Using wood with too high or too low moisture content will decrease the smoothness.
Accounting for the direction of the wood grain is also important when trying to achieve maximum smoothness. Sanding along the grain patterns will allow for a smoother finish. Going across the grains can result in small scratches or ridges that add friction.
Using Lubricants
Applying lubricants is another great way to improve the sliding of wood on wood. The lubricant minimizes friction between the two surfaces, allowing them to move more freely over each other. Typical woodworking lubricants include waxes, like paraffin or beeswax, oils, like mineral oil, and greases like petroleum jelly.
Wax is one of the most popular options for lubricating wood. It can be easily applied by rubbing or spraying the wax directly onto the wood surface. Wax fills in any remaining tiny pores and irregularities to help create an ultra-smooth barrier. As the wood pieces slide across each other, the wax lets them glide instead of scraping and sticking.
Oils are another good choice for lubricating wood before assembly. Light mineral oils can penetrate into the woods pores to reduce static friction when initially sliding the pieces. This allows for smooth sliding motion. Oils also protect the wood from drying out over time.
Greases and gels provide thicker lubrication than oils and waxes. They sit more on top of the wooden surface instead of absorbing. Greases like petroleum jelly prevent sticking and dragging between the wood pieces. Though they can sometimes collect dust and debris over time.
Some other specialty lubricants made for woodworking include silicone sprays and dry graphite powders. These all reduce friction by separating the two contact surfaces. Proper application is important to avoid over-application which can make things too slippery. Removing any excess lubricant helps achieve the ideal balance.
Friction Properties of Wood
The inherent friction properties of wood also enable it to slide across itself better than many other materials. woods like oak, maple, birch, and beech are moderately high on the Janka scale for hardness and density, but lower on the friction/adhesion scale. The ratios of their elasticity, surface energy, and stiffness provide some natural lubricity when interacting with itself.
The porous and layered structure of wood also contributes to its good sliding capacity. Mini air pockets within the wood grains act as a cushion to improve slippage across another wooden surface. The porous composition essentially makes it less “grippy” than surfaces like stone or metal when in contact with itself.
Wood’s natural ability to absorb and dissipate moisture is another important factor. While moisture allows more initial sliding motion, uncontrolled wetness makes surfaces sticky. Wood maintains equilibrium moisture content on its own for a balance of smoothness without excessive moisture buildup. This gives wood an advantage over materials like metal that are either too sticky when wet or abrasive when dry.
The friction coefficients for wood materials are also favorable. For example, the static friction coefficient of oak on oak is between 0.25 and 0.50. Meaning it doesn’t take much force for oak to start sliding across itself. The kinetic friction coefficient is even lower, between 0.20 and 0.25, allowing oak to keep moving smoothly once in motion. This is much better than many other stiff, dense materials.
Wood Sliding Mechanisms
Applying the concepts of surface smoothness, lubrication, and wood’s natural friction properties allows crafting wood joints and sliding mechanisms with minimal resistance. Here are some examples:
Drawers
Drawers are one of the most common applications of wood sliding over wood. The case needs to be ultra-smooth sanded, and drawer slides waxed or greased to enable effortless opening and closing of the drawer. Side-mounted drawer slides keep the interaction between the polished case and drawer sides.
Tables
Expanding tables utilize wood on wood sliding to increase the table surface. Buffing the table rails and applying paste wax lets the leaves smoothly extend outwards. The aligned grain patterns and lubrication prevent sticking when opening the table.
Doors
Interior doors with wood frames rely on the sliding friction properties of wood to function. The door hangs on hinges attached to the frame, and the latch side moves against the doorstop.ensure both the door edges and stops are finely sanded and waxed for noise-free operation.
Puzzles
Wooden puzzles like burrs involve intricate interlocking pieces sliding against each other. A light oil finish on the parts, along with smooth sanding lets the pieces shift and rotate freely to assemble the puzzle. Minimal torque is needed thanks to the wood on wood friction.
Factors That Increase Friction
While wooden surfaces generally slide well on each other, there are a few factors that can negatively impact the friction:
– Surface roughness – Any bumps, ridges or irregularities will increase friction and drag between the surfaces. Ensure sanding is done progressively from coarse to fine grit.
– Excess moisture – High moisture content causes swelling and stickiness. Keep humidity stable and avoid soaking or over-drying the wood.
– Dust/debris – Built up particles, grime, or sawdust can get lodged between surfaces and impede sliding. Keep wood clean and maintain any lubricants.
– Misaligned grains – Having the wood grains oriented perpendicular increases friction slightly compared to lining them up parallel.
– Insufficient lubricant – Not using any wax, oil, grease between the surfaces results in pure wood-on-wood drag.
– Wood type – Some naturally sticky woods like walnut can grip more than ultra-smooth ones like maple. Select wood species appropriately.
– Too much lubricant – Over-application of oils or greases can actually attract more dust and dirt over time. Apply judiciously.
Conclusion
The key factors that enable smooth sliding of wood on wood surfaces are proper sanding and finishing, using the appropriate lubricants, and the inherent friction properties of wood. Careful preparation of the sliding surfaces, maintaining cleanliness, and controlling moisture allows woodworker’s joints and furniture pieces to operate with ease indefinitely. Understanding these fundamental concepts provides the knowledge to craft stunning wooden mechanisms that will last generations.