|Nathan Schultz & Zach Caldwell
The condition of ski base material and the relief pattern or “structure” on the base surface are key factors contributing to gliding speed. While choosing the appropriate ski flex for the skier and snow conditions has the largest influence on ski performance, the ski base quality and structure contribute far more than wax.
Stone grinding is used to maintain the ski base and modify the structure pattern.
The base material of racing skis is sintered ultra-high molecular weight polyethylene (UHMWPE, common trade name P-Tex).
P-Tex is minute particles of UHMWPE pressed together under very high pressure, resulting in very small particles of crystalline UHMWPE surrounding amorphous zones of lower-density plastic and additive material.
UHMWPE is non-porous and is basically impervious to oxidation, so when we say that wax is soaking into the base of a ski, it is soaking into the amorphous areas (sometimes called “pores”), not the actual “P-Tex”. People (including us) generally say that a base is “oxidized”, but those white spots on a ski are generally dry regions or abraded fibers of UHMWPE, not oxidation.
With sufficient heat applied for sufficient time, wax goes into solution in the “pores” of the ski base. Physically, wax changes the hardness on the base surface providing the ability to tune the base hardness to the shape and aggressiveness of the snow crystals.
Chemically, wax adjusts the hydrophobicity, or water repellency, of the base and also lubricates the ski base, minimizing frictional forces.
Waxing only has a lasting effect when wax is absorbed into the base. Damage to base material prevents wax absorption by essentially closing the pores and sealing off the path for wax to enter the base. This can be caused by overheating, which melts the UHMWPE together, hardening caused by prolonged exposure to air or clogging with dirt, among other things.
After base material quality, structure hugely influences ski speed. Structure influences two major forces that slow down skis. First, the mechanical interaction of the base with particles of ice or water is called mechanical friction.
In general, mechanical friction is lower when the surface of the base is smoother.
The other force acting against glide is what we commonly call “suction”, (a misnomer, but common enough to remain in usage) the adhesion/cohesion of two surfaces with liquid moisture between them. Even at temperatures far below freezing, suction can be an issue, particularly when the track surface is prone to glazing.
In general, more structure results in less suction.
The design of grind and structure patterns is a matter of calculating the balance between frictional forces and suction-related forces. Priorities must be taken into consideration as well, and these priorities can be different for different applications.
For instance, classic skis spend most of their time in a track, which is almost always glazed, and classic skis seldom have to release the snow in motion. Skate skis on the other hand more frequently encounter unglazed snow, and must be very free as they release the snow in motion.
The stone grinding process strips away a thin layer of base material, exposing healthy polyethylene, optimizing speed and durability of glide wax. It is the industry standard for base preparation of all skis, from touring models to World Cup Race Skis.
All skis are ground in the factory, and factory stone grinding has improved dramatically in recent years.
Skis are often stone ground either to adjust the structure or to refresh damaged bases.
Stone grinding machines use a spinning abrasive wheel to cut a very thin layer (usually less than 0.04mm) of base material from the ski as it is fed through the machine. Machines vary widely in their capabilities, and results vary depending on the machine and operator.
Cross-country specific machines have softer grind wheels and lower feed pressures than alpine machines to account for the XC skis’ softer bases and narrower width. In general, a cross-country specific grinder will produce better results.
The grinding wheel is cut by a synthetic diamond which passes over the face of the stone as it spins. The shape of the cut determines the pattern that is cut into the ski base. Simple machines spin the wheel to a certain RPM and move the diamond across the stone at a specific speed, giving a small range of patterns and depths that can be cut into the stone.
More advanced machines are numerically controlled, using precise robotic movement similar to a CNC machine and will track the diameter of the stone and adjust the RPM so the stone speed is consistent as the diameter changes. These options provide infinite possibilities for structure patterns and generally improve consistency and repeatability of grinds.
Systems are cooled by pumping water onto the stone. Some machines are connected to industrial chillers, which keep the cooling water at a consistent temperature.
Applying too much pressure or not enough cooling can burn the base and seal the pores. The softer stones and lower pressures used on cross-country grinding machines reduce the heat and minimize base burning, and a skilled operator can consistently produce clean, hair-free grinds on these machines.
Step 1: Flattening
A layer of base material must be removed to flatten the base and expose fresh, undamaged base material.
We find it faster to use razor-sharp metal scrapers to peel paper-thin layers off of the ski, but it is also common to simply feed the ski through the machine repeatedly with an aggressive structure on the stone.
The amount of material removed depends on the base condition. Damaged and/or burnt bases require aggressive base removal. Under normal use, a ski should be able to stand up to 5-10 grinds or more.
Once finished metal scraping, we cut a relatively aggressive structure into the grind stone and feed skis through to flatten any slight imperfections created by metal scraping.
If we did a good job with the metal scraper, it may take 2-5 passes of a light structure cut 0.02mm deep. If it went badly, it takes 15-20 passes with a deeper grind of 0.04-0.06mm.
Step 2: Polishing
With a rack full of flattened skis, the next step is to “blank” or “polish” the base to remove all signs of structure from the flattening. The stone is cut completely flat, erasing all structure when skis are passed through. Depending on the flattening structure used, it takes between 4 and 10 passes through the grinder before a ski is ready to move on to the next step.
Step 3: Final Structure
The last grinding step of cutting the final structure into the ski requires the most precision and craftsmanship. The desired pattern is cut into the stone then a test ski is run through the machine.
We examine the result under a microscope to visualize the structure. While the machine provides astounding precision, there are several factors that can ruin a cut.
As the diamond cuts the stone, it too wears slightly, changing the shape of the diamond. This affects the shape of the cut the diamond makes into the stone.
If too much of the diamond wears away on one or both sides of the tip, the diamond becomes “pointy” and produces odd-shaped cuts instead of clean edges.
If issues are detected on the test ski, the stone has to be flattened and then re-cut until the final structure is correct.
Once the stone is cut correctly, the skis are passed through one last time to take the final structure. Some structures require two or more passes, with each pass a different cut on the stone.
Step 4: Post Grind
Generally, post-grind work involves brushing and lightly “combing” with special pads to remove any hairs created by the final structure.
The quality of the grind determines how much work needs to be done to remove “hairs” of UHMWPE that were not cleanly cut.
The skis are then ready to be saturated with a soft wax, followed by hardening with a harder wax.
The ski base is very fragile immediately after grinding because there is no wax in the base to absorb heat or protect it from physical damage.
Ironing should be done at the lowest temperature possible, keeping the iron moving quickly across the ski.
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