Post Job Assessment

After I finished the skis I decided that some stiffness testing was in order.  The stiffness of a ski is critical to its performance.  A backcountry or powder ski needs to be lighter and can be softer.  The soft snow is more forgiving and high speeds are not typically achieved.  An east coast carving ski needs to be relatively stiff to hold an edge at high speeds and avoid vibration.  To test the ski stiffness I placed weights on the ski and measured the deflection.  After searching my house for weights I only found two 5 lb weights.  Since a gallon of water weighs approximately 8 lbs, I filled a bucket up with two gallons to get an additional 16 lbs.

I tested all the skis that I have to get a good comparison.  The goal with the skis this year was to design them so that they would be stiffer that the first generation Goliaths and softer than the second generation Goliaths and Tomahawks. This goal was achieved.  In order off stiffness, here is how my skis rank up from stiffest to softest.

1. Tomahawk
2. Arlia (M)
3. Arlia (R)
4. Panjshir
5. Elan Silver Magic
6. Blizzard Titan
7. K2 Kung Fuja
8. Goliath V1

The graph below shows the test results when 5, 10, and 26 lbs was applied to each ski.  Note that I tested the Arlia and Panjshir prior to attaching the second layer of fiberglass and topsheet; these results are noted by a (PC) after the name.


The calculations that I performed prior to building the skis are summarized in the table below.  These calculations did not take into account the stiffening properties of the epoxy.  The test results show that the epoxy has an average 4.18 stiffening factor for non-metal skis and around 2.4 for skis with metal built into the core.  The skis I built with metal rods in the core were very stiff and therefore the epoxy had less of an effect on the stiffness.  Since I don't have a pair of the second generation Goliaths I could not test the stiffness (these skis belong to my brother).

Overall the skis turned out as I expected and have a stiffness within the range that I had hoped for.  The following is a summary of what I have learned this season:

1. Rather than packing tape, use contact paper to protect the base and top sheet as the application and removal would be easier.
2. The cost of printing out full size templates is worth while.  In the past I printed out the templates on 8.5 x 11 paper and taped the sheets together.  This worked, but the skis were never quite straight.
3. Assuming a mold springback of 15 mm is safe.  The Arlia mold was designed with a 30 mm camber and came out at 15 mm, as planned.
4. Paper is a good medium for appling graphics (acrylic paint) and when soaked in epoxy becomes 70-90% transparent.  Cotton cloth still works well for opaque solid colors.  Solid colored construction paper may work well also.
5. If a cap construction is used rather than a plastic sidewall, then the sides of the skis must be sealed to make sure the core is sealed after edge grinding.
6. 120 grit sand paper / grinding wheel is appropriate for edge grinding.
7. A course jig saw blade works best when trimming flashing off of the skis after pressing.
8. Isospeed 4400 p-tex base material grinds more easily than Durasurf 4001 p-tex base material.  This is likely due to the Isospeed material being slightly harder than the Durasurf material.
9. West Systems Epoxy, 150B Resin / 206B Slow hardener wets up easily and provides enough time for lay-up without hardening.  The pumps are worth buying to easily measure out the epoxy.
10. Mounting edges on the ski mold effectively holds the base in place during pressing and simplifies the process.

Bindings, Grinding, Sharpening, and Wax

The final steps in the ski building process are to mount bindings, grind the base, sharpen the edges, and wax the skis.  I have done all of these steps in the past, but didn't have time to do them all myself this year.  I took the skis to Cycle Sport and Ski in Greensburg, PA and had the skis finished off there.  Using a professional grinding machine and expertise does have its benefits.

One additional step I needed to complete once the shop was done with them was to seal the edges of the skis.  In the process of finishing the skis, I ground the tip and tail round and ground the edges.  This process exposed some of the wood core.  In order to ensure that the core was safe from water I mixed up a little extra epoxy and went around the edges.

The ground and waxed base:

 The finished skis:

The bindings I used are shown below; note that none of these were bought at full price.  There is typically a significant discount for bindings from previous seasons.

Marker Free Ten Binding (MSRP $210)

Head Gold Thang Binding (MSRP $215)

Marker Baron Alpine Touring Binding (MSRP $445)

A final cost assessment shows that for three skis it took approximately $816 and 101 hrs of labor.  Per ski this is $272 and 34 hours of work.  Some of the final cost of the skis went into building the molds and templates; this would reduce the cost of the skis into the $240 range.  If I were to make $20 per hour working on this skis I would have to charge $920 per pair.  Ordering materials in bulk and automating machining processes would save additional costs.  Note that bindings, grinding, sharpening and waxing are not factored into this price.  A full cost and labor report is below:

For price and ski style comparison two Volkl skis are listed below.  I'm not saying that my skis are just as good as Volkl skis, but just comparing the price for a similar style ski:

Panjshir:  Gotama $825
Arlia:       Kenja $775

 Three generations of HOLUTA skis:

Post Processing

Once the skis have been fully pressed, the flashing must be trimmed off and the edges sanded to remove excess epoxy, fiberglass, and topsheet.  Trimming the flashing can be done with a jigsaw or bandsaw; in this case I used a jigsaw.  Note that the blade will heat up quickly and will have to be cooled throughout the trimming process.  I recommend a coarse blade.

The easiest way to trim the flashing is to turn the ski over and cut from the bottom side, using the edge as a guide for your blade.  A dust mask is helpful if you want to enjoy a lifetime use of your lungs.  Protection for your ears and eyes is also recommended for the same reason.

Once the flashing is trimmed off the ski, the edges, including the tip and tail, need to be sanded.  To sand the edges, I find it easiest to clamp the ski on a work bench while supporting the camber of the ski.  A belt sander will do the best job in this case.

To sand the tip and tail I find it easiest to move the ski against the sander vice moving the sander about the ski.  If you have a bench top belt sander, this will work best; I do not have one (yet) so I clamped my belt sander to a work table.  I work the ski from the steel edge towards the tip or tail; this will maintain a nice curve.

Once this is finished, the only production work left on the ski is to remove the protective tape from the tip and the tail.  This is a brute force operation and can be time consuming, but it is always worthwhile to protect the top sheet and base so it looks clean at the end.

I have decided that although packing tape works well to protect the ski from damage during production, it is too much of a pain to use again.  I spent several hours removing the tape from these skis; fortunately, it was during the Steelers game so I did not mind too much.  In the future I recommend the use of a thicker tape.  The problem with packing tape is that it tears very easily which makes removal difficult.  A six inch wide piece of clear contact paper would work far better.  The contact paper is thicker and will not tear easily and is just as sticky.  If you buy an 18 inch wide roll I suggest cutting the whole role to size with a hack saw.

The production portion of the ski making process is now finished.  The base must still be ground, the edges sharpened, the base waxed, and the bindings mounted.  I will let Cycle Sport and Ski do this work for me.

The finished skis:

Graphics and Press stage 2

There are many options for graphics and most of them are relatively simple to create.  In industry they typically print the design directly onto the plastic top sheet or on a separate layer.  I don't have the technology needed to print directly onto a topsheet, although, I have taken steps toward this method. I just need a larger printer which is not in the budget...

I took two approaches to the graphics this year. The first method was to paint onto paper and directly on to the wood of the ski.  Painting on these two mediums is easily done with acrylic paints.  The paint on the wood will be slightly obscured due to the fiberglass layer between the wood and the top sheet.  The fiberglass becomes mostly clear when soaked in the epoxy.  The paper when soaked in epoxy will become ~75% transparent. The second approach, which I have taken in the past, was to get a sheet of fabric, purple in this case, and paint directly on it.  The advantage to this is that the fabric will be very opaque and you can get a solid color, but it absorbs a lot of epoxy and will add weight to the ski.

Some past designs:

2010 Goliath

 2009 Goliath, this is still one of my favorite graphics.

The Arlia's I am making for Megan have a cardinal as the focal point and are relatively simple.  I wanted the wood to show as part of the graphic as well.  Emily helped me paint the skis; I owe most of the credit to her.

I always place a horizontal line across the ski to indicate the binding center.  Note that this is not necessarily the center of the ski and will often be slightly back of true center.  Without this line it would be difficult to mount the bindings properly.  I also always put my JH symbol on the skis.

The skis I made for Matt, the Panjshir, use a similar technique in painting on both the wood and paper.  The focal point of these skis is the wolf graphic.

The Arlia's I am making for Rachel take a different approach using purple fabric and gold paint.  The fabric, when soaked in epoxy will become several shades darker.  The JH on this ski is made from gold leaf.

Once the graphics are complete the skis are ready to be pressed for the second time.  The built in alignment features on the mold makes this process as easy as setting the ski on the mold applying epoxy, fiberglass, the graphics layer and the top sheet.

A small paint roller can be used to remove bubbles from underneath the top sheet.  It works best to start the roller at the center and work towards the tip and the tail pushing the wave of bubbles out of the ski.

Since this is a cap design, the top sheet must fold over the sides of the ski.  The air bladder will do this to a certain extent on its own, but I use 2" of foam to help press the top sheet to the edges of the ski.

As always I use rubber gloves and a foam brush to spread the epoxy.

This is a picture of Megan's Arlia when first removed from the press.

The following video shows the layup and stage two press at 16x speed (complete with music!).

The images below show the skis as pressed with the flashing still connected to the ski.  The only steps left are to trim the flashing, sand the edges, and remove the protective tape from the top and bottom of the ski.

Ski Press Design Notes

SKI PRESS ONE

I designed my first ski press in early 2009; I was a mechanical engineer less than a year out of college.  I could not afford to build a press out of steel and determined that a press made from wood, if designed properly, would operate just fine and would by 1/10th the cost. The design of my first ski press is outlined below.

The ski press is used to react the pressure from the air bladder when it presses the ski into shape on the mold.  For example, the press is 10 inches wide by 84 inches long. At 25 psi, this is a total force of 10in x 84in x 25 lb/in^2 =21,000 lb or 10.5 tons of force. This is a significant amount of force to react, but illustrates the power and usefulness of an air bladder.

The air bladder is made from 6" diameter marine discharge hose rated for 70psi.  The ends are bolted closed with angle iron and are sealed with silicone caulking.  A pass-through-wall valve was used to attache the air hose.

I designed the press to work at 30 psi and pressure tested it.  I used it to press my first two skis and it worked well.  I then pressurized it for the third ski.  After pressurizing to 30 psi, I looked up and notice the second brace back from the front had cracked left of center and was bowing outward.  The press was stable, but I decided I should decrease the pressure and try to save the ski.  As soon as I decreased the pressure the brace snapped clean through, causing a chain reaction breaking all but the last two braces.

When I realized what happened I knew the ski was lost as the epoxy hardened it into a shapeless ski.  I few seconds later I could feel my mouth getting warmer and filling up with blood.  The pressure relief valve was connected to the ski bladder and when I relieved the pressure the front brace broke outward and struck my jaw. My teeth went into the side of my cheek and I ended up needing two stitches in my mouth.  I did have safety glasses on and eventually noticed bruising on the bridge of my nose where the glasses protected my eyes.

RECOVERY

I share this story so that the  people can learn from my mistakes.  There were several things wrong with my press which I corrected.

1. The cause of failure was a stress concentration in the second brace.  I had drilled a hole through the cross brace (see part 10 above) so that a bolt could pass through and suspend the platform (Part 3).  The stress around this hole was approximately 3X higher than in the rest of the brace.  This location fatigued after three uses and cracked on the fourth. To correct this problem I used metal straps to connect the cross braces to the platform.
2. The platform (Part 3) as shown in the image above was free to bow as the pressure increased.  Therefore the cross brace was in full bending, maximizing the stress.  To correct this problem I attached plywood to the top of the platform
3. The pressure gages and relief valve was attached to the bladder.  I moved this to the other end of the hose for future pressing to keep my body away from the press in the event of a failure.

In addition to the items noted above I redesigned the press for a much higher safety factor and took pure bending into consideration in my calculations.  I added two additional braces to the press and made the braces from 2x6 lumber in lieu of 2x4 lumber. The calculations I did are shown below:

 These calculations show a safety factor of 5.32 at an operating pressure of 22psi.  Additionally, I ran a finite element analysis of the bracing with the old design (left) and the new design (right).  This design change shows dramatic reduction in the stress levels.

To rebuild the press I salvaged what I could from the wreckage and started over.

The finished press:

 Comparison of the old press bracing size to the new brace.

I managed to salvage the core of the ski by tearing of the base and the top sheet and then sanding any remaining epoxy and the graphics layer away.



The redesigned press has worked now for 16 presses and 3 pressure tests and shows no signs of fatigue.  To prevent such a failure again I inspect the press for cracks prior to every use. There were some cracks that existed in the wood at the initial build, but those cracks have not propagated no significant cracks have been noted since it was designed.  I also pressure test the press prior to a round of pressing to ensure the press is in good shape as well as the air bladder.  Additionally, I put on my face shield prior to pressurizing the press after which my adrenaline always starts pumping.

A final note, whether or not you are an engineer, take care in the design of your ski press and always build redundancies into the system; if you don't know what you're doing, then get someone who does to review your work.