Flat bases. (new aussie member)

[SCHÜSS]

Hi there, first up i want to say excellent work you guys are doing here!! I am half way through building my own skis. I've got all the materials and cut them up to perfect size. now i just have to laminate the skis together.

I decided to make a small test piece (15/20cm in length). this test piece was an exact replica of the construction of the real ski. I made this test pice so i could practice and have a go at putting the edges and base on.

I cut the base to a perfect fit to the core and it lined up with the edges really well. all was good in the sizing department. my only problem after laminated my sample piece together was there was a slight dip in the middle of the base. (the bases were not completly flat.) it only dipps about .5mm but it is noticable from the cross section view.

I think it is because the sprongs on the edges sit above the rough side of the base. they do not extrude much, maybe .2/.4mm or so but maybe that is the cause.

-The core is perfectly flat
-I used plently of resin/epoxy.
-We plan to machine tune the skis.. would this remove the problem?

So yeah, because the base and edges are one of the most vital bits of the ski, i want to get it right. plus 2 sheets of base and 8 edges cost $160 to ship them to Australia!

So if you guys have any tips or advice that will help make the base perfectly flat? That would be a great help! i know these are my first pair.. but i still dont want to make any un-forced errors! hehe

thanks

[o0norton0o]

Of course any asymetry in any glue up will cause some non rectangular alignment. I would assume that the first tuning of the ski would take out any minor irregularitys.
In any lamination the thing to avoid is voids in the glue up. They will cause a weak spot in the ski and a concentration point for breakage to occur or possible delamination. You could rout in the teeth of the metal edges into the base material I supose to balance the geometry, but I haven't seen it mentioned here yet. If you hold a straight edge across your base and it seems excessive to you, fill the space between your edge teeth with say..... a thickness of carbon fiber shaped accordingly and add it to your glue up. Hopefully some of the builders here will add their opinions for all our benefit.
norton

[G-man]

Hi Schuss,

I route a .025 inch deep x 7/16 inch wide groove into the perimeter of the core base (into the bottom surface of the UHMW side wall strips in my construction). For this, I use a 3/4 inch router bushing in my router base and run a 7/16 inch straight router bit with it (a plunge router works best). I run the bushing along my regular base material cutting template, but I clamp the template edge inboard from the core edge just enough so that the metal edge relief groove ends up in the right place. A 7/16 inch wide groove is a tad wider that my metal edge and allows for just enough extra space for the layer of glass mat that must also be accounted for in the layup. The whole assembly comes out really flat, as long as the mold base is flat and beefy enough so as not to distort under pressure. And, once you get the initial set-up worked out (and make some reference notes for the next time), it only takes about 5 minutes per ski. BTW, of all of the many tools that I use to build a ski, a good set of router bushings are about the best trick in my bag. If you haven't used them before, do an online search and check them out... I use them for everything. Good luck!!

[SCHÜSS]

hey, thanks guys.

i will do a few more test pieces and see what works.

i will post up my results when i complete them!

thanks again

[SCHÜSS]

I know this is just a theory.. but do you think it would be correct?

[G-man]

It's a good theory. Like many other of us fledgling ski builders, you must lay awake at night and run these things over and over in your head. My sense regarding the approach that you describe and illistrate is that given the relatively small 'sealing' surface of the metal edge (really just the 2mm or so of continuous material, because the teeth portion of the edge will have no sealing ability), and the porosity of the composite fabric that will be sandwiched between the core and the edge, the pressure of the pressing operation will enevitably force much of the epoxy out of the space between the core and the base material. If this did, in fact, happen, it would be because the pressing pressure would have forced the longitudinal center of the core to deform/deflect downward into the space that you are wishing to preserve. Again, in theory, as the resin cured, and because the surface of the base mold is completely flat, the ski base should turn out flat and the top sheet/top surface of the ski should be slightly concave. I reality, though, I think that the ski core (after the ski is removed from the press) rebounds somewhat to its former flat cross-section, leaving the base also with a slight concavity. Your diagram illistrates the basic method that many of this forum's builders use, and many of those builders report problems with slightly concave bases, although most also report that a light surface grind takes care of the problem. I'm a bit more nerdy myself, and I like my bases to come out of the press pretty flat from the start... I don't like grinding off all of that base material (especially now that I know just exactly how thin it really is), and I really like a nice flat base to work with when tuning and waxing. It's also true, though, that once you've gone to all of the time and expense (and weeks of laying awake nights thinking) to set up for and build the first pair of skis (which may or may not be very skiable), you may find yourself 'hooked' and unable to wait very long before you build the next pair (even though it does cost a small fortune to get materials to Australia). So, it's possible that your first pair or two might not see as much time on the snow as you might initially think.

Anyway, and in spite of all of the above rambling, I think your plan of doing smaller test assemblies is the way to go to test your theories (as long as your excess parts hold out). That's realy the best way, IMO, to find out how a theory holds up in practice... and find out what level of imperfection you can actuallly live with. As my friends keep having to remind me... "It's still important to have time to get away from the shop to actually ski these things".

Cheers

[Alex]

I used a mixture of resin and cotton flakes that i spreaded carefully on the base with a squeegee. The result is an allmost flat surface above the base/edges setup. My base was perfectly flat after pressing.

Additonally you get a stronger compound edges/ski.

[SCHÜSS]

Hey thats not a bad idea..

My second test piece, in used a bit more resin.. and more pressure. the bases worked out to be pretty good without anything between the base and the core. so i spose there is a small elelemnt of luck involved aswell! i hope my baseswork out on the main ski when i press it on thurs!

[SCHÜSS]

i finally pressed one of the skis. (mold allows only one at a time) and the base isnt perfectly flat but its still pretty good. maybe .2 or .5mm dip in the center. I havnt tuned them at all and no wax on them yet. as there is still a month to go till our winter.

hopefully a tuning and some wax will totally make the bases flat.
Also as it is a race ski, we will spend most of the time carving on the endges. so in theory it will have good edge grip. but possibly wont be as fast in a strait line. Will have to see!

[hose-man]

I used a mixture of resin and cotton flakes that i spreaded carefully on the base with a squeegee. The result is an allmost flat surface above the base/edges setup. My base was perfectly flat after pressing.

Additonally you get a stronger compound edges/ski.

What are cotton flakes???

[Alex]

This is ground cotton:

http://shop.ezentrum.de/1727539/nJIohnS ... rtid=48697

Sorry - it's in german - but maybe the picture helps.....

[hose-man]

thanks!