Goat-Ham-Ah project

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vinman
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Post by vinman »

Ok I understand you problem better now with that pic. I guess I'm surprised that you get vertical deflection with those center posts there.

Do you think adding an addition horizontal tube under the frame would prevent the deflection you are seeing? It looks like you could weld a 2 x 2 tube that would run between the red parts of your frame without interfering with your internal dimensions. Could even add one to the top to stiffen hat up as well.
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ben_mtl
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Post by ben_mtl »

It might help a bit but it's not gonna be the solution... a long tube like that will bend with the rest of the press.

I just got into Finite element analysis (thanks to Jekul who made me some pretty nice analysis), so I took my notes from school (had to dig it pretty deep) and gave it a try with a CAD software I have at home, just analyzing the lower frame.

I tried different "easy" fixes and so far the best I found would be to weld 2"x1/4" flat steel on each side of each horizontal short member (the 2" will be vertical).

I tried the 2x1 between the existing members, as well as adding a 1.5x1.5 tube over members... not as good results.

I would be really curious to know if other people who have built a press like that experience the same deformation.

some pics for information :

Image

Image
A bad day skiing is always better than a good one at work...
doughboyshredder
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Post by doughboyshredder »

Personally, I would weld one inch flat bar on end to each of your horizontal members. Top of the top and bottom of the bottom.
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Post by doughboyshredder »

read, then post.

yeah 2" would be better.
ben_mtl
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Post by ben_mtl »

Done all the update (welding) on monday night night, rebuilt the press on tuesday night, bonded some planks for a new core on wednesday night, shaped the core + prepare edges on thursday night and finally pressed a new pair yesterday ! busy "afterwork" week !
I didn't noticed any significant deflection on the bottom side of the press @ 40PSI (and that's a lot of pressure as the bladder almost didn't inflated. The cavity was super tight so I might have had a 8+in width of contact on all the length of each ski (5in fire hose x2).
Anyway, I pressed 30min @ 70°C and took the skis out of the press 10 minutes after they were cured (still hot). I checked quickly for convexity, it's still a tiny bit convex, ways less than before and I think to eliminate that I now have to make a new bottom mold... later, it's still pretty good !
Another thing I noticed though is an important loss of rocker !
The skis I pressed are a new shape (for me) with regular camber underfoot and rocker on tip and tail, the loss in rocker is mainly visible on the tip. My mold was having a 40cm/25mm rocker followed by a regular tip going to a 55mm height.
When I took the skis out I noticed there was something weird there, laid the skis flat on my work bench only measures a 15mm rocker (maybe less) and the tip only goes up to 45mm. I took the skis back in the press and let them sit there, unheated but under small pressure, overnight. I have not checked them yet but I'm pretty sure I will just be the same and nothing will have changed.
I'll take pics later today.

Anyway I'm trying to understand the problem and I remembered on the previous pair (with regular camber) I had more camber than the mold on the final ski, which is exactly the same "problem" - too much camber in a regular cambered ski = loss of camber in reverse camber (or rocker in this case)
That made me also remember on this previous pair I noticed the bottom blanket was ramping up faster than the top blanket when I started the heating. And yesterday I think it was the same (bottom blanket being a few degrees more hot than the top blanket)

Do you guys think this can be my problem ? I made a perfectly symmetrical layup, no carbon or fancy materials.
Maybe I pulled the skis out too fast ?

If I push my thinking on the different ramp up bottom vs top a bit more I think it would also contribute in having a slightly convex base...
Maybe I'm going too far though :p

pics later !
A bad day skiing is always better than a good one at work...
ben_mtl
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Post by ben_mtl »

Here are the pics :

overall view, pretty satisfied of the result with this clear topsheet !
Image

Inserts as usual...
Image

Now for the bad...
tip rocker is almost half what it should be !
Image

camber underfoot is slightly bigger but not much
Image

tail rocker is also smaller than what it should be...
Image


Overall I'm happy esthetically but the loss of rocker is a major disappointment, especially as I modified my molds to increase the amount of rocker / tip height ! I'm already preparing new material for another attempt but still trying to figure out what has been wrong with this one.

My idea is the bottom blanket sits directly on the mold (MDF) which acts like insulation, allowing for a fast increase of the heat to the bottom cassette.
On the other hand the top blanket is only separated from the aluminum cat track by a 1/8in masonite... a big part of the generated heat might just go there, hence a slower temperature increase...
I might try to start my top heater before to lower one, with this kind of shape, if I end up with more rocker I don't think it would be such a big deal.

To the electrical gurus : I there a way to control that my 2 blankets are always at the same temperature ? For now I control my 2 blankets separately with 2 PIDs.
Another thing that might play against me is I use the thermocouple that is in the blanket... not some other that I would stick directly on the casette...
A bad day skiing is always better than a good one at work...
FrontierSkis
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Post by FrontierSkis »

Hey Ben, Skis look great! I like how you use inserts for your bindings.. Dukes at that!. I am curious to know how you calculate how far to set back your bindings from center? or are they mounted dead center? I would like to use inserts on my skis but not sure how to figure out setbacks? Thanks!
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brandt
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Post by brandt »

Nice work Ben_mtl! What bindings are you using? Something like Jester/Griffon Schizo which has a bit of margin for dialing in the standing position?
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ben_mtl
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Post by ben_mtl »

Those inserts are set to use Marker Dukes (or baron, same pattern)
I don't have any adjustability with that, which is not that good, especially to tweak with the bindings position.
I have 2 pairs of Dukes I switch from a pair to another, one set for my AT boots, the other for Alpine boots. I have Dukes because I go touring a bit (touring is a great word here..) to find some fresh snow but I'm really thinking of selling one pair and buy some Schizo, which will make things easier when comes the time to find the best position for bindings. If Only Dukes and Schiso had the same mounting points !!! damn non-standard within the same brand !!!

Right now I put my inserts so the boot center is exactly at the waist of the ski. I design my skis so the waist/boot center is 55% of the running length but I'd like to run some tests with a different bindings position.

My main concern is if I want non-over-crowded powder to test my skis I have to go away from a ski hill... so I have to walk up... sckizo means snowshoes, that's bad ! Well anyway for now I have to wait for snow
A bad day skiing is always better than a good one at work...
ben_mtl
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Post by ben_mtl »

So ! Here are some news from my latest built.

After my previous built where the rocker was ways less than expected, I made a new pair yesterday : same shape, same mold, same wood, same layup !
What I did different : I started the top heater a couple minutes before the bottom one and then I maintained a temperature difference top vs bottom (75°C top / 68°C bottom) for 20 minutes, I then set both at 70°C for another 30 minutes. For info my resin is supposed to cure in 30min@70°C.
After an hour with heating I just stopped the heaters and let everything cool under pressure till this morning.

Result :
Again I have less rocker than the mold. It's better than the previous pair but still, I only get about 70% of the rocker of my mold !
I used transparent topsheet again but this time I screwed up and put it upside down in the layup ! It peeled up right out of the press so I got a ski which is 300g lighter. We all learn at every layup I guess. It's no big deal though, I'll just apply some varnish and it'll be good to go.

No pictures this time as they would be the exact same as the last ones.

I'm really scratching my head to understand what makes me lose that rocker. Next time I'll increase the temperature difference top vs bottom, something like 15°C to really see how it affects the camber/rocker.
I'll have to wait though, no more cores are ready and no more resin in the drum...
A bad day skiing is always better than a good one at work...
doughboyshredder
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Post by doughboyshredder »

no need to equalize the heat at the end. Just keep the top blanket warmer.

I would guess that you may have a difference in top v bottom laminates?

As iggyskier pointed out recently your glass orientation needs to be the same top and bottom. i.e. 0 degree fabric towards or away from core.

Is the loss of shape noticeable immediately out of the press, or only after the board is cut out?
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SHIF
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Post by SHIF »

ben_mtl wrote: Another thing that might play against me is I use the thermocouple that is in the blanket... not some other that I would stick directly on the casette...
I think this last statement is the crux of your heater control issues. On my first couple builds I mounted tiny thermocouples on the ski top and also on the aluminum cassette surface. (I only heat from below). I originally used the internal TC that is built into my MEI blanket as feedback to my PID controller. I monitored the temps of the additional TCs using a Fluke digital thermometer. I was so surprised to learn that the built-in TC was totally wrong, especially in the 150-180 degree range. My lower press plate was actually overshooting by fifty degrees! I immediately switched the PID controller to use the TC mounted directly to the hot pressing surface. I never used the built-in TC after that first time and I plan to remove it with a knife at the first opportunity.

The TCs mounted on top of the ski were used to measure the heat transfer up thru the ski. At the tips, the delta was in the five degree range. In the middle where the ski is thickest, the temp delta was about 25 degrees. This was measured after heating the ski to 190F for and hour! I always do a long post-cure bake at high temps to really cross-link the resin. Then let it cool to ambient while under pneumatic pressure. So far my skis maintain their molded shape and perfectly match the press form later. I hope this remains true for my current ski build, double rockered, similar to yours.

Cheers,
-S
ben_mtl
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Post by ben_mtl »

I use the same fabric top & bottom : triax 22Oz, and I take care of having the longitudianl fibers away from the core on both sides (so the ski is slightly stiffer... it might not even be noticiable)

I don't even add a layer of fiberglass in the bindings area as I use inserts.

The loss in the shape is right out of the press, when both skis are still "attached" together. It keeps its shape after it's cut.
Next time I'll keep the temperature difference for all the curing time.

I'm pretty sure my skis will be good even with that rocker loss... I had a look at the '10 Gotama yesterday and the rocker is smaller than mine. It's just not what I wanted to try in terms of shape.

Thanks a lot for your help !

I wanted to take a break in my building season 'till the end of the winter but... well... I might just order some more resin and take a trip to the wood store... addicted you say ?
A bad day skiing is always better than a good one at work...
iggyskier
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Post by iggyskier »

SHIF wrote: My lower press plate was actually overshooting by fifty degrees!
happened to us too. We were seeing temps well past 230 degrees. Cooked a pair of skis like nothing I have ever seen.

Use a separate thermocouple for sure.
ben_mtl
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Post by ben_mtl »

New pair out of the press just in time for my last day this season.
The shape is an evolution of my original pair (Goat-Ham-Ah), I wanted a more "all-mountain" ski that would handle the hardpack and actually be fun on it but that would also be fun and easy on a powder day at the hill (meaning fresh untracked pow for the first 2 runs, then chopped/over-tracked snow). As I'm on the east coast, the skis have to be easy to handle in tight trees too.

The result : 178cm long, 134-96-121, 17m radius.
mini-early-rise in the tip (tip is going upwards 3in before the end of the sidecut), small tail.

The big change on this pair, except the tip profile (mini-early-rise) is the use of carbon, I put a 2" band of uni-carbon on top and bottom almost full length (I stopped a couple inches before the tip to keep it soft).

Layup :
- Red Durasurf 4001 + edges
- VDS (1in)
- Uni-carbon (2in)
- Fibreglass triax 22Oz
- Core (100% poplar, maple sidewalls, 2-11.5-2)
- Fiberglass triax 22Oz
- Uni-carbon (2in)
- Fiberglass triax 22Oz on the binding area
- White PBT topsheet

West system epoxy as I had some leftover.

I think I didn't made the best choice in the layup when I placed the fibreglass layers just against the core, next time I'll do as follow :
- base+edges
- VDS
- 22Oz triax
- carbon tape
- core
- 22Oz triax binding reinforcement
- carbon tape
- 22Oz triax
- Topsheet

Anyways, as expected the skis are pretty stiff I will see how it behaves on the snow.
I really like the shape and the camber/tips profile, can't wait to give them a try tomorrow !

I mounted a pair of demo bindings on them so I can have other people to try the skis and I also can play with the FWD/AFT position of the bindings.


Here are some pics :
Image
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Image
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A bad day skiing is always better than a good one at work...
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