uneven/lumpy mold: solutions?

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knightsofnii
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uneven/lumpy mold: solutions?

Post by knightsofnii »

dont know which category this belongs in really.

My first mold "strips" from my cnc, they look real clean. When I bolt them all together, they vary by as much as over 1mm in height.

I know why (mostly), because the work piece was not fully secure and moved around some while being machined. The last three pieces, i paid more attention in setting it up, and those three are really good.


Anyway, I'm hoping it doesnt matter so much, worst case I spend $150 on more mdf and build new strips. But I have this old BH heat blanket that is quite thick,
and i'm wondering if I laid it over the mold, would that provide enough of a buffer to even out the load?

Thoughts? Did I explain that well enough?
Doug
MadRussian
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Post by MadRussian »

just a thought.
Since you doing it of CNC why you have to cut all ribs individually? why not just bolt black ribs and cut camber profile all at once.... similar to profiling core?
I have not failed. I've just found 10,000 ways that won't work.
Thomas A. Edison
gozaimaas
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Post by gozaimaas »

Drill the holes slightly over size. Bolt them together loosely and flip them upside down and let it rest on a couple of boards or anything straight. This will align the top of your moulds anf make the bottom uneven. Now tighten the bolts and with a long piece of sand paper and a block the width of the mould start sanding away.
It might take an hour or so of hard work but the result will be perfect
twizzstyle
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Post by twizzstyle »

I did EXACTLY what gozaimaas just described, down to every little detail. I had the same problem you did, but for a different reason (my long axis on my CNC machine has built up a little backlash).

You really ought to be making the mold as flat as possible. Relying on the heatblanket/cassette to bridge the gaps is tempting, you you're setting yourself up to ruin some skis, and maybe even the heat blanket, doing that. I'd say do what goz suggested, and if it turns out being too much work, scrap it and start over.

It's always a bummer when there's hand-work required after a CNC did most of the work, but it is what it is, I've been there a few times.
gozaimaas
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Post by gozaimaas »

Yep. An hour sweating your ass off sanding will save countless hours and $$$$ down the track.
Im a huge believer in identifying problems and fixing them the right way first time. Cutting corners in this game results in heartbreak every time.
gozaimaas
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Post by gozaimaas »

Also, number the pieces so you can put them back the exact same way or better yet glue them together
Dtrain
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Post by Dtrain »

Yup. Been there too. Oversize holes for the rods, make sure it's on a flat surface. Then get busy with the orbital sander.
OAC
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Post by OAC »

When I have my tip and tail molds done in CNC(outsourced.. :) ) they are done, spread out, on a full MDF. Only that they are not cut all the way thru leaving 2mm for me to cut out with a jigsaw and finish it with a table router(a shaper that is in my case).
Perfect fit! Every time! :D
ben_mtl
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Post by ben_mtl »

OAC wrote:When I have my tip and tail molds done in CNC(outsourced.. :) ) they are done, spread out, on a full MDF. Only that they are not cut all the way thru leaving 2mm for me to cut out with a jigsaw and finish it with a table router(a shaper that is in my case).
Perfect fit! Every time! :D
Same here, except I leave less than 2mm as an "onion skin" (that's the specific term :p).. usually 0.5mm just so my vacuum table doesn't lose suction and nothing moves.
Also I don't glue the pieces together anymore, the changes in temperature and humidity made them split (within the thickness of the MDF pieces)... no problem so far with only a threaded rod through the stack, but of course the pieces have to be all the same..
A bad day skiing is always better than a good one at work...
OAC
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Post by OAC »

"onion skin"... :)
I will use that term!
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MontuckyMadman
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Post by MontuckyMadman »

Brads and glue work well
sammer wrote: I'm still a tang on top guy.
knightsofnii
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Post by knightsofnii »

MadRussian wrote:just a thought.
Since you doing it of CNC why you have to cut all ribs individually? why not just bolt black ribs and cut camber profile all at once.... similar to profiling core?
next mold will be done this way. Adjustable.
Traditional rib type tip and tail blocks. Flat mdf center.
Underneath, a camber "insert", 1.25" mdf machined out from above on the cnc

thanks for all the advice folks!
Doug
ben_mtl
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Post by ben_mtl »

For what it's worth : I now try not to use flat MDF anymore for the flat between the tip and tail molds. After a few pairs of skis are pressed I found out the MDF compresses under the action of humidity, pressure and temperature, leading to non-flat bases in this area. With a flat edge you could clearly see the bottom of the MDF was flat but the top was concave where the skis sit...

I'm not 100% sure of those conclusions but "Rib-style" molds seems to be more stable. Any info on how many pairs can be pressed on a MDF ribbed mold (I'm thinking about ON3P and other medium-sized manufacturers that use one-piece MDF ribbed molds) before the mold shows signs of wear/compression and has to be discarded ?


For the flat I now use birch plywood but I can't say if it solved the problem or not : I didn't press many pairs of skis since I changed the material... The bases turned out pretty much flat though, but it was the same with MDF at first...
A bad day skiing is always better than a good one at work...
rightsideways
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Post by rightsideways »

knightsofnii wrote:
next mold will be done this way. Adjustable.
Traditional rib type tip and tail blocks. Flat mdf center.
Underneath, a camber "insert", 1.25" mdf machined out from above on the cnc

thanks for all the advice folks!
That's how I do mine. i use 1/2" MDF for the top and profile MDF below using my router bridge. it seems to work pretty well. when i first did this i just profiled some MDF to the camber, and then put the hardboard "casette" directly on top. the bases weren't super flat, and inconsistent where they weren't flat (if that makes sense). i suspected an uneven profile from the router bridge method so that's why i made the switch to a piece of MDF, unprofiled, on top.
ben_mtl wrote: For the flat I now use birch plywood but I can't say if it solved the problem or not : I didn't press many pairs of skis since I changed the material... The bases turned out pretty much flat though, but it was the same with MDF at first...
when you do this do you profile the birch plywood, or do you use heat for camber? I'm curious if profiling plywood would cause tearout problems at the interface of the layers, but, i don't really have a whole lot to back this up other than issues when dadoing plywood.
knightsofnii
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Post by knightsofnii »

ah, if i could do the upside down thing and get them to line up like you all described,
I could then put in the cnc and deck the bottom.

Sooner or later i'll have a program that will deck the bottom and top side of the mold.
Doug
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