warped base?

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Danielbroski
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warped base?

Post by Danielbroski »

So. My latest pair of skis with my new complete press went great, I thought. Its sort of hard to explain what exactly happened, but one side of the ski has much less sidecut than the other. At first I figured I just totally screwed up on my template and didn't notice, but after looking at the template again it seems to be fine. I also checked out 2 other bases that I made in the exact same cut as I did the bases on these skis, and while there was a tiny difference in side cuts, it was not substantial as it is in these skis.

Image
In this picture the skis are base to base with the tip and tail of the skis lined up. You can see that on the right side of the ski, the base of the ski in the back over hangs that of the ski in front.

Here is a picture of the template I used. Not completely perfect, but seems pretty even with no extremely noticeable inconsistencies.
Image

Pretty bummed about that, and I want to figure out why that might have happened so I don't do it again. Any thoughts?

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falls
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Post by falls »

Don't cut your bases out ahead of time - absolutely just before is best
Don't cut them out at one temperature and then move them to another eg. outside to inside.
Your edges should be bent symmetrically L and R sides otherwise when you attach them they can pull the base one way.
Also attach your edges at the same time L and R working from tip to tail. ie. Don't attach the L one first then the R one
Make sure your core is symmetrical as well and that the edge rebate is actually allowing room for the edge teeth and not warping your base to accommodate the edge teeth.
Don't wait up, I'm off to kill Summer....

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chrismp
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Post by chrismp »

Make a vacuum template like many others on this forum have done and attach the edges while the base is still sucked down to the template. That way the base cannot warp after cutting out and will be held in shape by the edges once you release it from your template.

twizzstyle
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Post by twizzstyle »

I think falls and chrismp pretty much summed it up - I had this happen badly on my first pairs. My issues were two-fold - first I was cutting the material by hand with an exacto knife, using a paper template. I pulled the paper template tight with tape, but as I cut it, it released the tension and the paper warped without me realizing it. Then to make things worse, I would cut out my base weeks (sometimes months) before laying up the skis.

Now I save the base cutting until the very end, when everything else is ready for layup. Cut the base, attach the edges, pre-bend the tips (optional, I do it), then pile it all on and stuff it in the press.

It's very hard to confirm that your template is perfect by eye-balling - was it CNC'd, or did you cut it by hand? That could also be a factor. If you have some kind of laser level thing you could do some accurate measuring on the template to see if its off.

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vinman
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Post by vinman »

Tempuature distortion is the main culprit I see. I keep the material as close to a stable temp as possible. I use an air conditioner in my shop in the summer to keep things stable.

I've seen bases warp within minutes if the temps are not stable.

Also one more tip I can add is make sure you cut your base off the roll, split it for width and let it relax for a upto day before trying to cut out bases. Trying to cut material that is not flat will always end up with a warped base.

As for templates. I use Sammer's method. I print a paper copy, adhere it to some Masonite. Shape the Masonite on one side only. Attach the Masonite to the MDF with screws placed on the centerline. Route one side, then flip the Masonite template and route the second edge of the MDF. This should result in a symmetrical shape as long as you do a good job placing your screws on the exact center of the template
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Danielbroski
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Post by Danielbroski »

Awesome thanks for the info. As far as cutting my base material I have been using a router and tracing the template with the bad clamped on top.

twizzstyle
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Post by twizzstyle »

How are you making your template?

skidesmond
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Post by skidesmond »

Another tip... I use double stick tape to fasten the base to the template at the tip/tail,mid-section as well as a few spring clamps. Then I route the shape and attach the edges as mentioned above. I leave the bases attached to the template until I'm ready to press. I've left them attached to the template for days and even a few weeks w/o any warping problems.

As Vin said, do a rough cut off the roll and let it rest for a day or 2. Working in a room w/ a fairly constant temp is a plus.

gozaimaas
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Post by gozaimaas »

A hand made template will never be exactly the same both sides, the way to ensure it is the same is to cut one edge, flip the template and then cut the other.
This will ensure symmetry of the base cut.

I never have a problem with boards but I think the width helps a lot, skis are so skinny and are prone to movement.

Danielbroski
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Post by Danielbroski »

twizz,
I make the template in 3 parts. First I make a sidecut template, a single template I flip to cut each side as mentioned in comments above. I then make tip and tail templates. I trace all of these onto a single piece of MDF with a router. Probably not the easiest way, but its nice because I can use the individual sidecut, tip, and tail pieces for multiple skis.

pmg
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Post by pmg »

Hi,

if you build a second template (easy if you already have one), and then flip the second one over on the first and rout both again you have 2 perfectly symmetrical templates. Get what I mean? I posted it in one of my journals.

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falls
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Post by falls »

What are people's experiences with cutting bases out, attaching edges and then using them a day/multiple days later.
You see stacks of edged bases in the factory videos that they use through the day. Surely the layup masters don't start an hour later than the edgers. They must sit there overnight. right?
Once the edges are superglued on is base plastic warping going to be less likely. I have less time these days and trying to get bases edged and all the layup etc in one go is hard work at the moment.
Don't wait up, I'm off to kill Summer....

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vinman
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Post by vinman »

I have a few things I've learned about cutting base material.

1. cut the material off the roll and split it in half, several days before you want to cut it to shape. Let it lay on a flay surface to relax. Add weight to it if you want. Don't expect to have your bases stay true if you don't do this.

2. I lay the base material on a flat surface and place the template onto the base material instead. This eliminates any bubbles imparted to the base material trying to do it in the reverse method.

3. I've started "pre-warping" my raw base material. I cut it off the roll, split it into 2 strips for skis, let it relax a couple days as described above. Then I use my heat blankets to warm the base material, on a flat surface, up to 100 deg F for 10-15 min and let it cool back to room temperature for a few hours before cutting it.

I've started doing this after talking to my father-in-law who is a former chemist for GE and had a lot to do with their plastics division. He was telling me with many extruded or sintered plastics will have a "thermal memory" within the grain structure of the plastic imparted while the plastic is cooling after being formed. There was some other technical info in his explanation but long story short, he said to try to remove the memory by re-heating the material. He thought 100 deg F would be sufficient.

I've only used this method once so far but I can tell you that the result was a set of skis that was perfectly matched in shape.

YMMV but it seemed to fix the minor irregularity issues I've seen in the past.
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skidesmond
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Post by skidesmond »

Vin, great info. Just before I layup I put the bases together to make sure they match. If they don't, stop. Time to start over or re-work the base. Also the edges s/b bent as close the shape of the base otherwise the edges could pull the base out of shape.

Falls, In some of the videos I've seen of the big guys they layup the ski in a recessed template. I think that would take out any minor inconsistency in the shape.

I leave my bases w/ edges on attached to the template until the moment I build and make sure they are symmetrical. I've left attached to the template for days with no problem. Even when I've removed them from the template they have been staying true, so far. But I always check before layup.

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falls
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Post by falls »

OK thanks
I was looking at the moment skis videos and they had a stack of about 10 edged bases on a shelf which they would just spray glue to a flat metal casette sheet. No recessed cassette.
I haven't had any warping issues. I usually do the base cutting and edging in the morning then spray glue onto cassette then go on to layup that day.
I'd just like to be able to do the bases and edges the night before and layup the next day sometime.
I always cut the base material in half weeks before and lay between 2 sheets of 18mm MDF to flatten it out + use vacuum templates for hold-down while cutting and attaching edges.
Don't wait up, I'm off to kill Summer....

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