What Happen????-PLaner Profile mathe related

[MontuckyMadman]

So I did this several times before and didn't really have a problem now all of a sudden my brain has lapsed.

Gonna get in the shop for all the bag people TODAY!

If I am planing a core that is 166cm for a ski to be 186cm
I want to orient my planer jig in what fashion to have them 2.5 in the tip and 13 on center, no flat spot.

its this one:




T = S*Y/X
If S = 1700mm
If x = 830mm
and y = 13mm
then what is T?

26.62mm right? just the height of the shim at the end?-not the overall height of the planer jig.

So, its not planing the same as before. I made the same cores and some others.
I must have done something differently.

Does the overall length of the crib matter and for some reason should mine be longer or shorter?

Should my cores be oriented in another fashion rather than even at the end of the crib?
Why are the cores in this picture hanging off the end of, what appears to be the front on the crib?


Does point A not actually equal 2mm like in the instructions? and the 2mm spot is actually nearer point W and the excess is sniped off?



If I want my cores 166 should I start with 186 and cut after planing? I don't think so.

Even with a 21mm shim its still not cutting far enough back towards center.


I know we all did this in 8th grade which is why I am so disappointed in my ability to do this. Just goes to show anyone can build skis.

Please someone spoon feed this info cause I can't get it all of a sudden.

Good thing I have a stack of cores waiting to be ruined.

[doughboyshredder]

snipe.......

166 cm cores should be at least 20cm longer for snipe, and then cut to shape after planing.

[MontuckyMadman]

damnit. That's what my partner skibuilder was saying but I didn't remember that. That's what happens when you build 10 cores 2 years ago.

[falls]

I agree with your maths. Using tan you come up with a height of about 26.7
I think the trouble is that this process doesn't allow for the fact the core is 2.5mm thick at your starting point. It is actually calculating the angle to go from 0 thickness to 13mm over that distance. This creates a sligthly larger angle and therefore tapers your core over a shorter distance.
To relate your x and y values to the s and t values you need to extrapolate your hypotenuse in the core situation back to the point where core thickness is zero then work out how long x + the extra distance is. This is what you should really be using as your x value.

[falls]

Your second picture is what I am talking about. You need to define x as the distance from the centre of your core blank to its end and y as your desired height at the centre of your core. This can then be used to set the height of t on your crib. The trouble with this method is that it doesn't give you control over where the point w ends up in relation to the end of your core blank. And therefore no control over the actual length of your 2-13-2 core.
If core length is more important then you have to specify that the distance from w to the centre of your core is 830mm (for the 1660mm core). From this and knowing that the height is 13 with some fancy calculations you can solve x. It will mean you need to cut your core blanks to twice this x value. Proceed to thickness until the end of the core blank reaches zero thickness and it should then be 2mm thick at the point 830mm from centre and 13mm at centre.
Another option is to just do it on CAD.
Draw what you have in the picture with a and w in it but do it to scale for the 830mm from w to core centre, 13mm for height and 2mm height at point w. If you extrapolate a horizontal line outwards and a line from the top of the 13mm height through the top of the 2mm height it will intersect with the horizontal line. CAD can then tell you the angle required plus the distance equal to x. From that angle and knowing the S value on your crib you can define T.
High school maths vaguely coming back! I am on my phone at the moment but will try and draw it tomorrow on the computer and then post.

[jono]

When I used a wedge style crib I calculated the slope I needed from the 2mm point to the, in my case, 11 mm point. Those were my X and Y. I interpreted the skibuilder's diagram to be doing the same thing. Once I knew the slope then I chose arbitrary,but spread far apart, points on my wedge crib for my S and T numbers. I used cross multiplication to arrive at T since I knew X,Y and S.
Once I established the proper slope I attached my 11mm core blank so it protruded slightly from the thick edge of the wedge. This allowed me to measure until I achieved my 2mm thickness. I also shaded the flat area in the middle of my core with pencil so I could see where the sloped part of the core started.When the core was 2mm thick at the end I checked and I was pretty close in terms of where the sloped part of the core was.
The protruding core was not a problem. It did not explode or splinter and this approach made it so I did not have to cut the thin ends of my core.

Overall the wedge system was a bummer because I had to readjust the wedge for different slopes on the tips and tails. This meant running everything through the planer twice. The other problem is that the wedge didn't allow for a flat area on the ends of my cores.

[falls]

Here is the solution to the correct angle you need to use.
I added a construction line and used the known dimensions of 13 and 2 to create the second triangle. The angle theta in this triangle is the same angle as the one in the first triangle by the law of a line bissecting parallel lines.
The angle is about 0.76 degrees
You can use this and the known S value to determine T and therefore create the wedge with a 0.76 degree slope.

Then mark the centre of your core and the centre of your planing crib. Also mark the point 830mm from centre where you want the core to end. Centre your core blank on the crib and plane. keep measuring the thickness at your core end point and when it gets to 2mm the thickness at core centre should be 13mm. Spin the core around and repeat. Finally trim the ends off at the 830mm from centre points.

(NB. My knowledge of this is purely theoretical )

26.62mm right? just the height of the shim at the end?-not the overall height of the planer jig.

Yeah the T value is the height of the shim.
The S should be the distance from the end of the horizontal piece of wood to the point where the shim supports the upper deck of the crib.

[falls]

I also shaded the flat area in the middle of my core with pencil so I could see where the sloped part of the core started.When the core was 2mm thick at the end I checked and I was pretty close in terms of where the sloped part of the core was.

I think using the distance from the 2mm point to max thickness as your x value is the reason you got "pretty close" to matching the shaded area to your cut, not exactly on. But in reality it probably changes the angle of the crib by something like 0.2 or 0.3 of a degree. Probably not that significant. I think in Montucky's case where he wants the core to peak at the centre point he would end up with a very short flat section in the centre or if he kept planing til the peak ocurred the end would be less than 2mm thick
It's all going to be pretty marginal though I reckon.[/quote]

[MontuckyMadman]

Falls, That's tits on toast. Do you guys say that or just the kiwis?
Even I can figure it out now I think.

All this stuff is coming back to me but I had such a rudimentary limited understanding of it at the time I just swept it out at some point.

The flat spot was actually about 10inches or so, so not very small at all.

I can tell, as the cores are, they are to stiff.

We did beach before with poplar and the maple with poplar is noticeably more stiff.

[falls]

Just the kiwis I think!
Hopefully it works out well with the cores.
The other thing you could try is the two sheets of mdf separated by shims at the end, then you can plane in one pass. I think someone named mark on this forum had pictures somewhere of this.
Glad to see that some maths from school actually is useful in the real world! (that is if ski builders are living in the real world )

[MontuckyMadman]

Yes yes I saw this today!
http://www.skibuilders.com/phpBB2/viewt ... highlight=
Sweet! I think this is better! Glad to enlighten your vernacular.

[Jekul]

I used the method pictured above a two weeks ago and had great results. I didn't do any measurements to verify accuracy, but both of the cores turned out nearly identical, and it was very easy to control the depth of cut.

[shopvac]

This is what we do as well. Pretty good results but it isn't flawless. Our tolerance from our last pair of cores was around +/-0.2mm. I think that is acceptable for our building purposes but I would love to have a CNC to get +/-0.005"! I saw some cores that a guy in town CNC'd and they were nice to say the least.

Some more info on how we figured this out here:
http://www.skibuilders.com/phpBB2/viewt ... 2&start=75