Towers B/C

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baker
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Re: Towers B/C

Post by baker » October 27th, 2016, 4:37 pm

You can cut plexiglass with a table saw. Just move at a steady pace. Too slow, you'll melt a little... To fast you'll chip. I have a 50 tooth carbide blade on a good Sears table saw. Just be steady...

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Re: Towers B/C

Post by kinghong1970 » October 27th, 2016, 5:26 pm

Balsa Man wrote:
kinghong1970 wrote:tbh, unless you have a lot of tools and experience at your disposal, cutting plexi or any form of plastic is a massive pain in the rear.

to get accurate and dead flat cuts, table saw would be nice, or those new track saws... but with the speed these work at, it will melt the plexi and you will not have accurate cuts.
I absolutely agree, table saw is preferable; that's how I would do it (though I have done the hand cutting, and with patience and some practice, for sure, it can be done very accurately). I described the hand cutting approach for hearthstone because it sounds like he has neither access to, nor experience with table saws, and wanted him to know there is a workable way to do it without them.

Key is good steel straightedge, and firmly taping it in position, and not trying to cut/get too much depth w/ each pass of the cutting tool. You don't have to cut all the way through to get a clean break; if you get around half way through, put the piece with the cut line on a counter top/work bench top with a good sharp edge to it, with the cut line like 1/8" beyond the counter top edge, and good heavy pressure on the straight edge, snapping the piece beyond the cut line off is actually.....pretty easy/reliable.

From discussions with one of the coaches I'm working with this year, there may be an even better way than (old school) table saw- if you can get access to a laser cutter- program in the dimensions..... also, with access to a big enough 3-D printer, you could do it... ain't technology wonderful :shock:
this actually got me thinking...

if you can actually score plexi with a knife and bend it over a sharp edge, the "relief cut" that you get can actually serve as a a guide groove for the legs of the tower too... hmm...

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Re: Towers B/C

Post by baker » October 27th, 2016, 5:38 pm

If the score isn't deep enough, when you bend it, it's possible the plexi will crack to a different path and ruin the piece
Last edited by baker on October 27th, 2016, 6:20 pm, edited 1 time in total.

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Re: Towers B/C

Post by baker » October 27th, 2016, 5:48 pm

Just wanted to share a 'balsa stripper' I made several years ago.. Took a piece of 1/4 inch thick flat PVC, 6 inch wide and 20 inches long as a base plate. Then took another piece of 1/8 inch plastic 2 inches wide 20 in. long and screwed it to the right side of the base plate. Then mounted one of those Master Airscrew Balsa Strippers to the assy at about 14 inches. The stripper is flush with the 1/8 inch piece which serves as the edger. Cost was about $6.00 as I already have plastic and PVC laying around. I'm able to run 1/32 balsa thru it to make 1/32 x 1/32 sq pieces. +/- .004 accuracy. 1/8 in and 1/4 in balsa no problem. Also did 3/32 bass wood to make 3/32 x 3/32 sq sticks (it was a little hard on the stripper). Just keep a sharp blade in it. I use vernier calipers to measure. Use a scrap piece of 1/32 balsa to set up before running your good wood.
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Re: Towers B/C

Post by S4BB » October 27th, 2016, 7:55 pm

Another option, we have successfully used 1/4 in MDF to make Tower Jigs before, a 2' x 4' sheet is about $6.50 at Home Depot.

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Re: Towers B/C

Post by hearthstone224 » October 28th, 2016, 6:09 am

It won't be exactly 8x the BS right, for example on my sheet I calculated the BS would increase by a multiplier of 8.91x for a 1/2 bracing interval.
End of freshman season. Good luck to everyone! No state for us, but nevertheless great season. Regional was out of 12 teams. (CLC)

Mat Sci-> Second at regionals
RSensing -> First at regionals
Towers-> Third at regionals.

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Re: Towers B/C

Post by Balsa Man » October 28th, 2016, 8:24 am

hearthstone224 wrote:It won't be exactly 8x the BS right, for example on my sheet I calculated the BS would increase by a multiplier of 8.91x for a 1/2 bracing interval.
It sounds like you have it correct. Cool. :D
Leg length for C tower = 61.343cm
1/2 of that = 30.67.
The proportion of that to 36" (which =91.6cm) is 0.335
1 over 0.335 squared is 8.92 (very close to the 8.91 you're getting, just a tiny decimal difference)
And that's saying if the 36" BS on a stick you tested = X, then if you used that stick as a leg, with a single braced point at the middle, the braced leg would have a calculated BS of 8.91 times X.
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Re: Towers B/C

Post by Kenshi Takahashi » October 29th, 2016, 4:00 pm

When the rules say the tower must span a 20x20 cm hole, does it mean that the tower's base must have minimum dimensions of 20x20 cm, or that it simply needs to cross the 20x20 gap?
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Re: Towers B/C

Post by baker » October 29th, 2016, 4:45 pm

Kenshi Takahashi wrote:When the rules say the tower must span a 20x20 cm hole, does it mean that the tower's base must have minimum dimensions of 20x20 cm, or that it simply needs to cross the 20x20 gap?
You can rotate a 4 legged tower so you hit the center line (10 cm) of each 20 cm side. A tower base around 15 or 16 cm square should work.

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Re: Towers B/C

Post by baker » October 29th, 2016, 4:56 pm

Event Super Question - A tower built so it spans the 29 cm circle. Team places the loading block, chain and bucket on the tower, but before they are able to load any sand the tower collapses. Do they get the 2 kg bonus as their score?
For example a 2000 gm load (bonus) + 600 gm (bucket, block, chain) / 8 gram tower = 325 score
Last edited by baker on October 29th, 2016, 5:12 pm, edited 2 times in total.

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