Ski Builders Forum

So I'm 4 boards in and loving this new addiction. I think its time to build a real press and ditch my vac bag mold. I've been drooling over some of the ibeam presses but I came across a very good deal on 3"x6" x .25 web steel box tubing. So good, i thought i'd sketch something up.

Am I on the right track>?

Deflection look okay?
Dimensions okay?
Can i bolt this together?

Length: 8'
Ht: 6'
Width: 22.5"

Working ht: 14"
Working width: 16.5
Working length: 90"
Spacing between lengthwise beams: 1.5"

All frame is 3x6x.25" steel.

We built a very, very similar press and it does deflect at 50PSI. you will want supports in the middle. Maybe 2 a couple inches apart and tight.

Wait, spaces, no no bueno. You want those all welded together with some plate on there.
Just go ibeam.

Whoa really? Surprised about the deflection. Was your press out of 6x3x0.25 steel? The moi is pretty big and rough calc for max deflection at 50 psi is .009in. 'course, I'm taking notes and trust your guys experience more than my spreadsheet. Just saying...really surprised about the deflection comment...

OK-- I've done some more calcs and used some Solidworks express modeling-- any mechanical engineers out there??

I set up a single beam simulation, with the ends fixed and a pressure across one surface. I compared a 3"x6"x.25" box beam to a 10"x6"x.25 Ibeam. Max deflection results are really similar. Am I using too small of an ibeam for this model? From this, it looks like a regular ibeam press will deflect a bit-- about the same that a hefty box beam press would... Note that I am using the 3x6s turned sideways to get the max moi.

Thoughts?

Box beam
30 psig- .7mm
50 psig 1.2mm
80 psig 1.6mm

Ibeam:
30 psig: .6mm
50 psig: 1.0mm
80 psig: 1.6mm

An old subject, but in case someone is digging/searching - I beams will deflect a ton. As MM said, you need to support the centre.

My press is made with massive I-beams weighing around 600kg and at 60psi I see around a 3-5mm (measured, not calculated) deflection in the top beam. The bottom doesn't deflect so much because it's supported at the centre. It doesn't matter if the top deflects anyway, the bottom mould is what is important.

My press, for reference: http://www.skibuilders.com/phpBB2/viewtopic.php?t=2399

Cool! I love when people go through and add info for posterity!

probably a good time for me to say that my press matches my calcs and solidworks sim model. About 1mm deflection in the middle at 40 pisg in the bladder. I'll also add that I think this is reasonable and wouldnt be surprised if a lot of people with the 'ol ibeam press saw this amount of deflection.

Rainy weekend here in southern NH. Great day for building!

just out of curiosity how do you physically measure the deflection it seems like a measuring tape would be very difficult to see your deflection.

1. Measure the gap
2. Inflate and measure the gap
Easy.

1. Measure the gap
2. Inflate and measure the gap
Easy.

JSquare wrote:any mechanical engineers out there??

... yep!

Made a rough calculation and according to that the max deflection is 7.7mm at 50psi. So the gap opening would be 15mm, which is a lot… For higher pressures this will increase linear. It’s calculated on the conservative side, but it will be in this order of magnitude.

So either you put a support in the middle and it should be fine, or you weld a sheet metal plate a couple of mm thick onto both sides of the beams and so you basically created an I-beam yourself. But welding has to be done properly and not just on some spots along the beams…

Cheers, Reto

gozaimaas wrote:1. Measure the gap
2. Inflate and measure the gap
Easy.

Exactly. It's quite simple, I used a metal ruler though not a measuring tape. I left the ruler in place while I inflated. Then deflated and measured again. The before/after inflating was consistent.

I also dug through my data, I was incorrect, it was about 0.5-1mm not 3-5mm that I measured. Which is still too much.