I have two questions.

I think I would rather use 4" x 7.25# as the 1010302SB with the legs facing downward instead of theway the 3" x 4.1# are install in the plans. Would this be fine?

Also,... the plans do not cover bolting the 1010302SB's to the 1010322SA's. What size bolts are being used for that task? I was thinking about using one big bolt but then again, having two smaller bolts per end I think maybe would be better? Maybe two 3/8" bolts?!?!

I think the intent is to weld them instead of bolting. They also serve a role in preventing the table from going out of square, assuming a parallelogram shape, so for this reason, one bolt would be suboptimal. (Granted, the deck and the spoilboard will act to resist this deflection)

Also, a channel shape has most of its rigidity perpendicular to the plane of the flanges (the legs). The web (or the back) of a channel should be oriented parallel to the anticipated load, usually vertically.

If you place them with the legs down, they'll be less resistant to deflection when you load a sheet on it. Will they still be resistant *enough*?.... I dunno.

Short answer: I would weld them in the orientation shown on the plans.

I can't weld them. I need to be able to dismantle the machine for transportation.

This is what I'm going to do,... I'm going to use the 3" x 4.1#. I am going to bolt them to the xBeam but,... I'm going to weld the channels together as two sets of 4. This way when I bolt these 2 sets to the xBeams, I won't have to worry about my table going out of square.

Charles,
Your spoilboard will do the same function and save you some work. You will need a spoilboard anyway. :)

My thoughts exactly Heath

I am out of town and away from my data tables, but my gut feel is that you are reducing the strength by more than 50%

Oriented legs down (or up), a C4x7.25 50" long with a uniform 150# load will deflect .026" at midspan. (the kind of loading from very heavy sheet goods)
The same C4x7.25 50" long with a single-point load of 150# will deflect .11" (the kind of loading resulting from standing on the crossmember to change a light bulb)

http://www.engineersedge.com/beam_be...flection_2.htm

Structural steel has a modulus of elasticity of 29,000,000.

In the y axis (legs down) C4x7.25 has a moment of inertia (I) of .432. It has a section modulus (Sy) of .343. It has a radius of gyration (Ry)of .45. It has a neutral axis distance (Z) of .459.

In the x axis (legs horizontal) it has an I of 4.59, an Sx of 2.29, an Rx of 1.47 and a Z of 2.

It's about 1/8th as strong oriented horizontally as it is vertically.

By comparison,, a 2x6 placed on edge experiencing the same point load at the same span will deflect .014"

Charles, in your other thread, you show the crossmembers oriented the traditional way. Am I misunderstanding your question?

Well,... im changing things around. i am going with the 3" x 4.1#. At the time I made the change back to the 3" beams because of the weight. the 3" is about half the weight of the 4" according to inventor.

Im just going to build according to plan.

Charles,

I think that following the plans is a great idea. It is a proven design and you will be happy with the machine you build. Gerald has designed an excellent machine.

Jeff, thanks for fielding that question - nice to have someone else around to explain "strength of materials" :)

Back this morning and glanced through my tables. A channel typically loses 80% of its stiffness when rotated 90 degrees. That is a lot!