I need to find a low deflection rectangular steel tube

[iakobos]

What I need is a 1"x.5" rectangular tube .065" wall and 84" long steel tube that will have the least or very low deflection. I have been trying to educate myself today on the different grades of SS which would be preferable for rust resistance. If I understand things correctly 304 SS is the most common but apparently it is more deflective than carbon steel. I have tried what I believe to be a mild 1x1.5x.065 carbon steel in my application and found it deflects too easily. I am concerned 304 SS will be worse.

What I can't change is the requirement for 1"x.5" 84" long. I can use either carbon or stainless (prefer stainless) and increase the wall thickness to .125" but would prefer not to for weight reasons. Specifically, I need the tube to be more rigid and stay very straight and I need suggestions on which grades of carbon and stainless steel will work better.

What I am working on is a new invention. I install steel roofs. Roofs with hips and valleys require a miter cut with a circular saw on steel roof panels that are 39" wide. Miter cuts may easily range up to 60". I've invented a Miter Table (pat. pend.) that makes the task much easier. My prototype is entirely aluminum. The miter assembly consists of two rails under the roof panel that support the panel and two rails above the roof panel that support and guide the circular saw. The aluminum I used was stiff enough to not deflect but it creates too much drag on the aluminum base of the saw making it harder to push. That is why I'm switching to steel.

 

[SteamKing]

Metal on metal can have high coefficients of friction, making it harder to push one metal item over another. I assume your frame is not painted or coated.

I don't know much about your device, but would it be possible to coat it or lubricate it in some fashion? For instance, if the frame were coated with plastic or painted with epoxy, the change in friction might be such to reduce the effort required to push the saw.

 

[iakobos]

Painting or coating is certainly a possibility. That's another avenue I may experiment with. Part of the problem with aluminum on aluminum, in my application, is even if one surface is painted, metal shards will be thrown between the base of the circular saw and the rail it glides across. My concern is the steel shards will still dig through a coating and cause problems. To negate this issue would require a very hard coating. I've thought of chrome as one possibility but am not sure how feasible it would be to have it done.

 

[nvn]

iakobos: Any steel (including carbon steel) and any stainless steel (including SS 304) will have approximately the same deflection. Carbon steel is only approximately 4 % stiffer than stainless steel. No noticeable difference. You can call them equal. There is nothing else economical that will be stiffer than carbon steel or stainless steel 304. Either one of these is your best choice; and they are essentially equal in stiffness. Therefore, use whichever one you prefer.

 

[iakobos]

nvn, Thanks for the info. Since 4% is not that much different, that gives me relief to go ahead with SS. From what I can tell no other SS grade is commonly available in my size requirement.

 

[pantaz]

Use a piece of HDPE (high-density polyethylene) between the guide and the base of the saw. The saw will slide very easily.

HDPE is pretty easy to find, and it can be cut and drilled with common woodworking tools.

 

[iakobos]

Thanks pantaz, I'll look into it.