How flat should my 2.5" x 1.5" rectangular support tubes be?

[Chuck Keysor]

Hello Shopbot friends:

Per Brady Watson's recommendation, I have purchased two steel rectangular (1.5" x 2.5" wall thickness 3/16") tubes. These will be welded onto my X car to better support the rails and to stiffen up the X car.

Before I try and cut them to exact length and square up the ends, I checked out the flatness of both tubes. One has a crown, of roughly .016", mostly centered. The other has a few smaller waves, of about .008" to .010".

While I would assume that the most ideal situation would be to have both tubes be exactly the same and perfectly flat, I don't have either.

However, I don't know if this is enough to worry about. But since I am trying to improve my machine, I don't want to waste $140 of new steel by welding in something that isn't right.

Should I send these tubes off to a machine shop to have them flattened? Should I do something else?

Thanks, Chuck

[Brady Watson]

Chuck,
If they charged you $140 for 2 crowned pieces of steel, you're getting ripped off. My local steel yard has 2.5x1.5x.1875 wall tubing for $88 - that's for a full 24' length. There should be zero crown or twist in those lengths of steel across only 6 feet. I would expect there to be some across a 24' length, but no way across only 6'. You should be able to lay it on a true surface (table or edge of X rail) and not see any light between them.

In posing the question, I think you already knew the answer...garbage in = garbage out. If you put twisted/crowned structural parts on your machine, anything sitting on top will also be crowned or twisted. Not sure if you can take them back or not. The only other thing would be to try to true out the crown in a press - but again, steel is cheap and I would just get new ones that were straight to begin with...and not buy them unless they passed a straightness test at the yard.

-B

[Chuck Keysor]

Thanks Brady. I guess I have to search for another source.

I called a few local steel supply houses, and found either no stock in this material, or they wouldn't sell retail. I finally called the friend who will be doing the welding, and that is how I found the source that I used.

My welder isn't a machinist, so perhaps he hasn't had to worry about accuracy. And myself being totally ignorant of steel specs, didn't know what to specify for flatness/straightness. I guess I still don't know. Do I have to ask for something such as machine grade flatness (whatever that may be)? I am at a bit of a loss as to how to avoid repeating my mistakes.

Making it more frustrating, none of the 4 ends were cut square. (Though I did have each piece cut an extra inch long,,,,) And t

hey also originally wanted me to purchase an entire 20 foot long piece. I felt like I had won a concession by only paying for what I wanted to buy.

Thanks, Chuck

[steve_g]

“they wouldn't sell retail”
“wanted me to purchase an entire 20-foot-long piece”

Chuck…

In my experience, I’ve found that the metal building suppliers are most willing to sell in small quantities, but still expect you to buy full sticks of metal. Usually we can buy a full stick cheaper than someone else will sell a cut stick of any length!

I keep two business names, SG mfg. and Treasures from Trees. I do this because “wholesale only” suppliers are more willing to sell to the mfg. name while “craft” and “custom” purchasers hesitate at the mfg. name. In either case, it’s necessary to have the appropriate sales tax ID numbers…

SG

[Brady Watson]

You won't hurt anything going 1.5 x 3.0 x .1875/.25 wall. It'll just hang down another 1/2"...big deal.

OnlineMetals is convenient if you can't do B2B. They have 2x3x1875 precuts...I would call and tell them you need them to be straight...but any major city should have a steel supplier (more than one) - where you can order up what you want. You don't need to have an actual company to do B2B. Just make up a name - tell them you are paying cash & it's all good. Some will even do CC. You are not tax exempt - that's it.

If you have to take the drop from the 24-footer after they cut it (specify SAW cut & not torch cut), then so be it - it's good fodder for your practice welds... It's steel...there's always a use for it in the shop. It has to be steel because you really want to weld up the entire gantry to stiffen it up - Plus, you want the stiffness against deflection that only steel brings to the table.

Welders have gotten REALLY cheap in recent days...With a budget 14" chop saw, 4" grinder and MIG welder. - You could do all mods yourself, PLUS build a 2nd machine or further mod your existing, if you so desired...Sometimes you just have to take matters into your own hands because life is too short for other people's sad stories when it comes to getting what you want...or make your mods and sell off the welding stuff when you are done.

Failure is not an option. Be unstoppable. Don't take no for an answer. What one man can do, another can do. You need to remind yourself of that from time to time & just Git Я Done!

$300 @ Horrible Freight:
Flux core MIG $189
14" Chop Saw $97
4-1/2" Grinder $15

-B

[Chuck Keysor]

Thanks Steve for your insights on how to deal with different business entities. I never would have thought of it, but what you said makes sense,,,,,,, thanks...…

Brady, thanks again. Now, in the spirit of trying to move on and get things going, it occurred to me that the side supports that hold up my 14 foot X rails, are both crooked as can be, and that the X rails are carefully set up on shims and spacers to get the X rails as straight as can be, and parallel to each other. So why shouldn't I be able to do the same thing with my new rectangular steel tubing? I'll just have the tubing welded maybe 1/8" below the Y rails, and then use carefully selected shims to make the Y rails flat and straight. Any problems with doing this, even though it isn't elegant???? Thanks, Chuck

[Brady Watson]

Chuck,
You're going to want to weld a 1" long bead on the side of each Y rail to the top of the 3x3 after you've got everything 100% perfect. Make spreader blocks to keep those rails 22" apart, tack everything and check it six ways from Sunday. If you want more Z clearance, add a 2x3 (raising it up 2") on top of the 3x3 and weld it up. If not, keep it as it is.

I wouldn't personally use them being such short length and having a crown in them. They're only $3.70 a ft for me, so they'd get chucked on the 'farm use' pile. If you have to shim them - go for it. At the very least cut the shims back so it doesn't look like a beaver in a headlock... I'd also want to cut them myself because who knows how square their saw was...almost certainly not good enough for a CNC...but nothing a MIG can't fix in proper hands.

-B