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

  1. #1
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    Default How flat should my 2.5" x 1.5" rectangular support tubes be?

    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
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  2. #2
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    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
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  3. #3
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    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
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  4. #4
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    “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

  5. #5
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    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
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  6. #6
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    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
    Attached Images Attached Images
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  7. #7
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    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
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  8. #8
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    Hello Brady! Thanks again for your information....

    Relating to increasing my Z capacity..... I DO want a greater capacity than I have. (I once had a chance to process a bunch of 5 1/2" thick foam and was disappointed to find out that it was too thick for my machine.)

    Not wanting to go back to the place that sold me the previous rectangular stock, I called around again, and got stuck. So I found a local scrap yard that would let me pick through their piles. I found 2 inch square and 1 1/2inch square tubing, with 3/16th inch walls. I bought two 30" long pieces of each size. (Total cost for all 4 pieces was $15..... I clearly have push-over painted on my face...)

    Can I stack these tubes up, to raise my Y rails by 3 1/2 inches without messing something up? (If I hadn't taken my machine apart, I would have tried to figure this out, but I still would have asked to make sure...….). If not, I'll just use the 2" tubing. (I could sell the 1 1/2" tubing back for 6 cents per pound .)

    Thanks again,,,,,,,,,, Chuck!
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  9. #9
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    Chuck,
    Question: What are you expecting to gain, aside from increased Z height by doing this work?

    I try to just say things 'how it is' - and some people can't take that...while others appreciate the honesty. Hopefully you fall into the latter. Before you do anything to that gantry, consider how it will look adding the components that you mention. Personally, I think it would look like a hack job and there's no way I'd want my machine (or yours) to look like that. Not that the PRT wins any beauty contests...but, you gotta use the right stuff. I don't want to see your machine on There I fixed it

    My compelling motivation for welding up my original PRT gantry was two-fold. First, I was tired of the gantry going out of square and the arduous process of getting it square and parallel again. It is a nightmare (but not so bad for me now having cut my teeth on them) - the second reason was stiffness and resistance to deflection under the weight of a heavy Y car with twin Zs.

    If you aren't having any issues with Z deflection/sag, then you can probably forgo putting steel under the Y rails and just run what you have. I would take a good look and make sure it is secured properly. I would however, run a weld bead where the Y rails attach to the 3x3s. This will keep the gantry from turning into a parallelogram. I'd run a total of 4 one-inch beads per corner. Two beads on the flat part of the angle (along the 3" face - space in the middle) and the same on the outside of the rail - then repeat on each corner. This is of course after tack welding all points to keep the heat from racking your otherwise true gantry.

    In terms of raising the gantry, you don't want to go more than 2" high. You will run out of T-rail unless you have a 12" Z assembly. $25 a piece for 2x3x1875" here As it is, the Z barely hits the spoilboard with a small, short bit. Measure carefully.

    Use the right stuff...don't settle for less than the right stuff - unless you like sad stories and disappointment. I don't, which has made me be more patient & determined.

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  10. #10
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    Brady, my primary objective was to improve my gantry's mechanical stability. It always just bugged me that I had to re-square the gantry every time I turned on my machine.

    Maybe a year ago, someone suggested that I use red Lock-tite on all the X-car bolts, and torque them to some specified amount (might have been 80 ft-lbs). But even with that it still kept going out of square. (I sure didn't like that Lock-tite when I started to disassemble the X-car the other day. I had to use a wire wheel on each of those bolts to clean them up and get rid of all that gunk.)


    My secondary reason was to eliminate the little bit of sag that I got with my big spindle in the middle of the gantry. It contributed some to my problems with V-carvings.

    I had not planned to increase the Z-height until you mentioned it. And now, after looking at it, I have decided not to mess with that.

    Before measuring how long the 2.5" x 1.5" steel support tubes need to be, I had to square up the gantry after removing the aluminum supports. I had the two rails 22" apart at each end of the X-car, but I couldn't get the framing square to square up from the angle iron to the aluminum plates clamped to each end of the gantry. Then I discovered that both of the angle irons were bowed in, by about 1/8" on each one. So I wedged in a spacer, to force the middle spacing to also be 22". And, luckily, the bow was evenly divided between the two angle irons, and each angle iron then read flat along the side. And then the framing square squared from each angle iron to the aluminum plates. After getting it close that way, I fine tuned the squaring with two diagonal tape measures. Then I clamped the gantry into place. I'll double check it in the morning, then measure the spaces into which the two rectangular support tubes will go.

    Chuck


    Attached Images Attached Images
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

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