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  1. #1
    Join Date
    Dec 2007
    Location
    Melbourne
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    80

    Default Making a Steel Beam Flat

    Might be a question for the CNC Forum but I thought I would try here first.

    I am thinking of making a CNC plasma cutter that can take a full sheet (2440 x 1220). The X Beams will be about 3100mm long and will be something like PFC or I Beam to minimise deflection. The X Rails will be Vee Rails and will be bolted to the X Beams. I am trying to work out how flat PFC / I Beams would be over a 3100mm length but I think it will be less than I would like! - anyone know? Whilst I can shim the rails to some extent, I would like to start off with a "reasonably" flat beam. What are the options to take a length of PFC / I Beam and make it flat? I was thinking surface griding but I don't think anyone has a 3100mm bed surface grinder (or do they exist) and I am unsure of the practicalities of feeding in a beam and doing a short distance at a time. Are there any suggestion on this? Sorry about the vagueness of this question but it is early days on this project so I am still trying to work out what is possible / practical.

    Grant

  2. #2
    Join Date
    Aug 2008
    Location
    Oatley NSW
    Age
    64
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    254

    Default

    I don't know how flat and parallel PFC or I Beams are, a quick google didn't bring up any answers as well. To find out you may need to measure some using Laser type sensors to get the accuracy. What accuracy are you wanting. Another way would be to find an Engineering Shop that has a large Brake Press and get them to fold you some PFC type section out of suitable thick steel. It would I feel be straighter than the hot rolled process of standard PFC.
    The other considerations would be when you make this is how flat/level your floor is, how you will level the machine bed in relationship to the floor, design of the bracing for the bed. A lot of questions that will need to be considered.


    Keith_W

  3. #3
    Join Date
    Dec 2007
    Location
    Melbourne
    Posts
    80

    Default

    Thanks for the reply Keith_W. I am just starting this adventure and I have far more questions than answers!!

    I am assuming that for a plasma cutter the accuracy required would be less than for a spindle cutting CNC, particularly the Z axis. Having said that, I am not sure what accuracy I am looking for yet. I am basing my design on a six leg frame and each leg has an adjustable foot so that will take out most of the variances in the concrete floor. I was also thinking about having the frame bolt together to give some adjustment (but it would probably be at the expense of the overall rigidity of the frame). I am unsure whether a folded PFC would deflect too much - if I am reading my tables correctly I ill be looking at some 180 x 75mm PFC.

    Grant

  4. #4
    Join Date
    May 2011
    Location
    Murray Bridge S Aust.
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    66
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    3,371

    Default

    Pardon my ignorance, but what is PFC? I've been involved in the metal fabrication industry for 40+ years and have never heard of it before.
    Kryn
    To grow old is mandatory, growing up is optional.

  5. #5
    Join Date
    Dec 2007
    Location
    Melbourne
    Posts
    80

    Default

    PFC = Parallel Flange Channel

  6. #6
    Join Date
    Mar 2014
    Location
    South of Adelaide
    Posts
    490

    Default

    I can't remember the specs off the top of my head, but the allowable tolerance on hot rolled sections is pretty wide. We used to have a job at work machining slots into 200ub, the flanges were normally 3-4mm out of parallel, 3-5mm offset and overall beam depth 2-4mm on the imported beam. We sourced one batch of beam off onesteel and it was a lot better, it think it was maximum 2mm out. If i was making a plasma table i would use RHS, it is a lot more dimensionally accurate ( my guess would be max 1mm out over that length). You should be able to just shim your rail without machining if you use rhs. To get anything 3.1m long machined is going to be $$$, time on machines that size isn't cheap.

  7. #7
    Join Date
    Nov 2008
    Location
    Qld. Australia
    Posts
    780

    Default

    I have some PFC here and I remember it is very convex across the top. Not sure about length, as the piece I have is less than a metre long and don't think I checked that, or how parallel the flanges are.
    Nev.

  8. #8
    Join Date
    Apr 2013
    Location
    Toowoomba Qld
    Posts
    350

    Default

    Not I-beam or PFC but there are some specs for squareness, straightness & twist for structural tube & pipe, one would think PFC or I beam would be similar although some of the comments above suggest it may not be as accurate
    http://www.orrconsteel.com.au/sites/...-Catalogue.pdf
    Pg 29

  9. #9
    Join Date
    Feb 2013
    Location
    Laidley, SE Qld
    Posts
    746

    Default

    I've made these sorts of frames in the past. For what you are doing I would avoid I beams and channels and weld (not bolt) 150 x 50 RHS into a suitable size base frame (3100 x 1700?). For the loads the base frame will hold, 3 adjustable feet along each 3100 member will suffice.

  10. #10
    Join Date
    May 2011
    Location
    Murray Bridge S Aust.
    Age
    66
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    3,371

    Default

    Thanks for that information. A bit like I beam and Universal Beams. PFC is that made by putting through a roller system, similar to Lysaght's rolled sections.
    It takes a real good shop that can custom make PFC, I used to get it made for a roller drawer system that I used to make, as a bearing had to run inside it.
    There would be anything up to 3 mm difference end to end or in the centre. The pressings weren't square, either over or under, sometimes both on the same side.
    I ended up getting a steel block to the correct size and forcing that down the channel so that the bearing would run in it without getting tight.
    Kryn
    To grow old is mandatory, growing up is optional.

  11. #11
    Join Date
    Nov 2004
    Location
    Port Pirie SA
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    46
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    808

    Default

    Just an idea, what if you used some 3m long straight edges as your bearing surface? drill and bolt them to a frame so there's no warping.
    They are not cheap Blackwoods want $727ea plus GST if you buy two, they are 8x50mm
    precision ground on all sides one side being 35 angle, they should be within a few 1/10th of mm over the whole length.

    Or you could just buy one and use it as a master measure to shim your beams/tracks straight!
    ....................................................................

  12. #12
    Join Date
    Feb 2003
    Location
    formerly from Sydney (north of The Harbour), NSW, Oz
    Age
    63
    Posts
    280

    Default

    beside the issue of how flat the beams are, you also have to consider the foundations.

    The typical shed floor slab may not be as level and strong as you would prefer for a CNC.

    Another consideration is the accuracy of plasma cutting. How much edge clean-up is expected after the cut?

    Then what is the weight of the material being cut and the cutting bed. I recall seeing a plasma cutter operating in a water bath -- i.e. the material being cut was covered by about 5 cm of water. Your foundations and frame need to support this load as well.


    All up, I suspect that provided the material you adopt has enough "meat" you could get the frame supporting the material being cut "flat enough" with an angle grinder and laser level. But when doing the grinding, the frame and it's supports would need to be loaded with the design load.
    regards from Canmore

    ian

  13. #13
    Join Date
    Aug 2008
    Location
    Oatley NSW
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    64
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    254

    Default

    Lot of good responses with practicable suggestions that's comes from experience.
    The size of the sheet was mentioned but not the thickness which would have a huge bearing on the type of frame structure that has to take the material weight. The type of sacrificial bars in the middle for sheet support needs to be considered as well. I looked at a few sites that makes Plasma Cutters and Tables to get an idea of how they go about it, I suggest that you do the same and copy their ideas as they have already ironed out the bugs so to speak.

    Keith_W

  14. #14
    Join Date
    Dec 2007
    Location
    Melbourne
    Posts
    80

    Default

    Guys,

    Thanks for all of the information, thoughts and suggestions. As I said, this is just the sart of the project and this sort of feedback is invaluable. Some answers and thoughts:

    1. I have purchased a 80A plasma that I hope to use for this. Although its stated capacity is 30mm, I don't realistically expect to cut anything thicker than 12mm part sheets and 6mm full sheets. For planning purposes I am working on a 200kg sheet capacity. It will be a water table so there will around 300kg for that. Plus the weight of the actual frame. I am not too worried about the structural integrity of the frame - I have a Mech Eng at work looking at the steel sizing and that is where I got the three legs and 180 x 75mm PFC for the X beams.

    2. Good point re the floor in the shed where it is going. I think it is a 150mm reinforced slab but I will need to check.

    3. I am interested in the concept of using RHS. This was my initial thought but I am using a V rail with a rack gear underneath (so the rail and gear are always aligned and parallel) but couldn't work out an easy way of attaching the rail to the RHS. Nuts and bolts seemed a bit tricky and I was doubtful whether the RHS would be sufficient to drill and tap. Interested on thoughts about this. The rail overhangs the beam and the rack gear attaches to the raim from undereath. Might have to put up a drawing of this!

    4. Had been looking for a welder to do the welding (obviously) but most were "hobby" welders which I am sure were OK but I put myself in that category and I not sure I am up to it, particularly minimising warping. Met up with a welder (with cerificate of competency to prove it) yesterday who run me through the process he would used and showed me some of his completed jobs and I was most impressed! Importantly, most of his work is built to tolerances and he showed me how he measured the final product to ensure it was within specification. A bit higher tech than I had thought. Hourly rate not too bad and nearby. More impotantly, my welding ability won't be put to the test!

    5. I went to NMW a month or so ago and had a look at the CNC plasma cutters there. Most that were in a similar size range followed a similar design that I used as the basis for this project. I have looked at some of the machines that have been DIY built (although most are spindle designs) and they follow a similar style (ie MechMate, Joe's CNC etc). There are also some similar commercial designs (Swift-Cut etc). I am trying to track down someone that has built a MechMate / Joe's "full sheet" design that uses V rails and rack gears (rather than ball screws and supported sliding rails) to take a look at one in the flesh so if anyone knows of one in Melbourne that would be ok with a visit let me know (bonus points if it is a plasma cutter!!).

    6. Will drop in to my local steel yard today or tomorrow to eyeball some of the beams they have. PFC doesn't appear to be that common in the size I think I neeed but plenty of I Beams that size. Will also check out the RHS.

    I am sure I will spend more on this than getting a commercial plasma cutting service to do all of my cutting for the rest of my life but where's the fun it that?

    Thanks for all the comments & suggestions - keep them coming.

    Grant

  15. #15
    Join Date
    Aug 2008
    Location
    Oatley NSW
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    64
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    254

    Default

    Your Mechanical Engineer should be able to calculate the size RHS you would need and the wall thickness. Depending on what you will be bolting on either get the RHS with a thick wall or you could weld a piece of Flat on to drill and tap or use Nut Inserts.
    The welder you speak of sounds good and he will get it pretty square for you, in the final fit out a shim here or there maybe all that would be needed to get on exact size.
    Take plenty of pictures on your build so we can see your progress.

    Keith_W.

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