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  1. what size rsj were you planning to use and how long?

  2. #52
    irving2008;33697]what size rsj were you planning to use and how long?

    On the gantry the rsj is 178mm x102mm x19mm and is 1720mm long

  3. I'd use the box section.

    Reasoning:

    a) 1.7m of RSJ @ 19kg/m = 32kg, whereas 1.7m box section @ 7kg/m will weigh 11.9kg, thats nearly 2/3 less to move around...
    b) RSJ will deflect .004mm in centre under its own weight plus 0.0004mm per kg load (i.e. negligible), Box section will deflect 0.018mm under its own weight, plus .004mm per kg load. So a 10Kg z-axis will deflect the rsj gantry .008mm and the box section gantry .058mm. In the scheme of things this is negligible.

  4. #54
    Quote Originally Posted by irving2008 View Post
    I'd use the box section.

    Reasoning:

    a) 1.7m of RSJ @ 19kg/m = 32kg, whereas 1.7m box section @ 7kg/m will weigh 11.9kg, thats nearly 2/3 less to move around...
    b) RSJ will deflect .004mm in centre under its own weight plus 0.0004mm per kg load (i.e. negligible), Box section will deflect 0.018mm under its own weight, plus .004mm per kg load. So a 10Kg z-axis will deflect the rsj gantry .008mm and the box section gantry .058mm. In the scheme of things this is negligible.

    so even though both are negligible as you put it.. you would still steer him clear of the easier/cheaper option of an rsj that needs little work over him welding box section and possibly leaving room for him to create errors...

    or have i mis-understood... i'm watching this thread as the thought of rsj being an easy way compared to welding box section appeals to me

  5. Quote Originally Posted by wilfy View Post
    so even though both are negligible as you put it.. you would still steer him clear of the easier/cheaper option of an rsj that needs little work over him welding box section and possibly leaving room for him to create errors...

    or have i mis-understood... i'm watching this thread as the thought of rsj being an easy way compared to welding box section appeals to me
    Not sure i understand your logic... given a lump of i-beam or a lump of box section, you're still going to have to drill holes to mount rails etc. He was suggesting the rsj for the y-axis element of the gantry... not sure what welding is needed???

    Also he says he can get a section for a tenner... the cheapest 178x102x19 rsj on ebay is 19.50/m min order 4m.. plus carriage...

  6. #56
    ok maybe i've mis-understood the whole thing... i was under the impression that if he used box section then he would have to do some welding or atleast joining 2-3 pieces where as the rsj was a solution that didnt need any work other than attaching other parts?

    my thought are that he is using 2 pieces of box to make what would be the top and bottom of the beam... but i can now see you are talking about one big piece of box the same size of the rsj... sorry my bad

  7. #57
    Once you include torsional stiffness in the calculation the single box section is much better than RSJ:

    Click image for larger version. 

Name:	box section vs rsj.png 
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    The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
    Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  8. #58
    Quote Originally Posted by Jonathan View Post
    Once you include torsional stiffness in the calculation the single box section is much better than RSJ:

    Click image for larger version. 

Name:	box section vs rsj.png 
Views:	1752 
Size:	47.3 KB 
ID:	6517

    The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
    Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.

    Thank you for that, I thought that was what was said earlier, which is why I posted what I did, however I didn't realise rsj would be just as in accurate.

    How do you join the 2 pieces of box out of interest, if its welding surely this can go just as wrong no???

  9. #59
    Dean if your intention is still the same has before to only cut wood then ignore Jonathan he's talking over complicated bollocks again and it will easily do the job.

    If you can get the Large box section for a tenner then obviously it's a no brainier and go for it.! . . Not disputing Box section won't be stronger but no where near has cheap or easy unless you can drop on something like this.

    The RSJ will do the Job no problem the extra mass will help with cutting but does cost in acceleration and speeds.

    I would how ever change the way you have the X axis bearings and use wider plate than the RSJ which bolts to it. I would then have gantry end plates which bolt to the RSJ and the bearings plates dropping down so the ballscrews can connect. This will help stiffen things up a bit.!! . . . Pretty much like I drew before.!

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  11. #60
    Quote Originally Posted by Jonathan View Post
    The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
    Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.
    Rubbish.!! What makes you think both these will be flat surface or straight .? Single or 2 pieces make no difference the surface will still need some work and while 2 pieces will allow easy general positioning they won't be anywhere near parallel all along the length. If the section has any bend then it will still need grinding or shimming at several points and just because you have a rail on each piece doesn't make it any easier.
    It's not difficult with a decent straight edge and files,grinder to the bring the main reference surface upto scratch then it's just a case of shimming or grinding the other rail surface parallel.

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