what size rsj were you planning to use and how long?
05-08-2012 #52irving2008;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
I'd use the box section.
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
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...
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
Once you include torsional stiffness in the calculation the single box section is much better than RSJ:
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.
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.!
The Following User Says Thank You to JAZZCNC For This Useful Post:
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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