Thread: Why not use unsupported rails?
As soon as you properly fix down a profile rail it wont move, which is clearly an advantage, however if the rail is bent to start with you'll still have a bent rail.
I just checked the straightness of two profile rails and a SBR20 rail on my surface plate to check. Took readings 3 times for each over 400mm length in different places so should be reliable...
SBR20-400mm:0.03mm height variation.
Hiwin 15mm (760mm long): 0.06mm variation.
NSK 20mm (520mm long): 0.05mm variation.
The hiwin catalogue says +-0.1mm for normal precision height, but I don't think it states weather that is +-0.1mm for random rails, or +-0.1 over a single rail.
This agrees with my saying the the straightness is similar, but absolutely no reason to think SBR rails are better as they're clearly not!
Also your not 100% wrong about bent being bent just 50% wrong.???
Profiled rail won't tolerate any bend if it's to work correctly but that doesn't mean they won't come with a very slight bend to them over there length.? Hense the ground edge's so it can be registerd against a reference edge.
They are designed to flex ever so slightly so they can be manipulated perfectly straight and also why you have fixed master rail and a floating rail that gets adjusted by moving the gantry of what ever the bearings are attached too slowly across the rails length while tightening the rails fixing bolts, so tweaking out any bend or tight spots.! This is also why most profiled rails have 60mm centres between the holes.
Now if round rail is bent then it's bent and your stuck with it without resorting to off machine straightening. Often thou it's not the rail it's self but the rubbish base that's bent or twisted and often not flat on the mounting surface.
Profiled and round are very different beast's so can't and shouldn't be compared they are not in the same league.
I didn't fasten them down as the purpose of the check was to show that even before you mount the rails they are bent (I measured straightness for the profile rails) which is clearly true. The test was not intended as a realistic indicator of height accuracy in operation, as clearly either will conform to the mounting surface.
Sliding the gantry back and fourth and tightening the bolts to tweak out any tight spots also wont get the rails straight. It will merely copy any bend in the master rail to the subsidiary rail. Very few DIY builds rest the master rail against a reference edge, or indicate it to get it straight, hence for those any bend in the rail as shipped will remain (in one plane). For example if I just bolt down one of my 15mm rails to a piece of nice flat aluminum tooling plate, then use the sliding method to align and fix the other rail to the same plate, the error I measured will still be present so the measurement is relevant. That's what most people do and it's perfectly valid for supported rails.
I get the feeling Jonathan you haven't ever used profiled rails before.? Because if you had you wouldn't be so quick to argue with me about the bennifit of the master rail and having a straight reference edge. You would also know just how critical and intolerent the bearings are of miss alignment and poor mounting surface.!
If you have then you know I'm correct and pride is just making you deliberatly argumentative is all I can think.!!! . . . . either way your very wrong.!
I did not say they wont be accurate if mounted properly. I'm merely pointing out that most people don't mount them properly, and thus don't obtain the best possible accuracy. Nowhere have I said it's a good idea not to mount them properly.
We both already knew everything either of us has posted in this thread, so the only purpose in discussing is to let others know. Spreading the knowledge is the whole purpose of this forum, hence I discuss.
Your lasts post seems to be suggesting the reference edge of the master rail is perfectly straight? Which is surprising, as this simply is not true. The point of the reference plane on the rail is that is it parallel to the grooves the balls run in - you can't easily indicate off the grooves but can use a plane. You have to first straighten the master rail when mounting by attaching a bearing block and indicator and using a genuine straight edge as the reference for the indicator, or by clamping the reference plain to the straight surface.
Hiwin's document on the subject explains it all perfectly clearly, well worth a read:
I've read the Hi-win document before and like wise fitted many hundreds feet of profiled rails, like tomorrow when I'll be drilling and tapping 160+ mounting holes for the 10mtr's of profiled rail so I feel I'm more than qualified to comment on all aspects of fitting them.!!
All that said and my original comment was that your statement that round rail and profiled are roughly the same regards straightness is incorrect. . . BUT for clarity's sake I'll ammend my statement to read "Rubbish total and utter rubbish" . . IF CORRECTLY FITTED.!!
Now lets go to bed it's 1 am. . :whistling:
The Following User Says Thank You to JAZZCNC For This Useful Post:
You have both covered this topic very well and highlighted very good points, so thank you as i myself have taken away some knowledge from this discussion.
Keep up the good work guys as this is exactly what its all about - Education !.Me
Woooo who swallowed a dictionary then.!!!. . . . . Not me thats for sure. .:rofl:
Nah ME argumentative for the sake of pride. . .NEVER . . . Your wrong. . . . . . . I'm right thou has those who have ever fitted them "Correctly" will agree and wouldn't argue. :naughty:
Okay thats better. Jazz and jonathan had been agreeing far too much recently.:heehee:
BruceThe more I know, I know, I know the less. (John Owen)
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