Why not use unsupported rails?

[Jonathan]

Sorry Jonathan that's Rubbish total utter rubbish they don't compare in any way.!

Sorry I should have been clearer, I meant the accuracy of the straightness of the rail, not the tolerances they are machined to. Obviously I agree that overall profile rails are machined to much better tolerances and thus by far the most accurate rails. Round rail bearing blocks and supports are merely extruded.Sloppy - i.e. clearance not preload which is exactly why I'm replacing mine. Simulation of a realistic gantry, same as in first post except both rails and 10mm aluminium plate stuck on the back, forces in X and Z and constrained at the ends:

[JAZZCNC]

Sorry I should have been clearer, I meant the accuracy of the straightness of the rail, not the tolerances they are machined to. Obviously I agree that overall profile rails are machined to much better tolerances and thus by far the most accurate rails. Round rail bearing blocks and supports are merely extruded.

Ok but even this is not quite correct.? They are very different in how they made and meant to be used.

Obviously unsupported are just straight bar with no support other than the ends so will flex in what ever direction the forces are applied.
Supported rails have the base support which dicate the strightness, again in my experience this base is far from straight or flat so if it bends or twist's so does the rail follow.

Profiled rail is meant to align to a registration egde on one rail. The rail is ground on at least 2 surfaces and in most case's all four. The rail actually as a special edge that's ground straight and true, often indicated with a arrow or machined grove/pattern. This edge is used to butt agianst the ref edge and is classed as the master rail while the other rails is left to float.
Now I know you know this but my point is that yes they are more far accurate just by the very nature of how they are meant to be used and all the ground square and true surfaces's. This is also what makes them very intolerent of uneven surface's and sloppy workmanship.

[Jonathan]

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!

[JAZZCNC]

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.

So what you checking here Height or straightness.? Very different things.!! Also did you fasten them down along there full length.? Doubt you did and untill you do then the height check means diddly squat.!

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.

[Shinobiwan]

SBR20-400mm:0.03mm height variation.
Hiwin 15mm (760mm long): 0.06mm variation.
NSK 20mm (520mm long): 0.05mm variation.

When comparing the parts yep but a more real world example would be that whilst installing on a machine you can further level the profile rails providing you've got the measurement tools. So those figures you've hinted at aren't set. They could even be made worse if you make a mess of the fixings which is less likely with the round rail as its only fixed both ends and not at regular intervals. Its as Jazz said, they're designed for precision and will give more of that over round rail but you have to exercise another level of precision when installing them to reap that.

[Jonathan]

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.

Its as Jazz said, they're designed for precision and will give more of that over round rail but you have to exercise another level of precision when installing them to reap that.

Which is what I said originally; "the accuracy of a profile rail is not much different to a round rail - it depends how you mount them as how many of us correct the bend in a profile rail by comparing to an accurate straight edge reference?"

[JAZZCNC]

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.

In which case they have incorrectly installed them.!!!! . . . If the master rail reference edge is accuratly straight which it should be and is the whole point of having it then the subsidiary rail as you call it will be perfectly straight.!! . . If not used then you will have incorrectly installed them and defeated one of the main points of using profiled rails.!! Accurecy

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.!

[Jonathan]

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:

http://www.hiwin.de/DownloadCenter/f...9TE14-1006.pdf

[Lee Roberts]

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.

I dont see that Jonathan has challenged the benefit of having a Master Rail or a reference edge directly, he has simply discussed the credentials of both and what those credentials will mean.

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.!

To be fair Jazz i think the shoe of pride fits the other man in the room, arguments drive discussions and that is what this is "a discussion on the fitness of unsupported rails for an application". So for me it is well done Jonathan for making full use of the forums.

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 !