Why not use unsupported rails?

[Swarfing]

Your right john, it would be interesting to see what the equivalent would be of unsupported against...for example would 50mm unsupported be the same as 25mm supported?

The other thing to consider in all this is that one uses open bearings and one closed, what deflection do you get off those?

A lot of the British machines are using designs from eons ago with very little change which is a shame as the ones from afar seam to at least try new things. Now Mary Portas has the right idea of getting Britain manufacturing again. I'm sure we would we streets ahead if only we got our hands dirty. But for now we will have to stick with pants over here....

[JAZZCNC]

There are so meny variables when building a machine to it's accurecy. All this talk of unsupported or supported, profiled etc is a bit lost and pointless on 95% of DIY builds because of other areas of the machine that are completely overlooked or under estimated.!! . . .The BED and Z Axis being two of the main one's.

Most on here will Know by now I'm not into Calc's or simulated predictions so my view is based purely on personal experience and for machines over a certain length or width I just would not recommend or use unsupported rails. This Max length and width would be 300mm for X & Y Axis and I would NEVER use them on a Z Axis.
I've only ever made one Z axis this way and it was by far the weakest area of the machine and big regret, but alas and as is often the case it was down to cost saving as usual.!! This is why I URGE any one building to just spend the few extra quid and save them self's this problem and go for supported rails.

Profiled rails are very misunderstood.?? You don't build a machine with profiled rails for there strength, infact they would probably show a tad weaker than supported round if simulated like jonathan did.?
You use profiled rails for there accurecy, smoothness as well as there abilty to handle far far higher loads and in DIY usage for millions of years.!!
To be honest you don't have to be an engineer or an expert to see and feel the difference between round rail and profiled, the quality just ouse's out and by comparison round rail, supported or unsupported, are like baggy fanny's in comparison.!!

Another often unknown thing about profiled rails is there intolerence of poor design, shoddy materials or poor workmanship. They will bind and lock with the slightest of miss aliagnment or uneven surface so this again is the test of a well built machine.

John S test is a great test and if you want to test the accurecy and more so the repeatabilty then cut the Aztec calendar because this will soon show you how well your machine is performing. The Aztec calendar as very fine detail and with 855,000 lines of code It boucnce's about like a pogo stick so if your machine is out in the slightest degree it will lose position and show any error when recutting so removing any detail.

[Jonathan]

The purpose of this simulation is purely to compare rails, not to say one type of rail is better than another in all circumstances, clearly each of the 3 main types of rail has its place.
What the supported rails are mounted to has to be included, as nobody would mount them without any support, however including other factors such a bed strength is unnecessary complication and so should be considered separately. Clearly when designing a real machine you must consider how all the parts interact, so ideally one would simulate the machine as a whole. However since the rails are a major source of error it is reasonable to look at them independently to help with selection.

The main advantage with profile rails is they can tolerate high preload, so the stiffness is much higher and there is no play (so long as the preload is not exceeded), which is why I will soon replace my round rails. Round rails have much less, if any, preload so the deflection of the bearing block relative to the rail is non linear and greater. In the case of just supported rails the load rating/stiffness varies depending on the direction of the force which is a significant disadvantage if not mounted 'opposing'. 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? Not many as for the vast majority of machines it's excessive.

Your right john, it would be interesting to see what the equivalent would be of unsupported against...for example would 50mm unsupported be the same as 25mm supported?

Depends what you mount them to...I've run simulations to find the equivalent in supported rail of 30mm unsupported. Since you suggested comparing 40mm extrusion I have used 40mm *light* aluminium extrusion. Same forces as before.

Table, Ratio colum shows deflection relative to the equivalent loading on 30mm unsupported.



Pretty pictures attached:

Simulations.zip

So the 30mm unsupported rail is similar in bending to the SBR16 rail on weak 40mm extrusion, yet currently using the cheapest prices for each I can find the 30mm system costs 70% more. That's for the rails, bearings and end supports, or supported rails, bearings and extrusion. That implies that now the prices of SBR (and TBR) rails have decreased significantly since 2eopoz bought his it is no longer makes economic sense to use unsupported. The exception of course is a small machine (Jazz's 300mm estimate sounds reasonable) with lower forces, or laser and plasma cutters.

The thing to do now would be to run the simulation with two rails as that is more realistic, however I don't think it will tell us anything new.

[JAZZCNC]

The accuracy of a profile rail is not much different to a round rail -

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

Just the fact the bearing blocks are ground flat and true on profile bearings makes a massive difference. Bearing in mind 90% of round rail is cheap chineses tuff I've yet to see a round rail bearing block that's flat on top with sides machined 90deg. Only the very best brand quality are any where near and these can cost the same as profiled.

Then we get to the actual bearing it's self, the round rail bearings are sloppy from the begining just use a dial gauge and apply side pressure to see that, you won't find this on profiled bearings due to preloading and the precision machining of the rail.

They are night and day apart.!!

[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.