Thread: Why not use unsupported rails?
Our bouncy friend is correct in saying that we should also consider the worst case examples for supported rail.
1) Supported rail rigidly supported at both ends but without any underlying box section.
2) The deflection of the underlying box section sans rail.
Well, 1) gives us some idea of the contribution of the basic rail support structure and 2) gives us some idea of the contribution made by the box section itself.
I would love to see a similar analysis of profile rail. Just how significant is the support structure? 20mm plate, 40mm and 90mm section, aluminium and steel. After all, £600 worth of rail is meaningless if the £100 worth of extrusion allows deflection on a par with round rail.
Last edited by ecat; 01-04-2012 at 10:18 AM.
01-04-2012 #12Obviously not a great one since your using unsupported rails
now if you were going to span a 20mm rail over 1.2m then you are asking for trouble. It is all about using the right thing for the right job so comparing the same size of rails against each other is pointless.
As for my job i do system engineering not mechanical but my rail was spec'd by an mech engineer which is why it works.If the nagging gets really bad......Get a bigger shed:naughty:
Well we had better start looking on Ebay for some real cheap routers that only need upgrading to supported rails.
That will cover most of the Techno Isel range, Hiez, and all the Brit ones.
I'll grant you supported rails will be far stronger than unsupported, it doesn't take a white paper to work that out [ unless it's toilet paper - non coloured ] but how about doing a paper on what is allowable in a DIY enviroment.
Has any one else ever mapped the deviation on say an X rail as it travels full length due to bed twist, and then done the some on the other rail as it returns back?
Those results will be very interesting. A quick and dirty test on any machine is to do a very fine engraving in brass then run the code again and see what extra metal is removed on the second pas or if it's thinned the engraving down. This is a very good PRACTICAL test.
John S.John S -
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....If the nagging gets really bad......Get a bigger shed:naughty:
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.
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.
Table, Ratio colum shows deflection relative to the equivalent loading on 30mm unsupported.
Pretty pictures attached:
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.
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.!!
Last edited by Jonathan; 01-04-2012 at 08:04 PM. Reason: Wrong image
Actually Jonathan you have now shown a fairer account of the simulations, this shows a more realistic picture so well done to for that. The deflections shown though in the grand scheme of things show very little in the way of movement between them all looking at those figures. Now if i were to build again i would 100% go for linear rails supported on the biggest bases as possible, in a small desktop machine for pcb and plastic work then round rail would be fine (only if it was the cheapest and only option).
Thing is a lot of people expect their routers to be milling machines as well, only in this case you would need to worry about expensive linear movement. For most diyers the need to be honest about what they want to use there machine for and build accordingly. If they can not afford that then they have to live with the compromise or save up and wait a bit longer.If the nagging gets really bad......Get a bigger shed:naughty:
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.
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