SBR12 Linear Rails

[RickyBurton]

Hello everyone,

I'm currently designing my CNC on a budget and got most of the parts, the only problem I have is that I have been reading lately that I should be using SBR16 minimum, even tho I'd already brought SBR12 off a friend, so came up with a possible solution.
so the x-axis is going to be 700mm long on the linear rails, and the y-axis is going to be a metre.

the machine will mainly be cutting wood carbon fibre and plastics but will occasionally be wanting to cut aluminium too

the possible solution is too double up on the SBR12 as I have 4 700mm for x-axis and two 1000mm for y-axis upping to 4 if needed.



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[routalot]

I will be watching this thread with interest as I have just fitted the SBR12 rails to the Z axis of my homebrew router.

[NIC 77]

I realize that everyone's budget is different, and I'm OK with that. Making a CNC machine can be different things to different people. As long as what you make meets your expectations, then you are happy.

I have just heard so many bad things about SBR round linear rails for CNC machines.

I have to wonder if the MGN12 (square type) rails would be better than the SBR12 round rails. The MGN12 can be found for cheap on EBay as well, and I have been looking at them for a 3d printer application.

Yes, you can double up on the rails if you have them already. Shouldn't be a problem. You just have to be careful about the alignment when you drill your mounting holes. The way to do this is to mount the plate that your bearings are attached to for both rails, and use clamps with some wood blocks to position the rails, then you can see that the movement is smooth before you drill guide divits to help you position your through holes that you will tap. That's just one way that works.


As to whether 4 of them will work to your expectations, that I can't answer, but it would be interesting to find out your thoughts after the fact if you do this.

[RickyBurton]

routalot have you tested your z-axis yet?

And thanks, NIC 77 basically I bought the SBR12 rails cheap £20 for 2 1000mm, 4 700mm and 2 475mm. so not a bad deal. could you link me to some of the information on why they are bad id be interested in reading the issues faced?
I understand there are issues with the bearing block upright 100% load side on 70% and 30% don't hold me to the figures I've read it somewhere and I was possible thinking on the y-axis of doing a mixed setup of linear rails to accommodate for the multi-directional force.

Y axis

[routalot]

I haven't tested my Z axis yet.I'm almost there with the basic construction and need to deal with the electrics now.In fact this afternoon I hope to get the limit switches located.My experience may not mirror yours as I am building a very small router for model making and have no daydreams about moving vast amounts of metal at high speeds for days on end.

[RickyBurton]

Great have to let us know how you get on with your build, models what kind?

[NIC 77]

And thanks, NIC 77 basically I bought the SBR12 rails cheap £20 for 2 1000mm, 4 700mm and 2 475mm. so not a bad deal. could you link me to some of the information on why they are bad id be interested in reading the issues faced?

I've never used them. It's from what I've read on the forums. For a router, good quality square rails are the best choice, size 20 or 25. My current build is using size 25 Bosch Rexroth rails, and the quality of them is fantastic. I got them at a discount but it was still a fair amount of cash for my financial situation.

As far as I understand it, they are more prone to slop / play / less rigidity in the movement. Perhaps someone who has actually used them can give you some better insight.

I've seen a few people toss their supported round SBR's and upgrade to good square rails in these forums. Never heard anyone say they were happy with them. You'll have to do some searching on here. But it's apples to oranges, because people buy those crappy 6040 machines all the time, and those use round unsupported rails, which is worse than the round supported ones. So it just depends on your expectations and whether your machine will meet what you expect. That can be a difficult question to answer for an outsider, and like I said I have no first hand experience with the SBR rails, let alone the size 12 ones.

The question is how much work and expense are you planning to put into this? Because if you are going to spend the next year building a fantastic machine, then it isn't worth it to use cheap components, but if you are planning to knock this out rather quickly, and have it as a functional first machine that works and is a good learning experience, then use what you have and do it as cheap as you can. There's no point in going half way in either direction. You may end up building a 2nd one that is better later on. What are you planning to use as a drive system? Rack and pinion, ballscrew, belts, leadscrew?

It sounds like you are planning to make the longer 1000mm length as your gantry. This goes against convention, as you normally want to make the gantry the shorter of the 2 axis. This is because the deflection of the gantry is a function of the length cubed. I am breaking this convention on my build because I want to be able to load and unload some specific things I want to make easily from the front of the machine. I have compensated for this by using a robust gantry structure based on a large steel tube.

Your question about doubling up on the rails, yes, you can do it. Your alignment between the two beams will need to be better, but this can be overcome with some conformal (epoxy) shimming or stainless steel shim tape in practice when you are making it, so it is totally possible to add more rails, it is just more work on the fabrication side.

[routalot]

Well I did make a start on my electrics,but am far from completing the machine.In the interests of curiosity,I fitted a short piece of steel rod to the motor coupling and ran the axis to its limits using a battery drill as a driver.No problem with movement and no slack evident after half a dozen runs.Seems totally rigid and I started to wonder about the nature of the problems people have experienced.I have to wonder whether they were using just a single pillow block sliding on the rail and this could be the cause of the slack.The Round portion of the rail is hard steel and I would expect the ball bearing to be pretty hard too.I have two pillow blocks on each of my rails at 150mm centres and the two rails are mounted on 130mm centres,so four points of guidance and support.

The machine is intended for models of boats and cars and won't see much use with anything harder than modelling block.Of course I might find a need for a bigger and better machine in the future if this one is a success.