SBR16 or SBR20 linear rail?

[frige]

I am considering a DIY cnc router build using SBR linear rail.
300mm/700mm/1100mm are the rail lengths I plan to use.
16mm would be 3/4 the weight of 20mm.
I am going to start with an MDF design - but may rework it with 8020 eventually.
Overall price difference between 16mm and 20mm rail is not that great.
Is there any reason NOT to go with the SBR20 instead of SBR16?
-frige

[spoiledbrat]

Since most of us are not truly engineers, we tend to overbuild things. I can think of no reason not to go with the larger rail.

In fact, would the larger rail not stiffen up your MDF structure?

I say go big. Stand by for other arguments...

Rob

[mlabruyere]

While the advice to overbuild is well understood and many engineers go this route...especially when weight is not an issue. I would not, however, recommend using your linear rails to "stiffen" the structure of your machine.

You note that you may rebuild with 8020. I started a build a few years ago with MDF but stopped part way through due to issues with the actual MDF material. I now have a fully 8020/Steel machine and never looked back. If you want to experiment with MDF great but don't push the machine too hard.

I look forward to seeing your machine in action.

[HereinCS]

I would add that if you want to use the same rails on 8020 extrusions, and you want to bolt it directly to the extrusion, pay attention to the bolt size used on your rails. For me, I looked at 25 and 35 series rails, only to find that 35 series rails need m8 bolts, which is too big for my existing 25 series extrusions.

[DonFrambach]

I just made a router using series 15 8020 extrusions and sb20 rails on one axis. I bolted the sb20 to 3/8" cold rolled steel. Here's a couple of photos:

[frige]

I pulled the trigger, and ordered (ebay):

SBR20-300/700/1100mm linear rails(6 supported rails+12 SBR20UU bearing blocks)
3pcs of ballscrews RM1605-350/750/1150mm-C7(3screws+3ballnuts+3end-machinings)
3 SETS OF BK/BF12(with locknuts and cir-cplis)
3pcs of flexible couplings 6.35mm*8mm

I also ordered the motors/g540 from cncrouterparts.
also M12VC router, and bracket (K2CNC).

No turning back now.
Total investment is going to come close to $1500 - that's all software/electronics/mechanical.

The mounting holes in the SBR20 take M5 bolts, and are 30mm apart.
-frige

[Pplug]

Originally Posted by frige The mounting holes in the SBR20 take M5 bolts, and are 30mm apart. -frige

Don't plan on the holes being perfectly spaced at 30mm. I just got a shipment of sbr20 rails 1350mm long and about 10% of the holes are off by about 3-4 mm. They look like they were drilled and countersunk by hand.

[DonFrambach]

Originally Posted by frige No turning back now. -frige

I think you made good choices.

Have fun with your build!

Don

[gio666]

Originally Posted by Pplug Don't plan on the holes being perfectly spaced at 30mm. I just got a shipment of sbr20 rails 1350mm long and about 10% of the holes are off by about 3-4 mm. They look like they were drilled and countersunk by hand.

Yes all the rails are hand drilled,i bet if you look on the back,you will see pencil lines

[Pplug]

Yup they are there! If they could only put the drill on the line it would reduce a bit of work! :^}

[gio666]

Yes and also i bet the bottom is not perfect flat too

[dixonlv]

I've been looking at these rails for a couple of months now. I was concerned that they were so cheap (compared to other linear rails) that the tolerances might not be as close as necessary. Now I'm even more concerned to find out the holes are not correctly spaced down the length of rail..
I'd rather drill them myself so I know they would be correct.
Is there any side to side slack/movement between the rails and the blocks?