Z-axis with small 9mm carriages and linear rail (NSK)

discussion of design changes / improvements / suggestions
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orangezero
Posts: 76
Joined: Sun May 12, 2013 8:35 pm

Z-axis with small 9mm carriages and linear rail (NSK)

Post by orangezero » Mon Dec 07, 2015 9:23 pm

Is this a bad idea? I have some 200mm long, 9mm wide linear guide rail from NSK, with matching 9mm carriages (two per rail).

From the specs, it appears they have a dynamic load rating of 1490 N and 2150 N for static load for each carriage (photo attached). I have tried to find some comparisons to see if they would fit a shapeoko-type DIY machine.

RM2zz wheels: static 8260 N dynamic 2650 N
SBR16UU bearing blocks: static 774 N dynamic 1180 N
625 bearing: dynamic 1750 N
608 bearing: dynamic 3300 N

I haven't found any comparisons with a delrin wheel directly (dual bearing combo) so I thought I'd ask some who have already dealt with similar situations. I was surprised at the difference compared to a 16mm round rod bearing...

So, when trying to make a stiff, short Z axis, would these 200mm rails be appropriate? I would be using a nema 23 (around 250oz holding) attached to two 9mm thick aluminum 6061 plates, and a Makita 701c trim router (around 4lbs). It seems they are similar to the 625 bearings in load capacity, but would perhaps provide better stiffness due to their shallow mounting possibilities? The rail and carriage are maybe 10-12mm in height, just enough for my nut/screw to fit.

I already have the rails sitting on my desk, so it isn't about "buying the wrong ones." Another project fell through. I would have preferred the 12mm or 15mm rails, but the deal was too good to pass up. I assume these aren't as common because of the typical cost. Mine are official NSK and seem quite sturdy for their size. The closest comparison I could find was the pocketnc, which uses rails (but a blog update shows they upgraded the size before production).

A second set of rails (so two on each side of the screw, eight total carriages) would increase the stiffness, but is this overkill for most purposes (wood, acrylic, maybe light cuts of aluminum)?

I'm also asking because I may end up using them for a small, portable, cnc machine at some point in the future. Originally they were for a small 3d printer where heavy loads were not an issue. I don't want to ruin them if I'm going to be approaching their breaking point. As you may tell, I'm not an engineer, or I'd just solve the equations. :) Thanks.
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AnonymousPerson
Posts: 758
Joined: Sun Apr 26, 2015 1:16 pm
Location: 3753 Cruithne

Re: Z-axis with small 9mm carriages and linear rail (NSK)

Post by AnonymousPerson » Thu Dec 10, 2015 2:39 pm

Finding it hard to visualise what you're meaning. :(

Are you able to take photo's or something of the bits you have, to try and illustrate better? (apologies if I'm an idiot :))
Shapeoko 3 #516

Jimf
Posts: 233
Joined: Wed Feb 11, 2015 8:58 pm

Z-axis with small 9mm carriages and linear rail (NSK)

Post by Jimf » Thu Dec 10, 2015 5:25 pm

Profile linear rails can handle tremendous loads when properly mounted. Even smaller 9mm ones such as your NSK. They are the best type of linear rails to buy. Use them.

Always use atleast two bearing carriages per rail. The linear rail has lots of mounting holes, use them all for proper load handling. The rails must be perfectly aligned parallel to each other or else they will bind. This makes profile linear rails much harder to install correctly. Some bearing carriages do not have the ball bearings captured and will fall out if you remove them from the rail. Check with datasheet to verify.

I use plenty of profile linear rails most from THK but NSK are just as good or better. IKO is another brand that is very good. Very expensive if you had to buy them new.

samc99us
Posts: 270
Joined: Mon Nov 11, 2013 5:20 pm

Re: Z-axis with small 9mm carriages and linear rail (NSK)

Post by samc99us » Wed Dec 30, 2015 4:16 pm

Go with these and find a matching ballscrew. It will take some effort to design all the mounting plates. Recommend you attach the rails to the actual spindle carriage and the carriages to the x-axis effectively.
Modified Shapeoko 2: 1500mm fully supported y-axis, 1000 mm 4060 x-axis, ACME Belt Drive Z-axis, Dewalt DNP611, full aluminum t-slot table, 4x NEMA23 180 oz/in stepper motors driven by a G540

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