picking the best linear rail

[HMD]

Hi there.Im planning to build a 5x10 plasma table with servo drive motors on each side.Need 12' long rails.Do I go with 25mm linear rails and ball bearing ?blocks or would BWC dualvee wheels and track be better?My other pick was Techno series 300 rails and carriages.Table frame is going to be 8x2x1/4 tubing so imwanting to mount the rail and rack and pion on the 8" which will be drilled and tapped.Need advice please.Thanks,Brad.

[schbrownie]

Nobody can give you a straight answer at this point until we know the answer to this question:

What kind of tolerance do you want to hold on the part you are cutting?
a) +/- 0.001" (or better)
b) +/- 0.010"
c) +/- 0.031" (1/32")
d) +/- 0.063" (1/16")
e) +/- 0.125" (1/8")

(or are you looking for something inbetween the choices listed above)

Once you have this answered, we can help you with your choice.

~Jon

[HMD]

The tolerance I would like is around .005 to .010.I talked with the people at BWC about the dualvee rails and wheels and they sayed that I should not go that route.So im looking at Hiwin HGR25R3660H for the rails and HGW25CCZOH for the blocks.Is the right way to go?For the rack-n-pion should I go with a helical gear?Need help finding the right one here also.Thanks.

[Greolt]

Your talking about a plasma 5x10. I would not be so quick to dismiss Dualvee.

The accuracy your looking for is easily obtainable with these.

As an example have a look at the Mechmate site. http://www.mechmate.com/

Obviously this is not a plasma but the principals shown are relevant.

Plasma cutting is a dirty gritty environment. Dualvee will handle this OK.

BWC are expensive. Look at this site for better economy. http://www.superiorbearing.com/

Forget helical cut gear. The engineering required to constrain the axial forces on the pinion in this situation is way beyond what the application calls for.

Standard Rack and spring loaded pinion will easily give you better tolerances than required. A good choice for plasma.

Greg

[schbrownie]

I would also check out the following websites:

http://www.linearvguides.com
http://www.pacific-bearing.com

I've found both of these companies to be less expensive than BWC. From what I've been told, BWC heat treats and then polishes their rail. Pacific heat treats, grinds and then polishes which leaves a harder and more accurate surface...all for a lower price. (Always great to get more for less!)

Something else to keep in mind, as a general rule of thumb, if you are going to design a system which you want to hold tolerance of 0.005"-0.010", you'll want your machine base to machined to a tolerance 1/10 of what you want to hold. You just need to be careful if you want to go ahead and use the V profile.

You can also try the Redi-Rail product from Pacific... they hold their system to a +/-0.001-2" tolerance and it's less expensive than profile rails. Their rails are available up to 20ft long, so the size of your table will be no problem.

If you keep the weight of your Y-axis down, you might be able to get away with a belt driven system. For tighter tolerances, you might also want to consider using a linear scale for more accurate positioning.

Good Luck

~Jon

[harryn]

Hi Jon

I noticed from your posts that you are pretty happy with the redi rail product. Do you happen to remember what size the rail was and about how much load it is under. I am toying with using the RR65 series due to a fairly large torque load potential in my build, and this one looks perfect for it.

Thanks for doing the R and D.

Harry

[schbrownie]

Harry,

I think I've used just about every size of their redi rail product in one design or another. In general, I try to keep the system at 50% of max load or lower. I know that I could go higher, but if I keep it below this point, it avoids me having to pull out my college textbooks and do all of the engineering calculations! The nice thing about this system (as well as just about any other system I suppose) is that if you have problems with loads and/or forces, you just add another carraige.

A recent change is that they now have inch and metric series and the inch is a more low cost solution, but the carriages are not adjustable and there is no wiper on the rail. I've used this for some of my hand actuated systems. The metric system is adjustable and has the wipers on the end. I would really recommend this if you're going to put it into a motor-driven system. The wipers will help push debree out of the way and help to keep your maintenance down by lubricating the rails.

They also have a new product (it wasn't in their last catalog from 2005, but is in their 2007 catalog) called Steel Rail (or Steel Linear Guides) which is very similar to redi rail, but is all steel. This version costs a little more, but carries more load in a smaller package (so you won't need the largest size). I would suggest looking at both of these products.

Let me know if I can help in any way.

~Jon