Linear rails and bearings- overkill?

[pminmo]

I just really wonder if there is not a lot of over focus on high cost linear rails and bearings for home machines. By that I mean something that isn't cutting parts constantly on a daily basis. For example, wouldn't a 1" x 3" 6063 aluminum as rails with skate bearings running on it last for most home machines? The aluminum is machineable with carbide tipped wood working machines, you can lap it with fine wet/dry sandpaper for more precision if you wanted. It is dimensionally stronger and more stable than MDF. Of course its more expensive, but at the price people are paying for Thompson et all. it's cheap. Granted if you want precision below .006, but then the MDF doesn't make sense either.

Phil

[balsaman]

IMHO you are correct, unless you can get a hold of some real cheap, in which case why not?

Eric

[ballendo]

Hello, right idea wrong material choice, IMO.

Yes, there is an over-focus on high grade rails. But...

No, aluminum is not a great 2nd choice , when used as you suggest. Why not?

Because steel rod is cheap,widely available, much stiffer, and most importantly, has a higher surface hardness. Which matters because the skatewheel bearings are hard. And they're exerting a lot more force than you think. Remember that the .866 diameter means that the contact point is pretty small. And you don't want to increase it by using a flat surface because that invites "bumps" from rolling over dust and swarf...

Hope this helps,

Ballendo

P.S. As fro cutting, you're only cutting a few pieces to length. hacksaw-able, even

[pminmo]

I would tend to agree but how much force are we talking here, IMHO not a lot on the size of a machine that is just cutting wood. Tempered aluminum alloy such as 6061-T6 while not as hard as steel, will take some abuse. I would concur on the flat surface as a collection point on top, but not so much on the sides and bottom. Just thinking out loud some. Glad you replied.

Originally posted by ballendo Hello, right idea wrong material choice, IMO. Yes, there is an over-focus on high grade rails. But... No, aluminum is not a great 2nd choice , when used as you suggest. Why not? Because steel rod is cheap,widely available, much stiffer, and most importantly, has a higher surface hardness. Which matters because the skatewheel bearings are hard. And they're exerting a lot more force than you think. Remember that the .866 diameter means that the contact point is pretty small. And you don't want to increase it by using a flat surface because that invites "bumps" from rolling over dust and swarf... Hope this helps, Ballendo P.S. As fro cutting, you're only cutting a few pieces to length. hacksaw-able, even

[ballendo]

Hello,

More force than you think! Been there, done that, have the t-shirt...

Think about not only the weight, but the vibration, and the dynamic loads as the axis moves.

If you want to use aluminum rails, switch your bearing material to UHMW, or one of the exotic plastics designed for use with aluminum rails. But now we're back as expensive as what we were trying to avoid...

Of course, if you're talking 6IPM, and a 12x12 machine with a dremel, then okay. But you're not really saving anything, and you ARE giving some things "away",

Ballendo

Steel rod is cheap. pipe may be even less. Even angle iron's a better choice than 6061 or 6063 aluminum (when used with skate bearings); you'll just need to sand it first

[balsaman]

ballendo,

I have seen you on CAD_CAM... Good to have you here. Welcome.

Eric

[Konrad]

I just have a thought on this railing business, I'm not a router guy jet, still learning from you and have come to conclusion that you need the router table as light as possible for faster acceleration and less wear an tear on your threaded rods and nuts[I only guess], if that's the case, using lighter materials is the way to go.

So using aluminum extrusions, [not solid] is the way to go, and for wear, where your rollers run [scate rollers], you can simply tie a wearstrip, lets say about 1/16 x 3/4 or 1/2" wide springsteel.
I have seen it on precision machines.
All you do is, drill a hole at the end and screw it to the rail.
If you need high accuracy, then you could scrap it first.
Does that make sense!
Konrad

[jphagen]

I agree with Konrad, that it is better to use steel components for the wear surfaces and bearings.
Use aluminum for the structure where moving weight is a factor.

[steveald]

Hi,

I have to agree that some linear bearings are overkill on most homebuilt hobby machines. Accuracy and repeatability are a function of the entire system of components in your machine. Expensive linear bearings won't compensate for deficiencies elsewhere, so IMO money, design consideration and effort should be distributed evenly between all the components of a machine.
As far as weight goes, IMO the gantry should be as light as possible without sacrificing stiffness, but the table itself should be heavy to dampen vibration (assuming moving gantry style).
Back to the linear bearings, the Thomson bearings themselves aren't that expensive, it's the pillow blocks, case hardened shafts and shaft supports that are expensive, and with a little extra work you could use regular steel shafts and build the other parts. CAD files are available from most of the manufacturers, and you could end up with close to Thomson performance relatively cheap.

Steve

[limbo]

Its funny to see that indeed most people are thinking in the same direction. Its just as if there are a few standard designs you can choose from, and vary a little in implementation. (OK, me too ) Would be great to see some real out of the box designs. So pminmo, the floor is yours

BTW, somewhere I've seen a pic of grooved cams runnning on a steel wire, laid in a small nut or groove in a square tube. Neat, one wire on top, another on the bottom, bit of pre-tension et voila! I bet this could be simply made with hardware store materials?

greetings,
John

[twombo]

Don't forget HIWIN rails! I bought 20 mm lgh HIWINs for the X and Y axis' and 15 mm THKs for the Z my 36 x 24 router for roughly $1100 from bob campbell. The cost seems high but the quality is real good and I believe I have achieved that good balance that steveald speaks of:
1. Good solid extruded aluminum structure,
2.precision slides
3.minimal backlash in the drive screws using 3/4" zero backlash cam action Supernuts on the X and y axis' (z is 5/8").

Admittedly I have spent nearly 3500 dollars on the machine but it should provide steady, reliable, service since each component is quite robust.

Your goals , I guess, dictate your investment tolerance. I want a rugged reliable machine that will hold up in a light production environment with a failrly high degree of accuracy. I hope i madethe right choices.

The great discussions on this site are very valuable in this project, Thanks.

[pminmo]

TWOMBO, You are a fortunate person that can justify/spend $1100 for rails and bearings and $3500 on your machine. I guess I believe that's a little more than most hobbist want to spend. IMHO. But thanks for your input. Opinions of all varieties are welcome.