Low Cost Steel Shafting

[MrKal]

Hello,


I am new to this forum and would like to say hello, and also ask a question
or two about shafting used for linear guides using linear bearings.

While reading these questions please keep in mind that the intended
end product is a low cost CNC machine that has roughly these requirements:

* Low cost
* Not high accuracy, 0.002 inch per inch error is fine.
* Not high powered, the moving stage will have to support less than 1 pound.

1. Did anyone ever use cold rolled steel rod for linear shafts, rather than
hardened and machined steel shafting? The price is much cheaper but
do the standard linear bearings still work with these rods? If not, is
there any good priced web sites that sell the standard case hardened
steel shafting?

2. Do they still make bronze linear bearings, or do these have balls inside too?

3. What is the cheapest linear bearing that can be had? Any web sites
have them?


Thanks in advance!

[RICHARD ZASTROW]

Linear guide blocks with ball bushings will pretty much eat cold rolled steel guide rods. Less expensive bushed blocks will probably suffice if your expextancy isn't too high. Things get more expensive as you expect more.

Depending on loads, distances, accuracy etc. bushed linear guides without ball bearings should meet your .002" per foot accuracy. Weight of the load and speed of motion determines the shaft diameter and bushing material, both not mentioned at this time.

There are members far more knowledgeable than I who will be happy to help if you give more particulars of your project. I genarally work with crossed roller bearing linear way systems, so I'm of little help to you.

Hang in there, If you state it they will reply. lol

Dick Z

[blackbeard52]

Please give us your dimensions...some of us have ins to salvage and lower cost good shafting or linear bearing stock.

Bob

[neilw20]

Forget using ball type things on mild steel shaft. It will just eat holes in the shaft.
A cheap effective solution, provided you don't mind grease on the sliding surfaces is this.
As an example with 1/2" shaft.
1/2" bright mild steel is usually about 0.001" under size (in my experience).
Get a length chrome plated, and it would be nice if it is prepared properly with copper plating, and all the extra things the platers do to make it work properly. Talk to the electro platers about surface preparation as this is where you can save some money.
Once plated it is almost exactly 0.500" and has a hard finish. The hard finish is important. It won't work well if not plated.
Use standard off the shelf bronze bushes 1/2" ID, and that is the cheapest slide you will get.
Works for other sizes. This is just an example. You could also try nylon bushes. Chrome plated shaft still is a must. I have tried unplated, and you will eventually give up. It will just keep getting stuck.

[MrKal]

Hi again,


Thanks much for the replies/info. I ended up with a lot of
questions about what works and what doesnt, and you guys seem
to be giving me the answers i was looking for so thanks much.

The sizes, ideally, would be:
Two shafts, 1/2 inch dia., 16 inches long (Y).
Two shafts, 1/2 inch dia., 12 inches long (X).
Total 8 bearings, two on each shaft. I had a feeling
bronze was the cheapest but have difficulty finding
these on many manu sites, they direct me to ball type
or ceramic type or plastic type.

Other facts:
Speed of either travel would be 1 inch per second or maybe
even 0.5 inch per second might be ok. Speed is not essential.
Accuracy is not strict either, 0.002 inches per inch error is ok.
That is, 0.024 inches per foot error is ok.
Load is less than one pound, probably less than 6 oz.

Possible substitutes:
Maybe all 3/8 inch dia shafts, maybe both 12 inches long.

Another question about the bronze bearings:
Do the ones made for rotation also work for linear slides?
I've found many rotational types but not sure if they
work for linear too.


Dick:
Well, in the future i may wish to move to ball bearings
and hardened shafting, so you may be able to help there.

Bob:
I included some dimensions this time, is that what you meant?

neil:
It's ok if i dont use ball type bearings, the bronze ones
are ok. i dont think i'll want to go through the trouble
of getting the cold shafts plated however (that's a great
idea BTW) so does this mean i have to go to hardened
steel then? In your experience, did the bearings bind
because of heat or because of slight twisting motions of
the moving stage, or was the reason indeterminate?
Please see my question above too about the bronze bearings,
as im not sure what type to buy yet. What would i
order here for bronze linear bearings? Can i use the
standard type used for rotational purposes or no?
Dont mind if the shafts have to be greased.


Once again, thanks very much for the replies/info. Im trying
to build this thing and have a little fun at the same time.

[blackbeard52]

I may can help with some steel shafting and bearings...I will look in my salvage bin and I am goin back to check for more linear bearings. I know I have one shaft and two bearings the dimensions you require

[neilw20]

Rotational bronze bushes are OK.
If you don't have the shafts hard, then tiny bits of rubbish (from the soft shaft) embed them self in the surface of the bush and after a few moves it all turns to grippy jammy 5hit.
Have you ever noticed how a rubber hose can rub a hole in an aluminum or steel tube. Tiny fragments of the harder material seem to get embedded in the softer material, then the soft material wears out the harder piece.
By plating the shaft, nothing rubs off, so there are no bad bits to wear the original piece. That's how I see it, anyway. I use these slides in automatic test equipment, and they survive for years in a production environment.
At 12" you might find 3/8" gets a bit springy.
Have a look at and see why titanium or aluminum can be springy.
http://en.wikipedia.org/wiki/Young%27s_modulus

[ahren]

One other thing to keep in mind before jumping on bronze bushings -- even under ideal conditions, the sliding friction in these things is usually still 10X what you would get in a system with rolling elements. That's why rulon, frelon, etc. bearings have specs about how off-center you can apply the load.

For low cost, you might consider some of the other options presented on the zone, or go with some cheap linear rails off of ebay since your travel is short.

Ahren
www.cncrouterparts.com

[martinw]

Dear MrKal,

IMVVHO, given that the total length of shafting that you need, and given the small diameter, the total cost of even really good ground and hardened shafting is not going to be very great. Over in the UK it costs about $1 per inch, and you lucky people over there can probably get it is for 50 cents per inch. That is probably a very small percentage of your total budget. It you get the good stuff, you can upgrade your bearings susequently.

My 2 cents,

Best wishes,

Martin

[MrKal]

Hi again,

Since there were a number of replies i reply here one by one...


blackbeard52:
I am interested, how much would you ask for these
parts (USD)? How much does it cost to ship this stuff?
If it works out and you have part numbers for the shafts
and bearings i would be able to order more of the same
because i would know what to order. Right now im
not even 100 percent sure what to order, although i
am starting to get a good idea by reading these replies.

neil:
That's very interesting about the wear. That tells me
to stay away from the cold steel unless i plan to get
it plated. I guess because of the expense and time
of plating however i'll most likely go with hardened
steel, even if the cost is more. At least now i know
the con's of using plain cold rolled steel, before i
wasnt sure, so i dont have to go through the trouble of
experimenting with something that wont work. Your info
here has helped quite a bit. Thanks for the detailed
explanation about this wear and its effects.
That's good to know that they (plated) last a long time
too just in case i end up going this route at some point.
I had a feeling 3/8 inch would be too bendie at that
length, but i didnt have any beam deflection formulas
on hand either, which i am now seeking.
I also considered aluminum, but thought that would be
too soft anyway.
I'll be looking at the bending modulus and beam
deflection next, thanks. I had formulas long time
ago but not sure what happened to them. Now ill
have to rely on the web for info i guess unless
someone here has some formulas known to be good.
I could work in N/m^2 etc., i guess.

ahren:
That's something i didnt know either. I figured they
wouldnt be too bad but 10x does sound rather high.
Still, i dont think i want to shell out 20 dollars
per bearing (ball type) when i need (or should have)
eight of them to start.
I guess what i can do is start with bronze and one
i start testing it if it doesnt work well i'll move
to ball type bearings, hows that sound? I know
many of the decent printers these days use bronze
but not sure what kind, anyone know this?
When you say off center, do you mean off center of the
bearing itself? My intended drive is as follows:
A plate with four bearings, placed on bottom almost
like tires on a car except rotated 90 degrees in
the horizontal plane. The two shafts slide through
the bearings, one shaft through two bearings, making
a linear slide. This means one plate will be at
least 12 inches wide, with the four bearings on the
bottom, two on each side, and the drive force will
come from directly in the middle of the plate, 6
inches from each set of bearings. This of course
means the drive will be located 6 inches from each
bearing, but there will be two on the other side
too, also 6 inches away from the drive force. Does
this sound like a problem due to off center drive?
Cheap linear rails? Where might i find these on
ebay or other? What other options on the zone are
you referring to?

martin:
Yes, that's sounds like a good point too. Perhaps
im trying to build a mountain here out of air, you
need at least some dirt and rock :-)
Oh and yes about the upgrade too, if i have to do
that (move to ball bearings) i'll need the hard
steel instead as neil pointed out.
I guess the facts are clear now, hardened steel
is a must without hard plating.
Perhaps i was looking in the wrong place too,
is WM Berg a bit overpriced perhaps? Maybe
another source would be better?




Thanks again to everyone for their information which
is helping quite a bit with all this.

[MrKal]

Hi again,

Did i ask too many questions or something as everyone seems to have
disappeared?

[neilw20]

My brain is empty.

Cheers.