8 mm Rods: Shafting Steel vs. Stainless Steel vs. Mild Steel

I'm always intrigued by recommendations to use "stainless steel - any grade" in preference to mild steel rods. Is this based on experience and research or just presumption?

For the record, pretty well all grades of "steel" (mild, stainless, hardened tool steel, etc) have pretty the same stiffness / deflection characteristics as each other, because the Elastic Modulus / Young's Modulus of all grades of "steel" are pretty much the same. (Assuming we are talking about "small" bending deflections well below the yield strength of the material - and I think we can agree we don't want to see "large" bending deflections on any of our axes!)

If it's wear that you are concerned about, then hardness is what matters. Hardness depends upon both the alloy you choose AND its heat treatment / cold working, etc. Many common stainless steel grades are actually quite a bit softer than mild steel - if you really want to minimise wear rates, you are probably looking for a hardened shafting steel - which is not the same thing as common grades of "stainless steel".

For corrosion, stainless steel and bright shafting steel will outperform mild steel - but in a typical indoor environment, a light coating of sewing machine oil in conjunction with the constant motion of the bushes / linear bearings will keep even mild steel polished and "rust free".

I built my Prusa machine using "unidentified" 8 mm steel rod (definitely not stainless or shafting steel). I first fitted it with standard PLA bushes [julianh72.blogspot.com] - they were a bit stiff at first, but freed up nicely with a bit of machine oil and manually running them up and down the rods 20 or 30 times. I have since upgraded to LM8UU linear bearings on the X and Y axes [julianh72.blogspot.com] on the same 8 mm rods, and they are running REALLY smoothly! OK, I have only been running this way for a few months, but I honestly don't see any real NEED for stainless steel or shafting steel rods. I can always upgrade later if I see any problems develop.

Comments, anyone?

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

I agree. Although for 6mm rod, I would say stick to the harder stuff.

I use it first because it's a bit tougher and second hopefully no surface rust if you keep it in the garage like I do where there can be moisture in the air

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Experimenting in 3D in New Zealand

I'm glad to see this discussion as I'm gearing up to build a prusa based machine with a larger build envelope.

Earlier today, I placed an order with McMaster Carr for some of the materials. I wound up ordering E52100 alloy 5/16" rod even though I'm using LM8UU's (8mm). The diameter is only .0027" smaller than 8mm which is still a HUGE improvement over the PLA printable bearings.

This 52100 has a brinell hardness of 248, where 303 stainless is 262. I'm think that will be fine, but please let me know if not. The yield is 62,000 - 87,500 psi which tops the 303 by a long shot (303 being 30,000 to 50,000 psi).

Is there any reason that I'd have a problem with the E52100 alloy? Sure it's not stainless, but the machine will be in a dry warm environment. There are some other selections available in SHAFT instead of ROD that I'm sure are far superior, but they are also spendy. Still, I wonder if I should have paid the extra since I'm going to have a build size of 300x * 250y * 175z.

I know there are a lot of other considerations with a large envelope like this (heatbed and such), but this discussion is only concerned with rod/shaft selection.

I know my post reads mostly as a statement of what I'm doing, but please treat it as a question. I'm looking for any solid advice based on similar experiences.

Thanks Julianh72 for starting this topic.

Some interesting points here. My experience is that the stainless rods I bought (303) are a little harder than mild steel rod I bought from B&Q (Probably cheap EN1a 230M07) and also the surface finish is slightly better, but as you rightly pointed out it depends on grade and work hardening. Most bars are "bright drawn" which causes work hardening so the quoted hardness figures are a little misleading. For example EN1a is quoted at 115 -200 Brinell, Stainless 303 is about 160 Brinell depending where you look (I think some of the values quoted above may be a little high and are probably maximum values). So mild steel MAY be harder, but usually isn't.

Have a look at the Wiki page for Brinell Hardness. I work in industry and have done many Brinell tests in my time and these seem about right. For generally available grades mild steel is slightly softer than stainless. My experience tells me this as drilling stainless sheet or cutting stainless bar is much more difficult than mild steel. A good test is to just run a file across the corner of the material and you soon get a feel for how hard it is.

To put it all into perspective it probably doesn't really matter what you use as there are hundreds of people using mild steel, stainless and drill rod. My preference is for stainless due to the corrosion resistance. It works well looks good and not to hard to work with.

Edited 1 time(s). Last edit at 01/11/2012 06:05AM by martinprice2004.

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pretty well all grades of "steel" (mild, stainless, hardened tool steel, etc) have pretty the same stiffness / deflection characteristics as each other

True. Hardening or alloying steel (in the usual amounts) doesn't change its young's modulus.

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If it's wear that you are concerned about, then hardness is what matters.

Also true, applies pretty much to bearings only, though. Ball bearings have a very high pressure at the contact point, so hardness matters. For plastics bushings (PLA, Igus, ...), hardness is always high enough. Igus even recommends Aluminium rods for it's high performance bushings (they have several qualities).

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

I don't know anything about Brinell hardness but having cut both stainless and mild steel bars with a hacksaw I would say stainless is a lot tougher. It is very hard work sawing it and it destroys the blade in a few cuts. Surely this means it will wear more slowly?

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[www.hydraraptor.blogspot.com]

Machinability is influenced by more than hardness. Wikipedia confirms my guess for stainless, it work hardens quickly. The article also cites toughness (usually measured by whacking a sample with a notch in it to see how much energy is required to break it) and gumminess. I'm not sure what is meant by gumminess, but it might refer to a tendency of the cut material to stick to the cutter. Cutting oil would help with this. Ironically, the Wikipedia article suggests hardening stainless slightly to mitigate the gumminess.

Stainless is one of the materials machinists don't like. Mild steel (annealed) usually scores high on machinability.

The work hardening of stainless might make it a good candidate for low-load bearings like the LM_UU bearings in our applications. Any indentation of the rod will tend to make the rod harder. Mild steels aren't noted for work-hardening, AFAIK.


Why isn't the Firefox spell-checker working in these forums?

Edited 4 time(s). Last edit at 01/11/2012 10:36AM by Dale Dunn.

Skypuppet Wrote:
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> This 52100 has a brinell hardness of 248, where
> 303 stainless is 262. I'm think that will be fine,
> but please let me know if not. The yield is 62,000
> - 87,500 psi which tops the 303 by a long shot
> (303 being 30,000 to 50,000 psi).
>
> Is there any reason that I'd have a problem with
> the E52100 alloy?

Well the mcmaster page says the 52100 is shipped annealed, so won't be as hard as the claimed hardness (which they state explicitly). This will be good since you have to cut it yourself, but you probably don't have the ability to properly harden it (though maybe you do). you'd need to get the whole rod to temp and then quench, in oil or water. I don't think a common propane plumbing torch will do the job.

actually it makes me wonder how many users have annealed rods? maybe that is the cause of Lm8uu furrowing?

*you have to select the language for firefox to spellcheck these forums, for some reason. rightclick on the typing area, go down to languages, and select English, etc.*

Edited 1 time(s). Last edit at 01/11/2012 11:33AM by Buback.

What would happen if you used larger radius aluminum rod and tube with plastic bushings? Deflection would not be an issue, it's cheap, it's very straight, and it is just a much nicer material to work with. Sure it is not the ideal material for this application, but most of us are building practical inexpensive machines and not ideal machines.

I've been using carbon fiber tubes for the past year, so you can pretty much use anything you want just to get set up and printing. Though i am upgrading to steel rods now since i'm switching to lm8uu linear bearings.

Buback: Good call on the annealed material. I saw that comment and wondered, but it seemed that they made the brinell claims regardless of post treatment, where they stated that the annealed material could achieve Rockwell hardness of C39 - C65.

I need to read up on brinell hardness. I'm not really familiar with it. I assumed it was the hardness of the alloy in its annealed form. The hardness being a factor of chromium in the case of 52100. I'll definitely read the wiki page as suggested by Martinprice2004. Thanks for that.

Another thing to consider is that this is bearing grade alloy steel. I don't think this is really classified as mild steel per se. It was not in the "easy to machine" selection of materials. In fact, it is only regarded as "fair machinability" in its annealed form.

This is why I posted. I'm glad to hear from experienced botters. I had heard about lm-uu bearings wearing grooves into rods and I'd like to avoid that.
Of course, it will be an easy option to build this machine with experimental rods and then replace them with either better rods or even shafts at a later time.

I hope more people chime in. It will be interesting to hear from people that have had failures with specific grades of rod, or successes as well. The most important thing is that one knows the material specifically, since there are numerous grades of steel. I think off the shelf material is really what were looking at here. Hardening rods properly at home may not be an option for most, and I'm sure there are plenty of off the shelf materials that are working out there.

Just a thought, would increasing the rod diameter remove the necessity for harder rods as the load is spread over a greater area? I know that the pressure is not evenly distributed and have no idea about the raceways in the LM-UU bearings so this might not be a valid point at all.

yellow fish,

yes I'm sure going up in diameter would likely offer some better bearing choices. The lm8uu bearings are a 4 row circuit generally and that is the real issue with surface hardness. If the larger lm10uu bearings offer more circuits/rows and larger balls, then you will get a more even load distribution, which is good. Also a longer circuit will help but this can also be achieved by using the long version, or lm8luu bearings at 45mm oal.

There is plenty of shaft quality steel rod available in 8mm dia, but it goes up in cost accordingly. I would think that the general practice here is to upgrade an existing machine. It would definitely cost a lot more to bump up to 10mm rod in that the rod and bearings are both more expensive than 8mm rod and bearings. You could definitely get good 8mm rod for the price of mild 10mm stuff. Then there is the redesign and reproduction of all the plastic parts to make them fit the larger rod..

Even for a ground up build, I'm not sure that it's necessary unless the build envelope is huge.

All in all,I think the issue is easily remedied by proper material selection which doesn't necessarily mean big $. Just a better education when selecting rod.

I did just think of one good reason to use stainless which I haven't heard mention of; Stainless is not likely to become magnetic with the constant stroking motion of these machines. A magnetized rod will hold metal filings and could cause bearing damage, but likely very little unless the printer is in a harsh environment..

Edited 1 time(s). Last edit at 01/11/2012 08:30PM by Skypuppet.

Personally I would go with stainless because rust is a major concern where I live. In fact I think the smooth rods on my RapMan are starting to rust. Apparently they aren't stainless.

Of course, like you said, you can avoid rust with proper maintenance. But I am not one to maintain things very well . And I really do hate rust, so in this case the other factors are minor in comparison. I wish more things were made from rust resistant materials.

OK, at the risk of over-complicating the whole issue:

Atlas Steels provide an excellent "Technical Handbook of Bar Products" at: [www.atlassteels.com.au]

It goes into a great amount of detail on chemical composition, strength, toughness, ductility, hardness, machineablity, etc - there should be MORE than enough information for anybody!

(I'm not sure that all this information will actually help anyone come up with the definitive answer as to what is the "best" product to use. Having perused the document, I have to say I am much better informed, but none the wiser! )

For what it's worth, my 8 mm mild steel rods with PLA bushings (now replaced by LM8UU linear bearings) are holding up just fine so far!

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

julianh72 Wrote:
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> I'm always intrigued by recommendations to use
> "stainless steel - any grade" in preference to
> mild steel rods. Is this based on experience and
> research or just presumption?

Just today I visited the local hardware store and looked at the steel rods. The stainless ones were straight and smooth. The ordinary steel rods looked like they were made by the village blacksmith. So specifying stainless steel does seem like a sort of a guarantee of getting straight and smooth rods...

I buy from a steel wholesaler in 6m lengths of SS which I then cut up with a grinder using a jig I made for the complete kits I sell here in NZ

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Experimenting in 3D in New Zealand

Skypuppet Wrote:
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> Earlier today, I placed an order with McMaster
> Carr for some of the materials. I wound up
> ordering E52100 alloy 5/16" rod even though I'm
> using LM8UU's (8mm). The diameter is only .0027"
> smaller than 8mm which is still a HUGE improvement
> over the PLA printable bearings.

LM8UU's won't ride properly on 5/16" rod. I've tried. The ball bearings don't seem to engage well. It'll work, but considering that 8mm rod on McMaster is only marginally more expensive...

NewPerfection, You are correct. 100%, with the exception of "it'll work, but considering"... not for me anyway. It just wont do.

While the 5/16 rods are ideally only .0027" smaller, mine actually measure closer to .005" smaller, which is definitely enough to cause a rattle in the bearing circuits. I'm sure they'd be spitting out little BB's before too long.

I will say that the E52100 is definitely a very hard material tho. By no means is this run of the mill "mild steel". It is bearing quality alloy steel. High carbon stuff. That wont make it any more usable with LM8UU's (in 5/16"). And they don't stock it in 8mm so...

I plan to put that rod to use on other things. It could be used with bushings, but I'm not real interested in that either, Not for my redesigned/Larger prusa based machine.

My current machine is a prusa from lulzbot. It's a nice little machine, but I felt it necessary to convert it to LM8UU's. It made a HUGE difference on that machine which has 8mm stainless.

So after shopping McMaster Carr again (no this is not an advertisement), I found some reasonably priced "type 17-4 ph stainless" precicion ground rod. Not cheap, but not horribly expensive either. I'm not trying to make margins on my machines either, so a few extra bucks is money well spent.

On a side note, I'm not saying whether you need to have stainless, or such and such hardness or whatever. I'm just sharing my personal experiences. I'm mainly trying to fill in the blanks when I state where I'm buying and exactly what meterial I am selecting so that others might benefit from my experience.

I hope it helps somebody

I don't really make anything if I take my time into consideration too.

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Experimenting in 3D in New Zealand

I am building my for printer and it seems to me after spending a few weeks researching that to buy *smooth* M8 is a challenge. The threaded one can be purchased from Fastenal. I will be glad share my experience once I get into working with it. I think I will use a air grinder of a small diameter. Probably more work but like more accurate.

Can anybody share item munbers SKUs from the website where the hard Stainless Steel can be purchased? It would help a lot to ensure to get the by experience veryfied SS.

Thanks.

Ed

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Skypuppet
So after shopping McMaster Carr again (no this is not an advertisement), I found some reasonably priced "type 17-4 ph stainless" precicion ground rod. Not cheap, but not horribly expensive either. I'm not trying to make margins on my machines either, so a few extra bucks is money well spent.

Note that if you want to do this properly (aka as the datasheets recommend) you are not looking for precision ground 8mm stock. The bearings are designed for an h6 shaft which is 4 microns under 8mm in diameter and has specific surface roughness requirements.
[www.tribology-abc.com]
You will note the mcmaster linear shaft ( [www.mcmaster.com] ) is h6(-0.009mm to +0.000mm). If you buy regular 8mm stock it generally has far wider tolerances and may not meet the surface roughness specifications.

Pretty much anything will work okay but if you are going to go through the trouble of trying to pick the best shaft you might as well use the one called out in the datasheet and engineered for exactly this application.

Edited 4 time(s). Last edit at 01/24/2015 11:12PM by 691175002.

From my personal experience, I have learned to -strictly- use hard-chromed precision rods from now on. It was the best decision of my printing-life I'm not going to put in all kinds of theories in here, it just works.

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http://www.marinusdebeer.nl/

I've got a question for you all...has anyone considered hardened aluminum? On Mcmaster you can get aluminim rods harder than ANY of their steel rods (C70)

What would the drawbacks be? We don't have to worry about them being brittle really and as a plus aluminum is super light..

I think it's anodized right? I think Aluminium might have more flex over distances when comparing to hard chromed for example.

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http://www.marinusdebeer.nl/

Yes Aluminum just sounds like a very bad idea. I get my hardened chrome steel rods as well as linear bearings (and any other bearing you want) from [www.vxb.com] they are very high quality at a low cost. I wonder sometimes how they can have such low cost with the quality they have. I have been buying various bearings from them for many years. And no, I don't get paid to tell you this :-)

I desagree. Aluminium can be as good, it depends of the use. Appreciate there's some high precision aluminium or carbon guides, too. Most of them are better than chinese grade chromed rods. Fist, you cannot talk about guides, without talking about bearings. Ball bearings and aluminium goes wrong because the balls digs into aluminium. With self lubricating brass or plastic bushings, aluminium can work at last as good and last longer. What realy matters is the diameter and the admissible load matching your needs. If your carriage is light, aluminium can stay in better shape under the load, because it's lighter. For example, a long chromed rod will flex a bit under it's own weight while an aluminium rod won't. Some are lighter, some stands better vibrations, some are cheaper, each one have its pros and cons. As far they're machined with the same precision, no matter you use aluminium, carbon or chromed steel rods, you just want to calculate the best fit possible for your design.

There's no basic awnser to this question. The good rod/bushing type depends of many factors and needs to be estimated and calculated on each project. For my Reprap purpose, I choosed low-friction plastic bushings on my existing chromed style rods, but I calculated there's no need of chromed rods on Prusa i3 Z and X axis, and if you have a lightweight Y carriage, you can also deal with light rods on Y axis too. Attention, this is calculations for my printer. Your machine may have a different configuration, with or without a bowden system, light or heavy motors, and so on. On word : calculate.

Edited 5 time(s). Last edit at 01/26/2015 05:11AM by Zavashier.

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Collective intelligence emerges when a group of people work together effectively. Prusa i3 Folger (A lot of the parts are wrong, boring !)

Quote
Zavashier
Ball bearings and aluminium goes wrong because the balls digs into aluminium.

Not true. The rods I was referring to have a surface hardness of C70 and most bearings are C55-65. Hardened aluminum also doesn't flex enough to really ne a factor over the 300-400mm we're dealing with. The only drawback I can think of is that they're really really expensive. Hardened steel is the way to go.

Also on the question of stainless vs regular steel- stainless is much softer and much more expensive. The only real benefit is corrosion resistance, and since it's fairly easy to keep the rods coated in an oil/lube film they really won't corrode a lot. I personally don't feel that the benefits of stainless outweigh the price.

Specific materials always resist better the wear. There's some examples on CNCzone of bearings scratching aluminium extrusions. Simply because that aluminium types is softer than the bearings. I don't know what's hardened aluminium. I know aluminium alloys (7575, 2017A, 6061, etc.) which one are you refering too ?

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Collective intelligence emerges when a group of people work together effectively. Prusa i3 Folger (A lot of the parts are wrong, boring !)