How Smooth do smooth rods have to be on Prusa?

Hello,

Im just in the process of getting all the parts for my first ever Prusa Mendel, I was just wondering how smooth the rods have to be, not the threaded ones, just the smooth ones. They are used for X Y Z axis right?

thanks!
Schulz

A2 steel, straight and smooth.
Only the best quality should be used for the axis rods.
The same goes for your Z threaded rod, A2 steel.
The rest of the threaded rod can be cheap BZP as long as its straight.

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

AISI A2 tool steel or A2 stainless steel? How straight is straight and how smooth is smooth? What about size and roundness?

The SAE (meaning, inch) Prusa Mendel BOM specifies this rod, follow the link for the tolerances. See also the comments on this in the BOM. Something in that ballpark should be adequate, especially if using printed bushings. It you're using bronze, Acetal, or Rulon bushings, you'll have to look the inside diameter tolerances on the bushing you choose to see if they will be acceptable. Don't forget that the straightness tolerance for each pair of rods needs to be accounted for in the diametral clearance calculations.

Since I bought a kit (MkaerGear), I previously missed the fact that the design specification for these printers a rather incomplete with regard to tolerances for these mechanical components. You have to bring your own engineering to the table.

I don't totally agree totally with Bob. Straight is important, smooth is not too important for prusa with PLA bushings.
I just bought the super cheap zinc plated steel smooth rods, and it is working like a charm.
My z-axis threaded rods are far from straight, which absolutely doesn't matter (as long as the bearings on the z-smooth-rods work well).

I would say: just go to your hardware store, look at the cheapest stuff the have, take a bunch out the rack and roll them over the floor (or table). You'll easily see which ones are the straightest.

Don't worry too much! Things like backlash (play in the x and y direction) are far more important!.

Thanks for all the responses guys!

I went to my local hardware shop and got most parts: nuts, bolts, washers, even threaded rods. They did have some "smooth" steel rods, however it was really jagged, and and bumps and rough edges all over the place, going out by up to a centimeter or more! That can't be used can it...

Ill keep lookin around

thanks to all btw, you guys are awesome!

Schulz

If your in the US I suggest you go with 5/16 tool rod from fastenal. They keep it in stock, and it's plenty strong smooth and straight.

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repraplogphase.blogspot.com

rhmorrison Wrote:
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> A2 steel, straight and smooth.
> Only the best quality should be used for the axis
> rods.
> The same goes for your Z threaded rod, A2 steel.
> The rest of the threaded rod can be cheap BZP as
> long as its straight.

This is not /quite/ accurate. A2 steel is great, but ANY ground tool steel rods (usually called "drill rod" in the US and "Silver Steel" in the UK) will work equally well. A2 tends to cost about 2x as much as a similar W1 or O1 rod, and there is no practical benefit to the more expensive rods for our application.

At McMaster, the best choice is their part number 8890K41 for 5/16" rods ($4.05/3ft.) or 88625K67 for 8mm ($4.88/3ft). Compare that to A2 which is $8.53 for 5/16" and $13.02 for 8mm for the same lengths..

hmmm,

Well I'm in Australia, I'm not 100% sure if we use American or British, though I think it would probably be American. The stuff I could find was ANSI grade 316 stainless steel or the above mentioned O1.

I guess that settles that hopefully...

Thanks everyone!

north90ty Wrote:
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> I don't totally agree totally with Bob. Straight
> is important, smooth is not too important for
> prusa with PLA bushings.
> I just bought the super cheap zinc plated steel
> smooth rods, and it is working like a charm.

Any will work, but better smooth rods WILL give you better, more reliable performance. Considering how overpriced steel is at your local hardware store, springing for the $4 per rod for good drill rod is not going to break the bank (for people in the US at least). If you have no other choice, plain steel rods will work fine, but they really should be avoided if you can. At the very least you will wear out your bushings a lot quicker, a nuisance even if you can print a new set.

> My z-axis threaded rods are far from straight,
> which absolutely doesn't matter (as long as the
> bearings on the z-smooth-rods work well).

Correct, mostly. The smooth rods are what dictates the movement accuracy, not the threaded rods, so if they are slightly bent it should not matter. That said, straighter is still better, but don't stress if yours are not quite perfect.

There's a number of good steel suppliers listed on the Australian Suppliers wiki page.

BTW: 304 grade steel is fine for smooth/threaded drive rod use.

Awesome! Thanks Cefiar I guess all things stainless it is then

Thanks guys for all the help!
Schulz

Dale Dunn Wrote:
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> AISI A2 tool steel or A2 stainless steel? How
> straight is straight and how smooth is smooth?
> What about size and roundness?

A2 is tool steel, not stainless. For most users that is not important, but if you will be transporting your machine a lot, stainless may be preferable even if it is not as smooth and consistent. For normal use, keeping the rods lightly oiled will prevent rust.

By definition, tool steel is has a machine ground surface, so it will always be very smooth and round. In rare cases you will get one that is bent, even if your supplier has a straightness spec. They are all straight leaving the factory, but they do occasionally get bent during handling. Any good supplier should replace any that arrive bent.

>
> The SAE (meaning, inch) Prusa Mendel BOM specifies
> this rod, follow the link for the tolerances. See
> also the comments on this in the BOM.

I just updated the BOM with a link to the cheaper W1 rods ($4.05/3 feet, with the same straightness and diameter specs as the more expensive A2). For our purposes, there is virtually no difference between the two, so paying more than 2x as much for A2 is a waste of money. If we were using these for their intended purposes (they are typically hardened and used to make metal cutting tools and drill bits-- hence the name "drill rod") then there would be advantages to justify the extra cost of A2. For us there is absolutely no benefit.

I get my vitamins from The Home Depot and Lowes. The trick with the rods is to roll them on the top of the cart one at a time and observe the ends. If the rod is straight, it will show no vibration. When pulling the threaded rod out of the bin, be sure to pull them ALL out together. This will prevent damaging the threads.

Zinc plated smooth rod is fine on prusa. If you want to invest in precision rods you may as well toss out the pla bushings and get bronze, otherwise you don't really benefit much from the extra smoothness, since the pla bushings tend to flex on the shaft a bit.

I use A2 stainless steel rods and fasteners. They are sold as "stainless" and they are non-magnetic and do not rust.

---

[www.hydraraptor.blogspot.com]

I bought my A2 stainless steel rods over Ebay at a very reasonable price.

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

nophead Wrote:
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> I use A2 stainless steel rods and fasteners. They
> are sold as "stainless" and they are non-magnetic
> and do not rust.

I did some Googling, it appears there are two different metals called A2. A2 Stainless is roughly the same as 304 Stainless.

304 Stainless is most definitely not a tool steel, it is the most common grade of stainless steel and probably makes up most stainless steel parts you see.

A2 tool steel is a different beast. Tool steels are designed for heat hardening to make cutting tools. The two metals have completely different chemical compositions. You can compare the two at the links below:

[www.fullermetric.com]
[en.wikipedia.org]

Note A2 Stainless has .08% carbon vs. A2 tool steel at 1% and A2 Stainless has 17 to 20% Chromium compared to only 5% for A2 tool steel.

Either of these metals are suitable for use as the smooth rods in your bot, however A2 Stainless (or any other stainless) will typically cost more, and unless it is a ground rod, it will typically not be as precise as most tool steel rods (though in practice it is plenty precise enough).

If you have a source for it, W1 or O1 tool steel rods will likely cost less money and work just as well or better.

Edited 1 time(s). Last edit at 04/20/2011 04:04AM by kludgineer.

Sweet stuff, maybe we should add this in the wiki...

I decided on 304 grade stainless was only 6.55 a meter, which is ok I think...

Thanks for all the help guys!
Schulz