Tool steel vs. Linear Motion Shafts

im about to start build a printer that is approximately 24" wide 18" deep and 18" tall. I was planning on using 16mm linear bearings from ebay. my question is can 16mm tool stell/drill rod be used or should shafts made for linear motion be used?

thanks

When we were designing our printers we asked the same question.

Generally speaking linear shafting has a diameter tolerance, straightness tolerance and finish level (in RMS) and has been case hardened to 'file hard'. Drill rod generally does not have anything but a diameter tolerance and is generally a crapshoot as to exactly how straight and smooth it is. With that being said a lot of the drill rod we tested was straight but most certainly lacked a 9RMS finish like we are using currently. Also we found that the hardened balls in linear bearings do eventually wear small grooves into oil hardened drill rod which of course affects smoothness and accuracy. The only downside to linear shafting is the expense which can be significant. When dealing with especially long runs like you are talking about I wouldn't think you would get consistent results over that big of a span, although you may get lucky!

Chelsea - QU-BD

I built a couple of oversized mendels (12" x 12" x 10"). I used McMaster-Carr drill rods (8mm). After about 4 months of heavy printing I found that I wore groves in the rods. I just twisted the the grooves away from the ball bearings and continued. Drill rods (as least from McMaster-Carr) only have a surface hardness of about B80-100, about half the hardness of a true hardened linear shaft (~C60).

But the trade off is that the cost of the drill rod is way lower. So you can replace them 2, 3 or 5 times for the cost of a linear shaft.

I ordered 16mm rods, and EBAY 16mm linear bearings for a reprap CNC. I found that 16mm is WAY big. I think something around 12mm is more reasonable.

Anyway, hope this helps.

Edited 1 time(s). Last edit at 03/28/2013 11:24AM by pfinucan.

If you can live with somehow higher tolerances, I have found IGUS rails and bearings to be a moderately priced alternative (elements being anodized aluminium rails and plastic bearings, that can be cheaply replaced). Check Drylin series.

P.S. I am in no way affiliated with IGUS.

for the cost of stainless steel rod, it's not a bad way to go, i find that the bearings do eventaully wear a groove in them, however the amount of play that developes is insignificant by comparison to other factors,

somthing i have used in the past with some great success is this [www.thingiverse.com] printable pla lm8uu equilivants, they slide quietly and with bugger all friction on them, and go reallly well with a bit of silicone spray on the rods

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Taking this a step further in terms of cost effectiveness, take tool steel/stainless steel rods polished with a bit of brasso. Then for linear bearings buy some normal spinning bearings (which are dirt cheap), remove the cage and the balls leaving you with the bearing centre which is a perfectly sized bushing. You need to oil it with thin oil since grease creates too much friction, but it works really well otherwise!

konwiddak Wrote:
-------------------------------------------------------
> Taking this a step further in terms of cost
> effectiveness, take tool steel/stainless steel
> rods polished with a bit of brasso. Then for
> linear bearings buy some normal spinning bearings
> (which are dirt cheap), remove the cage and the
> balls leaving you with the bearing centre which is
> a perfectly sized bushing. You need to oil it with
> thin oil since grease creates too much friction,
> but it works really well otherwise!

you can use grease but high temp grease id prefer to use grease then oils as oils tend to evaporate or run off where a high temp bearing and bushing grease would be less too maintain

dont use stainless rods. It is costlier and inferior quality for that purpose.
You need high chrome steel, grounded, polished and hardened at least to 54 Hrc, preferably 60 Hrc. Stainless is way lower hardness.
Often marketed as stubb rod, technical euro designation is 100C6 or 100CrC meaning it is 1% Carbon and 1.5% Chrome.
No risk of rusting if oiled.

These rods have also a straightness guarantee you wont have in other cases.

Then, if you dont want linear bearings, IGUS bushings are not that costly and at worse use good old sintered bronze ones which will be both dirt cheap and precise. The only drawback of sintered bronze is that regular oiling is needed as well a cleaning the rods from any dirt.

just use open rail, way less friction, and much sturdier, make it as long as you want.

I have a question about drill rods and I've had a debate that never got settled. If you are using lme12uu linear bearings can you get away without to many issues in combination with 15/32" rod as its 11.90 or 11.93mm. My friend says that it's possible and doesn't create much of a noticable difference unless you're looking for super accurate prints.

He says this and he also says he uses 5/16 rods with lm8uu and it works great. I would like to find out if he is correct or if in general printing use it would name a big difference. Thanks in advance.

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5/16 works out to 7.937 mm, which is about the accuracy you might get from random 8 mm stainless smooth rod that many are using.

15/32 is only 11.9 as you point out, which is a bit loose for an LM12UU.

Either way that is not the key factor. Most Reprap style linear bearing assemblies are so inaccurate that the linear bearings are twisted or offset from the axis of the smooth rod. Contact between the bearings and the rod is a bit haphasard. As long as the complete assembly itself doesn't rattle much, prints will be just fine. Any further increase in mechanical precision may not be very visible because other sources of slop are still there!

Edited 1 time(s). Last edit at 11/02/2013 03:01AM by Yvan.

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Yvan

Singularity Machine