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Hi,

I'm just wondering if anyone on this forum knows a lot about T slot aluminum extrusion tolerances. It's difficult to find data on the internet about this kind of stuff, but I think it's important for our printers.

There are a bunch of things that can be measured;

Length of the extrusion
Cross-section width (I measured this to be 0.996 on several of the rostock towers)
Slot width
Straightness
Twist
Warp

The cross section widths I've measured seem to be very consistent across extrusions, they're always about 0.996 inches wide for a 1 inch extrusion.

I have no way to measure the length of the extrusions precisely. The surface finish on the cut ends looks very smooth (almost fly cut), but repeatability is the most important thing if a 3d printer relies on having precise alignment based on extrusion length.

Slot width: important if rollers are used like on the Rostock Max.

Straightness: (sometimes called "bow") is how much the rod is bent over its length. This is very important too.
Twist: is how many degrees it rotates from end to end, this can be reduced greatly be clamping the ends in a rigid frame. Unfortunately the Rostock Max doesn't do this.
Warp: I'm not sure about this, but I think it's similar to straightness.


I've seen quite a few delta printers that use cylindrical linear shaft instead of rolling carriages on the T slots. There are also a few printers now that use special ground linear rail bolted to the T slot extrusions. I want to know if these are necessary for precision before I finalize the design of my own large delta printer. T slots seem convenient and rigid, but if they have bad tolerances that affect print quality, it may be worth it to use precision linear rail in addition to the T slot extrusions.

Also, does the size of the extrusion affect its tolerances? My hypothesis is that larger extrusions will be more rigid than smaller ones, so they will bend less during manufacture. Maybe I'm wrong about that though.

I hope this doesn't come across as a ramble, I've just been thinking about it a lot because it's a very important part of any linear motion machine.

Short answer; I don't know.
Slightly longer answer; Contact a metals warehouse and ask for their specs.

DECADES AGO I built a utility trailer from steel tubing that I bought as off-cuts and rejects.
Barely perceptible bends and twists, but I was able to get long enough cuts out of what I bought where it was "straight enough".
That was STS in Marlborough Mass, they are no longer there.
Yarde Metals seems now to be the most local to me in New England, they do odd lots and small quantities.

Is this the sort of thing you're looking for?: http://us.misumi-ec.com/pdf/fa/2012/p2_0493.pdf
I expect that sort of data will vary by manufacturer and alloys used.

Precision measurement of long lengths can be a problem for simple tools. However, precision measurement of the differences in length is a much easier task, once you put them side by side in a square jig. As you said, the differences are what you really care about.

Perhaps you should work this in the other direction. What are your requirements?

That chart you linked to is pretty detailed, but it seems to give different results than what I found elsewhere. The chart says 1.3mm of bend for 300mm length, which is really terrible.

https://www.youtube.com/watch?v=3b18EixWRxo

I found that video a little while after posting this thread. From the video, the things look pretty straight! The chart also says that larger extrusions will be straighter than smaller ones (to an extent).

I don't really have an exact number for requirements, I just want to figure out if they're "good enough" for printers. There is a huge gap in price between buying T slot extrusions at 35 dollars each and having an alloy steel bar stress relieved, slot milled, and ground to flatness for several hundred dollars each (at least). The manufacturing methods are what add the cost in, since surface grinding is so accurate compared to everything else. And expensive.

Too bad nobody seems to make huge 4 foot long digital calipers!

When you start getting bit items, don't forget to add in temperature compensation. Pendulums on grandfather clocks normally have temperature compensation like a brass rod that shortens the iron rod pendulum to keep the length exactly the same over a wide range.