Interesting read for warping "TACO" bed

[Buurman]

I came across this and I wonder, I hear a lot about "taco" beds, but, it may not be your bed at all??

It may be your gantry, if I read this, the rails on your gantry expans different than the aluminium gantry itself, creating a bi-metal warping...

To me this sounds way more possible than the bed curving. I dont have the "taco" effect on my bed, but if you do, maybe a expensive new bed is not the answer?

https://github.com/tanaes/whopping_Voron_mods/tree/main/extrusion_backers

 

[ken226]

I have it.  This bed mesh was take at 110c, while printing some ABS parts.   It's very flat at room temp, and about half this much taco shape when printing PLA at 50c.

I might have to get some of those backers.   Though, they may require taller cable chain anchors.

 

 

Kind more of a bowl bed.  High at all 4 corners by .1mm 

Totally makes sense though.  With the steel linear rails attached to the bottom of the aluminum 2020 rails,  the aluminum would expand more than the steel and create a bow that is high in the center.   So, the probe would read closer to the bed at the corners and further away at the center.  On the Y axis anyway.

 

I'd be interesting in seeing the bed mesh difference with backers.

 

 

Edited November 23, 2023 by ken226

[YaaJ]

Also interested. Can source (or make) backers. Making backers is a least attaching some steel to the opposite side. Also been wondering if bolting the bed on the milling machine, heating it up for a little while, unbolting rebolting it, and then machining it with a large fly cutter could be a thing... And what about the fact that the bed on a v2 is subject to hyperstatism ? (as well as all kinematic and static parts in the chamber).
This was evocated by Stephan in his last interview video (CNC Kitchen), but no trade secret was revealed...

Something I was wondering : what about preconstrained extrusions : a 4mm steel rod could be mounted inside the extrusion. Like prestressed concrete.

[EDIT] you didn't tell. What X linear rails ? On front or on bottom or on top ?

Edited November 23, 2023 by YaaJ

[mvdveer]

18 hours ago, ken226 said:

his bed mesh was take at 110c, while printing some ABS parts

You can also have a look at this - may be helpful.

(Github direct link:  https://github.com/JiMcNulty/FDC)

 

[ken226]

Any of you using the titanium backers that West3d and others are selling?  Wondering how much of a difference they make in practice?

 

 

 

I ordered a set, and printed a new XY chain anchor that raises the cable chain 13mm.  That should make more than enough for the 3-1/2mm thick backers.

Academically,

Most carbon steels, and 400 series stainless steels have a coefficient of linear expansion of about 5.5x10^-6 inches per inch per degree fahrenheit.

Aluminum is about 13.3x10^-6 inch per inch per degree, so aluminum expands at more than double the rate of steel.     For perspective, a 350mm  aluminum rail if heated from 70f to 120f,  will expand by .0092 inches.    The same 350mm length of stainless linear rail will only expand by .004".

Titanium's coefficient of linear expansion is 5x10^-6 inches per inch per degree. Almost exactly the same as stainless steel.  A West3d titanium backer 350mm long would expand by .0035"   

So, though stainless backers made of the same material as the linear bearings would be ideal, titanium is very, very close.  And lighter.  Theoretically, titanium backers should do pretty well.

All of my values above were taken from:

https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

 

 

 

Edited December 7, 2023 by ken226

[chrisalbertson]

It's now a year later.  We know the Ti backers work.  They work a little better than you'd think because they are stiffer than steel.  The best-reported results come from using "junk" Chinese linear rails of the same dimension and material as the "good" rails as backers.

But. lately, I had another idea:  

1. The problem is not the rails but the aluminum material in the 2020 extrusions.   What if we replace the aluminum with some other material that has expansion to match the linear rails?
2. Or if we place a thin Teflon sheet between the linear rail and the aluminum and use nylon screws to allow the two different metals to slide past each other?