Bridge Gantry Profile

[EddyCurrent]

Well you seem to have made your mind up from the outset and the reality is it will most likely do your job so go ahead.

[~andy]

No. Not at all just exploring options. The base will be extrusion for sure, plate for the Z. Not fussed about the gantry, I just want something that is straight forward and easy to build.

Thought someone might have used that profile, or might be able to tell me how the numbers compare.

[EddyCurrent]

Did you look at that stiffness spreadsheet ?, at first I thought extrusion was not included but I found it in version 7 of the spreadsheet

[Boyan Silyavski]

No offence but this profile is a crap. Its not a heavy duty profile, just look at the design of it, with these holes and how the ribs are situated inside of it.

Of course depends how strong your machine you want to be.

Simply put: the ideal gantry alu profile is not born yet.

IMHO there are nice alu profile designs in the forums and the good ones have steel reinforcing plate on the gantry, one way or another.

The best so far alu implementation in a gantry i have seen is what Dean/JAZZCNC/ builds, so if you are for tidiness look there. And is not a profile.

This is the american way-bolting together profiles and having fear from some soldering. Shiny expensive useless machines. I was also a bit infected, thanks Dean that he cured me from the rack and pinion desire.

steel profile+square supported rails+ball screws=good machine worth spending some money for the build.

[routercnc]

Hi Andy,

In terms of how this section compares to others, I can only give you half the answer. The data they provide in the brochures is to allow you to then calculate bending deflection (from Ixx and Iyy) and bending stress (from Wxx and Wyy) depending on how you are going to load it. The G value is the modulus of shear (turning it into a parallelogram) which I don't think you can do much with for a gantry application.

They do not give you the twisting constant (which would allow you to calculate the amount of twist when you subject it to an offset load) because it would not give you an accurate answer as the profile is too complicated. There are values for simple shapes but all the others need FEA software.

The most useful thing for you is the Ixx and Iyy values. So to compare the performance I've shown the best calcs I can give you for 3 scenarios, one of which is your profile.

CONSTANTS (same for all cases):
Gantry length = 1m, Material = Aluminium, Cutting = Wood, Distance from cutting tool to centre of gantry = 150mm, Weight of Z axis 25kg

SECTION1 - RHS:
Width 80mm
Depth 160mm
Wall 6mm
Ixx 893cm4
Iyy 294mm4
X Deflection due to bending and twisting = 0.00117mm
Z Deflection due to sag from Z axis 0.007mm

SECTION2- Your Profile
Width 80mm
Depth 160mm
Ixx 944cm4
Iyy 183 cm4

I can't do further calcs on complex structure, but you can see that vertical bending will be no problem for 1 vs 2 (944cm4 vs 893cm4, but the horizontal bend and the twist (which is a much bigger factor) will be much worse for 2 (183cm4 vs 294cm4).

The best I can do is approximate your profile:
SECTION3 - Close approximation to your structure using I beam to achieve matching Ixx and Iyy
Width 120mm
Depth 160mm
Web 5mm
Flange 6mm
Ixx 989cm4 [ achieves similar to profile ]
Iyy 172cm4 [ achieves similar to profile ]
X Deflection due to bending and twisting = 0.041mm
Z Deflection due to sag = 0.006mm

Therefore you can see that in Z bending (due to weight of Z axis) it is very good and comparable to the RHS section (0.006mm vs 0.007mm).
But in X bending and twisting it is 35 times worse than the RHS (0.041mm vs 0.00117mm).

You might think that 0.041mm is OK for wood cutting, but remember that this is JUST the profile under ideal load transfer conditions (uniform distributed load) between all the bits attached to it. You will struggle to achieve UDL unless you apply very thick plates along the whole length, and then you are moving away from the simplicity. You also have to add all the other small deflections, plus the part that is impossible to calculate (for the DIY builder anyway) which is the effect of resonance effecting the finish.

If you ever want to cut Aluminium then the loads are about x10 compared to wood so your deflections will also be about x10. Thats 0.4mm in the X direction which is not really acceptable.

Hope this helps from a numbers point of view which is basically what you wanted to know.

From a recommendation point of view I would agree with the other posters.