Joe's 1000x600x200 steel and aluminium router started

[Voicecoil]

From plate I'm guessing I would form a channel 160x80 with the back open, question is would this be sufficiently twist proof?

Obviously it depends on how thick your plate is! BUT channels aren't generally that strong torsion (twisting) wise, a box is much stiffer weight for weight.

[devmonkey]

You are right!

I just used the demo version of shape designer, this is 16mm plate.

Torsion constants, for reference the 160x80 ITEM heavy profile 390cm^4

160x80x16mm channel is 39cm^4
160x80x16mm box is 1400cm^4

[Voicecoil]

Something else to think about..... 6" x 3" x 3/8" box section (available off the shelf): Ix = 352, Iy = 1152, torsion constant (J?) = 823 - and about 1/2 the price of Item 160x80, though you obviously have to drill some holes A lot of the slotted structural profiles don't seem to be that amazing torsion-wise, I guess it's because a lot of the metal is a fair way inside the extrusion, whereas in a box section it's all at the outside, where it does most good.

[JAZZCNC]

Something else to think about..... 6" x 3" x 3/8" box section (available off the shelf): Ix = 352, Iy = 1152, torsion constant (J?) = 823 - and about 1/2 the price of Item 160x80, though you obviously have to drill some holes A lot of the slotted structural profiles don't seem to be that amazing torsion-wise, I guess it's because a lot of the metal is a fair way inside the extrusion, whereas in a box section it's all at the outside, where it does most good.

Cheaper yes but it's not better unless it's prepped correctly and even then it's much more resonant. The profile is generally much flatter than box steel which isn't flat in any direction and it's less resonant, it's a lot easier to work with and does the job fine.

Torsion isn't an issue until gantry width gets wide and even then if sized correctly profile works fine for routers.

[devmonkey]

So I just did some maths, if 've got all my units correct, a 100N force on the cutter exerted at 300mm below the centroid of the gantry cross section and half way along the 1000mm Y axis would cause a deflection at the tool of approximately:

160x80x16mm channel is 39cm^4 - deflection 0.22mm
160x80 ITEM heavy profile 390cm^4 - deflection 0.022mm
160x80x16mm box is 1400cm^4 - deflection 0.006mm

I guess that 100N is unrealistic and would break the cutter first.

I'm going to have to drill all the holes anyway, is the bog standard 6" x 3" x 3/8" aluminium extruded box section flat and twist free with parallel sides?

I was looking at the Item profile extrusion since I thought it would give me parallel flat sides without having to process it. My mill table is only 700mm long, not quite enough to surface the entire gantry without re-locating the work which is difficult to do accurately in plane.

I'm also sure I've seem some pictures on here of peoples gantries constructed from plate?

[JAZZCNC]

I'm also sure I've seem some pictures on here of peoples gantries constructed from plate?

Yes probably seen gantry I've shown built from plate and few others used it or adapted to there machines. It makes for a very stiff gantry,

However, it needs a lot more work compared to HD profile. I've used all methods of constructing gantry's and HD profile is by far simpler with more than enough strength for any Router cutting materials up to Aluminium.

Aluminium Rect section doesn't compare to HD profile in terms of resonance and vibration dampening and this is should be more of a concern than twisting deflection you seem to be chasing if you want a good Router.
There is no cutter you will use that will withstand forces enough to twist a gantry made form any of these materials before it snaps. However, Resonance will affect any cutter and the smaller the cutter the worse it gets. My advice is to Stop chasing twisting etc and look more to lowering resonance if you want a good Router.

[Voicecoil]

Cheaper yes but it's not better unless it's prepped correctly and even then it's much more resonant. The profile is generally much flatter than box steel which isn't flat in any direction and it's less resonant, it's a lot easier to work with and does the job fine.

Torsion isn't an issue until gantry width gets wide and even then if sized correctly profile works fine for routers.

I was meaning box section aluminium, not steel, I'm aware of the problems with that. The stuff I got was pretty straight, the only error from square was a very slight (0.3mm) but even dip along the middle of the long sides, obviously the die designer didn't get his shrinkage allowance quite right

[devmonkey]

I was meaning box section aluminium, not steel, I'm aware of the problems with that. The stuff I got was pretty straight, the only error from square was a very slight (0.3mm) but even dip along the middle of the long sides, obviously the die designer didn't get his shrinkage allowance quite right

That is interesting, if the box is square to its corners then it is much simpler to skim it flat on the mill.I would need to buy a length in order to check it in the mill with a DTI, Aluminium warehouse if out of stock.

I'm still interested in looking into constructing the gantry from all this spare plate I have.

[Doddy]

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[devmonkey]

How do I make the images in these posts larger rather than just thumbnails?