Aluminium profile CNC

[BriceO]

Thanks, is it the same problem for the Y axis? Should I place the rails on top and bottom or is it fine?

To be honest, I can't really imagine how it will bend and vibrate. I was thinking that the weight/force is supported by all the length of the extrusion.

[Davek0974]

Thanks, is it the same problem for the Y axis? Should I place the rails on top and bottom or is it fine?

To be honest, I can't really imagine how it will bend and vibrate. I was thinking that the weight/force is supported by all the length of the extrusion.

Yes it is, but, the ends are fixed - the weight/force now tries to act upon the whole beam but cannot move the ends as they are fixed - it will then act upon the point where the force is applied, if say it is applied centrally then the force will try and bend or push the beam in the opposite direction to the force - the beam will bend. The skill in designing beams is to transmit those forces to the ends of the beam without letting it bend - this is done by using heavy enough section material or bracing thinner section material with struts or triangular trusses.

Vibration is the same really, just many more, smaller forces that need dampening out or preventing in the first place - mass is a good way of stopping vibration or at least moving the harmonic point to somewhere outside the range of expected vibrations.

[BriceO]

Thanks for these explanations, I used the Item site to calculate deflection of the profile (http://product.item24.co.uk/en/home/...ral-41665.html) for 1200mm and 400N. If I do it right I have a deflection of ~0.01 mm.
I was thinking that Aluminium was a good compromise between weight/strength and offer a good flatness. I don't have enough experience to determine how bad is it.

Sadly, I already have these "light" profiles. Should I go for steel box section instead for X and Y? Do you have any advice plz?

[routercnc]

Thanks for these explanations, I used the Item site to calculate deflection of the profile (http://product.item24.co.uk/en/home/...ral-41665.html) for 1200mm and 400N. If I do it right I have a deflection of ~0.01 mm.
I was thinking that Aluminium was a good compromise between weight/strength and offer a good flatness. I don't have enough experience to determine how bad is it.

Sadly, I already have these "light" profiles. Should I go for steel box section instead for X and Y? Do you have any advice plz?

The deflection calculation will probably be just simple bending, so either simulates a plunge in Z or simulates the sag caused by the weight of the spindle assembly. This is usually easy to get a low value.
What really tests the machine design is cutting in the X direction (confusingly many US builders call this the Y direction, so be aware if you are reading different forums) because this puts a moment on the gantry and tries to twist it. This is usually the limiting factor and will result in much more deflection at the tool tip than the Z direction loading.

However, based on the fact you have the extrusions already, and you mainly want to cut wood, then stay with them and build it as per the top picture in post #26.

Also be aware that the aluminium profile is not especially flat unless you have it specially machined.

[BriceO]

Ok, I will do the best with this and maybe add steel plate to the back to add weight and strength.

I just received an offer from Chai for these components:

-2 HIWIN HG20 rails + 4 blocks hgh20CA ZA 1800mm
-2 HIWIN HG20 rails + 4 blocks hgh20CA ZA 1380mm
-2 C7 Ball screw RM2010 total length end machined 1760mm with ballnut
-1 C7 Ball screw RM2010 total length end machcined 1360mm with ballnut
-3 2010 Nut Housing
-3 Jaw Flexible Coupling 6.35mm*12mm (Ballscrew to NEMA23)
-3 BK15/BF15 P5 AC

Chai price is 656 usd Total. Air shipping cost is included.
Fred from BST Automation is USD1155.00 for the same items.

Is it normal to have a big difference?

[Davek0974]

656usd is amazing!

[Nr1madman]

656usd is amazing!

I agree.. take it before he finds out that he calculated wrong :D

Just a reflection.. with that long ballscrews and direct couplings the motors will stick out in the back/front where you have mounted them.
Have you thought about using pulleys or shorter ballscrews?

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[mekanik]

Hi Brice0
Could you post a link to Chai's store.
Regards
Mike

[BriceO]

Here is the actual design


But I will add pulleys as you recommend, I could probably keep the same length to add pulley?
I was thinking to add something similar to the design of JoeHarris:


Hi mekanik, here is the link of Chai Store: www.ebay.fr/usr/linearmotionbearings2008