4' x 4' x 8" Router Build

[JonnyFive]

Neal / Jazz - thanks for the feedback.

I'll look at thickening things up, it does add up quite a bit - going from 50x50x4 to 50x50x5 to 60x60x5 each jump adds about £100 but I only want to build it once so want it to be right.

Jazz - I was only thinking of sand filling the static parts, I wasn't intending to fill the gantry - I'd be much happier not to have to do this. The gantry will be the same 160x80 ali extrusion whichever way I go with the frame.

Is the 50N loading a fair number?

Cheers!

[JAZZCNC]

The gantry will be the same 160x80 ali extrusion whichever way I go with the frame.

Is the 50N loading a fair number

The 50N loading in which direction.? However, to be honest, no matter the direction the correlation between simulation in SW and real-world when it comes to cutting and vibrations etc won't match.

What I can tell you from experience of building god knows how many steel-framed machines is that the extra cost of using thicker steel pays big time regards vibrations. The loads going into the machine don't come close to stressing the steel so it's not something worth fretting over.

Regards the Gantry then again most of my machines use HD ITEM Profile. If you want a much stiffer gantry then I'd suggest you change design slightly and go with L shape setup like I use. Over the years I've used both setups and the L shape is far far stiffer and two pieces of 120 x 80 arranged in an L shape gives much stiffer gantry with lower vibrations than single 160x80.

[JonnyFive]

The 50N loading in which direction.?

I'm referring to the reaction force of the machine pushing the cutter through the material so could be in any direction depending on which way the router is moving. My only reference point is using a hand router and it doesn't fell like it needs 50N to push the router along. I'm talking about the cutting force as shown on this diagram:

two pieces of 120 x 80 arranged in an L shape gives much stiffer gantry with lower vibrations than single 160x80

Do you mean 120 x 60? I can't find 120 x 80. I have run the calcs on the L section and I'm amazed how much difference it makes - you've convinced me! Do you have any examples of the machines you've made, I'd love to see them.

Cheers

[JonnyFive]

Do you mean 120 x 60? I can't find 120 x 80

Don't worry, I've found it - I was looking in the Profile 10/12 sections, it's in the Profile 8.

[JAZZCNC]

Don't worry, I've found it - I was looking in the Profile 10/12 sections, it's in the Profile 8.

Yes, the 8mm slot is better because gives more area for profile rail to sit onto.

[routercnc]

I've found the 50 N force in X and Y directions to be a good guide but it is only the start. If under this simple load condition if you are <0.05 mm then I would call that a fair start point, and you would expect a fair amount of accuracy on the finished dimension.

But wait, this 50 N is a simple static force, and does not account for the repeated impacts of the cutting action in the material which will cause a ripple on top of this average force, say from 45 N to 65 N (making the numbers up to show the point) with every rotation. As the cutter rotates very fast this will cause vibrations in the machine, not only at the main impact frequency but across a wide frequency range with amplifications where the machine frame resonances are and this may show up in the finished part.

This is where the wall thickness will help you as Jazz is advising, and this is money well spent. It adds significant stiffness (more than might be suggested is required by the simple static load analysis), which will lower the vibration levels as higher frequencies decay quicker and as they are stiffer they also deflect less initially with each impact. It also adds mass which will also lower the vibration (acceleration) levels as shown by re-arrangement of the famous F=ma equation ; a = F/m). So it is a win-win up to the limit of what your steppers can accelerate around.
Technically you would not have added much damping (energy absorption through friction, e.g. damping pads or sand) but you will still have reduced the vibration amplitude through the mass and stiffness which is all that matters.

If it helps I went through the same thought process on my mk4 machine for the upper box section members on the bed. I went with 5 mm wall in the end, as every mm costs £, but I know it makes a difference.

[Gustave]

Hello JonnyFive, finally have you done the assembly of 2 x 120x80 suggested in the discussion or do you stay with 160 x 80 ?
Gustave

[JonnyFive]

Hello JonnyFive, finally have you done the assembly of 2 x 120x80 suggested in the discussion or do you stay with 160 x 80 ?

Hi Gustave, I haven’t done any assembly yet, I haven’t been able to spend any time on the project recently and I’m still tweaking the design. I will be using the 120x80 when I get around to it, the maths says it will be a lot stiffer.

Cheers.


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[Gustave]

Hi Gustave, I haven’t done any assembly yet, I haven’t been able to spend any time on the project recently and I’m still tweaking the design. I will be using the 120x80 when I get around to it, the maths says it will be a lot stiffer.

Cheers.


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Thank you, I think I will also going on L shape based on 120x80.
I am designing a nearby machine and I have the same R&P and ball-screw dilem ! For the moment I think going on ball-screw for price (Fred BST), precision, easy to achieve. The X is 1800mm length and I plan to use 2 x 2510 ball screws. I bought servos motors (Simdrive AC system 750W, Holding Torque (N.m)
2.4, Peak Torque (N.m) : 7.2, Rated Speed (rpm) : 3000) and I hesitate between direct drive or the use of pulley with a ratio 2: 1. Do you have any advice for me?
Best Regard