
For those interested, I thought I'd post my updated stiffness calculation spreadsheet in a new post. This spreadsheet is intended for a typical moving gantry cnc routing machine, and may help with the design of the basic structure. X axis is the long axis which the gantry jogs along. Y axis is spanned by the gantry, and Z axis is up and down.
General process is:
step1. Select 'gantry top' tab and study diagram
step2. Enter required load, material and other data
step3. For your chosen section(s), enter geometry (e.g. RHS, tube)
step4. The deflection calculations are then shown below
step5. You can then select one of these sections to be used in the tool deflection graph
Repeat the above for 'gantry sides' tab
Finally, in the 'tool error' tab, enter the required accuracy and the section results selected in step 5 from the previous tabs are shown as an ellipse for the X and Y deflections. Z direction is just a bar graph. You may wish to add a safety factor to the required accuracy figures to account for parts of the machine not modelled.
New features in version 7 are:
Usual disclaimers apply.
 Stiffness approximations of commercial ali profiles (e.g. Hepco)*
 I value calculations for arbitrary shapes, not just RHS, round etc. This is a standalone calc and gives vertical bending deflection only.
 Gantry top bending in the X direction (previously ignored)
 Visual tool deflection error graph vs requirement
* method used is to create an RHS section with similar properties
Building a CNC machine to make a better one since 2010 . . .
Thank you very much :)
So much to learn.
Flash git.
Looking good mate , now I don't have to do it......:tup:
I see you have covered up the torsion calc bit, is that work in progress?
I like the bit about tool deflection and what is actually required, I have just got there from a spindle design point of view and it would suggest that the best place to look for weakness is the tool itself. Seems daft to design a really rigid machine with crazy fast rapids if you are only using small tools with a slow low powered spindle, theres so much to consider with this CNC game eh
Any way going off topic, save that for the spindle one....
:tup: thanks Ross and ecat.
Torsion  I don't know how to calculate the twisting (torsion) constant K for arbitrary shapes so it is work in progress. There's probably a parallel axis theorem equivalent. But all the regular shapes are done.
[EDIT: Bit of research and it's not possible to calculate the torsion constant for arbitrary shapes since they are not symmetrical and will warp, leading to large errors. Only option is to use FEA (Finite Element Analysis) software. Alternatively choose a regular shape which is closest to your arbitrary shape, and use Engineering judgement over whether your arbitrary shape is likely to be better or worse than this for torsion. Sections with more material distributed around the outside are the best for torsion e.g. sections approaching a tube.
The other work in progress is the router mounting plate tab  lots to do here. At the moment all the Z axis is assumed rigid, including the cutting tool.
The more I add to this sheet, the more I realise how much is missing, but hopefully it's a start.
Ross, I think you can still contribute. You were working on a Z axis weights sheet  that could be useful to tab at the front of this one because at the moment the Z axis weight (or mass!) is a user estimate.
Last edited by routercnc; 24082010 at 01:11 PM.
Building a CNC machine to make a better one since 2010 . . .
That makes me feel better, I thought I was just being lazy using FEA. :whistling: If you just want to calculate the K value and dont want full blown FEA then I tried this software and seemed very good. About £40 for a licence, if you go for it.[EDIT: Bit of research and it's not possible to calculate the torsion constant for arbitrary shapes since they are not symmetrical and will warp, leading to large errors. Only option is to use FEA (Finite Element Analysis) software.
or this is free working demo, cant save or print (think you can screen capture tho) aproximate k value but very quick for calculating composite beams.
sure paste it in, the spindle design bit has lead me to start at the cuuting tool so Im working on the feeds and speeds and required forces at the mo. so that should help as well.Ross, I think you can still contribute. You were working on a Z axis weights sheet  that could be useful to tab at the front of this one because at the moment the Z axis weight (or mass!) is a user estimate.
I just want to say "thanks" to routercnc for this very useful design tool. It has quickly helped me to make some choices on the gantry design. I was surprised at the efficiency of box sections over simple plate, which most aluminum deisgn home routers / millers use. I had trouble with commercial extrustion section, but found the formulae in the "item" (Bosch  Rexroth) catalogue for deflection and torsion. There is also a free useful program "Tech Toolkit" produced
http://www.8020.net/DesignTools26.asp by the American outfit 80/20 which calculates the deflection for their profiles and you can define your own sections in it if you know the material and moments of inertia. Overall well done and so useful!. Measure twice cut once eh!!!!
can some one do a quick video and post it, phew, too much to take in and i am having headache trying to work out haw to fill in
help please
What a awesome calculator. Thanks a lot !
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