Unsupported Precision Round Rail Calculator

[Ross77]

Hi Andy.

Try this version, I have back saved it in compatibility mode. It is best if you have a play with the figures as it with give an idea how the weight and length affect the deflection. and it might scare you.....:twisted:

You didn't state a bar size so a quick calc using 25mm dia. 1500mm span and 250N (25 kg) load the best case deflection would be just over a mm but worst case is 5mm depending on the restraints.

You really need additional cross rails to support the gantry sides as at the moment the rails are taking all the loads as well or you will have to assume simply supported.

I think the biggest problem you will have is the bounce caused by vibration. but that can be sorted by not letting the torch travel at resonant frequencies and keeping the acceleration and deceleration down.

Let know if you can open that one either.

[Normsthename]

Thanks Ross, that version loads fine
I have added a couple of lengths of 40mm aluminium extrusion to the model


I have one small problem of the unsupported rails being 30mm too short
I may need to move the X Axis rails on top of the box section to give me a bit more room to play with

I think the biggest problem you will have is the bounce caused by vibration. but that can be sorted by not letting the torch travel at resonant frequencies and keeping the acceleration and deceleration down.

One thing I want from the new machine is to keep the speed up on corners etc
My current machine has a really heavy steel gantry (I originally built is as a Router), and I have to keep the acceleration and speeds down on corners.
On straight sections it cuts parts to almost LASER Quality, when the machine has to slow down for corners it causes dross to build up on the corners :cry:

Andy

[routercnc]

I've updated the previous gantry stiffness spreadsheet with a few minor tweaks (one slight error corrected, factor 4 re-located). New version 3 below:

This will calculate the vertical deflection of various types of section, plus twisting and resultant tool deflection due to cutting forces (on routers etc.)

Instructions for use
1. Top left boxes:
Load applied to section
Cutting force in that material (see sheet for suggested values - thank you Irving2008 for these values, borrowed from stepper motor calc sheet!)
Length of beam

2. Next set of top left boxes are for the material.
Defaults are steel, but there is a lookup table off to the left for aluminium or wood (Douglas Fir). Copy over the 3 relevant values into Young's modulus, Density, and Modulus of rigidity.

3. Geometry
The shape inputs are next in rows 15 to 17. Purple boxes.

4. Results
The yellow boxes in row 25 and below show various outputs.

Enjoy !

[routercnc]

Hi Andy,

Your machine is starting to look really good. A suggestion (from a non-plasma expert!) is to consider running a simple bearing from the back of the Z axis plate onto the top and bottom of the upper 40mm rail. This would give a bit more support to reduce the sag, and possibly help with the resonance (if that is due to the round unsupported rails vibration). Just a thought . . .

In terms of running the x axis rails on top, rather than on the sides, I've always wondered if this was easier from a build point of view. As drawn the gantry has to be very precisely made so that it is a nice fit onto the rails. The rails or gantry might need to be shimmed to achieve this. If you put the rails on top then everything can be bolted together and settled before finally marking the holes into the frame. Maybe there is another way but this seems a bit easier to me.

Good luck with it,
Barry