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  1. #11
    Ross77's Avatar
    Lives in Devon, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 8-9 years. Has a total post count of 660. Received thanks 25 times, giving thanks to others 27 times.
    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.
    Attached Files Attached Files

  2. #12
    Thanks Ross, that version loads fine
    I have added a couple of lengths of 40mm aluminium extrusion to the model

    Click image for larger version. 

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    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

  3. #13
    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 !
    Attached Files Attached Files
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  4. #14
    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
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  5. #15
    Ross77's Avatar
    Lives in Devon, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 8-9 years. Has a total post count of 660. Received thanks 25 times, giving thanks to others 27 times.
    Glad it works and is of some use to someone

    Your machine is looking good. what CAD package is that?

    With regard the rail orientation then I don't think it really matters for a plasma machine. Mounting them on the side will make the most of their strength and equal out the load capabilities but as you are only moving a light gantry then you shouldn't be getting any where near the max strength of the bearings. There is also the point that the cheaper open bearings can bend and open up even more if mounted on their side.

    I've not had anything to do with plasma or even looked into it but it seems to me that if the z axis only has to hold the weight of the torch then there is scope to really reduce the weight of it. Which will help with the deflection and vibration.

    I would have thought that you could use sheet metal instead of the plate, get the smallest dia ballscrew (or use belts) and 8mm round rails. Really lean it out.

    Is that a double travel arrangement that you have on the z axis, again not a plasma expert but it looks like its just adding extra weight

  6. #16
    Ross77's Avatar
    Lives in Devon, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 8-9 years. Has a total post count of 660. Received thanks 25 times, giving thanks to others 27 times.
    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
    While this seems to make good sense it would appear to me to transmit any imperfections in the ali section to the rail and carriage. If you go down this route then there is no reason to use the precision rail and and I would just mount more bearings and run it on the Ali sections full stop.

    Barry, I see that is how you have done it on the x axis on your machine, do you get any problems with surface Finnish or the bearings compacting the Ali and causing a groove?

  7. #17
    what CAD package is that?
    It was drawn on Solidworks
    I am lucky in having use of Solidworks thru' the Company that I do work for
    It is a brilliant package for designing CNC Tables, you can animate all the gantry movements etc
    The picture I posted was created after I loaded the assembly into Photoview that creates a 'Rendered' view of the model
    This is what the model looks like when actually designing it.

    Click image for larger version. 

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    The weldments tool is brilliant, I drawn the complete framework for the table in 20 minutes

    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 . . .
    Not a bad idea
    Is that a double travel arrangement that you have on the z axis, again not a plasma expert but it looks like its just adding extra weight
    That is a vital part of a Plasma system
    It is a 'Floating switch'
    What happens is Z axis lifts up and moves to its pierce position, the Z axis then lowers until the Torch makes contact with the material.
    The torch is fitted to the second linear slide, and just rests using gravity.
    When the torch touches the material it trips a micro-switch, this then sends a signal to Mach3 and zeros the Z DRO
    I will start a new build thread for my new machine, keep the suggestions coming!

    Andy

  8. #18
    Hi Andy,
    I was also wondering about the double Z axis arrangement, thanks for explaining.

    Ross,
    Yes, there are some risks from bearings on ali section. My machine is still some way from running yet so I don't know how it will cope with bearing surface wear. I picked some reasonably sized bearings to reduce the surface stress. The bearings are on axles which can be adjusted in slots, so I can add a running surface at a later date.
    Since I'm only machining balsa and liteply, and all 2D (formers, ribs etc.) , my requirements are more simple than some and I think this will be OK for my needs. I know what I'd really like to do, but it's all a question of finance !

    Barry
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  9. #19
    Ross77's Avatar
    Lives in Devon, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 8-9 years. Has a total post count of 660. Received thanks 25 times, giving thanks to others 27 times.
    It was drawn on Solidworks
    Thought it looked expensive....

    That is a vital part of a Plasma system
    It is a 'Floating switch'
    What happens is Z axis lifts up and moves to its pierce position, the Z axis then lowers until the Torch makes contact with the material.
    I see, still seems a bit overkill :heehee:, couldnt you just use a circular spring loaded mount on the torch?

    Ross,
    Yes, there are some risks from bearings on ali section. My machine is still some way from running yet so I don't know how it will cope with bearing surface wear.
    I will be inrested to know how you get on as the ali beam i just bought uses a similar idea but with rubber bearings and I was wondering how well it would work if I replaced them bearings.

  10. #20
    Updated stiffness spreadsheet to version4, see post #3 in this thread.

    Now has much clearer layout, drop downs for some selections, and added Ross's end conditions for bending load case.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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