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  1. #11
    I'd expect that the round rail would to be bowing with all that weight on it and in the bottom edge of image 0277 it looks like it is :-) Of course that might well be the lens of the camera giving that effect?

    What sort of length is the rail and weight is the table?

  2. Assuming those rods are about 500mm between end supports (based on pic 0068 and the size of the platter) the deflection in the centre is around 2um (2 x 1e-6m) per kg

    Won't bow as much as you think because the supports are fixed not simple.

    I'd guess that table weighs 25 - 30kg with the slide mounted - so deflection <0.1mm in the middle...

  3. #13
    Quote Originally Posted by irving2008 View Post
    Assuming those rods are about 500mm between end supports (based on pic 0068 and the size of the platter) the deflection in the centre is around 2um (2 x 1e-6m) per kg

    Won't bow as much as you think because the supports are fixed not simple.

    I'd guess that table weighs 25 - 30kg with the slide mounted - so deflection <0.1mm in the middle...
    Thanks Irving, I was thinking 900+ (based on ruler in pic 0263) and 40-60kgs! The other thing that has thrown me is the feet are not secured to the base...

    Seem to recall seeing a spreadsheet somewhere that someone did for unsupported rail, was it yours?... (of course I cant find it!)

  4. Quote Originally Posted by mocha View Post
    Thanks Irving, I was thinking 900+ (based on ruler in pic 0263) and 40-60kgs! The other thing that has thrown me is the feet are not secured to the base...

    Seem to recall seeing a spreadsheet somewhere that someone did for unsupported rail, was it yours?... (of course I cant find it!)
    Yes it was probably and no i cant find it either...

    900mm would be 12um per kg, so 60kg = 0.7mm assuming the legs dont rotate which is a good point, but they're quite wide . My 300mm was the length the slide is on from the feet furthest from the headstock to the next set, not the whole length. Its a whole new ballgame if you want to include the full length and the other sets of feet and their rotation. But even then it'll be less than 0.7mm which is worst case (unless you assume simple supports but that would be massively incorrect as well).

  5. #15
    A bit of general information. The distance between the two supports on the headstock is 300mm and between the headstock and the end is 600mm. I just weighed the tool post and it is approx 25kg including the xy table top. The rails are clamped in the supporting 30mm deep x 25mm wide solid aluminium rectangular section. The support legs are solid ali 25mm diameter and bolted into accurately 15mm diameter housings bored in the rectangular section. I thought when making it that I could run some threaded rod between the legs for support, but it has not seemed necessary. The feet are anti vibration pads and sit very firmly on the work surface top. I made it like that because I wanted it to be fairly portable so I could move it into the house over winter as it gets too cold to work in the shed. There at least I can write the gcode and test the outcome using a pencil instead of a cutter (would not dare try to machine something in the house!!). It has got progressively heavier (and me weaker) and this has become a 2 man lifting job. The original design idea was to make the headstock and work holding parts as separate unconnected pieces (and I may do just that at some stage in the future). But I later decided on the current setup cannot remember why. Its final incarnation will be as a six axis machine used to cut patterns in wood and pewter and etch in brass plate. The headstock will move linearly in the X and Z directions and rotate around Z. The tool will move linearly in the Y and Z directions and rotate around Y. I want to keep the two Z movements because the aim is to cut patterns on a profile outline such as the side of a bowl. I think this will simplify my code as the profile outline will be traced by the Z on the headstock leaving the Z on the tool for depth cuts.


    Alan

  6. #16
    Quote Originally Posted by alanb View Post
    A bit of general information. The distance between the two supports on the headstock is 300mm and between the headstock and the end is 600mm. I just weighed the tool post and it is approx 25kg including the xy table top.

    snip

    Alan
    Alan, what's the overall length of the round bar between the feet? 900?

    The spreadsheet I alluded to earlier is here; http://www.mycncuk.com/forums/showth...lculator/page3

    The figures Ross mentioned in and around that post were for "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."

  7. #17
    The tool post is on a 600mm length supported at each end. The headstock is on a 300mm length supported at each end.
    Alan

  8. Quote Originally Posted by alanb View Post
    The tool post is on a 600mm length supported at each end. The headstock is on a 300mm length supported at each end.
    Alan
    So, using the actual figures of 600mm and 25kg, deflection is between 0.147 and 0.073mm...

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