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  1. #11
    Hi Johnathan

    personally I wouldn't go for that length of travel on unsupported rail. The calculation you need is for a cantilevered beam, just you have it mounted vertically. As a rule of thumb I look at third points, so for a cantilever of 400mm you would need a 1200mm rail with the mounts 800mm apart. (not very feasible)

    If this is a router then I guess this will then be mounted on a y axis gantry and needs moving as well so overall weight will be an issue. Is there a reason you need such a long z or could you just change the bed height

  2. #12
    Quote Originally Posted by Ross77 View Post
    Hi Johnathan

    personally I wouldn't go for that length of travel on unsupported rail. The calculation you need is for a cantilevered beam, just you have it mounted vertically. As a rule of thumb I look at third points, so for a cantilever of 400mm you would need a 1200mm rail with the mounts 800mm apart. (not very feasible)
    600mm is about as much as is practical allready. I read somewhere a rule of 2:1, i.e. if the bearings are 200mm apart then it's acceptable to have the force (of the cutter) 400mm away.

    Quote Originally Posted by Ross77 View Post
    If this is a router then I guess this will then be mounted on a y axis gantry and needs moving as well so overall weight will be an issue. Is there a reason you need such a long z or could you just change the bed height
    Yes it's a gantry router. I can think of things I'd like to make which need that long a Z, maybe not quite 400mm but close enough.
    With regards to accelerating the mass of big Z axis/gantry I should be ok as on the X-axis I'm using two 3Nm steppers @ 70V for the and spinning the nuts, not the screws. The nut is currently held between two tapered roller bearings with a pulley in between.

  3. #13
    600mm is about as much as is practical allready. I read somewhere a rule of 2:1, i.e. if the bearings are 200mm apart then it's acceptable to have the force (of the cutter) 400mm away.
    As far as I know thats for friction bearings like the igus or dovetails like a mill/lathe. and is to stop/reduce binding, not for resisting the cutting force.....

    The thing to remember is that all machine design is a compromise, and as such it will be designed for a specific task, if you need 400mm of travel then you will have to put up flex and vibration at certian points and reduce feeds and speeds accordingly.

    The other point is that it will only be as good as the weakest bit....theres no point in have expensive powerful motors and drives and then using unsupported rail... it just dosnt make sense.

    I hope I dont seem negative or put you off but rather offered some pointers to another route.

  4. #14
    Did you save the transformer out the microwave ?
    John S -

  5. #15
    Quote Originally Posted by Ross77 View Post
    As far as I know thats for friction bearings like the igus or dovetails like a mill/lathe. and is to stop/reduce binding, not for resisting the cutting force.....
    I've just checked the Igus datasheet and that is indeed the case.

    Quote Originally Posted by Ross77 View Post
    The other point is that it will only be as good as the weakest bit....theres no point in have expensive powerful motors and drives and then using unsupported rail... it just dosnt make sense.
    I've already got the expensive powerful motors and drivers on my milling machine. I intending to use those motors on the router and put some 1 Nm ones I've got lying around on the router.

    Quote Originally Posted by Ross77 View Post
    I hope I dont seem negative or put you off but rather offered some pointers to another route.
    On the contrary - you're not putting me off, yet!

  6. #16
    Quote Originally Posted by John S View Post
    Did you save the transformer out the microwave ?
    The microwave itself didn't catch fire - sorry I exaggerated a little there. Still it was pretty big flames! So yes the microwave still works fine.
    I do have a couple of microwave transformers. I did take the 2KV secondary out of one with the intention of rewinding it for my stepper motors, however I got a toroid cheap on eBay so I left it.

  7. #17
    Did you save the transformer out the microwave ?
    Do you want George to enter this thread????????? its been nice not seeing his name on every one:naughty:

  8. #18
    Based on a point load I get 0.07mm deflection with the two 20mm unsupported rails and the 150mm bearing spacing in the drawing. I'm not sure how to calculate the torsional deflection...this is getting a bit beyond what I've just done at school and I'm too tired to start reading up now so I'll leave it for tomorrow!

    Quote Originally Posted by Ross77 View Post
    Do you want George to enter this thread????????? its been nice not seeing his name on every one:naughty:
    I'm happy for anyone to make a valuable contribution :)

  9. #19
    this is getting a bit beyond what I've just done at school and I'm too tired to start reading up now so I'll leave it for tomorrow!
    Ditto. except I've had too much to drink, What point load and what end fixity are you using?

  10. #20
    Hi Jonathan,

    The drawing helped, thanks. Wow, alot of overhang.

    You could design a lower 'floating bearing'. This is where the lowest bearing block has two or more alternative mounting positions on the Z axis part which holds the router. For long reach work you bolt it where it is now, and accept the deflection. For shorter reach work you bolt it further down, spacing the bearings out so that the loads are taken towards the ends of the unsupported beam, rather than towards the centre. You need a moveable limit switch with this idea.

    Another thought, although it doesn't really work with your requirements, is to support the rail at the centre, as well as the ends. The bearing blocks then run in the 'upper part' above this centre support, and the 'lower part' below this centre support. Your rail deflection is instantly halved, but there's a compromise with Z range.

    It looks like you are using an 'I' beam for the router mounting plate. You're only considering the bending in one or possible two directions, which will give you good results on the deflection for these load cases. But since the router or spindle will be mounted off this, and the cutting forces will be offset, there will be a twisting force (Y direction cutting) and this section choice will give you poor results, particular with that overhang. Overall it might be worse than the original plate, all loads considered.

    For such an overhang, you might want to choose RHS for the part which attaches to the router/spindle. This is good for all the load cases you are requiring. Ideally any bolting into this section should go all way through with spacers inside to get the loads into the section, rather than locally deflecting the wall. The calcs all assume good load transfer.

    As Ross has already suggested, I suspect you don't need to mill down 400mm into things, but rather you want to be able to machine to a depth of perhaps 30mm into things which are up to 400mm deep. If this is the case you could limit your Z travel to say 100mm. You then block up the workpiece with spacers to suit. This would be fairly stiff for all cuts, but give you options in terms of workpiece sizes.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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