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  1. Quote Originally Posted by Vic66 View Post
    ...I'm not certain what you mean about controlling the racking with belts or wire ? do you have any links to something that has used something like that ?
    He means like this

  2. #12
    Thanks for the suggestions, I had considered using counter holes in box section but theres an awfull lot of holes, and I want to keep it simple and as quick as possible to build.

    I did run some stress tests when I went for the channel, cant remember the results but it didnt deflect to much, I've no idea what sort of forces are involved, I just put a force of 500N in various directions at the tip of the router, I will re run them and post the results.

    If the channel twists to much I have thought about closing the open end with some folded ally channel as a work around.

    Thanks for the link and suggestion to the cable anti-racking system, I like the look of that, it appears straightforward to do and could be done afterwards if I get problems so I think I'll stick with the single center ballscrew and use something like this if it becomes a problem.


  3. Generally cuttting forces can be estimated as 5N (light wood), 20N hardwards/light alloys, 75+N cast iron, 100N+ steels, 150N+ stainless steel/hardened steels. A first approximation can be taken by using one of the many web-calculators for cutter spped/feed rates/cutting power based on a given material. Knowing the required power and revs and the cutter radius the (max) cutting force that can be achieved without stalling the cutter can be estimated from Force (N) = Power(W)*60/(cutter-radius(m)* 2pi * revs(rpm))
    Last edited by irving2008; 12-01-2010 at 09:49 AM. Reason: fix formula

  4. #14
    Irving, thanks for that, I am unfortunately just a basic steel and ally basher and maths is not a strong point, I would appreciate it if you could help me out with the calculation, I currently use a rotozip for a spindle, I want to change this as soon as I can afford to, I think this is about 800w 20k rpm, I imagine the largest diameter router bit I would use would be 10mm and the worst material would be MDF which I understand needs a fairly quick feedrate.

    I didnt have much spare time last night but quickly ran a test applying a force of 500N in the Y axis direction at the tip of the router with it at its lowest point produced a deflection of about 2.5mm !! much more than I remember although I have changed things a bit since the last run, I think that the channels on the side of the machine will be ok, but I need to change the X axis channel to something more substantial, I think that the 500N force is possibly over the top it is a stab in the dark !!

  5. 800W rotozip, 60% efficient (guess but typical) = 480W cutting. Ity probably doesnt maintain 20krpm cutting, say 15krpm

    then Power = torque * revs, torque = Power/revs where power in Watts, revs in radians/sec (= rpm*2pi/60)

    so torque = 480/ (15000 * 6.28/60) = 0.3Nm... for a 10mm cutter (radius = .005m) max force therefore = 0.3/.005 = 60N although you'd never actually get to that in practice...

    500N as a test force (how did you measure it BTW) is probably overkill!

  6. #16
    Thanks again !

    I have applied a force of 100N in the Y axis direction and X axis.

    Visual representation is exagerated for clarity (I hope ! ) Red area denotes maximum deflection, arrow direction of force.

    I need to either replace the channel for the X axis or strengthen it somehow, I've no idea if .5mm deflection is acceptable on a router or am I being to fussy ?
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  7. 0.5mm is OK if cutting wood as you'll never get close to 100N, 10 more like it... .05 would be a problem for metalwork. I'd still use box section, but i like to overengineer anyway...

  8. #18
    Does anyone know how much forces those v bearings are able to withstand as they do not look taht capable.

    Good for taking thrust in one direction but not at 90degrees.
    Just an observation on the mechanical loadings put on the bearings as all side load will be dependant on the fit of the ball races in the v bearings, to my mind the design lacks something.

    Just my 2p worth.

  9. Peter,
    I agree, If this was box section I would put a couple of skate bearings at 90 degrees onto the side of the box to contain lateral forces. Or better still use pairs of v-bearings at 90 degrees onto the supported rail as per diagram... (obviously not to scale)

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  10. #20
    According to Hepco, The dual V bearings I have (W3) are 5900N raidal and 1701N axial, literature here...

    I aquired 12 of them from work for nuppence they were left over from a job, and based their use on Joe's Hybrid design here...

    I was a bit skeptical about running them on steel angle rather than the ground V they are intended for but Joe seems to have had success with his design.

    I originaly went for box, attached pics of the unfinshed box section model below, I could return to box section, I've been thinking about what Irving said in a previous post about cutting access holes for spanners etc, this would mean re thinking the position of the lead screw on the X axis though, which could make the X axis carridge more complicated.

    I would rather not buy any supported rail, financially I have to run with what I have but I take on board the comments made about the side load on the bearings.

    With the forces Irving calculated would this be an issue though ?

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