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  1. #1
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    Aug 2008
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    Down force from milling cutters

    I'm scribbling some designs for a Z axis conversion for my RF30m drill/mill.

    I can estimate the force I need to apply to the cutter to force it to cut, OK. But, what kind of force can the cutter, e.g. end mill, produce in the down direction? (i.e. as it tries to screw itself into the work piece).

    Can I rely on a spring (pulling the quill upwards) for backlash removal? (If I can, then I probably use the existing rack and pinion thus keeping manual use as an option)

  2. #2
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    Quote Originally Posted by BillTodd View Post
    ....Can I rely on a spring (pulling the quill upwards) for backlash removal? (If I can, then I probably use the existing rack and pinion thus keeping manual use as an option)
    Why not have the spring pulling down? This means you may need a stronger motor to drive the Z axis but it guarantees your cutting pull will never overcome the spring pull because they are both in the same direction.
    An open mind is a virtue...so long as all the common sense has not leaked out.

  3. #3
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    Quote Originally Posted by BillTodd View Post
    I'm scribbling some designs for a Z axis conversion for my RF30m drill/mill.

    I can estimate the force I need to apply to the cutter to force it to cut, OK. But, what kind of force can the cutter, e.g. end mill, produce in the down direction? (i.e. as it tries to screw itself into the work piece).

    Can I rely on a spring (pulling the quill upwards) for backlash removal? (If I can, then I probably use the existing rack and pinion thus keeping manual use as an option)
    In one word...No.

    It's "fairly" good but it cheeses you off when you take big cuts.

    I'm part way into fitting a ballscrew to mine to alleviate the issue.

    Is your quill flange at the bottom 96mm?
    Keith

  4. #4
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    Quote Originally Posted by Geof View Post
    Why not have the spring pulling down? This means you may need a stronger motor to drive the Z axis but it guarantees your cutting pull will never overcome the spring pull because they are both in the same direction.

    I think I know what you mean (the weight of the spindle does this to a small extent at the moment, but springing the whole axis down against a motor, will rule out manual use (something I would like very much to keep - I don't want to have to boot a PC to drill a hole)

    I've figured out a way to remove (preload) the backlash from the rack & pinion without losing use of the fast feed handles. If I can find a backlash-free way of getting a ~30:1 reduction I might just be able to do it.

  5. #5
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    Quote Originally Posted by Kipper View Post
    In one word...No.

    It's "fairly" good but it cheeses you off when you take big cuts.

    I'm part way into fitting a ballscrew to mine to alleviate the issue.
    What mechanism do you have at the moment (BTW any pictures of your conversion?)

    Is your quill flange at the bottom 96mm?
    96mm sounds about right (I haven't measured it)

  6. #6
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    Quote Originally Posted by BillTodd View Post
    What mechanism do you have at the moment (BTW any pictures of your conversion?)



    96mm sounds about right (I haven't measured it)
    I'm still using the "vanilla" conversion, but took a picture of the ballscrew roughly in the place I want it...I have to make the casting for the quill yet.....I need coolant and compressed air for my mister.

    An image of the first (coolant only) pattern is in my online gallery at http://www.cnc-conversion.com/gallery , time is getting the better of me on more pressing jobs to get stuck in

    I believe (could be way off the mark) you can fit the ballscrew behind the quill on the RF30 using a rotating ballnut....sadly the gearbox leaves too little for me.
    Attached Thumbnails Attached Thumbnails Z thought.JPG  
    Keith

  7. #7
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    Quote Originally Posted by Kipper View Post
    I'm still using the "vanilla" conversion, but took a picture of the ballscrew roughly in the place I want it...I have to make the casting for the quill yet.....I need coolant and compressed air for my mister

    An image of the first (coolant only) pattern is in my online gallery at http://www.cnc-conversion.com/gallery , time is getting the better of me on more pressing jobs to get stuck in .
    Nice conversion I love the cast parts (but why did you keep the horrible Y bearing/mount?)

    BTW since you're using the original lead screws with steppers, you've got to make yourself a pair of roller nuts. I'd bet the lower fiction would give you another 5"/min (If it weren't so *********** cold I'd be in the shed making the Y nut)

    I believe (could be way off the mark) you can fit the ballscrew behind the quill on the RF30 using a rotating ballnut....sadly the gearbox leaves too little for me.
    Yes I believe so. I just like to be different. If all else fails I'll follow convention (perhaps with a quick release for the screw)

    BTW the flange is 96mm

  8. #8
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    Quote Originally Posted by BillTodd View Post
    Nice conversion I love the cast parts (but why did you keep the horrible Y bearing/mount?)

    BTW since you're using the original lead screws with steppers, you've got to make yourself a pair of roller nuts. I'd bet the lower fiction would give you another 5"/min (If it weren't so *********** cold I'd be in the shed making the Y nut)



    Yes I believe so. I just like to be different. If all else fails I'll follow convention (perhaps with a quick release for the screw)

    BTW the flange is 96mm
    The original cast iron part or the Al casting? I made the Al casting as is because I had a belt that length......I'm a Yorkshireman! and when I made I told myself I'd make a neater one "later" it works so I've left it, although my new spectacles tell me the belt pitch is wrong! I may convert it to have the motor etc at the rear out of the way of the coolant.

    I'm not getting rid of my ballscrews for anything, I still have the originals (they wear out fast with CNC!)

    96mm....I thought they all looked the same....I may cast a run of those.
    Keith

  9. #9
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    Quote Originally Posted by Kipper View Post
    The original cast iron part or the Al casting? I made the Al casting as is because I had a belt that length......I'm a Yorkshireman! and when I made I told myself I'd make a neater one "later" it works so I've left it, although my new spectacles tell me the belt pitch is wrong!
    The Ally casting is great. I just wondered why you had not included the thrust bearing mount in your casting.

    I may convert it to have the motor etc at the rear out of the way of the coolant.
    That's a good idea. Although it would mean drilling holes in the already none-too-stiff base if I did that.

    I'm not getting rid of my ballscrews for anything, I still have the originals (they wear out fast with CNC!)
    OIC I thought you were using the lead-screws.

    96mm....I thought they all looked the same....I may cast a run of those.
    Well, if I can't figure a way to use the rack and pinion, you may have a customer for one

  10. #10
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    Quote Originally Posted by Bill
    That's a good idea. Although it would mean drilling holes in the already none-too-stiff base if I did that.
    I think a 20mm hole and 2 8mm threaded ones won't be too bad?

    I did consider making an eccentric shaft to take the backlash out ala the rotary tables.....I mainly want it neater
    Keith

  11. #11
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    Quote Originally Posted by Kipper View Post
    I think a 20mm hole and 2 8mm threaded ones won't be too bad?
    True.

    I did consider making an eccentric shaft to take the backlash out ala the rotary tables.....I mainly want it neater
    ?

  12. #12
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    Quote Originally Posted by BillTodd View Post
    True.


    ?
    On the pinion....an eccentric to make it contact the rack better.
    Keith

  13. #13
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    Quote Originally Posted by Kipper View Post
    On the pinion....an eccentric to make it contact the rack better.
    Ah yes I see what you mean now Good idea. If spherical bearing could be fitted on the right/handle/drive side then a single eccentric on the left would be enough.

    Have you tried the two parts together on the bench to see if the gears will function properly?

    I was thinking about applying a lifting force to the spindle (hence the question about milling forces). By using a cable wrapped around a mandrel on the left side (<2 turns) to a spring tensioner on the quill flange bracket, I can remove the backlash in the rack and pinion without increasing the force on the manual handle.

    It would actually be better to pull the spindle down onto the pinion as Geoff suggested earlier, but I can't see away to do this.

  14. #14
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    Quote Originally Posted by BillTodd View Post
    Ah yes I see what you mean now Good idea. If spherical bearing could be fitted on the right/handle/drive side then a single eccentric on the left would be enough.

    Have you tried the two parts together on the bench to see if the gears will function properly?

    I was thinking about applying a lifting force to the spindle (hence the question about milling forces). By using a cable wrapped around a mandrel on the left side (<2 turns) to a spring tensioner on the quill flange bracket, I can remove the backlash in the rack and pinion without increasing the force on the manual handle.

    It would actually be better to pull the spindle down onto the pinion as Geoff suggested earlier, but I can't see away to do this.
    I considered both approaches, alas I decided that to reverse the spring on the spindle return was counter productive when I actually want zero (or close to zero) backlash in both directions and no handle remnants cluttering up the machine. I have a theory that what I don't have....can't go wrong
    Keith

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