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Thread: Couple of questions about my new mini mill...

  1. #1
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    Default Couple of questions about my new mini mill...

    I got my new mini mill (LMS 3990) mounted, and started milling some slots etc, just for practice. Everything is going great, but I had a couple of questions before I start a real project.

    Let's say I'm milling a .40625 slot in some 6061. Would it make any difference If I used a 3/8 or 1/4 end mill to start, and just move over for another pass to finish the slot? Or would it be better to just get a dedicated 13/32 end mill?

    And I've been using the speed and feed calculator on the LMS site. But is there a rule of thumb to know how deep I can cut on a given pass?

    Thanks

  2. #2
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    Never want to cut the slot full width, want to use a smaller endmill for a better finish. Slotting is actually one of the more finicky things to do. With CNC, it likes flood coolant. Issue is flushing away the chips so they don't get caught between the workpiece and cutter.

    Sent from my SM-G950U1 using Tapatalk

  3. #3
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    I've found that on less than rigid machines that with deep slotting cuts the cutter can start walking sideways until it breaks. One suggestion would be to drill a starter hole at one end of the slot (smaller and shallower than the slot), take relatively shallow cuts with a 1/4" endmill, say 1/16" to 1/8" to nearly full depth, then start cutting the sides at near full depth. Last cuts will be full depth to full width.

    Another tip is to get a couple of roughing end mills, they work wonders in small mills.

  4. #4
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    Well, that's good to know...

    In this case, I'm not too concerned with the finish inside the slots, they're just for holding magnets. Would that change how you would do the job? I'd rather save a little time than have a perfect finish.

  5. #5
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    Quote Originally Posted by wdfwguy View Post
    Well, that's good to know...

    In this case, I'm not too concerned with the finish inside the slots, they're just for holding magnets. Would that change how you would do the job? I'd rather save a little time than have a perfect finish.
    My primary concern is not abusing the endmill. You could chain drill the slot then clean it up with an endmill as well.

  6. #6
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    [QUOTE=RB211;1238056]Never want to cut the slot full width, want to use a smaller endmill for a better finish.

    this.
    .

  7. #7
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    Quote Originally Posted by wdfwguy View Post
    Well, that's good to know...

    In this case, I'm not too concerned with the finish inside the slots, they're just for holding magnets. Would that change how you would do the job? I'd rather save a little time than have a perfect finish.
    You should be concerned about the finish, if you want the slot to be at all accurate. If the interior walls of the slot are chewed up by the endmill and thus have a bad surface finish, then the dimensions arent going to be what you expect them to be, theyll almost always be oversized by a fair bit. My mill, for example, if i try to cut a 1/4" slot with a 1/4" end mill ill end up anywhere from .005" to .025" oversized, with the width of the slot varying along the entire thing. Course, thats just the detriment to the work, it also beats the hell out of the end mill.

    For a lot of things in life, and machining especially, standard practice becomes standard for a reason. You might have to make one pass instead of 2 to cut that slot, but your mill will like it more since it takes less HP and tool pressure for the smaller tools, your work will like it more for the same reasons, and youll like the end results more because youll have a slot with clean, straight sides on dimension, instead of a slot it looks like someone cut with a chainsaw. Good surface finish and accurate dimensions make everything on a project easuer

  8. #8
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    The above is generally correct, a slot THAT YOU WANT TO HAVE VERY ACCURATE should be cut down the middle with a smaller cutter, and then "trimmed out" to size.

    That said, if the slot is non-critical, perhaps just a clearance slot for a bolt to clamp something in position on a mill table, you are perfectly free to cut it to size in one pass of an "on-size" cutter. My preference is a "corncob", or "roughing" cutter for that type cut, both for ease of cutting, and because that type cutter is less sensitive to the forces that push cutters off-line.

    In general, I have never seen a slot that looked really rough just because it was cut with an on-size cutter, but when cut that way, you definitely can have errors of straightness, size, etc. Not usually big ones, but certainly several thou. For many needs, that is too much error, and the error is usually not consistent either. Cutting with s significantly undersized cutter allows those errors to be cut away by the trimming passes. The problems do tend to vary depending on the basic stiffness of the mill. A large solid mill will not have the issues that a minimill may have.
    Last edited by J Tiers; 05-15-2019 at 10:40 PM.
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  9. #9
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    [QUOTE=Mcgyver;1238099]
    Quote Originally Posted by RB211 View Post
    Never want to cut the slot full width, want to use a smaller endmill for a better finish.

    this.
    I do the same and my Mill is pretty sturdy.

  10. #10
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    I think you should start by lowering your expectations a little. Five places of decimals is too much, if you can hold 0.001" you will be doing well.

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