Scratching my head, breaking my bits ... Any clues about what causes this?


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    Default Scratching my head, breaking my bits ... Any clues about what causes this?

    Hi, a 50 y/o newb here.

    I'm totally lost with a weird problem in my aluminum parts - hoping someone with experience can help me.

    I have a Probotix Nebula router with DWP611 Spindle, which I use a lot to route wood and plastic. Coupled with RhinoCAM Mill - great combo.

    I recently had to run a series of cuts on 1/4" Aluminum ... And spending days trying to figure out this weird issue I'm having with my cuts.

    I know the min speed of 16000 is too high for aluminum, but that's all I have. My part, measuring about 6" at its longest, has some holes, some pockets and the last operation is cutting a piece out of the bar - that's where I hit a wall ...

    I make 1mm (0.04 inch) passes at 800mm/min (32IPM) to cut the piece away (5 bridges to keep it in place). The first 90% of the depth - everything is fine. All passes are perfectly aligned on top of each other. I spray WD40, remove chips, etc. Then all of a sudden once I get to the last two passes (esp the last one!), my endmill engages the part at full height (2x diameter of the tool) near the end of each circle. That is - pass starts fine near the entry point - then starts biting into the part at full height.

    Here is the part. I have succeeded cutting a couple, with part fixed in different orientation, and the picture is exactly the same each time. I get a TON of hair-like chips on the last two passes, and the tool screams bloody murder





    * RhinoCAM shows toolpaths are perfectly aligned.
    * I'm using conventional milling
    * Stock is screwed securely to the spoilboard
    * I tried positioning the part vertical or horizontal (thought it might be the CNC backlash) - but same result
    * I cut a channel about 1mm wider than the toll width (so, two passes per level with 1mm (0.04") stepover), thinking it was chip clearance problem (it is not)

    neddless to say - I end up breaking $30 carbide single-flute tools.

    Here is an end part for reference


    How can this be explained? Will climb milling help? (I dont see how it would) Bug in RhnoCAM? Some weird physics going on on the last two passes?

    Really appreciate any thoughts on this!

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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    According to your arrow, you are climb milling.

    The issue is most certainly a lack of rigidity. When you have rigidity issues, climb milling can be worse, because the tool can grab and get pulled into the workpiece.

    I would try conventional cutting, and decrease the depth /pass by 50%, and increase the feedrate to maybe 40-45ipm.

    Gerry

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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Quote Originally Posted by kamocio View Post
    Hi, a 50 y/o newb here.

    I'm totally lost with a weird problem in my aluminum parts - hoping someone with experience can help me.

    I have a Probotix Nebula router with DWP611 Spindle, which I use a lot to route wood and plastic. Coupled with RhinoCAM Mill - great combo.

    I recently had to run a series of cuts on 1/4" Aluminum ... And spending days trying to figure out this weird issue I'm having with my cuts.

    I know the min speed of 16000 is too high for aluminum, but that's all I have. My part, measuring about 6" at its longest, has some holes, some pockets and the last operation is cutting a piece out of the bar - that's where I hit a wall ...

    I make 1mm (0.04 inch) passes at 800mm/min (32IPM) to cut the piece away (5 bridges to keep it in place). The first 90% of the depth - everything is fine. All passes are perfectly aligned on top of each other. I spray WD40, remove chips, etc. Then all of a sudden once I get to the last two passes (esp the last one!), my endmill engages the part at full height (2x diameter of the tool) near the end of each circle. That is - pass starts fine near the entry point - then starts biting into the part at full height.

    Here is the part. I have succeeded cutting a couple, with part fixed in different orientation, and the picture is exactly the same each time. I get a TON of hair-like chips on the last two passes, and the tool screams bloody murder





    * RhinoCAM shows toolpaths are perfectly aligned.
    * I'm using conventional milling
    * Stock is screwed securely to the spoilboard
    * I tried positioning the part vertical or horizontal (thought it might be the CNC backlash) - but same result
    * I cut a channel about 1mm wider than the toll width (so, two passes per level with 1mm (0.04") stepover), thinking it was chip clearance problem (it is not)

    neddless to say - I end up breaking $30 carbide single-flute tools.

    Here is an end part for reference


    How can this be explained? Will climb milling help? (I dont see how it would) Bug in RhnoCAM? Some weird physics going on on the last two passes?

    Really appreciate any thoughts on this!
    The round rail machines lack the rigidity to cut aluminum very well, if you can tighten up the bearing blocks you may get it a little better, sounds like you have the wrong cutter also, pieces of aluminum stuck to the surface is not enough cutting fluid and spindle speeds running to fast, if you only have a router for your spindle then that too is part of the trouble as well, the spindle Bearings in these are not suitable for cutting aluminum

    Last edited by mactec54; 02-09-2018 at 01:30 PM. Reason: added more content
    Mactec54


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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Spindle is kinda light for this. You may want to break up the passes and take several cuts ...don't forget a .002/005 finish pass.



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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Gerry,

    Thanks for tips!
    I am indeed conventional-milling. Sorry, the first picture has the part turned 180 degrees - was struggling to take a better picture. Below is a part in its natural position with all directions properly indicated.

    If this is a rigidity problem - should I see less of it on an MDF part? I just tried cuttin it out of MDF, and upon careful observation - it I have the same issue!

    Is there a rigidity "litmus test"?




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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Appreciate help, Mactec54!

    My cutter is this: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
    1-flute Solid Carbide Upcut Spiral O Flute.

    I understand I dont really have a metalworking CNC. I'd be fine calling it a rigidity problem and sleeping well. But I have a feeling its not that. What really bugs me is that this is so consistent regardless of how the part is positioned. It is always at the same EXACT position, almost like a g-code is telling it to go there. E.g. say its a backlash/slop in the machine - I would expect the results to be somewhat different between cuts, esp if part is oriented differently, no?



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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Quote Originally Posted by kamocio View Post
    Appreciate help, Mactec54!

    My cutter is this: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
    1-flute Solid Carbide Upcut Spiral O Flute.

    I understand I dont really have a metalworking CNC. I'd be fine calling it a rigidity problem and sleeping well. But I have a feeling its not that. What really bugs me is that this is so consistent regardless of how the part is positioned. It is always at the same EXACT position, almost like a g-code is telling it to go there. E.g. say its a backlash/slop in the machine - I would expect the results to be somewhat different between cuts, esp if part is oriented differently, no?
    Yes this could be a programing problem do you have settings in your cam program, check to see if you have tolerance settings or something similar set them to .0001 if you are using inch system

    Then I would look at the spindle Bearings, and see if you can adjust the linear rail Bearings, some you can

    Mactec54


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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Try a 2 or 3 flute HSS and decrease the feed and increase the RPM if possible and like i said take a final pass .



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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Quote Originally Posted by Par4par View Post
    Try a 2 or 3 flute HSS and decrease the feed and increase the RPM if possible and like i said take a final pass .
    HSS won't last 5 minutes at that speed ( 16,000 RPM ) he is already running

    Mactec54


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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Would a single straight flute work better than a spiral?



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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Quote Originally Posted by kamocio View Post
    Would a single straight flute work better than a spiral?
    No better, go with a 1/4" cutter if you can, won't be as wimpy as the 1/8" cutter, 3 flutes could be better than the 1 flute you have, find an Endmill that is designed for cutting aluminum

    This is what it should look like, can be uncoated

    Attached Thumbnails Attached Thumbnails Scratching my head, breaking my bits ... Any clues about what causes this?-3-flute-endmill-jpg  
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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Can't go 1/4", as I have to pre-drill and pocked small holes.
    Also, given I can not go below 16000 RPM on a potentially un-rigid machine, would 3 flutes require even higher feedrates (thus exaserbating the rigidity problem )?



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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    Quote Originally Posted by kamocio View Post
    Can't go 1/4", as I have to pre-drill and pocked small holes.
    Also, given I can not go below 16000 RPM on a potentially un-rigid machine, would 3 flutes require even higher feedrates (thus exaserbating the rigidity problem )?
    You could do it in 2 operations, a 1/8" cutter is not very good for profiling and doing large pockets, the 1/4" cutter could also be a problem, because of speed and rigidity

    Where you able to change any settings in your cam software

    Mactec54


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    Default Re: Scratching my head, breaking my bits ... Any clues about what causes this?

    After lots of trial/error/broken bits and painful discoveries - the verdict is clear, and its exactly what you guys suggested - the gantry router is simply not rigid enough to mill this part cleanly. However, by hook/crook I got it to churn out usable and good looking parts.

    I take 0.5mm cuts, cut a channel (basically two passes per level 1mm apart - my theory being that it ought to help with chip evac.

    It still produces clumps of fine hairline chips on the last couple of passes, literally, looks like glass fiber (with straight hair) - but the bit seems to take it quite well, at least it does not scream as before. I think there is a positioning accuracy problem here as well, since on the last pass, the bit is only engages with soft MDF spoilboard (hardly any force on the bit) - but it still creats clumps of hair.

    Programming issues were resolved by taking it easy on large radius curves. For some reason, around those curves - the machine would slow down significantly and start burnishing the bit (I broke another one when going too slow around the curves).

    Thanks guys!



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Scratching my head, breaking my bits ... Any clues about what causes this?

Scratching my head, breaking my bits ... Any clues about what causes this?