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  1. #1

    bearing bores

    I'm a bit new to cnc. I'm trying to cut a bearing bore in steel tube (heavy walled) and am having a hard time with concentricity. Does anyone out there have a pretty good idea of depth of cuts for best accuracy? I've got sharp tools, rigid holding and so far have best luck with 2 finishing cuts at .002 depth, after my .050 roughing cuts. But I'm barely holding .001 tolerance.
    I know on my lathe, I get best repeatability with ~.010 cuts, but this is a different beast!
    The bore is 1.350 and is .440 deep.
    Any suggestions?

  2. #2
    Join Date
    Mar 2003
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    4826
    Concentricity has to do with the accuracy of chucking the part. But I'll assume that you're saying that its just generally not a good bore.

    What style of insert are you using? Are you having chip wrap nest problems that scuff the surface up? What feedrate and sfm?
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  3. #3
    I'm using a 1/2" 4 flute coated HSS end mill,climbing, finishing at 2500 rpm and slow 5 fpm feed
    At this slow feed rate, the chips are nearly non-existant!.

    I am holding it in a very rigid lathe chuck, mounted to the table. No flexi.

    I guess I'm really looking for some general advise about interpolating circles accurately with cnc. Its a Haas vfo

  4. #4
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    eschless it sounds like you know what you are doing and I cannot say I do, but .001 ain't shabby for a interpolated cut with an end mill. I would think it would be extremely difficult to get much better without using precision boring tools.

    Ken

  5. #5
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    Mar 2003
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    Here I had the notion that you were turning the part in a lathe!

    Okay, for really accurate holes in a mill, you would be best to use an adjustable boring head. If you have many holes to do, this would be the best way to make lots of good holes.

    HSS tools cannot be run at any old speed... you're going much too fast for it. The keen edge will be ruined. The dullness of the tool causes loads of cutter deflection. I'd recommend that you try to finish with a carbide endmill, if you must interpolate the finish cut.
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  6. #6
    That's what I'm trying to find out. Thanks.It sounds like I should use something like a Criterion boring head. I needed to hear it from someone who does this more than my limited experience. Have you ever heard of a Cat 40 Criterion head adapter, or a good way to adapt R-8 to Cat 40?

  7. #7
    Sorry, I missed that last post before I replied.
    It sounds like maybe I'm asking a lot from HSS tools. Problem with cnc is ya can't feel when a tool is cutting poorly. A friend suggests using indexed carbide insert tools only, and especially Iscar. Maybe I shoulda listened!

  8. #8
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    Apr 2003
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    Checking some software, the feeds and speeds are coming in at about 500rpm using a HSS .5 4 flute end mill cutting .100 deep at 6 IPM.

  9. #9
    Wow. I probably toasted the mill. It still feels sharp, tho. Why so slow with all that coolant?
    Any advise on tool speed software?

  10. #10
    Join Date
    Mar 2003
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    I don't think an insert cutter will do a good job. Buy a high helix carbide endmill and save it to do only the finish cut. It should last a good while. That's not saying that it will be as good as a boring head.

    I don't know what kind of mount a Criterion adapter uses, might be threaded? I imagine most anything is available in Cat40, though.
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  11. #11
    Join Date
    Mar 2003
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    HSS runs at about 100 sfm max. Maybe slightly more on a very light cut, but there is not much leeway. Carbide is more forgiving on light cuts
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  12. #12
    I just ripped my criterion apart. Its got a 7/8 thread , so there's a start.

    Found an adapter for $26 at Travers. Thanks for all the help!
    I guess I felt that you can interpolate anything perfectly...a bit idealistic there..
    uhhm. whats sfm? (proof I'm not a real machinist)

  13. #13
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    Apr 2003
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    I do not have the link, but just do a search for "ME Consultant"

    Not perfect but it is free, I use it all the time, it seems to be conservative in feeds and speeds which I like.

    Carbide, in my limited experience is a no brainer, I have found it always much better then HSS, big, big time improvement.

  14. #14
    Thanks. I'm cheap, usually to a fault, when buying tools. I guess there's a point where its 'false economy'!

  15. #15
    Just downloaded ME consultant. What a great simple program. Thanks. And thanks from my end mills....

  16. #16
    Join Date
    Apr 2003
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    Well they say there is :
    A time for war and a time for peace
    A time for laughter and a time for mourning
    A time for cheap tools and..........well there ain't no time for cheap tools

    You can take that to the bank

  17. #17
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    Apr 2003
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    I purchase most of my basic carbide stuff here:
    http://www.american-carbide.com

    OK, I am a little bit cheap as well

    great prices, excellent cutters.

  18. #18
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    SFM= surface feet per minute
    These numbers are the rule of thumb for machinists:
    HSS (high speed steel tools) run up to 100 SFM (in soft steel)
    Carbide 400 SFM roughing (in steel)

    To calculate SFM, determine the distance that a point on the circumference of the rotating element travels in one minute of operation. Whether the tool is rotating (as in a mill), or the work (as in a lathe) the same principle applies.
    SFM = (rpm * Pi * D)/12

    Note the division by twelve to convert the distance to feet instead of inches. Mmmm, I love the Imperial system!
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  19. #19
    Ok, so if the tool was a wheel, its how far it went in one minute? But how does the depth of cut get calculated? How do I know if I should take .100, .010, or .001? The cutter speed would not affect this would it?

    I would normally use experience to make the call, based on broken and blue tools from my past. Must be a better way?

  20. #20
    Join Date
    Mar 2003
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    Yes, you have the correct analogy.

    Depth of cut has no appreciable effect on SFM. Motor horsepower limits the feed and depth of cut, based on how many cubic inches of material you are removing per minute. This is called unit horsepower. You may find tables somewhere that give you this value. I don't really know them off the top of my head, I rely on experience more than anything. Usually, the workholding method or the vibration of the part itself prohibits cutting to the maximum unit horsepower on a powerful machine.

    In your case, you should operate just below the breaker trip point
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

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