Bridgeport head movement
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  1. #1
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    Default Bridgeport head movement

    I have a Bridgeport Series II Special which I do various milling and boring projects with. Yesterday, I was milling a partial slot into annealed 4140, using an 1-1/4", 6-flute helical HSS cutter mounted in a good R-8 toolholder (not import). 150 RPM, 0.25" DOC, 2.5 ipm feed, 50% diameter cut. I made one pass in conventional and one in climb. The conventional cut was about 0.010" too deep except at the end of the cut, where is was on the mark. The climb cut was about 0.010" too shallow and tapered.

    It seems to me that the movement had to come from the head assembly rather that a flex in the cutter or tool holder. I have had trouble maintaining verticality in the spindle and have trammed the head many times in both axes. I have been careful not to over-torque the head bolts in accordance with manual instructions. I intend to make a test cut with an appropriate indicator mounting to observe any movement in the head.

    Does anyone have any thoughts on this issue or some advice to find the culprit and a cure? I realize that the cutter size may be too large for this machine and therein may lie the entire problem. Is it possible that the former owner(s) over-torqued the head bolts and warped the surfaces to the extent that the bolts don't do their jobs anymore? Ordinarily, I use 3/8" - 3/4" HSS and carbide endmills but, even then, I have experienced the problems with maintaining head alignment.

  2. #2
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    If the cuts are in the correct locations (i.e. dimensionally where they should be) then the problem is the head or ram and not the bolts that are used secure it and provide for tram.

    However, if there is some noticable "dish" in the resultant cut, there's a chance that the head is flexing left-to-right

    If it were the bolts (and the tightness or lack thereof), the location would be lost.

    Some things to check.

    Make sure the ram and swivel-to-column bolts are tight. Tighten the rear-most bolts first.

    Make sure the quill lock is holding.

    Often times when taking heavy cuts, it's a good idea to run the quill stop nut up against the stop bracket (dont know the correct name) and leave it there. Even tho' the spindle lock may be as tight as practical, the spindle can still move under extreme conditions. Use the knee for the depth settings.

    Although less likely, check to make sure the spindle bearings and retainer flange are tight. Sometimes the retainer flange can get loose.

  3. #3
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    I am no expert on how hard to push a BP but based on comments I have seen in this forum, you may be pushing the machine a little more than its capable of.
    Tom

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    I should have been more specific regarding the errors, they occurred on the vertical surfaces - into the piece on the conventional cut and away from it on the climb cut.

    The quill was locked in the full up position, as I always do when using a large cutter and/or a heavy cut.

    I have noticed some dishing in surface cuts when using face mills, which then leads me to yet another tramming.

    I can't feel any play in the quill but that may not be possible by hand anyway.

    I intend to continue analyzing the possibilities and will check the ram and center bolts. The irregularities in the cuts lead me to believe that there is movement in the head around its axis that is parallel with the x-axis.

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    The 2J head is not the stoutest thing made. IMO, anything over 3/4" is pushing it. They were not designed for larger cutters and big insert face mills. The Series II Special was designed more for holding the weight of larger parts than taking huge cuts.
    Just my $0.02
    JR

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    Default Cutting Parameters

    Quote Originally Posted by Carl Douglass View Post
    I have a Bridgeport Series II Special which I do various milling and boring projects with. Yesterday, I was milling a partial slot into annealed 4140, using an 1-1/4", 6-flute helical HSS cutter mounted in a good R-8 toolholder (not import). 150 RPM, 0.25" DOC, 2.5 ipm feed, 50% diameter cut. I made one pass in conventional and one in climb. The conventional cut was about 0.010" too deep except at the end of the cut, where is was on the mark. The climb cut was about 0.010" too shallow and tapered.
    Parameters as i understand it
    SFPM about 49
    RPM 150
    Diameter 1.250 hss end mill
    6 flute
    Length Sticking out 3.0" ?
    R8 collet so shank is <0.9" or you have a long toolholder
    cutting 4140 annealed
    DOC 0.250
    Stepover 50% or 0.625
    Feed 2.5 ipm
    .
    you depth of cut is 250% to 500% higher than recommended (250% DOC at 50% stepover)
    your feed 0.003" thick chip
    Horsepower at 50% stepover 0.66
    if you had a longer toolholder that extends from where normal R8 ends (spindle nose) so you could use a 1" shank tool you would be at 700 to 1400 % over recommended depth of cut. your cutting edge is too far from spindle nose and this leverage is probably pushing things around.
    .
    at least thats my opinion using the Excel calculator i use.

  8. #7
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    Before tearing into the head, look over the lower areas of the machine first.

    A quick check of the workholding method for deflection.

    Also check gibs (all 3) and the condition of the adjusting screws for the table and crossfeed nuts. They are only ¼" and all the cutting force can deflect the head allowing unintended motion in either or both axis. The table pushed left or crossfeed pushed out press against the screw heads.


    Just my 2¢.

    Bill

  9. #8
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    Quote Originally Posted by DMF_TomB View Post
    Parameters as i understand it
    SFPM about 49
    RPM 150
    Diameter 1.250 hss end mill
    6 flute
    Length Sticking out 3.0" ?
    R8 collet so shank is <0.9" or you have a long toolholder
    cutting 4140 annealed
    DOC 0.250
    Stepover 50% or 0.625
    Feed 2.5 ipm
    .
    you depth of cut is 250% to 500% higher than recommended (250% DOC at 50% stepover)
    your feed 0.003" thick chip
    Horsepower at 50% stepover 0.66
    if you had a longer toolholder that extends from where normal R8 ends (spindle nose) so you could use a 1" shank tool you would be at 700 to 1400 % over recommended depth of cut. your cutting edge is too far from spindle nose and this leverage is probably pushing things around.
    .
    at least thats my opinion using the Excel calculator i use.
    Would you be willing to share your Excel calculator?

    Actually, the toolholder extends past the spindle nose about 3" and the tool has a 1-1/4" shank.

  10. #9
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    Default Excel Speed, Feed, Depth of Cut calculator

    i have posted it before but here is latest version. improvements are

    1) asks for end mill length sticking out of tool holder

    2) asks for length of tool holder. for example some tool holders stick out much farther than a R8 collet would. This tool holder length decreases max possible Depth of Cut. I have this effect also adjustable as not all tool holders are the same. A 6" straight shank extension for holding a 3/8" end mill in a 3/4" collet is not as rigid as a 1" shank extension

    3) asks for end mill shank diameter and or tool holder shank diameter which ever is smaller. Having a 1.250" shank in a tool holder with a R8 shank you are really limited by the R8 shank of roughly 0.9". The same thing limits a 6" facing mill with a R8 shank, obviously the tool holder shank has an effect.

    4) The machinability rating column (examples are given) is used to calculate chip thickness and depth of cut. Aluminum can obviously be cut with a thicker chip and greater depth of cut than 304 Stainless. I use a Logic function called If Then Otherwise to limit feed and depth of cut from getting too extreme. For example milling plastic the depth of cut could be several inches deep.

    Some machinist consider using an Excel calculator a waste of time. I find it has increased my metal removal rate 2-20 faster by better understanding what is the reason for not machining faster. For example by using shorter stub length end mills often it is possible to take 2x the depth of cut. Sometimes it is a horsepower limit of the motor, or the machine rigidity / strength limit to resist vibrating from high horsepower. Horsepower estimates I consider an extremely important feature but you should realize they are an estimate of a sharp cutter and a dull cutter can need considerably more hp.

    It is a work in progress. I add improvements as I discover weaknesses. For example a 3" dia end mill with a 3" shank can take much more than a 3" dia facing mill with a R8 shank.

    The Excel file is in a zip file (compressed folder) and once copied from the folder can be used in Excel or the free Open Office Calc program.
    Attached Thumbnails Attached Thumbnails calculatoraug19.jpg  
    Attached Files Attached Files

  11. #10
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    Thanks DMF_TomB,

    I had started building my own Excel calculator for milling and turning but got discouraged with all of the input work, copying from Machinery's Handbook.

    I really appreciate your comments, as I am trying to learn the proper relationships to avoid tool loss and messed up work.

    I will open and study your file.

    Thanks again,

    Carl Douglass


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