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Copycat Mill Build.

Discussion in 'MACHINE BUILD LOGS' started by Blackhawk, Dec 12, 2012.

  1. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    yes that is excessive
    did you take the bearings apart and shim in between them?
    turn the shaft back and forth by hand and find what is moving before the table starts to move.
    I have mine down to .0014 and .0013 and that is in the ballnut,
    Steve
     
  2. Blackhawk

    Blackhawk United States Active User Active Member

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    Roger that, did not shim the bearings, I'll take it apart and see what's moving.
     
  3. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    make sure the bearings are in facing correctly also
    the outer races are thinner on the side that faces out.
    I have had some assembled wrong.
    Steve
     
  4. dracozny

    dracozny United States Active User Active Member

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    I have issues with the ball nut mount sliding against the saddle on the Y which causes that. might be something to look at. even with the bolt torqued down it still gives some play so you might have to consider shimming the mating point to take up that slack.
     
  5. Blackhawk

    Blackhawk United States Active User Active Member

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    Thanks dracocny, I'll check that

    Steve,
    Bearings were installed wrong and are bone dry, should I pack them.
    When you say shim, you mean between the two bearings?
     
  6. Blackhawk

    Blackhawk United States Active User Active Member

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    Kind of a funny thing I noticed was the blocks with two bearings are crummy and the block on the end of the ballscrew has a really nice sealed bearing with no noticeable play, I would think it should be the other way around
    Maybe I should pop out the nice one and put it with the crummy one
    Lanham
     
  7. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    The two bearings in the fixed block are angular contact bearings (AC) and need to be used in pairs and installed with the correct orientation to each other. The other single bearing in floating block is a standard roller bearing and not suitable for the fixed side.

    The AC bearings need to be greased. These bearings also need to be shimmed to remove the excess play and set the pre-load. If Steve does'nt jump in with an explanation on how to do this I can later when I get home.

    Jay
     
  8. Blackhawk

    Blackhawk United States Active User Active Member

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    Thanks jay,

    So if I have .007 play on x and .004 on y I need to find a shim of equal value?
     
  9. Blackhawk

    Blackhawk United States Active User Active Member

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    Threw in a .026 washer (smallest I had and seems a little tight) but no play
     
  10. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    Lanham,

    This is a quick and dirty explanation of shimming the AC bearings.

    In this image is the various bearing arrangements. Because the ball screw shaft is constrained on only one end we need to use either the back-to-back or face-to-face arrangement. I use the back to back arrangement so I will be explaining that setup.

    AC Bearing.gif

    The next image shows (approximately) the typical Chinese bearing housing with spacers and seals.
    ACBearingBlock.gif

    This image shows where the spacer (shims) will go.
    AC Bearing2.gif

    First thing we need to know is how much play there is in the housing without shims. There are two areas where play can exist, the housing bore depth and the bearings themselves.

    First is to measure the housing depth. Install both bearings, back to back, without any spacers, fully seated in the housing. Do not install the cap. Measure the distance from the bearing outer race to the face of the block and record it. Now take the cap and measure the depth of the round boss that fits in the housing bore. Every Chinese block I have dealt with has always had too much room which will allow the bearings to slip back and forth. Subtract the cap depth from the housing. Example the housing is 0.100" from the face to the bearing and the cap is 0.090" deep. 0.100" - 0.090" = 0.010".

    Now we need to measure any offset in the bearings. Support the bearing on its back by the outer race only. 1-2-3 blocks or parallels work well for this. Using a height gauge or dial test indicator measure the face of the outer race and the face of the inner race. Record the difference. Example on each bearing the inner race is 0.002" lower than the outer race.

    So now we know we need 0.010" of shim to take up the housing play and 0.004" shim (0.002" for each bearing) to take up the bearing play for a total of 0.014".

    Now we need to add some preload. Some people just use a thicker shim and set the preload with the ball screw shaft nut. I don't like this method because with out doing a complicated setup to properly measure the preload you really have no idea if the load is too small (slop) or too much (potential damage or premature wear to the bearings.

    I prefer to use shims so I have a reference of what the preload value (compression distance) is to make adjustments later if needed. When I first seup my mill I added an extra 0.002" for preload which seemed good at the time. This left me with bout 0.001" of backlash under real time use. After using the machine for a while and adjusting the ball but preload, I increased it to 0.004" and now my backlash is around 0.0005".

    Your values may be different so I would start low, 0.001" to 0.002" and add as needed after running the machine.

    For shims I use these and turn the OD down to slightly under the bearing OD. The shims are 0.002" thick, don't pay attention to the photo. Otherwise you can make them from stainless or brass shim stock.

    I can explain how to turn down the shims if you are interested.

    Jay
     
    Last edited: Mar 27, 2015
  11. Blackhawk

    Blackhawk United States Active User Active Member

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    Thanks jbolt,

    I did the shims last night and will test today

    Lanham
     
  12. Blackhawk

    Blackhawk United States Active User Active Member

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    Jay, thanks for that, could you tell me how you turned the shims down. Mine will be in today

    Lanham
     
  13. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    Lanham,

    Sorry for the delayed response. I'm in the middle of a remodel.

    I make an aluminum mandrel to turn the shims. I start with bar stock with an OD larger than the stock shim and turn a boss on the end to the ID and to a depth of the thickness of all the shims stacked. Make a cap to clamp the shims to the mandrel. Turn the clamped shims with a carbide insert for aluminum in about 0.010" doc.

    Jay
     
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  14. JimDawson

    JimDawson Global Moderator Staff Member Director

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    :+1:
     
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