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Spindle deflection, how much is too much?

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  • Spindle deflection, how much is too much?

    Been trying to get a good 4 jaw chuck setup on my lathe, but that's a whole different story :-)! In the meantime, I've noticed that I can get a couple of thou deflection with only moderate thumb pressure on the side of the chuck. If I put a lever under the chuck and pry up, I get around .005" deflection with only moderate pressure. The machine is an older (80's vintage) Grizzly belt drive 12x24, and I've replaced spindle bearings, belts, and motor in the last year or so.

    I've never been totally convinced that I got the bearing preload correct, but finding a definitive answer on what constitutes proper preload is a challenge. One "test" I've seen is to give the chuck a spin and count how many times it spins. Mine won't make even one full rotation, which suggests that I may have too much preload, based on that info. I did the initial setup based on the instructions in the Grizzly manual, but then discovered a math error in those instructions, so I made the needed corrections, and that's where it's stayed since.

    So is this simply a case of stuff wearing in, and I need to readjust the preload? Or do I already have too much preload, based on the spin test, and I've got a different problem somewhere? Or is this normal, and I'm just being anal :-)? BTW, I've also checked for deflection with the spindle warmed up, it really doesn't change much, maybe half a thou. Any thoughts, advice, or suggestions would be much appreciated. Later.


  • #2
    My G4003 has about .0005" deflection at the chuck, with ~5# push. My bearings run with oil flow, since it's a gear head. I would check the pre-load.


    • #3
      One of the hardest things to do is to tell somebody how to load a pair of bearings. Be it wheel bearings or lathe bearings. I have never messed with my lathe bearings (no deflection at the spindle but a little at the chuck and I don’t know why, and it isn’t enough for me to worry about.)

      But I have done a bunch of auto wheel bearings and I am told it is just like that. So if you have ever done that, then you know what I mean. I will try to explain that.

      The best way to tell if they are too tight is to check if they get hot when running. You should be able to tell in 4-5 minutes. If you can’t hold your hand on them then they are too tight. If you have an instant read thermometer then I would gestimate about 120-140 degrees.

      Really snug them down good and tight, what you would probably think is over tightening them. Turn the spindle for drag and check the deflection. Then back off about a half a turn, and then tighten back about one quarter. You will feel the torque building, and then stop. Check for run out. Run the lathe for 4-5 minutes and see if it gets hot. If they get hot then they are too tight, if they get warm with no deflection, then they are just right.

      How much torque? I don’t know. If you were going to close a mason jar full of moonshine and lay it down in your kitchen cabinet and walk away, how tight would that have to be? (without breaking the glass or stripping the cap) That’s it.
      Last edited by Ron of Va; 02-11-2013, 04:11 PM.


      • #4
        Probably a silly question, but are you certain all the deflection is in the spindle? You didn't say what type of chuck mount you have (threaded spindle, Cam Lock, etc.), but if the chuck isn't well fitted to the spindle it alone may contribute to the deflection you are seeing. It might be worthwhile to put an indicator on the spindle itself, sans chuck, and apply side pressure to see what you come up with. Also worth noting, any deflection you see will multiply as you get further away from the spindle. Therefore if you are measuring from the portion of your chuck that is farthest away from the spindle (toward the tailstock) your readings will be greater than actual deflection at the spindle.

        Grizzly may have an online manual for your lathe that explains spindle bearing preload adjustment. Here is one for a 12 X 37 belt drive machine, Grizzly 9249.
        Page 58 tells how to adjust bearing preload on that machine. Whether this is the proper procedure for yours I can't say. You can look here for your exact machine:


        • #5
          Why bother?

          If the spindle deflects under a reasonable load it may not matter but if its OK when taking a fine cut I'd be inclined to leave it alone unless or until it really matters.


          • #6
            That sounds like too much deflection to me. I'd want to know where it's coming from.

            I'd be trying to set up the indicator to read the bore of the spindle at about the point where the bearing sits. If the indicator is mounted on the headstock, you'd see whether or not the deflection is in the bearing area, which would include spindle to inner race, inner to outer race (which is where pre-load comes into play) and/or outer race to headstock bore.

            If the indicator is mounted on the ways, then you could also be seeing a possible flexing where the headstock mounts to the ways.

            When you take a reading anywhere towards the right of the headstock bearing, then you are also including the possibility of some play existing in the bearing areas on the left side of the headstock. It might be wise to see how much flex or play exists around the left bearing. I can see there being a problem here because the spindle may have to be loose in this bearing in order for a pre-load adjustment to actually work. If the bearing is tight to the spindle, then the bearing OD would have to be loose in the headstock bore to allow this. The potential for flex or play here is the same.

            It should be obvious that the further out from the spindle you take a measurement, the more the flex would be. It should be just as obvious that the closer to the headstock that the cutting tool contacts the work piece, the more constrained the relative positions are, and the better the chance for good results.

            It gets a little mixed up here because the chuck to spindle interface can give problems, as firbikr suggested. My chuck is a thread-on, and if I don't get an easy threading followed by a firm seating, then I can't expect an optimum rigidity out of that. I'm sure that virtually any type of chuck mounting would have roughly the same type of problems if the mounting isn't 'crisp'.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-