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Bent spindle

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  • Bent spindle

    I have finally discovered the cause of the vibration in my Bridgeport since fitting a BT/QC30 spindle - the splined section is bent.

    It runs true at the tool end but the drawbar wobbles badly at the top causing the vibrations at 4000rpm, well anything above 1000 really.

    You can see it looking down the shaft from the top, with the quill fully extended its ok but anywhere else is bad.

    Is there a recommended way to assess and straighten these at home??

    It took a long while to find the QC spindle and i don't want to risk wrecking it.

    If it does'nt fit, hit it.

  • #2
    You need to dismantle the spindle and check the errors between centres. A lathe would work for this as most people don't have actual inspection centres. Finding the point where the bend is will help in making a decision of how to proceed. You may need to get advice from a machine shop.


    • #3
      Remove the spindle from the quill and remove the bearings from it.
      Support it on V blocks at the bearing locations. (Using the bearing locations allows a nice repeatable rotation when measuring the runout).
      Use an indicator, preferably '10ths if you've got one, and measure the total runout at locations about an inch apart from one end of the spindle to the other. You can use the tops of the splines for that part of the spindle. As you do it, put a dot from a whiteboard marker pen at the high point on each measurement.

      Either in your head, or on a bit of graph paper, mark out the graph of the runout. You will probably find that there is a single point where the bend occurs, but there might be a couple. You will probably also find that the high spots you marked are all in line, but if there are a couple of bends, they might not be.

      Now that you've located the bend(s), you can use what you have available to straighten the spindle. It helps to use some aluminium supports, turned to fit the spindle either side of the bend and at the bend, to avoid marking the spindle. It's also useful to have a substantial support for the bending operation. An RSJ or Square box section work very well. It needs to be stronger than the spindle is.

      Support the spindle either side of the bend location and press the bend out with your choice of flypress, hydraulic press, car jack, G cramp or custom made rig. Having the supports spaced further apart reduces the amound of force you need to apply, but be sensible about not supporting the spindle at the weak drawbar end and trying to bend the strong bottom end etc. Use the indicator on the spindle close to the bend as you apply the force. This gives you an idea of the amount you are moving the spindle and how much effect each application of force has on the bend. Start reasonably gently and you'll not get any permanant movement at all. Keep increasing the movement each time and you'll get to the point where you've exceeded the elastic limit of the spindle and you've got a permanent change. Once you've got a feel for that, you can reduce the bend with more confidence.

      You'll need to repeat the survey and possibly work on other locations on the spindle, but it's not too hard with a bit of care.
      Last edited by Mark Rand; 04-21-2018, 05:38 PM.
      Location- Rugby, Warwickshire. UK


      • #4
        I see the same thing when I look at the hex part of my draw bar when it's running. I have the long hex for use with the RA attachment, so there is about 6" of hex sticking above the top of the variable drive head when I'm using a regular R8 shank or collet. The hex is pressed and pinned to the rod and apparently not in perfect alignment so it appears to wobble. It doesn't cause any vibration.
        I would double check if I were you....... I would hate to see you tear the head down for nothing. I can't see how a spindle wold get bent.



        • #5
          The spindle comes out easy if you remove the set screw in the back of the quill and then unscrew the end of the quill , lock the quill in the up position and gently tap on the end of the draw bar block the end of the spindle so it doesn't drop out to the floor, carefully remove it try not to move the outer races of the bearings I lay the spindle on the table (make sure it's clean) and use a rag with some acetone wipe a stripe on the bearings then draw a magic marker line on the bearings so you can keep them in order (outer races have a high point marked on them some times it can be seen others not) rotate the spindle and see if you can see run out even my old eyes can see .002"- or less if it's got a wiggle fix it gently . As an aside when you put the set screw back in the quill body if you tighten it to much the quill will go to the home position very hard the screw is only to keep the quill cap from unscrewing. Been repairing machines since 1968, can almost take a BRPT head apart with my eyes closed if some one doesn't move the tools LOL.
          Last edited by duckman; 04-21-2018, 06:10 PM.


          • #6
            Thanks all,

            yes its the splined section that is bent - you can clearly see the banana shape if you look down from the top with the spindle running slow. I have checked the drawbar and its runs fine in the lathe and on the bench on parallels.

            This is not the original spindle - i fitted it a couple of months back as i wanted QC/BT30 instead of R8, it took a long time to find one and cost a fair bit of cash too. I should have checked it more thoroughly i guess.

            Yes its not too bad to remove but not as easy as a R8 as the nose cap is retained behind the nose so you have unscrew a little, tap the top, unscrew, tap, unscrew........

            I'll have a look this week and get some measurements then have a go.

            I have also emailed a company that does this sort of repair - will see what they say Monday hopefully.
            If it does'nt fit, hit it.


            • #7
              I agree with Mark Rand regarding straightening, its a slow careful process, I would add one thing, put some kind of shimming under the spindle to limit the movement. Decrease the thickness of the shim a little at a time until the spindle starts to straighten when the elastic limit is reached. Go a few thousandths of an inch past straight, rotate 180 degrees and a gentle final push will stabilise the spindle.
              When I was working on aircraft parts, I kept a recipe book of the settings and fixture dimensions required to straighten part machined components. Then when another batch of the same parts arrived to be straightened, we saved 90% of the time taken.
              Getting a specialist firm's opinion is a good move.
              Last edited by old mart; 04-22-2018, 11:04 AM.


              • #8
                good post Mark


                • #9
                  You say the splined section is bent. Is it twisted? Are the splines straight?

                  Is there any indication that someone already tried to straighten it and were not successful?

                  Any idea how it became bent, especially in that section?

                  This does not seem like an ideal piece on which to to learn to straighten. Perhaps you should bend some other shaft and use it for practice?

                  I'd be inclined to find a very well regarded specialty shop and see what they would charge.


                  • #10
                    I have not pulled it apart yet, it has not been overloaded in my ownership - i run mostly aluminium and small tooling. It didn't look twisted when i fitted it or i would have retuned it to the seller.

                    I have a feeling the only way to bend this section is by careless stripping down of the head.
                    If it does'nt fit, hit it.


                    • #11
                      if you look on youtube at Turnwright machine works, keith fenner, he uses heat and then rapic cooling to straigten shafting up to 2 1/2 dia! he goes into detail about the method. hope this helps!


                      • #12
                        Originally posted by Davek0974 View Post
                        I have not pulled it apart yet, it has not been overloaded in my ownership - i run mostly aluminium and small tooling. It didn't look twisted when i fitted it or i would have retuned it to the seller.

                        I have a feeling the only way to bend this section is by careless stripping down of the head.
                        I don't know how they get bent but my Beaver VBRP mill also had a bent spindle and a bent X axis feedscrew. That's how I learned how to straighten them .

                        Old Mart's comment about going a bit past straight and gently coming back is a good point that I forgot to mention. You'll find that it'll spring back with much less force than it took to move it in the first place. Presumably, after the straightening efforts, the metal has got a rare old mixture of built in stresses and the bend back relieves some of them and leaves the residual stresses more balanced.
                        Last edited by Mark Rand; 04-22-2018, 05:53 PM.
                        Location- Rugby, Warwickshire. UK


                        • #13
                          This video is a pretty good description of shaft straightening. The guy uses a pull-down press but the principle is
                          the same...

                          Just one project too many--that's what finally got him...


                          • #14
                            Thanks all, good videos there.

                            I measured the runout at around 1.2mmTIR so pretty bent really

                            Located the high spot, got it in the press and gently increased pressure, measuring each time, until we got some movement, then a few more times.

                            I now have a TIR of 0.04mm which i think is far enough.

                            Thanks for all the tips.
                            If it does'nt fit, hit it.


                            • #15
                              I recently bought a substantial but smallish bench drill, usual design, vertical motor at the back. Motor spindle was bent.

                              The obvious way for some heavy handed PO to bend it would have been to try to lever off the four step flat belt pulley. With the widest part mounted low, a good bit of leverage could easily have done that.

                              As you take it apart, imagine how a gorilla could have abused it.
                              Richard - SW London, UK, EU.