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  1. #1
    Join Date
    Apr 2004
    Posts
    99

    Regrinding spindle taper hole

    Hi all.

    I bought a cheap benchtop milling machine and then found out that the spindle runout is more than 0.1 mm at the internal taper hole of the spindle, bad luck of me! Checking the bearing looks good, re-adjusting the preload nut didn’t help, now I think the taper hole itself is out of center.

    Thinking to re-grind the taper hole, here’s the plan attached.

    I’m not a good machinist, need some advice from the wiz here.. Will it work? Any input appreciated.
    Attached Thumbnails Attached Thumbnails GrindingSpindle01.jpg   GrindingSpindle02.jpg  

  2. #2
    Join Date
    Mar 2004
    Posts
    761
    I would add more support or shorten the grinder spindle length for less flex

  3. #3
    Join Date
    Oct 2003
    Posts
    192
    Nice job modeling this!
    It sure will work....but with allot of skill!

    It looks like a morse taper??...only taking out the very least amount will work, otherwise your tooling will sit too deep!

    One other alternative...take the spindle apart and have a grinding shop doing it... probably save you money at the end.
    Konrad

  4. #4
    Join Date
    Mar 2005
    Posts
    1136
    That seems like a crazy amount of run out - .004 thou! Can you take the mill back (is it new?). How are you measuring the run out - indicator? Can you get the indicator on a part of the spindle other than the taper hole and is the problem definitely the taper bore?

    That’s an interesting set up and idea (x/z to make the taper). I don't think that I’d have the nerve try it, but admire the ingenuity ). Wayne is right, it needs more support. Make sure the axis of both spindles lie in the z/x plane. Have you an internal grinding spindle? If not, acquiring one is not trivial.

    Just be really sure that's the problem before you start removing material, and sending it out is not a bad idea.

  5. #5
    Join Date
    Dec 2005
    Posts
    43
    A couple of things for you to consider before trying to grind the taper. Machine alignments. If your spindle is not parallel and perpendicular to the axis you are grinding with you will absolutely have problems. An out of parallel condition will cause the taper to grind in what we call a bow tie condition. Tight in the middle and loose at both ends. That is as bad as it can get. I don't even know a way to program around that one. You can also cause the angle to be wrong which you probably could program around, but it would be some trial and error to get it right. Morse tapers are really tricky to regrind on the machine. A small amount of stock removal will cause the tool to sit quite a bit deeper in the taper. If you have .004 TIR you could move the tool as much as .040. There is not a lot of room at the bottom. We grind spindle tapers every day, and after 25 years, we decided that we would no longer do morse tapers. Too big of a pain in the a--.

  6. #6
    Join Date
    Apr 2004
    Posts
    99
    I would add more support or shorten the grinder spindle length for less flex
    Thanks, I’ll put both on my notes.

    It looks like a morse taper??...only taking out the very least amount will work, otherwise your tooling will sit too deep!
    Yes, Morse taper no. 4, from the web: large dia. 1.231”, small dia. 1.02”, depth 4.0625”, included angle 2.975 degree. The tool holder that comes with the machine still has a spare about 1 cm in length and there’s a big bore above the taper, so I think there’s enough space to play with.

    That seems like a crazy amount of run out - .004 thou! Can you take the mill back (is it new?). How are you measuring the run out - indicator? Can you get the indicator on a part of the spindle other than the taper hole and is the problem definitely the taper bore?
    Uhh.. The quality of this machine already makes me sick, and I can’t send it back too!
    I measure with DTI, even without any measuring tool I’m sure it is very bad, I can see the wobble with my eyes. One thing I just learn: Never buy the cheapest one! :nono:

    If your spindle is not parallel and perpendicular to the axis you are grinding with you will absolutely have problems. An out of parallel condition will cause the taper to grind in what we call a bow tie condition.
    Nice advice from the expert! So if I will do this plan than I have to start to work one step back, aligning the table if possible. How if it’s not possible? Can I just align the grinder support plate?

    Other things come to my mind:
    How to align the grinding stone on the right starting point?
    The diameter of the grinding stone will be getting smaller along its way, right? How to compensate this thing? Any guidance?
    Thanks to all advice, need a lot more… :cheers:

    CI

  7. #7
    Join Date
    Dec 2005
    Posts
    3319
    With regard to grinding spindle tapers:

    Grinding it oneself makes as much sense as doing one's own heart surgery....

    Several pieces of advice:

    1. Reread post #5
    2. Pull the spindle,
    3. find a local spindle repair shop,
    4. take it there
    5. let them grind it and provide Q/C certs thereafter.
    6. If you think about doing it otherwise, reread post #5.

    Yes this is a DIY hobby but you can't do EVERYTHING yourself.

  8. #8
    Join Date
    Mar 2003
    Posts
    4826
    The actual error is .002, or one half of the total runout observed with the dial indicator. I would strip the spindle completely down, remove the bearings, and set it up in V-blocks to try to see whether it might be bent, or whether it is actually poorly made. Removing the bearings may also reveal an assembly fault, such as a scored journal, interference between the fillets on the bearing race and the shoulders, etc. Also, the fit of the bearings inside the housing should be checked. They need to be a very close slide fit, or wobbling will ensue.

    I know that the hobbyist most likely does not want to spend $500 to fix up a cheap mill. But to fix it up cheap requires a lot of careful work, even in a home shop. You first need to determine the actual fault.

    If you were to make up a test mandrel by turning a male morse taper in your lathe (and never remove it after turning it until the tests are complete), then you might have a basic starting point to slide your spindle onto this test taper, and then determine whether the runout is real, or the shaft is bent, etc.

    The best would be to make your test taper as a double ended taper, one end fitting the spindle taper accurately, before you turn the 4 Morse to test with. A very accurate center hole then needs to be drilled into the outboard end of the test mandrel before turning the final test taper, because it is entirely possible to machine a crooked shaft that has been chucked with a mis-aligned center. If everything is kept very clean, and the spindle taper was in excellent shape to begin with, you could hope for a repeatable setup, as it may be necessary to remove the thing in the course of doing the repair.

    You need to constantly check everything in your setup for runout, there is no room for assumptions.

    So if the bearing journal turns out to be actually eccentric, then you could do a 'lame repair' by knurling up the journal, then turning it back to size. Or, undercut it and built it up with epoxy and then machine it to size. I know, we are all cringing If you have a welding capability, then build it up, straighten it as close as you can, then remachine it.

    It will provide several days of entertainment for you, and a friend with a lathe. It is certainly a job for a very patient and thorough worker, and maybe you could hope to get the runout down to half a thousandth of an inch, or so.
    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)

  9. #9
    Join Date
    Dec 2005
    Posts
    43
    There is so much that can go wrong here. Even us so called "experts" don't work on these in the machine anymore. And the tapers we do work on, we would never do by programming the machine. Again way too many variables that can make it worse rather than better. Seeing as this is a public forum, I hesitate to explain exactly how we fix them. I don't own the company, just work for them. And it is how I put food on my table. But let's just say in the hands of an inexperienced person it's just asking for trouble. You can try it if you wish, but a very high probability that you will end up pulling it anyway. And it might be cheaper to just bite the bullet and let someone else fix this one.

  10. #10
    Join Date
    Aug 2004
    Posts
    2849
    have to support NC and Hu.....ya ain't gonna make it better with your approach......well you might make it different....is that what you're looking for....

  11. #11
    Join Date
    Oct 2003
    Posts
    192
    To give you a good idea, if your spindle and bearing is half descant, then all you need to do is stick in some sort of tooling, lets say a milling arbor, instead the cutter, a chunk of round steel, then clamp a lathe tool bit in the vise, cut od, then check for run out on the cut piece of steel.....good run out = sound spindle, good bearing, bad run out = spindle possible NFG
    Konrad

  12. #12
    Join Date
    Dec 2005
    Posts
    43
    Cutting a piece of round steel with a lathe tool won't tell you much. If it is a bad bearing, you might see some runout on the piece. You will see more runout on the side of the piece opposite of the side you cut. If the bearings are good and it's just plain old spindle runout, you willl remove the runout in the workpiece. The clamped part will still runout but not the end you cut.

  13. #13
    Join Date
    Oct 2005
    Posts
    1145
    Dont forget to double check inside the taper itself. It may be only a small ding or dimple in one tiny spot that is causing the problem. A good strong bore light and a visual inspection has solved a few of those problems around here. Terry Parker

  14. #14
    Join Date
    Nov 2003
    Posts
    459
    Be sure to use some prusian blueing on a known good holder taper.
    Clamp the holder in your spindle, remove it and the heavy blueing spots show the imperfections. This is a 5 dollar quick check on any taper.
    Scott_bob

  15. #15
    Join Date
    Apr 2004
    Posts
    99
    Hi, thanks to all.

    There’s a strong suggestion here to send the spindle out to the spindle repair shop, I can’t do that because I don’t know if there is one here in this exotic land where I live.

    Following some suggestions, for the last couple of days I did some close inspections on the spindle. Pulling it out from the machine, put it on v-blocks and making some measurement. Then I disassembled the spindle, pulling out the bearings.

    The hole taper itself is out of center but not that much, about 0.03 mm depend on the depth. The worst thing is on the bearing journals, one for the tapered roller bearing is already “knurled” manually with hand prick-punch, and the one at the other side is very lose. This machine is sucks!

    Grinding it oneself makes as much sense as doing one's own heart surgery....
    Fortunately I have no HEART to her (ZX50F) anymore I just have another girl (ZAY7045) not as pretty as JFC-45 (the famous RF-45?) but sweet enough to me. Anyone experienced with the last one? She looks strong and tight… in tolerance I mean.

    :cheers:
    Attached Thumbnails Attached Thumbnails BenchtopMill.jpg  

  16. #16
    Join Date
    Apr 2004
    Posts
    99
    Hi,

    Just found the picture below at http://www.daieijapan.com/grinder.htm about an hour ago, check it please... more pictures there and a nice demo video too.

    Looks very similar to my idea, right?

    edit: not sure why I can't attach the picture, just follow the link.

  17. #17
    Join Date
    Aug 2005
    Posts
    1622
    Quote Originally Posted by Ch_Irawan
    Hi all.

    I bought a cheap benchtop milling machine and then found out that the spindle runout is more than 0.1 mm at the internal taper hole of the spindle, bad luck of me! Checking the bearing looks good, re-adjusting the preload nut didn’t help, now I think the taper hole itself is out of center.

    Thinking to re-grind the taper hole, here’s the plan attached.

    I’m not a good machinist, need some advice from the wiz here.. Will it work? Any input appreciated.

    Just from what I can see by the pictures of this mill.

    Is this CNC that you could do an X,Z move?

    No matter if not.......If the head tilts right and left, you could tilt the head to indicate zero off the taper, then just do a Z move on the column. Less things in motion the better.

    The video of the spindle grind is ok for theory, but 4mt will not leave much room to get a decent size stone spinning on a very rigid shaft. The stone max diameter can only be a tad bit less than the smallest portion of the internal taper. Typically the stones are mounted on a 1/4" shaft. The average grinders that I have used do not support the end bearing very well. It needs to spin at 15,000-20,000 RPM.


    Having a slow power feed on the Z will keep the feed, and thereby material removal constant. Dwells in the feed can dig in. Mostly due to spring in the grinder cutting pressure. Which, by rights should be MINIMAL. As well as the contact of the stones actual cutting point.

    Regardless od how well it is set up. The chances of success on the first grind are not all that promising. It very well may be on center, but will the taper match? If you can compensate the tilt on the head without overshooting the target and regrind again, you'll probably be OK.

    You could think of it this way......If it gets screwed up too big, you might still be able to take it out to an R8 like it should be in the first place!

    DC

  18. #18
    Join Date
    Mar 2005
    Posts
    1136
    or just take the 2-3k for an internal grinding spindle and buy a new mill . IF you did have a internal grinder (and i don't mean a dremel tool), and IF you've absolutely certain that’s the problem, you could do it in-situ. Unless you’ve got a lot of confidence in the accuracy of the cnc, I wonder whether the x accuracy/resolution is going to be enough to turn a proper taper – error is magnified with the taper. You’ve also got to make sure they’re in the same planes.

    Also, are you certain it needs a grinder? Do you know specifically that its too hard for carbide?

    I was thinking take a cross slide of the lathe and bolt it to an angle plate to get the angle and feed, but I like One of's tilt & indicate the head better. make sure the indicator is in EXACTLY in the middle (same with the cutting tool) - each cross section of the cone has a different angle.

  19. #19
    Join Date
    Aug 2004
    Posts
    2849
    Ch_Irawan....well you found his....and he probably found yours.....so, this could be a case of the blind-leading-the-blind....is you believe in your approach....just do it!

  20. #20
    Join Date
    Dec 2005
    Posts
    3319
    The CONCEPT is the same as what your IDEA is, Ch_Irawan. In fact, that is probably the ONLY way to do it.

    BUT the PROPER EXECUTION of the concept is where the difficulty lies.

    Prior posts have outlined the procedures and difficulties. Depending our your ability to interpret and execute them EXACTLY correct, you may have success. Failure to execute the procedure properly will only create more junk machinery.

    Good luck in your endeavors.....

    PS: I think you're in over your head if you try to do it yourself....

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