Bent lathe spindle, need ideas for repair

snapatap · Oct 7, 2017

I have had a bit or an oops and the back of the spindle on our little hitachi seki lathe is bent. The diameter the the finger clock is on in the pic has about .05mm runout. Didn't have time to check the runout on the flange, i will when i pull the cylinder for the power chuck off. So does anyone have any ideas on what i can do to fix this? or is the machine a writeoff? The spindle motor also appears to integral to the spindle.

azmachining · Oct 7, 2017

Try to straighten it, I would, there are multiple threads on straightening spindles on here, just do a search.

barratt · Oct 7, 2017

It must have been one hell of a crash to bend the spindle?
Did you take the chuck off and clock up the spindle nose?
I'm trying to imagine how the spindle can get bent.
What happens when you try to spin the chuck. Is it seized?
The bit you are clocking on does not seem to be particularly precision machined.
I guess I am asking are you sure it's bent?

art.h · Oct 7, 2017

You may be over reacting, your clocking way to far off the actual spindle. Too many items could have been shifted. How do the bearings sound, an action that would probably cause bearing damage. Best of luck!

snapatap · Oct 7, 2017

Its defiantly bent, you can see the cylinder for the power chuck wriggling around. I had about 400mm of 38mm 4140 hanging out the back of the spindle and it whipped. I had a bush in the spindle and i thought it would be ok to about 2000rpm but it wasn't. The bar that was hanging out the chuck bent about 5 degrees, I think the lathe walked along the floor a little, and I nearly had an uncontrolled bowel movement.

Limy Sami · Oct 7, 2017

A thought - is the actual spindle bent or what's mounted to it as in the power cylinder itself and not the spindle?

PS FWIW I'd be surprised if 400mm of 38mm bar (under 8lbs ) bent the main spindle - even if it was doing 2000 rpm ( but that's where my maths runs out

)

gregormarwick · Oct 7, 2017

snapatap said:
Its defiantly bent, you can see the cylinder for the power chuck wriggling around. I had about 400mm of 38mm 4140 hanging out the back of the spindle and it whipped. I had a bush in the spindle and i thought it would be ok to about 2000rpm but it wasn't. The bar that was hanging out the chuck bent about 5 degrees, I think the lathe walked along the floor a little, and I nearly had an uncontrolled bowel movement.

Unlikely it's the spindle.

I did a similar stupid many years ago, I thought it bent the spindle because the actuator was wobbling around visibly. Spindle was fine, actuator just needed reseating on the end of the spindle and all was well.

Try just loosening the bolts on the flange that holds the actuator on. With a bit of luck you might find that the wobble is massively reduced. Requires tweaking to get it perfect though.

The only way to rule out the spindle for certain is to take the chuck off and indicate the spindle nose, but if you do that, make sure and do what I said with the actuator first as if that is running out badly it will influence the spindle runout to some degree.

MattiJ · Oct 7, 2017

Limy Sami said:
A thought - is the actual spindle bent or what's mounted to it as in the power cylinder itself and not the spindle?

PS FWIW I'd be surprised if 400mm of 38mm bar (under 8lbs ) bent the main spindle - even if it was doing 2000 rpm ( but that's where my maths runs out )

38mm bar hanging out only 400mm and starting to whip around also sounds suprising.. is there something else going on?

gregormarwick · Oct 7, 2017

MattiJ said:
38mm bar hanging out only 400mm and starting to whip around also sounds suprising.. is there something else going on?

It happens. When I did it it was 50mm 316 stainless, hanging out about the same distance, maybe even less. Good tight fitting delrin liner. No wobble or any warning anything was going to go wrong. Just about had a heart attack.

Close to ten years ago, been extremely cautious about overhang outside the spindle ever since.

machtool · Oct 11, 2017

Ash.

Did you get that sorted? Appolgises for the delay. I'm trying not to come here. I'm reasonably close to Mel of H.S Machine Tools. He might have even sold that machine. I could extract a drawing, if he has it.

I don't count your test, you need to dig down to the pure spindle. Your clocking an adaptor there. Half that shit is aluminium, it can take a bruise, and run out like a bastard.

Regards Phil.

snapatap · Oct 13, 2017

machtool said:
Ash.

Did you get that sorted? Appolgises for the delay. I'm trying not to come here. I'm reasonably close to Mel of H.S Machine Tools. He might have even sold that machine. I could extract a drawing, if he has it.

I don't count your test, you need to dig down to the pure spindle. Your clocking an adaptor there. Half that shit is aluminium, it can take a bruise, and run out like a bastard.

Regards Phil.

I haven't had time to have a good look at it yet, its low on the priority list. Hopefully this week I can pull all the stuff off the back of the spindle and work out exactly what is going on. I'll let you know how I get on.

snapatap · Oct 23, 2017

One of the other machinists pulled the hydraulic cylinder off the back of the lathe today. The mounting flange (Shown in pic below) had about 5 thou of runout on the face.

We undid the nut, removed the flange and to my relief the spindle has 0 runout on the diameter the flange mounts on. The flange is also ok. When the flange is refitted without the nut done up it has about half a thou of runout, when the nut is done up it goes up to about 2 thou. We then did some tests and came to the conclusion that the thread on the very end of the spindle is slightly bent. I believe that because the nut is now out of square it is pulling the flange sightly off square when it is done up, but there is some debate in the workshop about this.
What do you guys think?
The thread is quite tight, would skimming a bit out of the nut help? or maybe shimming under one side.

onecut · Oct 23, 2017

Skimming the nut wouldnt take long and wouldnt do any harm if its done properly or get a replacement obviously skim the nut locating on the thread and eliminate this thought,must be a relief to you and you can solve it with a bit of exploration.Look for minute high spots or burs

SIM · Oct 23, 2017

If the nut is already out, squaring it up cannot do any harm...its already out.
As I say, you can't break broken.

Glad it was not the spindle.

adama · Oct 23, 2017

If you have a surface grinder its realy easy to pack a nut up and grind a slight taper on it to alleviate the symptoms. Have never done it for a lathe spindle, but have for other things with out any issue. Simply work it out mark the side to reduce and slap a packer under that side of the part on the magnetic chuck.

FYI i too am surprised that a 38mm bar (would take some serious effort to bend that!) that far out was a issue at that speed. That said, even with only a slight bend, that bar then weighs a lot lot more than Sami, forget his 8lbs! But personally i would not have blinked a eye lid at running that kinda setup. Probaly should adjust my speed - stick out ratios a bit lower!

huleo · Oct 23, 2017

Would there be harm in obtaining a precision ground washer to step the nut back onto to some hopefully more square threads? I think I might also check that nut pretty well to see if it is square with the world.

Richard King · Oct 23, 2017

Back in the old day when I rebuilt a lot of Crank grinders those spindle nuts has tapped holes around the perimeter with set screws so we could adjust the spindle bearings spindle to .0001 run-out. Instead of shimming. Drill and tap some 6 or 8 32 holes(not sure in mm sizes) every 1/2" on the diameter and adjust it until it reads zero, then take out one screw at a time and loc-tite the set screw and replace it.

I'm not sure if this will work on your application, but it worked great on those spindles way back then.

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