Mill spindle truing - set up problem.

Terry Keeley · Jan 11, 2019

The OD of the nose of my spindle is out a couple thou., I'm trying to true it up. The MT2 taper is also out (only a couple of tenths) but it has a nice score mark where something spun in it so I might re-grind it.

I'm having a problem setting it up in my lathe. It has two bearing shoulders to indicate from. If I get one running true the other is out 0.015" or so and vice versa. I'm pretty sure I need them both running true to the lathe spindle to true up the nose and taper? Then I was going to move the steady rest into position being sure not to tweak the shaft away from the lathe's center line.

The lathe has been leveled and aligned with a test bar, it cuts absolutely straight over that distance and more.

What's causing this? Is my 4 jaw out?

eKretz · Jan 11, 2019

In short, yes. The jaws are probably not perfectly square. Not unusual in the least. Put a stop in your chuck so that the jaws are only grabbing by 1/4" or so (or use some 1/8" copper shim on each jaw or wrap a copper wire around the part) and tighten the jaws only just a little bit more snug than it takes to keep the tailstock end from falling, then tap the tailstock end true. Snug the jaws a bit more while rezeroing the chuck end, then tap the tailstock end true again. Reiterate until both ends are zeroed and chuck is good and tight, then add steady rest.

Tyrone Shoelaces · Jan 12, 2019

I agree with eKretz, you're gripping too much of the spindle in your chuck. As he says, holding it by 1/4" with some softer material is the way to go.

Regards Tyrone.

Milland · Jan 12, 2019

You might want to do an initial test of the spindle to ensure it's not bent. If you have a surface plate and two good V-blocks you can set the bearing surfaces on, you can then carefully spin it and observe (or use a DTI) if there's any wobbling or other signs of lack of straightness.

adama · Jan 12, 2019

Is the spindle your trying to re-machine bent? Have you sat the 2 bearing race surfaces on V blocks and then used a DTI to confirm straight?

You need to straighten or confirm straight before you move to the step you appear to be on

If it is straight, yep you need less in the chuck jaws so it can be pulled into true hat far out. its common to get the chuck end running true then tap the other end lightly with a mallet to get it running true.

Milland · Jan 12, 2019

adama said:
Is the spindle your trying to re-machine bent? Have you sat the 2 bearing race surfaces on V blocks and then used a DTI to confirm straight?

Rumors prove true, Adama and I are the same person...

mnl · Jan 12, 2019

I believe this is referred to as “spooky action at a distance.”

michiganbuck · Jan 12, 2019

If the part has a center on the chucked end between center and steady might work.

A soft slug center put in the chuck and tool bit turned to 60*.

Some times a single strand of soft wire perhaps 1/32 - 1/16 diameter can be wrapped around the chucked end and the then part indicated in to take the chuck jaw wobble out of the truing.

*I see some have said hold only the very end of the part.. a good method also.

Terry Keeley · Jan 12, 2019

The spindle is straight, I checked it in V blocks on my surface plate and all good.

Thanks eKretz and Tyrone! Now I remember reading about this but forgot it somehow.

I've decided to try and lap the nose instead of grinding since it's so close already and am making a brass mandrel/lap to fit the spindle taper. Without taking it off the machine I'll try fitting the spindle to it with a dead center in the tail stock. Had to add a 0.001" shim to my tail stock so now it's dead nuts, hopefully it'll work. If the tail stock pulls the shaft out of alignment I'll just add the steady.

If not I'll cut some copper strips and tweak away!

michiganbuck · Jan 12, 2019

The lathe spindle can be straight perfect to a test bar...but still a 4 jaw (any chuck) even with the part indicated to dead zero at one place may have wobble inside the chuck.. so a small diameter part will take on some of that wobble... with it showing up some place down the length of the part.

QT: [Is my 4 jaw out?] Yes nothing is perfect..Even the best chuck brand new is not perfect..

and still one has to make near perfect parts..

michiganbuck · Jan 12, 2019

That part might be best finished held in its own bearing as assembled..

Terry Keeley · Jan 12, 2019

michiganbuck said:
That part might be best finished held in its own bearing as assembled..

Thought of that but was leery making swarf right next to a $500 pair of matched spindle bearings!

michiganbuck · Jan 12, 2019

I was talking about in the mill (or what) yes with having extra protection like oiled spindle nose and a tight tape wrap to ride on that nose so as good/tight as mist grinding spindles

*But yes, grinding and metal machining machines should be kept far apart.

I agree with checking the part on a plate...perhaps in two v blocks or two good parallels...just rolling on the plate.
Perhaps the part needs straightening not machining.

Some times a part checks odd when taken from its assembled position..or seems to check odd.

other: [score mark where something spun in it so I might re-grind it.] some time a blue-up and hone can restore a injury..

Matt_Maguire · Jan 12, 2019

Terry Keeley said:
The OD of the nose of my spindle is out a couple thou., I'm trying to true it up. The MT2 taper is also out (only a couple of tenths) but it has a nice score mark where something spun in it so I might re-grind it.

The only thing I ever stick on the outside of a little mill spindle is a rig to sweep bores??? Unless it’s spinning at some insane speed or you really want to fasten a tool to it, it doesn’t matter.

I agree with bearing dia's in V blocks & testing the bore since you have it out. If they all agree then bending it is not needed as the bearings to nose taper is straight.

I’m gonna have to check my B’port knock off now...

Thanks,
Matt

Terry Keeley · Feb 10, 2019

Just a quick update. I used some brass shim stock wrapped around the spindle (1/4" wide) and was able to get both bearing seats running very close, thanks guys!

Trued up the shoulder where the spindle bearings rest as well as the nose.

Now to re-grind the MT2 taper with my Falcon G-1...

michiganbuck · Feb 10, 2019

Qt:[The MT2 taper is also out (only a couple of tenths) but it has a nice score mark where something spun in it so I might re-grind it.]

I take a round hone to hone a bug/score..then with an old taper tool or center with a bit of fine rouge or compound see if I can
lap restore a little off-center ID taper...doesn't always work but if it does it saves a lot of work.

Used to use much the same method to move a part center .00005/.0002 (or more) with putting the off end on a abrasive center lap to put a heavy hand on the part and rotate it perhaps 5 to 20 degrees back and forth to lap the part center the way/direction it needed to go. yes the good or other end on a machine center.

That falcon G-1 is a very nice device...

Terry Keeley · Feb 10, 2019

michiganbuck said:
Qt:[The MT2 taper is also out (only a couple of tenths) but it has a nice score mark where something spun in it so I might re-grind it.]

I take a round hone to hone a bug/score..then with an old taper tool or center with a bit of fine rouge or compound see if I can
lap restore a little off-center ID taper...doesn't always work fut if it does it saves a lot of work.

Used to use much the same method to move a part center .00005/.0002 (or more) with putting the off end on a abrasive center lap to put a heavy hand on the part and rotate it perhaps 5 to 20 degrees back and forth to lap the part center the way/direction it needed to go. yes the good or other end on a machine center.

That falcon G-1 is a very nice device...

Ya, thanks for the suggestion, my toolmaker buddy had the same advice.

I made a brass lap and gently tried to true it up but the lap just followed the existing run out which is actually about 6 tenths near the big end.

I got the Falcon about 20 years ago to ID grind some hard chromed cylinders for my R/C boats and have used it very little since, seems they're discontinued now. Same company that makes the Planet line of air grinders to do "jig" grinding on a mill.

We made a cast iron holder with a precision lead screw and are able to in feed in tenths, it'll straighten it out!

hanermo · Feb 10, 2019

Not to be negative ..
but usually grinding will get to 2 microns in overall straightness, taper, tir, from true, or better on industrial setups.

Endless examples of skilled people doing VMC taper fixes, etc.
Running a VMC spindle into a mounted point will knock off the high spots.

And many people doing BP-type fixes with some lashups, often to very good results.

Rigid, proper, lapping should deliver good results to much better than 1 Micron tir / 1mu // 100 mm out.

Terry Keeley · Feb 10, 2019

hanermo said:
Not to be negative ..
but usually grinding will get to 2 microns in overall straightness, taper, tir, from true, or better on industrial setups.

Endless examples of skilled people doing VMC taper fixes, etc.
Running a VMC spindle into a mounted point will knock off the high spots.

And many people doing BP-type fixes with some lashups, often to very good results.

Rigid, proper, lapping should deliver good results to much better than 1 Micron tir / 1mu // 100 mm out.

I think if I had a good solid mount for the lap it mighta worked out better but I dunno, I also see how a lap would follow the existing run out as it did in my case.

If I get 2 microns (0.00008") TIR by grinding it I'll be a happy camper!

Screwmachine · Feb 11, 2019

If it's not bent I would have not touched the bearing seat. And would have tried a reamer on the Morse taper before grinding. And if grinding was required would do it in the machine like buck said.

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Mill spindle truing - set up problem.

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