Headstock Spindle

Last Updated: 11 January 2008 (added the bull gear section at the bottom)


This photo shows a complete headstock spindle assembly with names for each important part labeled.  It will help the reader understand what I mean when I use various terms related to disassmebly and refurbishing.

In my opinion the two most important parts of any lathe are the headstock spindle and the bed ways.  On this page we address the former.  When evaluating a lathe for purchase it's important to pay close attention to the condition of the headstock sprindle, particularly the bearings.  These bearings are singularly critical to the performance of the lathe.  If they are worn or damaged the lathe cannot produce top-quality work.  It is absolutely essential that these bearings are clean, oiled, and smooth.

From what I've read there are two main types of headstock bearings - roller and plain.  Most (all?) South Bend lathes use plain bearings, which apparently offer a better surface finish on machined parts than roller bearings.

As such, the headstock bearings are particularly costly to replace.  As of this writing, a new bearing for this lathe costs ~$400, and there are 2 on the spindle.

This particular lathe had very smooth bearings, with good clearance (~0.002" as measured according to this procedure).  The oil cups were full of clean oil, and the bearings ran silently and generated little or no heat.

Nevertheless, I wanted to dismantle the headstock to check the condition of the bearings, the expander screws, and the oiling system.  In my opinion, this is part of regular lathe maintenance to ensure the bearings are getting proper oil, at least.

Removing the spindle is straightforward, as described in detail in this document.  We begin here by removing the bearing caps AFTER REMOVING THE EXPANDER SCREWS!  There are small bearing "expanders" installed in slots in the bearings that are used to spread the bearing slightly so that an oil passage between the spindle and the bearing is created.

In this photo I've removed the pipe plugs, expander screws, and the cap screws that retain the bearing caps.  South bend says to use a rod to jar the bearing cap loose, and I agree with them - mine were stuck in place by the factory paint (I think).

This photo is showing the rear cap, but the front is exactly the same (although slightly larger).  Notice here I've backed off the takeup nut several turns, as instructed in the document I linked to earlier.  According to the above document the takeup nut is threaded on to hand-tight, then backed off 3/8" and clamped down.  I'm not sure why there's so much clearance.

Here I've removed the front bearing cap and noted the various parts.  I labeled the bull gear pin even though it has nothing to do with this discussion, merely because it's clearly visible in this shot.

It's dirty on the outside here - lots of oil "goo" - but otherwise appears to be in working order.  It's hard to see but there's a tiny letter "F" stamped at the front of the bearing expander.

Here I'm also showing the rear bearing with the cap removed, because there's an interesting problem - the bearing expander has (apparently) been installed backward!  The little "F" stamped into it should be toward the "F"ront of the headstock, right?

I suspect it was installed this way from the factory, although I could be wrong.  It's not clear to me whether the orientation really matters on these, but there is a very subtle difference between the front and rear of the expander.  The rear of the expander has a round radius to it, while the front has a tiny flat section.  It'll be more clear when I show a closeup of the expanders...

With the bearing caps removed the spindle can be lifted out of the headstock.  This isn't necessarily hard, but there are two things to bear in mind:

1. The assembly is heavy - probably between 35 and 45 lb, and
2. The presence of the flat belt makes it difficult for one person to remove the assembly.

I suggest, therefore, that an assistant be employed when it's time to remove the headstock spindle.  It's not strictly necessary (I did the job alone), but it really makes things easier.  This isn't something you want to risk damaging in any way.  I had to lift it out and set it back down 3 times before I got myself in a position where I could get it out through the flat belt.  If you have someone else to hold the belt out of the way it would make the job much easier.

With the spindle sitting out on the bench it can be disassembled.  We begin by removing the takeup nut and the rearmost gear.  The gear is not press-fit, so it simply slides off.  It engages the spindle via the key shown.

Here's the spindle with the takeup nut and gear removed.  Once that little key is pulled out, both thrust bearings and the spindle bearing slide off the spindle.  This is followed buy the pulley cone, which also slides right off.

Here I'm showing the condition of the spindle in the area that was under the bearing.  There are no score marks or damage of any kind.  This is in excellent condition.

Here's what the spindle looks like with the pulley cone removed.

Here's a shot inside the pulley cone bearings.  They look excellent.  No visible damage.  There's some tiny oxidation spots here and on the spindle, but nothing significant.


Removing the Bull Gear





 The bull gear is held to the spindle with a press fit, and locked in using a key.  South Bend says to use an arbor press to remove the bull gear, but my press's throat is about 5 inches too short (the spindle is 16.5" long).  So I had to come up with my own puller, pictured here.

The lower picture shows the relevant parts of the press without the spindle.  Notice I had to cut a notch in the PVC for the bull pin.  This probably could be avoided by using 6" PVC instead of 4", but I used what was to hand.  PVC is nice because no matter what you do you won't harm the steel parts.

Make it at least 6.5" long to have sufficient room to press the gear until it can be moved by hand.  The gear has a press fit over the section it's sitting on from the factory, and a very close fit through the area where the cone pulley mounts.  But I was able to twist it by hand down the section where the cone pulley bearing surface is, once the gear cleared the keyway of the shaft.

The fit isn't as loose as I would've liked, but gave no real difficulty to the tool.  I tightened the press, then smacked the gear with a rubber mallet, which would jar it forward along the shaft a little.  Then re-tighten the press and repeat.

Be sure to wrap the rear bearing area and takeup nut threads with tape before sliding the gear off!  You'll be glad you did.  You can see a nick in the tape here, but it didn't penetrate down to the metal because I double-wrapped the tape.

Bearings

Here's an extreme closeup of the rear sleeve bearing as it was removed from the spindle.  Note the scoring in the photo, which can be felt if you run your finger across the bearing.

And here's the front bearing as it was removed from the spindle.  Note the scoring, which doesn't appear as dark as the rear bearing because this one is simply cleaner.  As with the rear bearing, you can feel these grooves with your finger.

Damage

I thought, given the condition of the paint on this machine, that the bearing caps had never been removed.  I now understand I was quite mistaken, as this photo proves.

When removing the bearing caps, for the love of all that is sacred, first remove the expander screws completely!  Here we see first-hand the carnage that occurs when this advice - which South Bend took the time to print clearly on a brass plate affixed to the inside of the headstock cover - is not followed.  Honestly, how dumb do you have to be to do this?  Whoever had this apart knew enough to mark the front of the bearing with an "F" (which is in the instructions from South Bend), but didn't know enough to remove the expander screws first?

The rear bearing shows no such damage, which leads me to believe whoever did this tore into it, broke the bearing, then went back and read the instructions before continuing.  This kind of senseless cruelty to machinery really makes me angry.

When you don't first remove the expander screws, you end up pulling the expander out from the bearing by force.  Something has to give, and in this case it was the bearing shell.  It would have been more convenient if the expander had broken - it's a lot cheaper than a new bearing.

Fortunately, I don't think this bearing is trash.  I've seen this machine run, and the bearings are silent and smooth.  The only issue I had with it is the clearance, which measures out to 0.002".  I should be able to properly set clearance when I reassemble it despite this damage, but we'll see.

It turns out the rear bearing, which I thought was completely undamaged, has suffered from someone reassembling the bearing cap with the expander sitting atop the bearing (rather than in the slot cut in the bearing).  This has the unfortunate effect of placing indentations in the bearing which precipitate corresponding raised sections in the slot.  I've tried to show one of these raised areas in this photo.  It's difficult to see, as the camera doesn't have a very effective macro mode.

Suffice to say there were two raised sections in this slot, and two corresponding indentations on the top of the bearing.  Much better photography of this situation on another machine may be found here.

I used a smooth mill file to gently file away most of these protuberances.

Reassembly

To reassemble the spindle we begin by putting the front bearing in place, ensuring the "F"ront of the bearing and its expander are properly oriented.  Once the back gear has been installed it's not possible to remove the bearing, so it's important to get it right the first time if you don't want to have to press the back gear off again.

But before I show the back gear going back on, I thought I'd show a couple closeup shots of the spindle.  We see here the spindle has been discolored with what looks like grooves in a record.  These lines are so shallow that I can't feel them with my fingers - neither the skin nor the nails.  The spindle comes "superfinished" and hardened from the factory, which means the surface roughness is under 0.00005" (I think).  Obviously something has happened to the spindle in this area, but it's not clear exactly what.




Anyway, moving along we slip the front bearing shell and expander onto the spindle and install the back gear press.  As for the removal, this press was made from a bit of 4" PVC, notched in the appropriate areas to clear the bull pin mechanism and a large counterweight cast into the back gear for balancing.

This press worked remarkably well, even though it's pretty much impossible to get it precisely centered on the spindle.  As I commented before, the bull gear isn't a particularly tight press fit, and it didn't take much effort with the wrench to get it to slide back in to place.

It does annoy me, however, that this gear is pressed to the shaft.  On the whole, I don't like press-fit parts.  In this case, there's little reason for a press fit here, since the gear is keyed to the shaft.  The presence of that key, in fact, makes the pressing operation that much harder, since you must ensure the keyway on the gear will clear the key during the press.  The only reason for the press fit, then, is to prevent the gear from walking off the shaft.  But since this is a straight-cut gear, the axial load is going to be tiny (if any), and the thrust bearings can take up any such load with no problem.

If I had another precision lathe I might've bored 0.001" or so off the gear so it could be slid on/off by hand.

Note the proper orientation of the bull gear, with the bull pin facing FORWARD.  That's very important.  The gear butts up against a shoulder on the spindle when it's been properly positioned.

Note: in the following text, when I refer to "sides" of the bearing I mean the parts closest and farthest from the camera in the photos.  The closer side, which is the side where the operator stands while running the lathe, will be called the "operator side", while the opposite side will be called the "far side".

Here's the spindle reinstalled, but lacking the bearing caps.  For this part you definitely want 2 people and a long dowel rod of some kind placed in the spindle to help carry it.  It can be a little tricky to get the oil wick hole on the bottom of each bearing shell to slide onto the oil wick tube.  The danger lies in accidentally pressing the oil tube into the casting with the weight of the spindle.

The bottom picture here shows my method for reducing the chances of pressing the oil tube in as well as retaining the oil wick during assembly.  I used a 3/16" rod through the top oil port, which runs right through the hole in the oil tube I showed on the headstock casting page.  Before installing the spindle, you press the wick down and put the rod in to hold it.  The rod will also retain the oil tube in case you make a mistake and drop the spindle into the casting.  After the spindle is installed you pull the rod out and the wick springs up to contact the spindle.

The next step is to install the bearing caps and set the spindle clearance using shims. On the headstock casting page I describe and show pictures of my initial shims, which were 0.015" brass.  When I got the lathe it had ~0.0025" clearance (as measured per the South Bend procedure) using ~0.017" shims on both sides of both the front and rear bearing.

South bend says to run 0.0007" - 0.001" clearance in the bearings.  This is measured by pulling up on the spindle with a force of "approximately 75 lb".  It is virtually impossible to be precise with that 75 lb specification - I tried it standing on a bathroom scale and found it extremely difficult to do accurately.  I'd prefer to run close to the 0.001" specification to keep the bearings from heating up too much.  I'm willing to sacrifice some precision in the lathe in exchange for assured bearing life.

With only 0.015" shims the spindle locked.  With 0.016" shims on both sides the clearance was something around 0.002" (front and rear).

Front bearing.  0.015" on the operator side and 0.016" on the far side gave an indicated clearance of ~0.0012" - 0.0015", depending on how hard I pull up on the spindle.

Rear bearing.  0.016" on the operator side and 0.015" on the far side gave an indicated clearance of ~0.001" - 0.0015", again depending on how hard I pull up on the spindle.

These clearances are larger than the maximum specification, but my inclination is to leave them alone.  As it stands, it seems like there's a lot of drag on the spindle.  If I spin it by hand without the chuck in place I get between 1/2 and 3/4 revolution before it stops.  With the chuck installed, I get around 3 turns before it stops.

More disturbingly, I've noticed the drag decreases for about 1/4 of the rotation, suggesting something is binding up for 3/4 of the rotation.

I guess I won't really know anything until I power up the spindle and see if the bearings heat up.







It was requested that I show some more detail of the bull gear itself, with particular attention to the locking mechanism.  The first photo is the back side of the bull gear.  The only part missing in the photo is a thin metal plate that helps retain the sliding block.

The second photo shows an end-view of the pulley cone behind the bull gear.  Notice the pockets where the sliding block engages.



After getting the electrical system sorted out, I powered up the spindle again.  I was pleased to find it runs silent and smooth, with minimal heat generation.  I ran it for about 10 minutes at maximum speed and the bearings remained below 110°F (warm to the touch).