Apron

Last Updated: 6 Mar 2011


The apron is attached to the carriage and contains the gearing necessary to bring power from the leadscrew to the power feed on the carriage and cross slide.  This photo shows an overview before removal.

Half nuts (which ride the leadscrew for threading operations) are engaged by pulling up on the half nut lever.

Cross feed is selected by moving the power feed selector to the upper position.  The lower position engages power longitudinal feed.

To remove the apron, the four slotted screws which secure it to the carriage must be removed.  This can be done after the leadscrew has been removed, or the entire carriage assembly (with apron attached) is slid off the end of the lathe (if the leadscrew and gearbox are not going to be removed).  I found the screws very tight, and you need a minimum 3/8" flat head screwdriver to avoid stripping the slots.  A large "drag link" socket might be even better.  I had to use a wrench on the handle of my screwdriver in order to get sufficient torque.

There is an oil sump in the bottom of the apron which is drained via a plug at the bottom.  It's probably best to drain the sump before removing the apron (I didn't and ended up with a minor oil spill).  If the sump is completely full (as it should be), there's quite a lot of oil.



Begin by removing the threading dial, if equipped, by removing the single hex head cap screw that retains it in the apron..  This device has a small gear that rides the leadscrew and is used to position the carriage properly when making a thread in multiple passes.

Shown here is the thread dial cleaned up and disassembled.  A small oil port is provided in the top of the shaft for oiling (not shown).

With the apron off, remove the oil sump cover from the back to get at the innards.  There's a gasket at the bottom to prevent leakage, so you might need to strike it to remove.  Be ready to catch oil coming out.  In fact, it's best to do this in some sort of drain pan.

To remove the sump cover, remove the 5 screws as shown in the first photo.  The one in the center is longer than the others.

The lower photo here shows what's behind the sump cover.  Power enters the apron through the leadscrew, which passes through the worm drive as shown.  There's a key in the worm drive that engages the keyway on the leadscrew.  The power feed clutch acts on the worm drive gear to transmit power into the rest of the apron.







In order to remove the worm drive assembly, the half nuts must be removed so the worm drive cylinder has room to slide out.  To do so, remove the gib on the left side secured by 3 hex-head cap screws and located by 2 dowel pins.

The dowel pins are probably too tight to permit removal of the gib by hand.  I found it necessary to drive the pins into the photo, which causes them to drop into the apron lower cavity where they're easily retrieved.

In the middle photo is shown the cam which actuates the half nuts vertically through a rotary motion of the control lever at the front of the apron.  Pins in the back of the half nuts engage the cam slots.  To remove the cam, first remove the handle on the front side of the apron by driving out the taper pin that retains it.  Once the handle is removed, the cam will slide out.

The lower photo shows the inside of the half nuts.  It may be difficult to tell from this photo, but the thread forms are heavily worn.

Update - 12/2/10
Since originally writing this section I have cut some threads.  I found, quite surprisingly, that resulting thread form is very good, in spite of heavily worn half nuts and leadscrew.  I was cutting some 7/8-9 threads to fit a nut for a spindle arbor on a grinder.  A 7/8-9 nut is not a precision thread form, but it was critical that I get the threads on the arbor just right to avoid scrapping the part.  I had no significant difficulty doing this.

I've heard it said (and seen it done) that to help compensate for slop throughout the drivetrain, leadscrew, and half nuts, one should hold a light pressure on the apron handwheel when making a thread.  This takes up the slack and runs the half nuts tight up against the leadscrew threads.  I want to point out in the above example with the 7/8-9 threads that I did not do this, but I still got a good thread.

My point in saying all this is, even if your leadscrew and half nuts are almost worn away to nothing, don't assume you can't make good threads.

Removing the worm drive can now be accomplished.  The first photo here shows the collars which secure the worm drive to the apron casting.  Each collar has a straight pin, which is removed by driving into the picture.  The collars are threaded onto the end of the worm drive cylinder, which runs in bushings as shown.

Once the collars are removed, the bushings are clearly visible in the second photo.  The locating pin is longitudinal, and can't be removed with the bushing in place.  To remove the bushings, drive them out (they're lightly pressed into the case) as shown in the third photo.






The clutch assembly is removed next.  To do so, begin by removing the star knob (or lever on later model lathes).  There's a hex cap screw that retains the knob, which was missing on this lathe.  I suspect the screw is #10-32 left hand thread.  The star knob is also threaded on to the shaft behind it.

The second photo shows how the clutch shaft is retained in the case.  There are two shafts - an outer and inner.  The outer shaft provides a bearing surface for the worm gear while the inner shaft actuates the clutch pack.  A straight pin retains the inner shaft to the outer shaft, which in turn is retained by the hex nut.  When the pin is removed, the clutch return spring may release the clutch pack on the back of the apron!

The third photo shows the clutch being removed.  With the star knob loose, the clutch pack is decompressed by the return spring, which permits the main drive gear to rotate independently of the outer shaft.  When the knob is tightened, it squeezes the clutch pack together, which forces the outer shaft to rotate with the main drive.

The fourth photo shows a partial breakdown of the clutch assembly.  The outer shaft is retained in the main drive by the external retaining ring as shown.

The fifth photo shows a partial mockup of the clutch assembly.  The inner shaft is positioned concentric with the outer, and the clutch spring is in place, as well as the end bell.  There are two types of clutch disks: one whose inner diameter is splined to match the outer shaft, and the other whose outer diameter is splined (the semi-circular protrusions that can be seen in the pack as shown) to the main drive.

The last photo shows what I thought was an anomaly.  According to form 910D the part labeled "Oil Washer?" should be an oil washer, but on this lathe it's a thin gear.  The inner diameter of the gear is larger than diameter "A", and "C", but smaller than diameter "B", which means when it's mounted on the shaft in the position shown in 910Dit just flops around on the outer shaft.  It turns out, it was designed this way.  This washer, with it's gear teeth, is actually an oil slinger, used to throw oil on all the gears in the vicinity.  It doesn't rotate at the same speed as the outer shaft; instead it will rotate at some slower speed, but is apparently quite effective nontheless.








Next remove the cross-feed drive gear.  This is the only gear that protrudes above the apron casting, and in so doing engages the cross feed leadscrew.  To remove, punch out the pin as shown, and loosen the set screw.  The shaft can then be driven out in either direction, and the gear removed.

Next remove the traverse gear assembly.  This is what engages the rack on the lathe bed to move the cross slide assembly when you rotate the handwheel.  Note the taper pin - it's easy to tell the big end from the small end here.

The gear that engages the rack is permanently affixed to the shaft, so the shaft must be removed by driving it out toward the bottom of this photo.




To remove the handwheel, punch out the taper pin that retains it to the shaft.  The shaft can then be pushed out in the direction shown.

Next remove the gear shift mechanism, which is simply a gear on an offset cam actuated by the gear selector lever.  First, remove the taper in as shown in the first photo (note the apron has been flipped upside down to gain access to the pin).

The clearest way to show the gear shift assembly is laid out in pieces, as in the second photo.  All of the gears in the apron are shown here.  It's difficult to tell from the photo, but the stub shaft on the gear selector cam is off-center, so when the drive selector lever is moved the central drive gear engages either the cross slide gear, nothing (neutral), or the rack drive intermediate gear.





Lubrication System
The original lubrication scheme used a series of long felts running from the oil sump at the bottom of the apron casting up to the various shafts that require oil.  I don't particularly like this system, especially after seeing the condition of the old felts.  Nevertheless, the system worked pretty well, so I intend to use a similar arrangement, with a couple minor changes.

Of the 5 shafts, 3 of them are too far from the oil slinger to receive oil from it.  These are the handwheel shaft, the rack drive gear and shaft, and the cross slide gear shaft.  The gear selector lever shaft is splash-lubricated from the main sump, and the half-nut lever is lubricated automatically when oil is added to the half-nut sump via the oil cup on the right hand side of the apron.

The handwheel shaft, near as I can tell, had no way of obtaining fresh oil from either the lower sump or the upper reservoir.  I suspect this apron was apart in the past, and not reassembled correctly (more evidence of that to come).  To remedy the handwheel shaft, I drilled a small (7/64") hole in the casting directly over the felt keyway, as shown in the first photo here.  This hole penetrates directly to the felt keyway, and permits feeding oil directly to the felt.  To test the idea, I loaded the handwheel shaft completely dry and pumped the recommended oil (Mobil Vactra No. 2) into the hole.  As expected, the wick soaked up the oil and provided an abundance of lubrication.  The wick I used here is a strip cut from a 12"x12"x1/8" F10.  F5 would be equally good here (perhaps better, since it's more resilient).  It's interesting to note it would be possible to remove the handwheel and shaft with the apron still on the lathe, so I can make quick inspections easily if necessary.

Internally, I modified the felt arrangement for the rack drive gear as shown in the second and third photos here.  This photo shows the wick for the rack drive gear shaft runs up (toward the top of the photo), around the gear, and then through a hole in the casting to the upper oil reservoir.  Originally, South Bend had that wick running straight down from the outer bearing (the right side of the photo) into the main oil sump.   This resulted in one unused access hole to the upper oil reservoir.

A bit of research revealed why that extra hole existed: in prior years, South Bend routed these wicks exactly the way I've done here!  The fourth photo here is courtesy Mr. G. McLane, showing his South Bend Heavy 10 apron (very similar to the 13" apron), which is of an earlier vintage than my 13".  Notice the unfinished cross brace, which doesn't contact the saddle when the apron is installed; contrast that with my 13" apron, which has a finely ground surface atop that cross brace which contacts the underside of the saddle when the apron is installed - a fact which precludes use of any sort of attachment (such as the wire shown in the photo).

To facilitate holding the wick away from the gears, I decided to grind a very small slot in the cross piece so that a small-diameter wire can be installed similar to the Heavy 10 arrangement.  The fifth photo here shows the slot with a piece of 20-ga. wire installed to retain the felt.  I used a piece of stranded copper wire (insulation removed), twisted and tinned with solder to make the retainer.  The ends were then soldered together to make a ring without the need for twisting.  The photo was taken just before snipping off the excess wire ends.

It's interesting to note that South Bend specifies two different types of oil for the apron: the upper reservoir gets Mobil "Vactra Heavy Medium" (since superceded by DTE Heavy Medium - see the lubrication page), while the main sump gets Mobil Velocite No. 10.  The Velocite is substantially less viscous than the Vactra.  If the outer rack gear drive bearing were run down to the main sump (as it came from the factory), it would receive the thinner Velocite.

I'm not clear on why South Bend changed the routing of that wick.  It could be for economic reasons - it may have taken slightly less labor to install the wick down into the sump instead of across to the upper reservoir.  Or perhaps there is some flaw in this routing that I don't see right now.


The wick I'm using here is from a 12"x12"x1/8" sheet of F5 from McMaster-Carr.  I've simply cut strips of it long enough to reach from each bearing over to the upper reservoir.  The 1/8" thickness is perfect for the depth of the keyways in the bearings.  F5 or F10 would both work in this application, but I'm using F5 because it's more resilient than F10.  F1 wouldn't work here because the felt needs to be compressed in service and F1 isn't compressible.

















With the rack drive arrangement reinstalled, we move over to the main drive assembly, which must be installed before the cross slide drive gear due to access considerations.  This is where main power enters the apron from the leadscrew, and it has an interesting wick arrangement, difficult to explain and show adequately with photos.  If you have questions, email me.

The first photo shows the routing of the wick before the worm gear itself is installed.  Note the orientation of the wick keyway in the worm drive bushing!  On this lathe, the assembly had been apart in the past and the bushings were swapped left/right when reassembled.  This caused the keyway to be pointing 180 from where it is in the photo, which is bad.  Fortunately, there was no damage (which leads me to believe the "repair" was relatively recent).  Actually, the wick routing I'm showing was completely absent on this apron, and it took quite a bit of research with other lathe owners to figure out what the correct wick arrangement was.

The wick I'm using at this location is 1/8" diameter F1 corded felt, purchased from McMaster-Carr for a very reasonable cost.  F1 is quite durable, and fits nicely into the keyways and around the clutch gear.  It would also be possible to use F5 here, but in my opinion the sturdier F1 is a better choice.

The second photo here is showing the inside view of the felt routing from the first photo.  The loose end of the wick on the left side of the photo is just laying there temporarily for assembly.

The third photo is showing the way the wick lays with the worm drive gear installed.  The collar that retains the worm drive isn't shown here, but when installed it keeps the worm bushing pin from falling out and holds the wick against the its keyway in the bushing.  Since the bushing is stationary and the collar rotates with the worm drive gear, the felt provides lubricant to the interface.

The fourth photo shows what I did with the ends of the wick that drop into the main sump.  Originally, South Bend didn't extend the wicks into the sump, but I figure why not?  In so doing, we make the felt triple-redundant, by which I mean it obtains oil from three sources.  The two walls should keep the felt from engaging any of the mechanism.  This photo notes a few other items that are visible and important for the next section.


Next the central drive gear must be installed.  The first photo here shows it installed (during disassmbly - I didn't take a picture at this point during reassembly).  Although the worm drive is clearly absent in this photo, I installed the central gear after reinstalling the main drive assembly.  This gear is mounted on an eccentric shaft that is rotated by the gear selector lever on the front of the apron.  As the selector is rotated, the central gear engages either the rack drive gear or the cross slide drive gear (or neither for neutral).

This is also a good opportunity to look at the interlock mechanism that prevents the operator from engaging both the half nuts (for threading) and the power feed simultaneously.  The second photo shows the interlock with the main power feed engaged.  Notice the interlock has been pushed from the notch in the gear selector and the end of the shaft is now engaging the lower half nut.

The third photo shows the interlock in the opposite configuration - the half nuts have been engaged, which has forced the interlock mechanism to engage the notch in the gear selector.

This is a very crude mechanism, and at first seems odd without some kind of spring to hold the interlock in one position.  But it works, and is foolproof (near as I can tell).






This photo shows the worm drive installed, which is rather unremarkable.  There are two 1/8" pins that keep the end caps from backing off the worm gear (not visible in the photo).  The clutch is also partially assembled here - the plates have been inserted but the pressure plate has not.

Also visible are the half nuts on the left edge of the photo.



The last gear I installed was the cross slide drive gear.  This also happens to be the most difficult felt to run, due to the tight clearance.  There's a groove for the felt in the gear shaft, but due to the way it's put together you have to install the gear shaft through the gear at the same time as the felt.  What I mean is, you must position the felt in such a way as to drive it into the keyway as the shaft is being driven into the case!

If that's confusing, you've just about got it right.  It took me awhile to plan out a method, and 3 or 4 tries to get it right.  The felt has to be thin, as indicated, because the keyway is shallow and the clearance between the gear and the case is tight (around 0.075" or so).

The red arrows in this photo are attempting to show the path that the wick takes from the upper reservoir to its termination at the end of the keyway in the shaft.





One of my oilers - the one that feeds the half nuts - had a missing cover.  Initially my intention was to replace it.  But i discovered, rather by accident, that a 3/16" drive-in shouldered oil hole cover presses nicely into the end of the original!  Doesn't look quite as "clean" as the original, but it's good enough for me.

If you would rather replace this altogether, a good part number from McMaster-Carr is 1239K15, "drive-in elbow-style oil-hole cover, 5/16" hole diameter".  This is a Gits-brand oiler, so it's consistent with the original manufacturer, but it looks nothing at all like the one I've pictured here.  The problem you'll encounter, however, is the metal used on the modern oiler won't accept solder.  In fact, if you heat it up to solder it you may inadvertantly melt it.  Believe me, I know.  So if you buy such a replacment be sure to consider the fact that you'll have to come up with another way of attaching a short piece of brass tube to it.

The last thing to do was cut a new gasket for the sump cover.  I decided to use a rubber/cork blend, which I got at an auto parts store for about $5 (much cheaper than McMaster-Carr, for some reason).  I applied a little Dykem blue to the gasket area on the cover, then transferred that to a piece of paper and made a template.

Of the 5 screws shown here, only the one in the center and the far right end (on the handwheel side of the apron) actually penetrate the casting into the oil sump.  To those two I applied a little thread sealant, but to be quite honest I figure this thing is probably going to leak since I didn't bother to seal the gasket.

I also stuck a little neodymium magnet on the apron drain plug so I can collect any metals that find their way into the oil when I drain the apron for periodic maintenance.  You could either put it as I've shown here, or you might want to insert the drain plug in the Apron casting, then attach a magnet (or two) to the outside of the plug if you're worried about possibly losing the magnet in the casting.  Initially, I did it as pictured, but later changed my magnet to the outside of the plug because it was difficult to keep the magnet attached to the drain plug when reinserting it.  Sticking the magnet to the outside of the plug should accomplish the same task, but in a more convenient way.