Reversing Gear Train

Last Updated: 11 April 2009

The reversing geartrain is used to control whether the gearbox (and hence the leadscrew) rotate in the same direction or opposite direction of the headstock spindle. As shown in the first photo, if the selector level is moved up to the upper detent, gear "A" engages the gear on the headstock spindle, which causes the gearbox and leadscrew to rotate in the same direction as the headstock.

On the other hand, if the selector lever is moved down to the lower detent, gear "B" engages the gear on the headstock spindle, which causes the gearbox and leadscrew to rotate in the opposite direction as the headstock.

This is useful for power feeding the apron regardless of the direction of the headstock spindle.

In the photo the selector is positioned at the center detent, which is neutral. Neither "A" nor "B" are engaging the spindle gear.

As I note in the photo, the nut on the bottom gear (aka the stud gear) is threaded onto the gear shaft, and rotates with the gear. The nut shouldn't be very tight. To remove it you'll need to lock the spindle using the back gear. To do this, simply engage the back gear with the lever and don't pull the bull gear lock pin out.

If, like me, your back gear isn't installed when you get around to working with the reversing gear train, you'll need to find a way to hold the bottom gear fixed whilst loosening the nut. I clamped a pair of Vise Grips to the gear on the front and rear faces (back away from the teeth) to wedge against the central casting. This held the gear while I loosened the nut. I did this with the reversing gear assembly removed from the headstock.

To remove the reversing geartrain, simply remove the two slotted screws that retain the reverse assembly retention bracket (bottom of photo above). The whole assembly then slides right out of the headstock casting. You must remove the takeup nut on the spindle (if installed) to clear the reverse gears!

This is what it looks like removed from the headstock. The nuts that retain gears "A" and "B" don't rotated with the gear, so they're easily removed.

Dismantling this is straightforward. Remove the stud gear nut (as discussed above), and the two nuts that retain "A" and "B". Drive out the small pin that retains the selector lever to remove it and the pin. There's a strong spring around the pin, which will shoot the pin across the room when the selector lever is removed!

The second photo shows gears "A" and "B" removed and sitting in the background on their shafts. It's a good idea to mark which side each gear and shaft goes. If the lathe is old the gears will have worn according to their position, and it's probably worth putting them back where they were. Don't panic if you don't, it'll probably be alright.

I removed the smaller of the 2 stud gears using a gear puller, because there's plenty of clearance between them to get the puller jaws in there. Piece of cake.

To remove the larger stud gear you'll need to drive the shaft out by pounding with a hammer into the picture. But there is great danger in this. I slightly damaged the key in the shaft and the reverse casting because the key was in just the right position relative to a felt wick groove in the casting. The third photo here shows how I damaged it. There was no way to see what was going on with the stud gear in place, so I had no way of knowing what I was doing. Thank God the key is made of soft steel and the damage to the casting is almost undetectable. To avoid doing this, I suggest:
1. If possible, pull the key out before driving the shaft off. This wasn't an option for me, because the key fit too tight with the stud gear.
2. Rotate the shaft so that the key is in front of a section of the casting away from the felt wick groove. This should cause the casting surface to push the key out as you drive the shaft.
3. Another method that was used with great success on another machine was to drive the larger gear off the shaft by striking it from behind. Use a few hammer blows where the gear can be accessed from the back, then rotate the gear 90 degrees and repeat. This method avoids any possibility of damaging the key or the shaft. Use a brass punch to avoid damaging the gear.

The key cleaned up nicely with a file, and the damage won't harm the operation of this assembly.

The reverse gear casting has a series of oil rifles drilled through it, which form a sort of oil reservoir for feeding oil (via wicks) to the reverse gears. The first photo here shows the position of the oil rifles and the wicks that I found installed. The semi-transparent green lines in the photo are indicating the oil galleys drilled into the casting. This is a fairly complex felt scheme. The top felt carries oil across each of the two holes behind gears "A" and "B", as shown. At each of those holes, oil is transferred to another felt that carries it out to yet a third felt embedded in a groove in each shaft for "A" and "B". There are a total of 7 felt wicks in this assembly!

In order to remove the wicks you'll need to remove the plug in the casting as shown in the second photo. To do this, drill a hole and tap it for a small screw which is then threaded into the plug to pull it out. I used a #10-24 machine screw to do this, but anything between a #6 and a #12 should work fine.

The third photo here shows my setup on the drill press to bore a small hole (for a #10-24 screw) in the plug. Notice I'm nowhere near center, which didn't have any effect on my ability to pull the plug out. Truth is, I thought the plug was thinner than it turned out to be, and started drilling it with a hand drill. When I realized it's a little thicker than you might expect, i moved to the drill press to do it right.

The fourth photo in this series shows my crude method for pulling the plug. I threaded a machine screw into the drilled/tapped plug, then held the head of the screw in a vise. I then struck the casting as shown with a hammer until the plug was freed.

Conveniently, the hole size here is compatible with 1/8-27 NPT threads. Actually, it's a little oversized for 1/8" NPT, but since we're only trying to seal against an extremely light pressure it doesn't matter that the threads are exactly the right dimensions. All you have to do is run a 1/8-27 NPT pipe tap in the hole 3/8" or so and you can insert a 1/8 NPT pipe plug. This will permit easier maintenance in the future.

With the big plug removed you can fish all the felt out of the cavity with a small pick and pliers. However, there is another plug that you may consider removing as well - one that is substantially more difficult. The fifth photo here shows the location of the plug, with the location of gear "B" noted to help the reader understand the orientation of the casting in the photo. This plug is closing off the upper oil rifle, and has been cut flush with the casting. It's not a plug in the same sense as the larger one - it's merely a piece of 3/16" steel rod pressed into the end of the top oil rifle (which is 3/16" diameter).

The sixth photo here shows the setup to drill out the flush-cut plug. Actually, it shows the operation finished, with a 1/16" NPT pipe plug installed.

To begin the process, I used a 3/16" drill bit, which is the same size as the top oil rifle shown in the first photo. There is danger in doing this! I failed to properly orient the casting with the drill bit by assuming the plug was perpendicular to the casting surface. In reality, it moves at an angle as shown in the first photo in this group. This caused me to bore my hole offset from the oil rifle, and nearly penetrate the hole for the gear "B" shaft! If you decide to drill out this plug, may I suggest the following:

1. A better way might be to drill a small hole in the plug, say for a #6 machine screw. Tap the hole and try to pull the plug using a similar method what I've shown for the large plug.
2. Or, if you decide to drill out the plug with a 3/16" bit, carefully orient the bit so that you're drilling concentric with the oil rifle! It's okay to be off a little, but too much will result in penetrating the hole for the gear "B" shaft.

After drilling the plug out it's easier to install felt in the top oil rifle, since you have direct access to it. I attempted to install the wick before removing the plug, and found it too difficult. But I believe with a little patience it might be possible to stuff new wick in the top oil rifle without drilling out the plug. For example, it's probably possible to thread a wick in there via the main (vertical) oil rifle.

I was left with a rather large non-round hole when I got done drilling (since I drilled at the wrong angle, as mentioned). I tried a few different ideas for plugs and finally settled on a 1/16" NPT plug. To do this, I had to drill a 15/64" shallow hole in the casting centered at the top oil rifle. If you do this, you can only drill to a depth of around 3/8" at most, since any deeper would risk penetrating the hole in the casting for the gear "B" shaft. Simply tap the new hole and insert the plug, as shown in the last photo.

I used 1/8" F1 cord for the felt in the upper oil rifle and the little holes that feed the upper gear shafts. For the bottom where the stud gear shaft runs, I used 3/16" F1 cord at the oil rifle.

The felt grooves in this assembly call for either F10 or F5, since it must be compressed. I decided to use F10.