Sunday, June 14, 2009

Replacing Atlas/Craftsman Part 9-11

The Atlas/Craftsman 12" metal lathe is a decent lathe for what it is - an entry level machine for the home shop hobbyist. This lathe was manufactured to meet a price point, not to be a professional quality lathe. One of the innovations employed in manufacture of this lathe was the use of a material called Zamak for many of the secondary parts such as change gears and handles. The idea was that die casting these parts from Zamak would save money and machining time. And it did.

Zamak, being an amalgam of zinc, aluminum, manganese, and copper is a reasonably durable alloy. It's stable, easy to cast, and plates well. Many common parts do very well when made of Zamac. But machines get bumped and banged. The Zamak parts broke much easier than their iron counterparts. When Atlas stopped making these lathes, inexpensive Zamak replacement parts largely disappeared too. Now the main assumption of Zamak - that replacement parts would be readily available and inexpensive - has become invalid. It's common to see Craftsman lathes will all manner of innovative replacements for broken Zamak parts. For example, when I bought my lathe the cross slide handwheel had been replaced with a water spigot knob.

But really, for the most part, the Zamak is ok. If a handle breaks after 70 years of use, well, you have a lathe, make another. There is one part, however, where the use of Zamak was downright egregious. And that is part 9-11, a housing that holds secure the pinion that connects the handwheel on the apron to rack. When you turn the handwheel, the carriage should move up and down the lathe bed. When part 9-11 breaks, and it will, parts fall out the bottom of the apron and your carriage gets all sad.

Here's a link to a document for my lathe. Refer to the "page 2" link - you can find part 9-11 at the bottom.

Here's a few images of part 9-11. This one doesn't fit my lathe, unfortunately. The lathe's original owner bought it without knowing that it fit a later model of the lathe.

The problem is this is a spindly part that gets a lot of lateral pressure applied to it. It never should have been made of Zamak, or at least not with this shape. The "legs" tend to break.

When I saw the part was missing, I checked Ebay, the easiest source for many Craftsman lathe parts. Since there is such a demand for 9-11 replacements the price is pretty high. I decided I would not pay top dollar for an antique Zamak part with a history of breaking. The only solution was to make a replacement.

Here's the diagram I worked from. All measurements are in decimal inches. Almost all measurements are multiples of 1/16". It was a simpler time.

The first image shows the bracket in place. There are three important things to note. First, notice the disruption of the aluminum where the axle passes through. That's because there is only a few mils of aluminum remaining there. Second, notice how the handwheel's small gear nestles in the angled area milled out of the bracket. Third, notice the poor fit of the large gear to the handwheel gear. The reason is in the notes below. Don't do what I did - I am still trying to remedy this.

Here's a view from the above front right, if it were in the apron. You can see the angled relief where the handwheel gear will fit.

Finally, here's a view from below. You can see the channel for the leadscrew.

Here are the instructions. The operations are all straight-forward. I had access to a mill. This would be hard to make otherwise.

  1. Acquire the large gear, small gear, and axle you see in the 9-11 pictures above. If you have a broken 9-11, you can salvage these. You will need to get one of the gears off the axle. Try an arbor press. On the 9-11 in the pictures above, the gears are peened on.
  2. Make the aluminum block. The width is extremely important. If it's too wide the gears will pinch the bracket when pressed on.
  3. Mark the location of the 1/2" hole. This is likely the most important measurement.
  4. Bore the 1/2" hole.
  5. Mill away the relief for the large gear. The depth must be at least the thickness of the large gear and the handwheel gear.
  6. Mill the "ears" through which the 1/4" bolts will pass. The depth of these was determined by the length of the bolts I had available. YMMV. You may not even need to add the ears at all if you use long bolts.
  7. Mill the leadscrew relief channel. When the channel intersects the axle hole you can stop. Removing more will not help.
  8. Put the axle in the bracket and carefully press the gears onto it. Check for a good fit before doing so. This will be hard to lubricate so add some ├╝ber grease to the axle now. NOTE - the axle is NOT symmetrical. Be sure you orient it correctly.
  9. Put some layout blue on the bracket where it touches the apron, where the 1/4" bolts will pass. Put spacers between the mating gears so they will mesh correctly. Many people use paper scraps. Push the bracket firmly into position on the lathe. The gears on the bracket should engage the rack and the handwheel gear. Have a helper mark the bolt hole locations using a transfer punch.
  10. Bore the two holes for the 1/4" bolts.

Notes, in no particular order.
  1. I don't know how long this part will last. I am an amateur. Caveat Emptor.
  2. The part I am showing is a replacement for part 9-11 from an Atlas/Craftsman 12" lathe made in about 1937. It has no powered crossfeed and a 5/8" leadscrew. If your lathe if newer or different, you'll certainly have to make changes. If you have an identical lathe, you'll probably have to make changes :-D
  3. As far as I know, the design is correct. But you can probably find 100 improvements.
  4. I made mine from aluminum scrap. If you have iron or steel, all the better.
  5. The biggest mistake I made was drilling the 1/4" holes too soon. This should be the last thing you do. Drilling them last allows you to position the bracket on the lathe in exactly the right spot. I drilled my holes first and had the unenviable task of making sure both gears and both bolts all lined up. They didn't. So I got out a file... It was a big time-sucking mistake.
  6. When you prepare the part for the final fit, pushing it upward will make it engage the rack. Pushing it back and forth will make it engage and disengage the handwheel's gear.
  7. From the front view, you see the channel at the bottom. That channel is necessary for leadscrew clearance. It channel will just intersect the 1/2" hole you bored for the axle. Yes, the part is nearly cut in two. The Zamak part also has a divot out of the axle area. Anyway, if you can reduce the width of the channel, you'll strengthen the part. I didn't have the tooling.
  8. Most of the dimensions are non-critical. You need to get the 1/2" axle hole pretty close. The width of the part is also critical or the gears will pinch it or slide back and forth. The milled area must be deep enough to accommodate the large gear that attached to the axle.
  9. Go slow when you press the large and small gears onto the 1/2" axle. Mine fit great - I got lucky. It's possible that if your bracket isn't wide enough the gears will be pressed against the aluminum and turn very stiffly. To be avoided.

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