The Reverse Tumbler Project

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Posted On January 19, 2012

       Hello again folks! This week I decided to work on my new Grizzly G0602 - 10 x 22 metal lathe which I just go a few weeks ago. The lathe itself is a very robust machine having the capability to cut 33 inch threads and 26 metric threads. The lathe has a 1-horsepower motor which is coupled to a 6-speed belt transmission that can produce 150 to 2400 RPM. This machine has a lot of features and yet is a simple machine compared to a toolroom class lathe. Basically all of the features a hobby machinist would normally want.
       I have done some cutting with this lathe and I am impressed with the accuracy and sheer power of the unit although I am coming from a limited previous machining experience. The machine I am used to is a 7 x 10 mini lathe so anything larger is an upgrade. Although there are a couple of features that my little lathe has that the G0602 does not and one of them is a reversing tumbler for the lead screw.
       A reverse tumbler is a set of gears in the drivetrain of the lead screw gearbox that allows the operator to change the direction that the lead screw pulls the carriage during forward rotation of the spindle. Normally a lathe will pull the carriage toward the headstock during forward spindle rotation which allows the operator to use a “right-hand” (from right to left) tool bit to turn down the workpiece. When the lead screw is reversed you are able to cut the opposite direction away from the headstock using “left-hand” cutting tool bits.
       The other handy feature of a tumbler is the ability to cut left-hand threads. Although left-hand threads are not commonly used they are still useful in certain applications. A reversing lead screw is a must to cut these types of threads and the G0602 does not have the ability unfortunately.

       Before I bought the G0602 I did some research on modifications that would improve the unit’s function and stumbled upon a few sites that were very helpful. One of these sites was run by a gentleman named Dennis Atwood which has an article on his experience building a tumbler for his G0602 lathe. I studied his article and learned that it was possible to add a tumbler to this machine with very few parts.
       I appreciate Dennis’ effort to post the article as he already did the research to find the necessary gears to build the tumbler which saved me a bunch of time and streamlined the build. He also had a good plan to align the gears in relation to the existing change gears which is basically what I used for this build. This is a prime example of why I post my builds as it helps out other builders and saves them time and money so my hat is off to you Dennis!!!
Note: If you would like to know a little more about the development history of the 10 x 22 tumbler design you can visit this build thread on the Projects In Metal forums which features the pioneering work of Norman Leonard.
       To start off my build I removed the top 60-tooth Nylon spur gear from the side of the headstock by unthreading the gear post from the cast iron housing. You can see the before (above) and the after (below).

       According to the Atwood article this Nylon gear (below) is a metric gear having a module or pitch of 1.5 and a pressure angle of 20°. This is the gear that we will want to reverse with the tumbler gear set.

       Using the information in the Atwood article I was able to source the two - 30 tooth acetal tumbler gears required for this build (Cat# A 1M 2MYZ15030) from SDP/SI which specializes in such gears. These gears have a 10 mm bore which I will be widening a bit to fit on a 13/32” W1 tool steel post (seen below).

       I layed out the gears on paper and drew up a design similar to Dennis’ tumbler design for the tumbler backplate. This plate will support the two tumbler gears and allow the operator to change the orientation of the two gears in relation to the spindle drive gear.

       Below is a rough draft of the backplate design followed by a high resolution drawing of the final product. You can right click on the high resolution drawing and click “save picture as” for future reference on your computer.

       Using my preliminary dimensions I was able to lay out my centers and cut lines on some 1/4” plate aluminum which just like in the Atwood article I had “laying around” :0) In fact this piece of aluminum was embedded in the ground on my fence line as the previous owner of the house used it to keep their dog from digging under the fence! I knew I would be able to use it one day ;0)

       After some cutting with the band saw and trimming with my belt sander the piece was ready for drilling.

       Using my new G0619 mill I centered up the backplate pivot hole using a “wiggler” center finding tool.

       I drilled out the pivot hole with a 9/16” step drill and the hole came out to about .560” in diameter.

       The next step was to cut a shoulder on the existing spur gear post to accommodate the new backplate.

         I used my 7 x 10 lathe to cut a .285” wide shoulder at a .560+” diameter on the post’s base as seen below. The .285” wide shoulder is allowing a little extra room for a 9/16” ID washer over the backplate (.250”+.035”). Of course adjustments can be made to accommodate any thickness of washer. The key is to minimize play in the backplate so snug is best.

       I installed the backplate to the post for a fit check. The plate was held on temporarily with a M8 x 1.25 nut and spacer (1/2” nut as a spacer) to simulate being installed on the lathe. The fit was snug which is perfect...

       The backplate is now ready to be drilled out for the the tumbler gear posts.

       At this point I wanted to fit the gears to the 13/32” (.406”) tool steel rod that I had. This rod is slightly larger than the 10 mm existing bores in the stock gears. I decided to use larger gear posts because I had the material already and it is easy to machine the acetal plastic gears. The first step was to cut down the shoulder of the gear to make the gear about .635” wide. This will line up the gear with the outboard edge of the 60-tooth spur gear and provide clearance for the change gear set later on.

       Now that the gear is the right width I can bore out the center to fit the .406” tool steel rod. I used a custom HSS tool bit I ground to bore the gears (seen below).

       I bored the gears to about .408”....

       After a quick fit check I was ready to make the gear posts. Note: the W1 tool steel I am using for this project is a “water hardened” heat treatable steel that when hardened and tempered is super tough. This will make the gear posts last for a long time, in fact they will probably outlive the lathe.

       To hold the gears on the posts I will be using some e-clips that I got out of an assortment set I had in the shop (I believe that they are 1/2” e-clips). I cut a .040” wide groove about .040” deep in the ends of both sides of the tool steel rod with a ground down HSS parting tool. I set back the groove so the inside edge was about .075” from the end of the post...

       I cut off the ends of the rod to make two posts that were 1.04” long. I then chamfered the bottom ends a bit to help the posts be press fit into the backplate assembly (not shown). They are now ready for heat treating....

       To heat treat the tool steel I used a MAP gas torch to heat the posts up to cherry red. I then quenched the steel into a water bath which makes the steel super hard.

       At this point the steel posts are very hard but brittle so they need to be tempered so they don’t shatter. I cleaned up both posts with some Scotch Brite pads so they were shiny again and then slowly heated them with the torch until they acquired a straw color as seen in the post on the right (below). This post has been tempered and is ready for use. Note: The darker the color the softer the temper so heat slowly and evenly for the best results. It is easy to overheat the steel and loose the hardness.
       Note #2: I noticed that the diameter of the tool steel rod had increased about .001” after hardening but reduced .0005” in size after tempering. It may be beneficial to bore your gears after the posts have been heat treated for a proper fit should you opt to use the same materials...

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