Back Gearing for a Grizzly Lathe
For years, I have done instrument work on a 3″ UniMat; it met my needs. The Grizzley 9″ belt drive lathe was a concession to my hobbies; took a long time too. A larger machine could not be justified for casual’ use. So; to make a small, inexpensive, lathe do things it wasn’t meant to do. This is how I’ did it. There are other ways. I defined the parameters thus; it must be cheap(!), be reasonably reliable, and simple to build. There was also consideration that as a “one off” job, it should work even if things did not come out exactly right the first time. I chose planetary gearing, the low gear package from an automobile automatic transmission. The sun gear MUST have a bore large enough to accommodate a bushing between the lathe spindle and the gear. This is the only real consideration. I recommend no less than 6 mm larger.
I give measurements in english units for the most part, but these machines are actually both metric and english. Like most of my gizmos, unless specified otherwise, tolerances are TLAR. ( That looks about right ) That is an attitude that WILL come back and bite, though.
Tooling: Outside of the lathe, a drill press is the only other machine that I used. Most of the cuts are “to fit”; calipers will yield sufficient accuracy. A wiggler will do for alignment. Dividers, a good scale, and a center punch about covers it.
Drill bits: wire sizes Nr. 36, 27, 25, 7, F , fractional 3/16 and 3/8″
Taps: 6-32, 10-24, 1/4-20
Materials: You will need brass (or bronze) for the sun gear bushing, and the pins the planetaries run on. Aluminium plate (0.300″ m/l) for the pulleys. I used 1/4″ plate and offset the belt grooves. Flat-head socket machine screws: #6-32 x 3/8″, #10-24 x 1/2″ & 3/4″, 1/4-20 x 1/2″ & 3/4″. Two pieces of 11 Ga steel sheet (11 Ga = 0.118″ = 3 mm, less a fuzz) 6″ or so diameter. My stock came out of an electrical enclosure. Several pieces of aluminum round and flat stock for jigs or fixtures. Doesn’t have to be aluminum, that is what I had on hand.
Some years ago, I traveled for a computer company in the Pacific Rim region. I learned much about “Life, the Universe, and Everything” from the locals. One of the most important lessons was how to carve a statue of an elephant. Start with a stone. Chip away anything that doesn’t look like an elephant. What is left is an elephant. But there has to be an elephant in the stone to start with. That’s what machine tools do. If we want a gear, we start with a hunk of metal. Chip away everything that doesn’t look like a gear. What is left is a gear. But you must be able to see the gear when you start.
Remove the planetary gear package from the transmission, and dismantle it. Brute force, a piece of 3/8″ drill rod and a three pound hammer did quite well. The gears are hardened. I made a small firepit with refractory brick and used a Natural Gas / Oxygen torch to anneal. Carefully heat the gears, taking care to bring them up evenly, to a medium red to a low orange. Avoid bright orange. A dark corner is good for judging temperature color. Hold them at this color for a few minutes, then allow them to cool slowly. The slower they cool, the softer they will be. I maintained heat on the gears, bringing them below dull red, by adding less and less heat until they went below visible. Use a file; if the gears scratch easily they are soft enough to machine.
The target thickness is 3/4″ rough, 5/8″ finished. Split the ring gear carefully and there should be enough stock for two. Cut off a piece of the sun gear. If the hacksaw doesn’t cut well, the gears may not be sufficiently annealed. See the previous paragraph. If the planetaries are long enough, split them as well. The sun gear will probably have a bushing inside. Cut this bushing gently at several places with a hacksaw and then use a small chisel to remove it. Be careful, the inside of the sun gear is a bearing surface.
Face off the gears. I used the three jaw chuck to grip the inside of the ring gear. Place the finished side toward the headstock, and true it up as well as possible. This is where a dial indicator will earn its keep. Face it down to 5/8″ (16mm). Break the new edge with a heavy chamfer, about the same as the factory side.
Chuck the sun gear by the inside. Check the bore of the sun gear for true. This will be a little harder to true up. I finially got it the way I wanted it (read: I got tired of fooling with it ) and faced it off also. Face off both sides, so that there is no hub.
Do the same for the planetary gears. For these, the desired thickness is a few thousandths (running clearance) less than the sun gear.
This will be a good time to clean up the gears. 400 grit emery did a nice job. The gears will run on brass bushings, so use finer paper on the bore if you have it. There is probably scale left from the annealing. Clean that off too, until you are pleased with the appearance. Mine came out a nice blue from the annealing.
There are several jigs that must be made ahead of time. Any piece of scrap of an appropriate size will do. I happened to have on hand some 2″aluminum round. You will need a piece cut to the ID of the sun gear and another the ID of the planetaries. Both should be at least 1/2″ thick and bored wire size “F”. Make a transfer punch that is 1/4″ OD. This will permit marking the working center of the gears.
Cut a piece of all thread long enough to hold parts on the face plate. I would recommend against using anything smaller than 3/8″-16.
The next part is a shoulder bushing, spindle OD for a half inch, and the shoulder as large as the ID of the sun gear; again about a half inch long. Bore to fit whatever size all-thread you use.
We also will need two pieces bored to spindle OD and 1/4″ or so in length. OD should be 10-12 mm larger. These will later be cut down to finished length, so for now just get the ends square enough to use for jig spacers.
Use corner pieces of the 11 Ga sheet and make a couple of fender washers, ID to fit your all thread and OD around an inch and a half. Regular fender washers will do, just double them up. Although it is not necessary, I made a shoulder bushing to fit the outboard end of the spindle, to hold the all thread near center.
Start with the 11 Ga steel. Lay out the center on one piece, near as practical. From this, layout a circle close to the outside diameter of the ring gear. Strike a line through the center and layout six points by swinging the radius from where the circle and the line intersect. Sorry, I know it’s basic geometry, but this is a just a reminder. Three of these points to the center will give us thirds of the circle. If the planetary package you got has four gears, you will only need three. BUT, they may not line up exactly on thirds. It depends on the number of teeth in the ring gear. The transfer punch marks are the last word for spacing.
On the plate, layout the ring gear, the planetary, and the sun gear, and center them up. The lines at 120 deg intervals will provide coarse alignment. Place the larger jig in the bore of the sun gear and true it by feeling for the center mark with the transfer punch. Clamp the sun gear in place. Using the smaller transfer jig, mark the center of each planetary. This assembly was a little loose on mine, and I tried to find a loose center for each gear.
Clamp both pieces of the plate together. I used several 6-32 screws outside the periphery of the ring gear. Step drill the center to 3/8″(size of the thread). On the drill press table, clamp a bolt or pin such that the plates will rotate and the center for one of the three marks falls under the drill. Step drill the three marks for the planetary gears to an “F” drill. This little dance keeps all three holes at the same radius.
Clamp the plates to the face plate. If your face plate will accept it, use three bolts through the three peripheral holes. Mine wouldn’t, hence the screws around the edge. I had to clamp through the center. Center it up using the scribed circle as a reference. Cut the outside round. Finished size will be to the bottom of the chamfer of the ring gear. A piece of 20 lb paper behind the plates will let you know when the tool is close to the faceplate. You will likely lose the small screws.
With the plates still in place on the faceplate, clamp around the outside edge. Remove the through bolt. Verify that the circle describing the center of the three holes is still centered. This and the next cut MUST be concentric. Bore to match the lathe spindle OD (30 mm?) This bore will transmit torque to drive the spindle; a snug fit will pull smoother and wear better.
The next steps define the follower plate. The follower plate is the one with layout markings. Separate the two plates; set aside the drive plate.
Chuck up a piece of brass about four inches long and cut it to a running fit as bushings for the planetary gears. Length is a fuzz less than the 5/8″ gear, call it 0.610″. Drill Nr 7 and tap through 1/4-20 in the chuck. By drilling and tapping through, a starter hole is passed through to the next bushing. Drill a Nr 36 hole into the threaded hole through the side of the bushing, about half way up. We will use this later as an oil hole.
Countersink the 1/4″ holes in the follower plate to fit the 1/4-20 flat head screws. Mount the bushings on the plate. It will be necessary to countersink the back of the bushings a little. Use the 1/2″ length screws. Again install all five gears. There is a little play in the mounting screws that will allow small adjustments of the drive bushings. Once you are sure everything will run smooth together, reassemble with “Loc-Tite” on the screws.
I spent a lot of time on the follower plate. The bore MUST be concentric with the running center of the bushings. Check the setup to the n’th degree. Check again after the clamps are torqued up. Check again just to make sure.
The driver plate is probably the most tedious, but the tolerances are not nearly as touchy. The drive plate carries the sheaves on the outside and the sun gear on the inside. The screws I used were 6-32, and I question the torque they will transfer, so I used eight screws to ensure strength. Use ten if you like. Hardened screws here will make a difference. Using a larger screw will interfere with a later operation.
Insert the stepped bushing (the one that is spindle OD vs sun gear ID) into the bore of the driver plate. Scribe around the step to indicate the ID of the sun gear. Slip the sun gear over the bushing. Scribe a few marks between the teeth to indicate the root OD of the sun gear. The space between the two described circles is the body of the sun gear. It is here that we will drill and tap. Flip the drive plate, then flip the entire assembly. This to expose the scribed circles on top, with the sun gear underneath, and the step bushing holding them in registration. Adjust the 3/8″ pin on the drill press such that a Nr36 drill will fall between the two circles.
Drill Nr 36, one time only, deep enough to allow tapping to 3/8″ depth. Tap to 6-32, deep enough to accept the 3/8″ long screw. It probably will make life easier to start the hole, then take the driver plate off. That would keep a loose piece out of the way.
Countersink the drive plate to fit the 6-32 flat head socket screw. A fuzz below flush is desirable. There is a shoulder that will run against this surface of the plate. Drill it through with a #27 clearance drill. Fasten the driver plate to the sun gear with a screw through the countersunk hole. Punch a witness mark on the sun gear and the plate. Refit the bushing with the gear and the plate and tighten the single screw sufficient to hold the assembly in registration. Drill #36 again opposite the one screw. Drill deep enough to start a hole into the sun gear. Remove the plate and countersink the second hole. Drill it out to Nr 27. Finish drilling the sun gear with a Nr 36 drill. Tap to 6-32. Reassemble the driver plate to the sun gear, this time with two screws. Reassemble on the step bushing, then drill the six remaining holes. Remove the plate; again. Countersink the other holes and drill through with a Nr 27 drill. Finish drilling the holes in the sun gear with a Nr 36 drill, then tap to 6-32. Make one last pass to debur everything, then assemble the sun gear on the plate, using all eight screws.
The driver plate is ready to bore. Mount the sun gear in the chuck and true it up for runout just inside the screw heads. If there is runout in the plate near the outer edge, there may be a chip or a burr between the gear and the plate, warping it. The plate must run in the same plane as the sun gear. Find it and fix it. Bore the plate a fuzz larger than the bore of the sun gear. Debur the edges well, and take care to not scratch the bore of the sun gear. You will find that the cut runs into the heads of the Nr 6 screws about 25%. This is acceptable, provided the screws are well tightened before hand.
The pulleys on the Grizzly machine are 0.300″ thick, for each sheave. There is a 0.625 hub behind them. This makes for a total of 1.525″ that we must accommodate. There is an undercut on the pulley, where the spindle nut fits. We will deal with that later.
The assembly that we have fabricated is comprised of the follower plate with three gears running on brass pins, a ring gear around them, and a driver plate with the sun gear. Assuming you used my dimensions, the gears run 0.625″ thick and the plates are 0.118″ each. That gives a total of 0.861″. Subtracting the thickness of the gearing itself from the full thickness of the pulley, we get 0.664″. That is only enough room for two layers of 0.250″ to 0.300″ material. I took the easy way out and eliminated the inside sheave.
When you shim off of the back of the headstock to get the belt lined up, there is clearance behind for the threading gears (change gears) to run clear. If you have installed my version of the tumbler for reversing the lead screw, well, I flubbed it. There is a modification necessary. This modification is covered later. If you are using the original change gear arrangement with an 80 tooth gear, everything should clear.
Start fabrication of the pulleys by cutting out two discs, roughly a half inch (1/2″) larger than the existing pulleys. If you want, you can make them larger, but the original belt won’t fit, nor will the cover over the back end. Layout and mark the center, near as practical. We have enough room to true it up, . . . a little. On the smaller blank, draw a line through the center, and a circle about three inches in diameter. Layout six divisions by swinging the radius. We will drill here later. Center up this blank on the faceplate. Use a spacer behind, about 1/2″, to keep the tools off the faceplate.
I used a large drill bit in the tailstock chuck to drill a starter hole. The aluminum is a bit more tenacious than steel. Gouging out a starter hole was a hand full. I drilled the second one. Bore the pulley blank to 1-1/2″. The exact size isn’t important. Debur the hole. Make another piece with the larger blank, the bore size the same. Remove the face plate and mount the three jaw chuck.
The following step really is necessary, as the small pulley will not be accessible for truing and cutting the groove after it is assembled.
Chuck the smaller pulley blank with the jaws gripping the inside of the bore. True it up carefully against wobble. Cut the OD of the blank to the size of the center pulley of the existing drive. Use a standard threading tool to cut the belt groove. If you use 1/4″ plate, cut the groove off center as far as possible, away from the layout circle. Break the sharp edges around the outside. I used a single cut rasp to round over the edges. Try to eliminate anything that could snag a finger.
We will need a plug ( fixture? tool? ) cut for a smooth fit into the bore of the smaller pulley blank. Take the end down to the ID of the sun gear for about a half inch. Place the smaller pulley blank on this plug, then insert it into the sun gear through the drive plate. It should be assembled such that the layout done earlier is to the outside. Clamp the blank to the drive plate, leaving the plug in place. Punch a witness mark on the drive plate and the sheave.
Center punch and drill Nr 25 at the six points laid out earlier. Drill through the drive plate. Take note that these holes do not fall too close to the three existing holes in the driver plate. Rotate the two pieces relative to each other to achieve this.
Remove the smaller pulley and replace it with the larger blank. Use the plug to align the center. Use the drive plate as a guide to drill the six holes through the blank. Make a witness mark on the large blank to correspond with the mark on the drive plate.
Drill out the holes in the smaller pulley to 3/16”. Feed slowly on these so they cut as round and smooth as possible. Drill out the drive plate as well. Tap the holes in the large pulley blank to 10-24.
Assemble the three pieces with the smaller pulley sandwiched between the large pulley blank and the drive plate using 3/4″ screws. Assure that the groove on the smaller pulley is offset toward the drive plate. The screw heads will extend out from the drive plate side. When you are satisfied with the assembly, remove the screws one at a time and countersink. Cut slowly. The screw heads must be below the surface of the drive plate by a couple thousandths. However, because the drive plate is so thin compared to the screw, there is little metal left to draw up against. These screws pull quite well on my machine. If you are not comfortable with them, add some more around the layout line. The first six will hold everything in place while you work.
At this point, you have a stack of the large pulley blank, the smaller pulley, the drive plate, and the sun gear. Drill through the three 1/4″ holes in the drive plate. Use an “F” drill. Deburr heavily on both sides; use a light countersink if you like.
Chuck up the sun gear and true it up for concentricity of the sun gear bore. To my way of thinking, a few thousandths eccentricity of the pulleys won’t matter. However, wobble in the belt groove will cause all kinds of misery. The belts must run true to the gear bore. Cut the OD for the large pulley. Again, try to duplicate the size of the original. The large pulley is rather large, you may find it necessary to remove the toolpost and set up the grooving bit with a clamp. If you are using 1/4″ plate, the groove should be offset toward the outside. The target dimension is 0.300″ center to center for the two grooves. If you invert the original pulley over the work, this dimension can be eyeballed.
Trim up the inside of the pulleys. I cut wide enough that I could access the small screws in the sun gear and tapered up to about 3/8″ from the larger screws that hold the pulleys and the dogs. The only stipulation here is that we must have clearance for a wrench on the spindle nut. Break the corners. Remove the assembly to the bench. This is a good time to smooth up all the edges. The pulleys form a handwheel when you are checking a setup. Trust me, if you leave a burr or rough edge, you WILL find it when you least expect it.
I note again that the drive plate should be as smooth as possible, as it must run against the shoulder on this bushing. Another consideration is lubrication. I have yet to figure out how to reliably lubricate this bushing. At this point, I just dismantle the drive whenever I feel the urge to oil it.
A real concern is running speed on a bushing. I calculate that with the input speed at around 1000 RPM, the output speed is 315 RPM. The differential is 685 RPM. I believe a brass bearing should stand up to that, even under a moderately heavy load. Remember, too, that is the worst case. The whole point to this exercise is to get 40 RPM out of 130. The differential there is under 100 RPM.
Clearance behind and around the gear case will vary depending on what size gears you could get and which model lathe you have. Using Ford C-6 gears on a Grizzly lathe, the outside of the gear case will clear the stock 80 tooth idler gear. If you have installed a reversing tumbler here, you may need to modify it for clearance. If you used my design, it is easy enough, my stuff tends toward simplicity.
Remove the tumbler plate from the machine. Acquire two pieces 1/4-20 x 3/4 flathead socket capscrew. Countersink the outside ends of the brass pins. Don’t worry about restraining the gears, the overhang of the flathead screw heads will hold them in place. Reinstall the gears on the plate with the flathead capscrews. Grind off the end of the screw where it penetrates the back. Reinstall the tumbler plate on the machine. It’s done.
We will need a spacer behind the drive plate. Earlier, we cut two spacers to the OD of the spindle and to a nominal length as jig spacers.
Remove the drive belt pulley from the spindle. NOTE: there is a set screw in the spindle nut. Remove the change gear drive and the key. Leaving the key out, replace the drive gear, slide on one of the spacers, the follower plate, the drive plate (with pulleys attached), and the bushing. There will be a shoulder on the spindle, use the other spacer to set back the spindle nut. Run it down fairly snug. Hold the chuck and rotate the pulley by hand. The ring gear will rotate backward. Assure that the mechanism runs smooth and free. When you are happy with the operation, lay a straight edge along the side of the outer pulley to the countershaft. A carpenter’s square is fine. Determine the amount of misalignment; this is the amount you must remove from the back spacer.
Dismantle the whole contraption, then reassemble with the original parts. Be sure to put the key back in. Set up the spacer and trim it to size. Repeat the process above; if you got it right the pulleys will line up. Install the belt and sight along it to make sure. When you are happy with the alignment, note how much should be removed from the front spacer. The goal here is to have at least one thread showing outside the spindle nut, yet avoiding bottoming it on the shoulder. When the spacers are sized, use a file to cut a keyway on the inside spacer and the follower plate.
When I devise a contraption such as this, I avoid permanent modification whenever possible. I want the option to restore the machine to its initial configuration. This is a valid point in that if I make a mistake and get beyond the point of no return before discovering so . . . Let me put it this way; How long did you lust after a decent lathe before actually getting one? And are you willing to put it at hazard? The POINT ?!?! We need a key. Make one, don’t destroy the factory key. The key will drive the threading train and the spindle from the follower plate. It must be trimmed off to allow installation of the sun gear bushing. I used a piece of 5/16 x 1/4 key stock and ground it down to the same dimensions as the original. Assemble the key, the threading drive gear, the back spacer, and the drive plate. Mark the key, then grind it down to the mark so the OD of the spindle is intact. The thin area will serve to keep the key in registration.
When the key is ground down, there will be a sloped area. I undercut the inside of the bushing enough to accommodate it. This probably will help prevent spinning the bushing later. A punch mark on the outside of the bushing shoulder opposite the keyway will make assembly a little easier.
Rotate the pulley to permit the three dog holes to line up with the planetary gear bushings. The bushings were tapped through and mounted with short screws. That leaves a threaded hole some 1/4″ deep in the end of the bushing. Use three pieces 1/4-20 socket cap screw through the pulley into the bushings. I used inch and a half screws, and cut off all but 5 pitches, so the screws would bottom. There is a reason as will be apparent in the next step. This forms the dog for direct drive.
The third inside sheave is not used, so we have lost the 600 RPM and 2000 RPM speeds. I have paid attention throughout this project to the speeds that I used, and I have yet to use the 600 RPM setting. If you positively, absolutely must have 600 RPM, it takes three minutes to reinstall the original pulley.
The dogging mechanism for reduction drive is one of the 1/4″ bolts threaded into the ring gear. Drill Nr 7 and tap 1/4-20 at the center of the ring gear. Use a small file to clean up the teeth inside the ring gear where the tap came through. The screw should bottom just below the root diameter of the ring gear; grind it off if needed.
Cut a piece of 1-1/2″ by 1-1/2″ by 3/16″ angle about 3″ long. Centered along one edge cut a notch a quarter inch or so wide, about halfway through. In the other leg, drill two Nr F’ holes. Drill Nr 7 and tap corresponding holes in the backing plate that carries the countershaft, belt tensioner and such. Alignment is such that the dog pin on the ring gear catches in the notch. If there is any slight misalignment in the planetaries, the ring gear will try to follow it. If the locking dog is tight, the gearing will bind up, causing irregular speed. Too, with angle cut gears, the ring gear will track left or right depending on the direction of rotation. This arrangement allows the ring gear to ride loose.
I drilled and tapped two extra holes in the angle bracket. It makes a convenient place to store the extra bolts removed from the pulley when changing ranges.
To run direct (1:1), remove the bolt threaded into the ring gear. Install three bolts through the pulley face into the planetary gear bushings. Spindle speeds are the stock Grizzly numbers:
BC1 130 RPM AC1 400 RPMBC2 300 RPM AC2 1000 RPM
To run reduction (3.2:1), remove the dog bolts from the pulley face. Rotate the ring gear such that a bolt may be threaded into the ring gear through the notched angle. It is crude, loose and simple. And it works very, very well.
BC1 41 RPM AC1 125 RPMBC2 95 RPM AC2 315 RPM
For finishing, I used “Old Dutch Cleanser” to make a lapping compound. That is the brand recommended by an old timer as having the most consistent grit size. If you have some lapping compound, by all means use it. Mix a thin paste of light machine oil (5w) and the cleanser. Using clean oil, lubricate the inside of the gears where they run on brass. Use a light coat of the paste on the gear teeth, and around the ring gear.
Remount the reduction gear assembly on the spindle. The sequence is: the key, the threading train gear, the inside spacer, the follower plate with ring gear, the driver plate, and the outer spacer. Run the spindle nut up snug. Clamp the chuck to a piece of 1×2 softwood, with the end down between the shears into the bed of the lathe. Turning the pulley by hand should cause the ring gear to run opposite the pulley. Make a full rotation to make sure there are no binds, then belt up to the outside (slowest) sheave. Let the machine run for a solid two minutes. Probably would be a good idea to get a couple minutes in reverse as well. Add some light oil to the joint around the ring gear whenever it starts to complain. We have left a lot of marks on the soft gears during machining, and this will help clean them up.
That was a rather traumatic experience for me, but the gears managed to survive it. I used electronic spray cleaner to remove the lapping paste. Soap and water and an old toothbrush finished the job. “Safety Clean” or even kerosene would probably work better than the electronic cleaner. Be sure that ALL traces of the paste are removed.
Lubricate with clean oil or white lithium grease. I personally use a good SAE 30, single grade, gasoline engine oil. It is a good high-pressure lubricant and I am a cheap sort. But I do lubricate the machine EVERY time I use it. Keep in mind, too, that this mechanism is open to the environment. Chips and dirt can enter the gap around the ring gear. Regular cleaning is probably a good idea. Keep a watch, too. At higher speeds, oil from the gears will sling off, making a bit of a mess. I guess I’ll have to devise a spatter shield. The good side is that drips fall on the change gears, keeping them well lubed.
The dog bolts for direct drive are made hollow, with a zerk on the end, sort of like a banjo bolt. Forcing oil in here will feed through the Nr 36 holes in the brass bushings, providing sufficient lubrication. Now to figure out the sun gear bushing. I haven’t made the hollow bolts yet, I still oil by hand. Have to open up anyway to get to the sun gear bushing.
Hindsight is always 20-20, right? I started out with a single dog pin for direct drive. That tended to pull the drive plate out of true. The fix is covered in the drilling instructions for the drive plate. The back spacer might be made of steel and dowelled to the follower plate. That would provide a wider key surface to transmit torque. I have not experienced a problem, but most of my work is light. Most automotive gears are cut at an angle, they run quieter. The angle causes the ring gear to pull to one side or the other. No ill effects that I can see, beyond a little noise where it rubs on the plates. Straight cut gears would eliminate that and probably allow a thinner profile too, with room for the third sheave in the pulley.
What this means in plain English, is that:
Anybody and everybody that wants to build this gizmo for themselves and/or educational purposes is free to do so. If anyone tries to make a profit, so much as a penny, I want a piece of the action; a big piece.
Who was it that said?
“We didn’t need all these damn laws till all those damn lawyers came along”
Have fun, enjoy it
Author: Bill Hudson