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Cummins 7x12 miniLathe.jpg

The motor belt for the older green Grizzly 7x12 lathe was missing, and the newer yellow Cummins had a broken speed controller and missing compount slide leadscrew, so I moved the electronics box and compound slide Grizzyly to the Cummins, and it worked in low gear, but not in high gear. I turned a few test pieces in low gear and it seemed fine. Took the headstock off and the plastic gear is missing a number of teeth. We might be able to pull a gear from the Grizzly, and there are plastic and metal replacement gears available, but I can't seem to press the shaft out of the bearings in order to replace it. An arbor press might be necessary. I have a project to do on it, so it will be back together next week (the week of May 2), but it'll only work in low gear. If we can get the shaft out, here's the link to the metal replacement gear that won't shatter like the plastic one did. Let me know if you have ideas or want to help. Adrian Kelly, - full instructions for using and maintaining our lathes and others like them.

Tooling: Takes 3/8" x 2.5" tool stock

Reference Videos

Open Source Machine Tools has a number of useful videos, in particular, these three from MIT:

Essential Machining Skills: Working with a Lathe, Part One

Essential Machining Skills: Working with a Lathe, Part Two

Essential Machining Skills: Working with a Lathe, Part Three

Rotating Tool Holder

The lathe has rotating tool holder that allows you to hold up to 4 tools at once and rapidly change between them. The bar on top loosens by turning it counter clockwise. The entire tool holder then can rotate counter clockwise by quarter turns. Turn the bar clockwise again to tight the tool holder in place.

Tool Holder.png

Lead Screw Control

On the back of the machine, between the high/low control and the gearbox housing, there is a control lever for the lead screw, to engage the transmission gears. There are three settings, stop, left feed, and right feed. The settings have three dimples that hold the control in place. There is an accidental fourth dimple, smaller than the others, between the stop and left feed dimples. This dimple can be accidentally used, and doing so will cause the transmission gears to not fully engage, causing them to become stripped or break. Be sure to move the control all the way up to engage left feed.

Lead Screw Control 1.jpg
Lead Screw Control 2.jpg


The lathe has a gear box that controls the relative rate of the chuck and the lead screw that allows you to move the workpiece automatically using the power feed. Changing the gears allows you to change this relative speed. This section explains how to remove all of the gears. Partial removal allows the operator to change gears. Reassembly of the gears can be done by following these steps in reverse order.

The gearbox is under this housing, which can be removed by unscrewing the two bolts at the top
Gearbox 1.jpg
After removing the housing, you will see the gears. Here the gears are shown with labels, for reference when re-assembling the gears.
Annotated gearbox
Remove the gear labeled D by unscrewing the bolt in the center. The washers must go on with the larger one touching the gear directly.
Gearbox 3.jpg
Gearbox 4.jpg
Gears 1.jpg
Remove the gears labeled B and C by unscrewing the bolt in the center. The smaller gear is connected to the larger gear but can be removed.
Gearbox 5.jpg
Gears 2.jpg
Remove the gear labeled A by unscrewing the bolt in the center.
Gearbox 6.jpg
Gears 3.jpg
Remove the transmission receiver by unscrewing the two bolts on either side. This is only necessary if you're cleaning the gearbox or otherwise need to access all of the parts.
Gearbox 7.jpg
Gears 4.jpg
Remove the gear adjuster. This is only necessary if you're cleaning the gearbox or otherwise need to access all of the parts. The nut and bolt that connect to the curved slot control the rotation of the adjust. The piece that fits into the straight slot controls the radius of the adjuster, and has to fit in as shown.
Gearbox 8.jpg
Gear Adjuster 1.jpg
Gear Adjuster 2.jpg