Mini-Lathe Steel Gear Retrofit

This is a plastic to steel gear conversion that will fit on the following machines:

  • 03911 Homier 7x12 Mini Lathe
  • 33684 Harbor Freight 7x10 Mini Lathe (shown here)
  • 5278 Cummins 7x12 Mini Lathe
  • 82500 Micro-Mark 7x12 Mini Lathe
  • G8688 Grizzly 7x12 Mini Lathe

I never broke a gear in the Grizzly mini lathe I previously owned. However I have read many stories of broken gears on various chat boards. A jammed tool or using larger chucks can lead to this problem. Until this gear set became available the only thing that could be done is re-install plastic replacement gears.

These gears are imported, as a US manufacture would probably make the gears cost more than the lathe itself. The quality is very good. The set I used had a very small amount of surface rust (small spots) that was easily cleaned away. It was of no consequence. The gears are delivered in rust preventive grease and paper wrapping.

One gear had some minor tool marks on one side that looked like it had been dropped into a machine. I touched up the marks with a file. None of the other gears have any blemish so I am sure this was an exception. THMS will replace any defective parts. There is no guarantee that these gears are unbreakable, but steel is much stronger than the plastic.

The tightest fit was the large transmission gear on the main spindle. The bore seemed to have a slight taper, maybe < 0.001". A little abrasive cloth (or a small flap sander) will clean up a tight fit. I used plumbers cloth abrasive. Again, a minor detail any machinist should be able to handle. I knew there will be variations in the various manufacturers tolerances even on the lathe itself, so I was expecting to do a little adjustment.

I highly recommend that the complete set be purchased and all the plastic gears replaced at the same time. The tear down is not all that difficult, but it is best to only do it once. The mini-lathe was designed and built to meet a price point. I am sure the plastic gear choice is a part of that decision. With the steel gears the mini-lathe is more expensive but is also much more capable to take on the loads the hobbyist tend to put on their machines.

Follow along below and see just how the conversion is performed.

This is the familiar HF 10" bed Mini-Lathe of fame and fortune. This lathe was borrowed from a friend who had placed this machine in storage. You may notice in some photo's the generous coating of rust preventive grease.

Here is the business end of the Mini-Lathe. The gears you see here that look black are plastic. This lathe has non stock dual 80 tooth gears in the reduction train to obtain a very slow feed rate.

This is the back side of the Mini-LAthe showing the two speed gear shift lever and the reverse tumbler lever.

Here I have pulled the upper gear cover/cluster and I am in the midst of disconnecting the electrical. This machine will be taken down to the bare bones at this end to replace all the plastic gearing.

Here is the spindle head taken off the base casting. It is held on with three screws. Also notice in the background that the feed screw must be removed.

This is a peek inside the spindle showing the stock plastic transmission gears and the shifting fork.

A closer look at the now bare spindle end of the bed. Note the crud that fell out of the spindle box. I of course cleaned this area before re-assembly.

This is the outside spacer on the main spindle. It is made of plastic but will be reused as it is used only as a spacer. Note the tiny key that holds the exterior spindle gear. It must be removed first to remove the spacer from the spindle shaft. The spacer is not keyed.

This is the spindle box after the main shaft has been pressed out. The bearing came out with the shaft and that is just fine. The other bearing was removed later.

This is the inside of the spindle head after the shafts have been removed and the interior cleaned up.

The original plastic gear is on the left. The new steel gear is on the right. The steel gear is an exact duplicate. A little light sanding was required to get a slip fit onto the main shaft.

This is a trial fit of the large gear on the main shaft. A (required) small plastic spacer is not installed in this test fit. This gear needed slight interior cleanup work but you can see it fits very well.

Here is the main shaft installed. You can see the spacers at each end. Make sure there is no end play between the shaft and the spindle box.

This is the drive shaft and shift fork. The plastic gear is shown to the right of the replacement steel gear.

This is the motor (of course). It says it is 3/4 HP. The label also says 3 amps at 110 volts DC. In Ohm's law, power = voltage x amps. Thus in this case 330 watts or about 0.44 Hp. Hmmm...

This is the reversing gear cluster. The plastic gears here will be replaced with steel gears. The conversion is not too hard but it makes sense that if I am going to this much trouble, I should change all the gears.

Here I am showing a three gear reduction set-up. That is an 80 tooth on the feed screw.

I hooked up the power supply to do a test run with a four gear reduction train set-up.

I included this corner view so readers will have a better look at the gear installation.

Flash Video version 8 viewer required.

 

This is the entire operation required to install the steel gears to replace the original plastic gears. Again, I recommend that anyone desiring to strengthen the gear train in their mini lathe, install the full set. This entire operation can be performed in less than one day on the weekend. That includes some cleaning of a well used machine. The Hobbyist Machine Store is the only source for these steel gears. Anything else is available at the Little Machine Shop.