Lathe Trick Out – Gear Box Cover Release

One of the two most ignored and discarded safety features of the Central Machine 33684 lathe, is the gear box cover.  The other is the clear plastic spring loaded spindle chuck safety guard, but will save that discussion for a later time.

Now this machine comes in many variants under different names and color schemes, but they all came off the same design.  Anyone who operates these machines will tell you the same, changing gears is time consuming in no small part because of the cover box.

Central Machine 33684 lathe gear box uncovered.
Central Machine 33684 lathe gear box cover.

Now it may sound like laziness, because it is.  To get into the cover you have to do the following :

  1. Find your Allen wrenches.
  2. Unscrew the cover and place the bolts aside (and not lose them).
  3. Remove the cover.
  4. Remove gears.
  5. Insert desired gears.
  6. Replace the cover.
  7. Find the cover bolts.
  8. The bolts started by fishing around for the holes and turn a couple times.
  9. Tighten bolts the rest of the way with the Allen wrench.
  10. Put your Allen wrenches were you can find them again.
  11. Resume operations.

If you have already selected and laid out your gear set before hand, this whole process takes about 15 minutes. Without the cover it knocks it back to about 10, so most people just leave the cover off.

Why you may ask, would an operator omit a piece of safety equipment which protects fingers from getting sucked into gears?

It’s about time, and getting back to work.  Lets take a look at the gear changes for something simple like a round bar.

  1. Change out current set for roughing cut set.
  2. Change out roughing cut set for finishing cut set.
  3. Repeat for next.

In here is 10 minutes wasted (and hassle) with just replacing the gear box cover.  Multiply that by how ever many changes you have to do, and it’s easy to see why the cover is left off.

So how are we going to fix this?  Well besides replacing the entire back end with a transmission, we will just make the cover easier to remove.  Replacing the Allen screws with studs and simple knobs, will greatly simplify cover handling.


First task is to model the original components so we can work out proper dimensions for the new.

Central Machine 33684 lathe back cover model.
Central Machine 33684 lathe spindle head left, with 5mm-0.8 Allen screws installed to normal insertion depth.

Note : To figure out the normal installation depth, you replace the cover and tighten screws till secure while keeping track of the number of turns.  Remove cover then install screws to same depth and measure distance to surface.


Next up is modelling the replacement studs and jam nuts.

New 5mm-0.8 studs and jam nuts.

The studs work out to 2.25″ inches long, with 0.65″ inch threading from each end.

Now you may notice that the jam nuts are not flush with the mating surface.  This is to model a mounting bracket which goes between the nuts and back plane for another project.

Also, the nuts at the tip of the studs are for integration inside the operator knobs.


So rather than trek to the fastener supplier or wait for delivery, we used some 3/16th” (0.1875) round stock to make our own.  Since 5mm equals 0.19685″ and the stock had an average size of 0.1900″, this would work out just fine.  Also helped that the lathe was between projects, so we could jump on for some quick cuts.

Layout and strike 2.250″ line on 3/16th” bar stock with a caliper and sharpie.
Parting tool and stock. Remember, the smaller the stock the closer to the lathe chuck you have to get to prevent flexing.
Once your 2.25″ blanks are parted, measure for excess material to be removed. Then perform a facing cut to remove the parting nub, and excess.
Blank tip after facing and sizing.

Now that we have our stud blanks, time to add the threads.  Decided to use a die rather than the lathe for these simple cuts.  While it would have been a good demo of small diameter threading, will save that for later.

Using a machinist vise, 5mm-0.8 hand die, and light oil to manually cut threads. Be careful and patient not to cross thread or bend the shaft.
Completed threaded studs.
Once complete. test fit into lathe using same number of turns as the original screws. Note the black sharpie stripe to keep track of turns.

Next up is the knobs.  A simple form built to mate with the bolt wells in the cover and contain a 5mm-0.8 nut.

.STL ready for the printer.
Printed product with integrated 5mm-0.8 nut.

It took a couple iterations to tweak the sizing for a user pleasing stand off from the back cover.

Completed studs, jam nuts, and knobs for installation. Note original Allen cap screws for comparison.

Next step is to modify the inside of the gear box cover.  The two stand offs are designed to mate up against the back plane of the spindle head.  Since we will have jam nuts and a future mounting bracket, they need to be shaved down by 0.325″.

Central Machine 33684 lathe gear box cover interior.
Using a stand off and bolt through the end stock opening, firmly secured to the milling table.
Using a medium end mill, shaved 0.325″ from cover stand offs.

When done, shake out shaving into a trash can then blow remaining out with compressed air.  Clean as well as possible so you don’t get muck in your gear box.


Finally assembly of our new quick access studs and knobs.

Remove the studs and spin on the jam nuts.  Apply some grease to the threads, this prevents them from becoming permanent via rusting in place.  Tighten the jam nuts against the back plane, doesn’t require much torque.

New studs and jam nuts installed.
Cover installed with knobs for easy access.

So with the job complete we have enhanced the access and safety on our Central Machine 33684 lathe. Reducing the access time to a few seconds will inspire use during run time and increase productivity.

Side note : This addition is part of an overall modification project, which will reduce or eliminate the need to access the gear box.  Until then, this is a great addition.

To the main project page