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

Ear Mold Container

First of all.. ew.. Ear Mold?

Actually it’s not what you think, and technically it’s an impression with the ear itself being the mold.  Which would be a good topic for later discussion.

So here’s the project.. Create a pocket sized container for a pair of custom fit hearing protection plugs.  It needs to keep them together, yet with separate compartments for sanitary purposes.  Small and light weight, fitting easily in a shirt pocket.

Left and right custom hearing protection.

Quick measurement allowing for finger clearance yielded for each pocket:

  • 1.000″ wide
  • 1.500″ long
  • 0.625″ deep
1.000″w x 1.500″h x 0.625″d pocket requirements.

To start off with, you really only need to design half a box and mirror it over.  Here we see the exterior plan form with edges rounded for comfort.

Half container plan form with rounded edges

Then create lines set 0.0625″ (1/16th) in from the exterior plan to create the well and side walls. From that generate an extrusion 0.1250″ (1/8th) shorter than the main box, and subtract leaving the final well.

Hollowed structure with 0.0625″ (1/16th) side walls and 0.6250″ depth. Creating a 0.1250″ (1/8th) thick floor.

Take the formed box structure and mirror it along the center of the separating wall.  The result will be the final box form with the required two wells.

First structure mirrored then added together for final box result.

Add the mirrored sides together leaving the final product ready for production.

Final well box design, ready for printing.

Off to the printer we go.

.STL sliced, checked and ready for printing.
Print complete. Run time 40 minutes.
Well box checked for sizing and fit with ear molds.

Now that we have a box that fits the items, it’s on to the lid.  Nothing truly special here, just a simple 0.0625″ (1/16th) cap with tangs to grip the interior wall surfaces.

Underside of cap design to highlight grip tangs.
.STL checked, sliced, and ready for printing.
Printed well box cap ready for checking. Run time 10 minutes.

Now that all our pieces are off the printer, time to check for fit and last minute changes.

Box, cap, and ear molds.

Cap fits snug with the tangs keeping a firm grip.  Subjected container to shaking while holding the sides and facing down, simulating shocks with ear molds impacting the cap interior.  Cap did not fail under load.

Completed container ready for delivery.

Designed, printed, tested and ready for delivery in just under two hours.  No surface treatments added due to possible reactivity and health concerns with ear mold contact.  Client satisfied with results and will report back with any issues or desired changes.

Go to original project page

Fat Gecko Fix

So here’s a project that came in the door with a 24 hour turn around time.  It’s a Delkin Fat Gecko suction cup mount rigged for iPad holding.  Problem, the turn screw closest to the iPad holder had become frozen and would not tighten securely.

Delkin Fat Gecko iPad mount, in standard position of use.

Now based on the position that it is usually mounted as you can see above, most of the stress is torsional.  Also this unit is subjected to near continuous vibration, so requires regular tightening.

Failed turn screw and knob.

Combine regular tightening with a fixed position and the result is thread wear, spalling, freeze up, and finally failure.

While not a complete failure of the turn screw, it was pretty close and required significant working to release.

Required channel locks to loosen for release of rest of unit.

Once freed from the rest of the unit, the threaded side was clamped into a machinist vice for complete removal.  This required careful application of force, as there was a significant risk to the remaining internal threading.

Secured head in machinist vice, and channel locks required for complete removal.

Finally free for layout and component inspection.

Layout of components.

As you will see below there was thread destruction about .18″ in from the screw tip.

Inboard and outboard turn screws, showing thread damage on frozen item.

Luckily the extraction did only minor damage to the internal threads on the head.

Frozen screw base, threads with evidence of spalling and minor damage from removal.

Since a knob of that type using 1/4″ x 28 threading is not a common hardware store item, the first choice was to refresh the threads.  This was also driven by the short 24 hour turn around requirement.

Attempted thread refresh with 1/4″ x 28 die.

Cutting damaged threads requires careful attention and patience.  Slowly lubricating and cutting a 1/4 to 1/2 turn at a time, with chip clearing along the way.

Lubricate. Twist on. Cut a half turn. Twist off. Repeat.

Unfortunately, things don’t always work out as you plan.  The result was the removal of to much material from the threads, creating a loose fit and low clamping force.  Also the stress popped the shaft from the knob head, revealing it to be a 1/4″ x 28 hex drive bolt.

Knob and hex drive screw after thread refresh failure.

Now what is common to the local suppliers are 1/4″ x 28 range hex head bolts.  When compared to the original, a 1/2″ length bolt would give sufficient clamping force without bottoming out.

Speaking of bottoming out, based on a cursory inspection it may have played a role in the original bolt failure as well.

Comparison 1/4″ x 28-1/2″ hex bolt for comparison with undamaged screw.

Now with a proper bolt, we need a way for it to be tightened by hand.  Can’t tell the client to carry around a wrench, so we need to make a knob to fit from scratch.  This also opened an opportunity for a better fitting solution, since the client expressed a desire for a lower profile on the unit.

Surprisingly enough, there wasn’t a 1/4″ hex screw / knob model in our project collection.  It’s a simple enough object, so the total modelling time was negligible.  Started with a basic round for draft printing speed, test fitting of bolt and head spacing.

1/4″ bolt hex head knob model.
.STL processed and ready for printing.
Draft printed 1/4″ hex head bolt knob.

With the clearances checked, time to move on to a more user friendly form.  For this we added indents to the circular construct, creating leverage for tightening.

1/4″ hex head bolt knob, with finger grip indents.
.STL processed for printing.
1/4″ hex head bolt knob, printed but not surface finished.

Once printing was completed, staged assembly of the replacement parts for comparison.  Added a nylon washer between bottom of knob and top of head casing, easing tightening and reducing surface wear.

Replacement knobs for comparison.
New assembly layout, showing knob height profile reduction.
Arm assembly showing knob height profile.
Finished assembly and layout in standard use position.

Also added a healthy dosage of grease to the screws prior to assembly for life extension.

Client declined surface finishing of the knobs as the tactile nature of the rough print provided better grip surface.

Unit returned to client ahead of schedule and received positive initial satisfaction of results.  Awaiting testing and feedback for any desired changes.

Side notes :

While this was the best solution given the time constraints, it wasn’t what could be considered the best solution for longevity.  The shorter bolt length could exaggerate the internal thread issues, leading to freezing failure again.   Optimally we would have preferred a machined insert that passed completely through the head, creating a greater clamping force and allowing replacement as needed.  Processing would have exceeded the return to service deadline, and was put on hold for client consideration.

Go to Original Project Page