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

Updated: April 18, 2017 — 5:36 pm

Leave a Reply

Your email address will not be published. Required fields are marked *