SmartWheel CDROM Repair

Due to a manufacturing error the delrin hub on which the cdrom is mounted causes the cdrom to quickly crack and break. I describe a method for repairing this problem below. If you have SmartWheels that have not yet been repaired please encourage your students to be gentle!

• Cause of the Problem
• Fixing the SmartWheel

Thank you for your cooperation. Such is the world of prototypes.

Cause of the Problem

The problem with the cracking on the SmartWheel CDROM wheels is caused by a radius left on the inner edge of the delrin hub after machining. This corner should be a sharp edge with an imperceptible radius.  The radius causes the plane of the CDROM to assume a slight cone shape which causes a tension strain at the inner edge and also causes the CDROM wheel to be deflected towards the circuit board which causes scraping when the wheel is rotated.

Cracks starting on CDROM

This cdrom has a slight cone shape, the outside edges are deflected down.

Looking at the same cdrom from the other side shows the scratching that happens when the edges of the cdrom are deflected so that they rub against the circuit board

In the future the delrin hubs will be machined with a sharp corner in this position.

Fixing the SmartWheels

First you will need to drill three mounting holes in a new cdrom. Then use the longer screws and o-rings to mount the new cdrom. This should make the cdrom much more durable. If you haven't done it yet you will also need to reinforce the jacks the flexible cable plugs into on the probe card and the probe itself.

• Drilling Replacement cdroms
• Adding O-rings
• Reinforcing the Connector Jacks on the Probes and Probe Cards

You will need the following tools and materials:

Tools:

• Hot melt glue gun and hot melt glue sticks
• #1 phillips-type screwdriver
• 1/8" drill bit
• drill press

Materials:

• replacement cdrom
• 3 AS568A-004 buna-N o-ring
• 3 4-40 x 1/4 phillips panhead
• 2 #10 nylon shoulder washer
• masking tape

The o-rings, screws, and shoulder washers can be purchased from a good hardware supplier. The supplier we use is:

McMaster-Carr, 473 Ridge Road, Dayton, NJ 08810-0317, 732 329 3200

part#	description	cost/100
9452K12	AS568A-004 buna-N o-ring	2.16
90272A106	4-40 x 1/4 phillips panhead	1.00
90061A011	#10 nylon shoulder washer	5.65

Drilling Replacement cdroms

If your cdrom cracks and breaks and you need to replace it locally here are some tips for drilling the mounting holes. Use a cdrom from a smartwheel as a prototype. It is OK if there are cracks appearing as long as it is whole. Put it on top of a stack of cdroms to be drilled. Make the sides align and tape the top and bottom cdroms together with three pieces of tape. Use a 1/8" drill in a drill press or hand electric drill to drill out the holes on the new cdroms. It is easier to be precise with a drill press. Make sure and clean out any plastic bits from the holes.

Note: We have found AOL cdroms to be much stronger than other cdroms.

The cdrom on the top of the stack above is one that has already started cracking. I am only using it for a template for drilling holes in the correct locations.

Adding O-rings

After removing the cdrom wheel put the 3/16" long screws away. You will need to use the longer 1/4" screws to attach the new cdrom. Place the longer screws through the holes and put the o-rings over the screw threads as shown in the picture below:

Start each screw a couple of turns into the black delrin hub. Make sure that the cdrom is centered and that the edge is not going to get caught on the edge of the delrin hub.

Now tighten the screws carefully

Here's a picture showing the repaired SmartWheel.

---

Questions and comments: stephen@concord.org