Hi,

I was lucky to get a DD30 in near mint condition last week
(for equiv. of 10$ in my home town .
But as it soon revealed - the GEAR was cracked,
So it was my turn to face that problem.

First thought was to check what was written on stereo2go,
but I didn't like the solution presented (although the work was
exceptional), because filling the gap makes the wheel not circular
any more (it's only almost circular), the wheel is not as strong as
the new one, because of the very limited surface of gluing,
it changes the original number of teeth, and last but not least
reproducing the proper geometry of a teeth is not possible,
so it wears mechanism more than original wheel.

So I've studied the wheel, and tried to understand what causes
the problem. And I think it is a design flaw. And a very special one,
because it comes from japaneese insane precision.
They matched the size of the metal disc with the plastic rim so good,
that there was no place for shrinkage.
Molding of a plastic part around metal disc needs to be done in
a certain temperature (approximately 230-250F), so there is
thermal shrinkage factor, which (because of the lack of space)
builds initial stress inside the plastic rim, after it cools down.
We have to take into consideration also, that almost every plastic
shrinks a bit with age.

So it does not take much  - a dust, a grain of sand (thats my case
I guess  as I found a few inside my DD30), heavy duty
or using in a low temperatures - to make the gear crack.
So much for my theories.

When I disassembled the wheel, I could not get the two ends of the plastic rim touch one another, so I took the rim off, and cut the edge of the metal disc (reducing it's diameter of a size of a gap) to the point I could easily get the to ends of plastic rim back together.
Be sure to remove the other part of the clutch before edge cutting,
to prevent other plastic gear from damage.
This way I restored the original geometry of the gear.
Than I had to join the two ends together, and I decided to for
a plastic welding, instead of gluing - does not look very nice,
but it's a strong and stable connection.

As you can see on the image, I didn't weld it to the very edge,
as I did not want to ruin the teeth geometry. I also sacrified one trigger (please put the right word here - the little thing on a wheel that drags the head and a roller in place when you press the play button), because it was too close to a crack.
But there are two of three triggers left, so it still works as it should,
but I think that next time I'll do it better - and I'll save all three.

The whole operation can be done within few hours,
so it's a quick one, and it works flawlessly.

Great job Pawel. This is what I call 'Think outside of the box'. All these while the attention is given to the plastic wheel itself and now you have a different approach, fixing the metal rim to compensate the plastic wheel shrinkage. Hats off to you, Pawel!

Thanks for a warm feedback

the good thing about filing the metal disc is that you don't really have to be that precise, because even if your metal disc will not be very cicular after reducing - the plastic rim will keep the geometry and do the job.
Of course - if you can do something better - you should, so big thanks for a hint DocP.
WM D3 is on the way, so maybe I'll report soon if I can make it better.
Strange thing is  - I almost wish the wheel is broken.

I don't think there is much to talk about -
it's just melting the plastic to the point its a bit liquid and sticky
so it "glues" perfectly the cracks of all sort.
So the idea is very simple.

The real trouble is:
a) not to damage thin elements (gear wheels in this case)
    by touching the good parts with hot iron - or just keeping it too
    close for too long, so there is no burn, but the gears loose
    proper geometry because of just heat.

b) proper applying of additional plastic - to make the connection
    stronger than original, because if you use it too few - it will brake,
    and if use it too much you will not be able to melt it properly,
    so the joint will not be strong enough but may be too heavy

c) avoiding overheating the plastic to the point when it burns
   and changes into an ash based stiff mousse
   (which is a quite easy thing to do)

d) with small elements you have only one try to go -
    and you have to be quick - or you'll destroy the whole element.

I'm using a 20W soldering iron for circuit boards.
the temperature of the iron is still too high for plastics,
but because of the low power it does not burn the plastic immediately
so with a practice one can master it to acceptable point.
with thick elements its more easy - with thin elements it's tricky as hell
so it would be a great idea to use a pro soldering iron with
preselected temperature, and practice a lot before trying to
repair the gears.

I'm using this way to repair toys, laptop shell, car interior
basically all elements made of thermoplastics.