Stuff I shouldn't have to do..

I try to buy higher end tools not just because I like using them, but you usually get what you pay for.
I really can't afford to spend over $1500 on a fancy rotary tool such as an NSK electric or similar. So I did what most modellers do and bought a Dremel.

The basic thing any rotary tool should be able to do, is spin true. Well my most recent Dremel purchases, fail in this regard.  I bought one of the new Dremel Micro's a year or so ago and a Dremel 7300 a bit after, because the Micro had terrible run out. The 7300 was better, but still not what I was expecting.

The run out was so bad on the Micro, drilling with it was impossible. (which is what I bought to do)   I ended up leaving a wire brush in it, to clean paint off wheel sets.  The 7300 was better, but still not what I would call acceptable.  One day I got pissed off and took them apart.  The problem was easy to figure out.  Inside the motor shell halves, is a groove for the spindle bearing.  Well the groove  is much bigger than the bearing diameter. The bearing can be moved by hand about 0.010" in its bore in each direction. The pocket is basically 0.020" to big. I fixed it by adding styrene shims to the bearing pocket in the motors shells. The green knife is pointing at one of the shims.

Problem fixed.  I did this to both.  The Micro is still not as good as the 7300, which I account to the the 7300 having a shorter shaft which means less vibration from flex or whatever..

Now why Dremel can't make the bearing bore close to the right size is beyond me. The parts are injection moulded with glass fiber filled thermoplastic.  This process can yield highly accurate parts. So much so, that the bearing pockets shouldn't need 0.020" clearance, may be 0.002 tops..

I broke at least a few $20 drill bits because of this manufacturing issue.

About the chuck and drill bits I use.

I use the Dremel keyless chuck (not shown in the picture), so I can hold 1mm shank Cobalt drill bits from Richards Micro Tool. These are higher end drill bits, much better than any wire drill bit you can get from a hobby supplier. I get mine from McMaster Carr.  Here is a link..

Cobalt Inch Size Drill Bits with 1mm shank

With this combo I can drill hundreds of holes in resin in a fraction of the time I can by hand.

Don't try it on styrene though, the Dremel's lowest RPM is still to fast, to not melt the plastic with a #79 drill.