This jaw type half-coupling can be used to connect a 6mm shaft to another shaft 10mm. It is held in place by two set screws, and fits together with another CL8-25 series jaw coupling of any shaft size with a hard elastomer spider in between.
Jaw type couplings are ideal for high-torque applications and applications where shafts of two different sizes need to be coupled. This series of couplers are rated for torques as high as 60 kg·cm and speeds of up to 16000 rpm.
If you're having trouble with slipping, you can file a groove into the shaft where the set screws hold it in place. You can also use a Loctite adhesive on the set screws to ensure that they remain in place.