Assuming cast is just taken off and surgeon gave clearance for PT , start with these two exercises
1. Lying supine in bed, push your knee down in to bed, hold for 5 sec and relax. Repeat 10 times.
2. Slowly slide your heel back, bending it as much as you can and straighten the knee.
Cpm machine won't be of much help, your surgeon probably is a good person to ask.
I have not seen very good results with cpm, majority cases, it slowed the progress.
Health Tips
Ice your knee after the exercises