How to make a knee joint?

Answer 1

if you're building an actual realistic model of a knee joint, that won't be easy at all since there are floating parts (patella) and such.

but anyway, you know that poplar wood is very close to the actual density and strength of human bones? they proved it on mythbusters. what other kind of material do you think you're going to be able to use, if you can't use wood? it's perfect, since it can be screwed into, nailed into, glued, etc. there will be NO suitable replacement that you can obtain easily. for example, you could use natural nylon/delrin, which is a common plastic, which comes in 12ft long bars, ranging from .500" to 6.000" diameter. you wont find that "around town."

the only other bone material i can think of would be clay. shape it, then fire it to harden it.

obviously, you can get rubber for tendons / ligaments. go to either a Home Depot, or an arts & crafts store. check for large rubber bands, rubber tubing, even bicycle innertubes would function.