Loose Arbor

So I was using the tormach arbor to saw some stuff, and the arbor nut kept on unscrewing. Due to the way it was set up, I have to cut from left to right, using M4 and spinning the spindle counterclockwise. The Tormach saw arbor works ok. I fixed it by using a 7/16″ lockwasher from Home Depot. It holds up for 100-120 cuts before it needs a bit of tightening.