Question: I have a 240 volt dust collector with an NEMA 6-15 plug. I want to plug it into an NEMA 14-50 receptacle. The 6-15 has three leads, while the 14-50 has a fourth (neutral). Can I change the plug and if so how do I wire it?

Answer: 
You can't do that. You can't put at 50A plug on a 15A appliance. 

What you can do is change the NEMA 14-50 to a NEMA 6-15. You will also need to change the breaker in the panel, and will need to pigtail the wires because #6 wires will not fit on either a 15A or 20A breaker or a NEMA 6-15 recep.

Now, if you need both sockets to be active, you can install a small subpanel at the NEMA 14-50 site. That subpanel will have 2 breakers: a 2-pole 50A and a 2-pole 15A or 20A. The 50A will supply the NEMA 14-50 recep. The 15A/20A will supply the dual NEMA 6-15.
You need to change the breaker or add some kind of fusing, then you can skip the neutral.

Receptacle configurations aren't just different to indicate what circuits are big enough for large tools, they also intend to protect by tripping when current too high exists. 

If that dust collector draws 8 amps and some bearings go bad and the motor slows way down and the motor draws 5x it's rating it won't trip the 50A breaker that is suppose to feed that receptacle. If you're in the room and hear something is wrong you can shut it off you could be just buying a some new bearings. 

Let's say maybe you can't hear it binding up or you left it on and went to the bathroom, and that motor completely binds up and draws 10x the current the cord, motor insulation, motor windings were all designed for. **Yes, 80 amps should trip the 50A breaker, somewhere between [90 seconds and 5 minutes](https://lh3.googleusercontent.com/aX9MD-ZA7HJIrcMix6SyC3AJpQ5Zx1j47PyYHBgqHaW5AR-evHCqrgGlDvXZKUQW2azy5LxXE5i7JecP-w6hvcZ0ay80WMy-BrE_Nr41YHpkWDMseciKcpmXbAaCnsiGAACHLhs) !!!** Take a look at that cord and ask yourself how long do you think it will take for that cord to melt and maybe start a fire if it has 80 amps running through it. 

So one option is too tap the 50A circuit, install a $12 [fusible disconnect](https://amzn.to/3cKTqat), and add a proper NEMA 6-15 receptacle.