My toughest job was turning double lead internal ACME threads for MOV’s (Motor Operated Valves), when the stem to try it on, was out in the field. The fit on MOV’s is critical to their operation, so you would err on the side of too tight, then put it back in the lathe and take another cut if required. We didn’t have collets that big, or 3-jaws chucks that ran that true, so all the work was done in 4-jaw’s. Indicating and and picking up your thread again was always fun. We had so many different valve’s, it wasn’t cost effective to buy ACME taps to finish them off.
Sticking on course with the OP’s, with grinders you get what you pay for. The cheaper it is the less power it will have and the more vibration you will get. Vibration is the Achilles heel for grinders as it shortens the bearings life. If you google for bench grinders, and check the ratings you will see that the biggest complaint is vibration. In some cases you can decrease the vibration by removing any imperfections on the shaft or on the supporting collars and finally, dressing the wheel. The better grinders have cast machined collars and the cheaper ones will be pressed steel.
If you can locate a used Baldor, buy it.
I have two recommendations for new both with 4 star ratings.
Shopfox
https://www.amazon.com/Shop-Fox-W18...er-hand-tools&sr=1-8#customerReviewsJet
https://www.amazon.com/Jet-577128-G...880157&s=power-hand-tools&sr=1-3
