A lot of the errors in surface grinding, as near as I can tell being new to it, are entirely in the operator. Somebody good can make great parts on the same machine that I am making scrap on. All I can say is to pay a LOT of attention to what you are doing, what is happening, and what are the differences between when you get good work and when you get poor work. Some of it does not seem to be all nuts and bolts, but also feel, paying attention to the sounds and the sparks, and trying to figure out what influences which other things.
Here are some things I 'think' I might 'know' about MY surface grinder:
If I run my table fast, it raises up and cuts deeper, probably riding on a wave of way oil.
If I move the table slower, it cuts shallower. (pushes the oil out of the way?)
If it has been some time since properly oiling the machine (hours), the consistency falls off. It improves again when I add more oil to the scores of oilers on the machine.
The vertical travel hand wheel and ways work pretty nice and smooth, but if it does not come down under load with the same constant tactile feedback, and feels light, watch out! It has hung up a bit, and will get even with me later when it finally slides down properly. The cure seems to be lifting it back up and then letting it down to my number until it gives the right feel.
Wheel hop, too much heat in the part, poor surface finish. If I dress the wheel properly and often, and do not cheap out by not dressing enough off, I get much better results.
Wheel selection is also a big issue. If I do not pick an appropriate wheel, I get PPP (**** poor performance). I have enough wheels to pick something close, but laziness bites me in the butt. If I have not studied enough to know what types and grits of wheels do what on what materials, I deserve the surprising results I get.
There is a LOT more of this, but you get the idea...
The good news is that, without a pro grinder hand looking over our shoulder, it is still possible to learn from our mistakes, and I make plenty... We also need to keep tweaking our old grinders little by little until they are reliable and repeatable enough to do good work.
I have worked on several surface grinders, and have the same types of issues on all of them...