Could be a number of things.
- Wheel balance.
- Poor or no dress.
-The wheel not wrench-tight and so tightens at starting or engaging the part.
- Hardness of the wheel or of the part.
- Having a bow/hollow on the opposite side of the part.
- Using the wrong grinding technic such as down-grinding* or incremental cross.
for starters.
*But no point in guessing until we know what you are doing.
QT: (a 5 degree turn of the handle is enough to plunge the wheel.)
Do you mean to say 5 thousandths?
QT: as the machine was set up by a professional. and he made a decent grind?
*Good to post wheel specifications and part thickness and hardness -> File soft, or too hard to file...and your grinding technic.
(*down grinding .005 and even .0005 may be way too much depending on the wheel or the part.)
I am new to surface grinding and have been experimenting with our Proth surface grinder on a test piece of steel.
I have the feed rate pretty slow and the cut depth is very shallow (although the steel stock is not perfectly flat with high and low spots of a few thou). When I go to make a pass, I keep getting these ripples in the surface finish and no matter how much I tweak things, they seem to persist. The depth adjustment seems very sensitive to me; a 5 degree turn of the handle is enough to plunge the wheel into the stock and cause the spindle to slow down. I am almost certain this is due to user error as the machine was set up by a professional. Any input on this is greatly appreciated.
-Justin
View attachment 385908
Most people don't understand that the complexities of grinding are a numerous as any other machining operation
So, by the pic I see how the material has burned, surface grinding should be done with LOTS of coolant. Long thin plates have the tendency to heat up really quick and they get "sucked into" the wheel by the material rapidly heating up and expanding into the wheel. I would put an indicator on the wheel head and turn the wheel down until you get .001" downfeed just to get a feel for how much travel looks like on the dial. I am really surprised it doesn't have graduations in at least .001" or maybe the dial could use some 7447(maroon) scotch brite polishing to see if there are actually numbers on it.
Unless you are doing some serious production grinding keep your down feed at .0002" to .0005" max, and again the coolant thing mentioned above, the more the better. The ripple effect could be one of many things, balance usually being the most prevalent cause for that issue. But there could be a bearing issue on the spindle, or there is also the possibility that the machine isn't lubricating the ways properly, or the wrong oil is in the machine(stick slip). Just like machining with any tool, the instant the feed is interrupted the tool leaves a mark where it was stopped . Most surface grinders use the hydraulic oil as the way lubricant as well, but there are a few exceptions I believe.
Balance is an issue with larger wheels, but usually smaller wheels with the 1-1/4" hole do not need to be balanced, just dress them smooth and take a test cut. Stop the spindle, re-start it and see if the problem comes back, it could be that the wheel flange is bad, or just not tight enough to keep the wheel in its proper place, and when you turn the spindle on the wheel actually shifts on the hub or the hub on the spindle taper too. Usually if there is a chatter issue you will be able to feel the machine vibrating if balance or bearings are an issue. Simple test for that one, put a fine test indicator on the chuck, turn the spindle on and the needle will show the vibration.
Wheel grade is very important as well. General practice is softer steel harder wheel, harder steel softer wheel. The wrong grit, bond, or even the grain spacing can dramatically effect the performance of grinding operations. There should be a wheel manufacturer's rep somewhere in your area that can come by to give a recommendation based on the job you want to do. I stick with the old favorite I or J bond. When the leading edge starts "loading" with material the bond will let that area break down, thus eliminating the loaded area and moving into fresh grit. I prefer smaller cross feeds, .050-.100 per pass, but if you need a really fine finish drop to .015-.030 cross feed and don't be afraid to let the machine do a long spark out period. I've done thousands of gages that had to have a very fine finish(mirror 0.8) without lapping. Just takes time, experimentation and patience.
The color of the burn can tell you a lot too. If I recall correctly, the first stage of burning is around 600° at the point of contact between the wheel and material. Blue is at around 800°, and there is actually a silvery type of burn that starts around 1000° at the POC. Keep this in mind if the hardness of the material is critical to the integrity of the part you are making. You can alter the mechanical properties of the material by rushed grinding.
Dressing can be an issue too. If you look at the diamond and see a flat spot you will need a new one. The larger the wheel the larger the carat size needs to be. It needs to be rotated every once in a while to keep a sharp dressing edge on the wheel. Most machines use the dresser at an angle off the centerline of the wheel to be able to rotate and get a fresh edge.
This seems like a lot of info to digest at once but this is only the tip of the iceberg in the grinding field. I would say that if you can get a rep to come over they will be able to help you a lot better than any of us could on this forum, the info we get can be so limited...
