Knowing how to grind a magnetic chuck on a precision surface grinder is one of those injection mold making skills that is often taken for granted, or totally overlooked. But think about it for a moment, how can you ever expect to obtain a perfectly flat surface on your part if you have a bowed or warped chuck?
The first time (actually the second) time this hit home was when I watched a cocky apprentice attempt this. He totally ignored the mold making training offered by the mold makers, refused to read the instructions that came with the chuck, and just did it the way he wanted. There is a reason the surface grinding process is exact, you can’t just make it up as you go along.
After he went home I checked it out. Unbelievable! It was .007 inch out of flat and looked like it had been done with a torch! I showed him the next day and he mumbled some incoherent excuse and avoided me. He didn’t work there too much longer though. I guess he wasn’t cut out for precision surface grinding after all!
Follow these directions to grind a magnetic chuck for a surface grinder. If you don’t you will take much longer and become very frustrated.
How to grind a magnetic chuck
- Find a coarse wheel that has an open structure. Personally, I use a 46 G or H grit, aluminum oxide wheel with an open structure. There are coarser grit wheels available, such as a 32 grit, but these are uncommon.
- Set the dressing diamond off to the left side of the wheel, far to the left. This will ensure that you are using a sharp facet of the diamond, not a dull, rounder edge. You want nice sharp particles in the grinding wheel to cut the chuck.
- Dress the wheel quickly so you have a coarse finish on the wheel. This is also known as an “open” dress.
Since the magnetic chuck is made of steel and aluminum (or brass), you will want to use flood coolant if you are using a wet grinder. Open the valve so it is flowing freely and you will have no problem with heating.
On a dry grinder you need to use a kerosene based oil, such as polishing oil, or old style EDM fluid. Kerosene is great for non-ferrous metals such, as are found on the chuck. Simply brush the oil on the chuck or use a squirt bottle, covering it completely. You won’t use much anyway, so there is no need to skimp. Do this every pass, if you do not, the wheel will quickly load up and burn the chuck.
Patience is the keyword. If you do this right you might be able to get the job done in 2 hours, or even less. Then again, it can take longer if you try a shortcut.
Grinding the chuck
This is the hard part, at least on a manual, dry grinder. Before you apply the kerosene, draw marker pen lines in strategic areas so you can tell where the low spots are.
Turn on the magnet. Do not attempt to grind it without having the magnet on. You will fail if you do.
Make sure you take off the back rail! You cannot grind the chuck with the rail on. Start grinding from the back of the chuck, in the area below the back rail. This is just in case you leave a divot when picking up the wheel after all the re-dressings.
Patience, patience, patience. Do not attempt to remove more than .0001 inch per pass. This will most likely mean that you need to make a lot of passes, that is the problem and many people try to take heavier cuts and up taking longer in the end. Take too much and the chuck heats up and leaves a low spot in the center.
Just keep at it and eventually you will have a perfectly flat chuck. You will need to re-dress the wheel several times as well, since the aluminum will clog the pores eventually. Surface grinding is difficult enough, make it easier by doing this the right way.
Testing for flatness
Take 5 small, identical pieces of steel and grind them flat and to the same thickness. Then place them in the four corners of the chuck, as well as in the center. Carefully grind both sides and check them on the surface plate with a dial indicator. Be sure to mark the pieces so you know where they were on the chuck.
This will reveal exactly how flat your chuck is. If it is not flat, you need to regrind it. This is a better method than indicating the chuck because it is in real-time and more exact.