Moldmaking Resource Plastic Mold Makers, Mold Designers, Mold Repair Guide 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.
The coolant
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.
First let me thank you for this article.
I am green as grass with grinding and have just rebuilt a small hand grinder.
It is unfortunately a dry grinder. So I’ll use you tip of using paraffin (kerosene?)
Your article explains everything but the advance?
How much would you cut on each advance (Y axis) ?
Is there any advantage to light cutting the depth? You state never go more than a thou.
So would half a thou give any advantage to less clogging of the wheel?
Regards
Rod
Rod,
.0001, not .001!
Kerosene is the best fluid I know of. Some hand polishing fluids, such as the one from Gesswein, work well.
Advancing in the Y axis (across the chuck) takes trial and error, but, once you are convinced that you are actually removing metal don’t advance quickly. When you first begin grinding you are only hitting high spots, so go across as quickly as you can.
The cutting depth is not a thou, it is a tenth! Big difference! It is painful to only take one-ten thousandth or less, like a half a tenth, but it is actually the fastest and best way to get a truly flat chuck with no dish in the center. Good luck!
It is a flash grind, if i had to take more than a 10th i would have a heart attack. I do have one issue with the procedure, the traverse should be allowed to run fulll length for at least an hour to warm the machine up.
Very good article. If the chuck is on a “wet” grinder and seems to require frequent grinding to keep it flat, take the chuck off. Check for rust build up underneath the chuck. Rust is not static, it grows and shrinks with moisture, the rust can alter the flatness of the chuck daily under some conditions. Clean the table surface under the chuck. It may become necessary to actually grind the table surface if it is badly pitted and no longer flat. Please remember a flat surface can have pits in it, don’t over grind. If this is necessary, grind it just as you would the magnetic chuck, .0001 at a time. Once the table surface is flat turn the chuck upside down on the table. Turn the chuck on and grind the bottom of the chuck just as you would the top, .0001 at a time. Flip the chuck back over, clamp it down, turn it on and grind the top, .0001 at a time. This is not necessary very often, but can enhance the quality and precision of your surface grinding. If you are working with a “wet” grinder always use coolant when grinding the chuck or table.
A note on checking the flatness of the grinder table surface on a surface grinder. The contact of the .0001 indicator needs to be directly under the center line of the spindle in order to eliminate picking up the pitch and yaw that may be present in the grinder. This will also make sure that the indicator will reach the full table surface as it is moved back and forth.
Unfortunately kerosene is very difficult to get into a plant with strict EHS guidelines due to the “link” to cancer. I have found that a 50/50 mix of either liquid WD40 or LPS-1 and 10 weight lube oil will do the job pretty well. As a side note I have found LPS-1 works almost as well as kerosene for stoning and polishing mold steel, and does not “clog” the stones if they dry out between uses.
Thanks for the tip on a kerosene substitute!
I did search for the Magnetic Chuck first so I could fully understand it. Now, I am already aware that it is a magnetic force from a magnetic material. Aside from the Kerosine based oil, is there any other fluid that could be used as a replacement?
Regular coolant works well, if you use is as a flood. Hand applied polishing fluid works well. Paraffin, or candle wax also works.
Rod,
You mention to make sure to turn the magnet on. What is the benefit or purpose of having the magnet on vs. not turning it on.
Thanks,
Ed
The idea is that the internal parts of the magnet move and might change the flatness if you grind it with the magnet off.