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There are so many subtle things to be aware of. I have been concerned with safety, and more so ever since I tossed my square.
I was doing the "five block test" which seems to be a thing. I just thought of it on my own after not getting my square "perfect" the first time. Some will say just toss work on there and grind it, and you'll know if it's flat or not, but I could not tell where the error was, hence my test.
I learned that when you grind the chuck, it should be ground with the magnet ON. Who would ever think of that?
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
ctwo wrote: ↑Tue Jul 12, 2022 9:39 pm
I learned that when you grind the chuck, it should be ground with the magnet ON. Who would ever think of that?
I would. It's obvious that you want to duplicate the actual conditions of use when you dress a mag chuck. That's particularly true when the chuck is used in grinding, where tenths matter.
High precision work is unlike common machine work. One with experience knows to not even touch a grinder when it is in operation. Touching handles is the exception, and even then you must use caution, depending on where the handle is mounted.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
One isn't generally inadequate, but often inexperienced. Before you know to not lean on the table of an active grinder, someone must bring it to your attention. The average person has no clue what's going on and can't imagine that a heavy machine tool is subject to deflection.
I'm no authority on this subject but have worked in that environment and understand the ramifications of actions of those who don't understand. I had to constantly remind one of the inspectors where I worked to NOT lean on the table while I was grinding. Remember, a .000050" deflection will manifest itself as a tenth on a round piece---which, typically, was half of the tolerance we had on ground diameters.
Be happy to answer any questions, assuming I'm able.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
ctwo wrote: ↑Sun Jul 17, 2022 5:51 am
So, when you grind your magnet in, you grind the table, then grind the magnet's bottom, with the magnet OFF. Because...
I would suggest that the bottom would be ground only once, and that would be BEFORE the top is ground. That way you grind the top parallel to the installation when built. I would use the magnet to hold the chuck to the machine table. That duplicates how it is applied when being used.
It is unusual for the magnetic chuck of a surface grinder to be removed once installed. If, by chance, you remove yours regularly, it may become necessary to regrind the base, but careful handling should limit that need. Draw filing the base when re-installing should be more than adequate.
I think my magnet has an aluminum bottom. My experience with grinding aluminum is that it fouls the stone.
That might be your problem. Most folks don't use stones to grind, they use wheels. Stones don't make very good wheels.
Recommended abrasive for grinding aluminum is not aluminum oxide, which I suspect is what you're using. For aluminum, a hard silicon carbide wheel would be the choice. Please note that the green aluminum oxide wheels are NOT that choice. The wheel you'd benefit by using would most likely be shiny black in color. The green wheels are usually very soft, which is exactly wrong for aluminum.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
I milled the bottom of my magnetic chuck before grinding the top. After milling, it hinged appropriately on the surface plate and did not deflect when clamped on the grinder’s set pad. I could have taken a very light pass on the grinder but saw no benefit to come from that.
Steve at Solid Rock did an aluminum-grinding video and recommends undiluted coolant on the aluminum. It’s a good video. He uses a couple different wheels.
All depends on what you expect from a surface grinder. It's extremely rare for a milling machine to be able to provide a truly flat surface, so not grinding the base may or may not be a good idea. Keep in mind, grinding is usually applied when close tolerance is demanded. The minor irregularity created in milling may not be evident, and may not even matter, depending on one's objective, but anything that provides the least opportunity for instability should be addressed when working to tight tolerances. The slightest thing can result in failure. Grinding the base would have been prudent. Regrinding the working surface if the chuck has been moved is almost mandatory. Again, depends on one's objective. It may not matter for some applications. Working to a tenth? I suspect it would matter.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
You really don’t need full contact on the base, you need even overall flatness. The point is to avoid inducing distortion in the chuck body when it’s clamped down.
My grinder produces parts flat within a tenth, from a chuck milled on bottom.
I agree with your comments. The point is, a mill may or may not provide that condition, while a grinder is far more likely to. It stands to reason that a saw cut may work as well, but it is most likely not to. One grinds to assure the end result. One then scrapes to assure an even better result, although that is not likely in the example of the base of a magnetic chuck.
When fly cutting a large surface with my Bridgeport, it is obvious that the table is not projecting a straight line. The deviation isn't excessive (far less than a thou), but it's not within the range of flatness that can be accomplished with a grinder, nor should one expect it to be. If it happens to fall within the same parameters, it's unusual. It's a mill, not a grinder.
It's entirely possible that the milled bottom of your chuck is not flat but dressing the chuck after it's installed should eliminate the minor distortion created by clamping. After all, that's the reason one dresses the chuck. Best scenario is that there is no distortion to eliminate, however.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
SteveM wrote: ↑Mon Jul 18, 2022 3:45 pm
Would it be appropriate to scrape the underside of the chuck for flatness, then flip it over and grind the top?
Steve
It would be my opinion that that would be a waste of time and effort. No harm in doing so, but even if the chuck isn't dead flat, mounting it should create a stable condition. That would be particularly true if it was making contact in the center, so tightening the end straps would cause full contact. Mind you, I'm speaking in terms of a few tenths, maybe even a thou, but not something that can be seen. Best case scenario, is, of course, a flat base and a flat surface upon which it mounts. Minor error in either one isn't fatal due to the final grinding of the chuck surface, which should then offer the same level of flatness as the ways of the machine offer.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
