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Surface grinding- Deeper cuts for better finishes ?

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*Posted By: Anonymous on Chaski forum

In Response To: surface Grinder depth of cut (Len Tate)

If you intend to grind steel, your wheel of choice will be of an aluminum oxide variety. Use silicon carbide for grinding non-ferrous and cast iron. Not the green variety, but the shiny black wheel, typically. THe nomenclature on the wheel should begin with a C for silicon carbide, and, naturally, an A for aluminum oxide.

Grinding soft steel mandates a harder wheel, hard steel a softer wheel.

I see lots of recommendations for using a 46 grit wheel, but my choice rarely included wheels that coarse. I like nice finishes, and 46 grit wheels don't give them to you. On the other hand, fine wheels have difficulty taking larger cuts. A nice compromise is a 60 grit wheel.

I have seen no mention as to actually how to grind a surface here. I believe there is a broadly held misconception that grinding involves lowering the wheel on a part as it goes back and forth. While that is the way most tool & die people sharpen punches and do other tool grinding operations, that is not exactly how grinders should be operated when you are trying to grind a flat surface, particularly when dressing your magnetic chuck.

When grinding, you will be well served to look at your grinding wheel as if it was a milling cutter, for it actually is. Big difference is this milling cutter has lots of teeth. With that in mind, now think of your milling cutter as doing all its work on a corner. If you plunge grind on everything, you have no idea if and when your wheel breaks down. The idea that the wheel will keep breaking down uniformly makes a nice story, but it doesn't work like that.

Depth of cut has been overly discussed here, which tips me to thinking that none of the respondents use a grinder as it is capable of being used. This all ties in to the side cutter cutting on a corner as stated above.

When grinding a surface, you can take two approaches. You can use a light downfeed and advance your cross slide rapidly with each pass. That's hard on the corner of the wheel, for it creates a large area of the wheel making the cut. Too much of the wheel that you rely on to do the sparking and maintain your uniform thicknes is damaged by the broad cut. The alternative is to take a relatively deep cut (.005"/.008") using rapid table movement, and advancing your cross slide around .010" or so with each reversal of the table. This may sound crazy to you, but what you now get is a wheel that cuts on one edge only, leaving the balance of the wheel to spark the already ground surface. By grinding in this fashion, you can see your wheel break down on the cutting edge only, and you can grind large surfaces knowing that your wheel has not changed diameter on you.

If you doubt this concept of grinding, I ask you to consider how centerless grinders take their cuts when through feeding. All the work is done on the corner of the wheel, the balance of the wheel, which can be as much as 8", does nothing more than spark out the part, leaving a very nice surface when properly run.

When grinding like this, when it's time to dress your wheel, you will have to remove the amount of the depth of cut from the wheel, for you will have pushed the corner of your wheel towards the center of the wheel as you did your work. That's exactly as it should be. You'll be surprised at the amount of work you can get done that way.

When I read comments of ginding too much (depth of cut) and heat checking surfaces, all that did for me is tell me that the person grinding was using a poor combination of properties. Hardened steels grind far better than soft steel does, you just have to make sure that the wheel is suited to the job. The surface that comes from grinding heat treated steel is superb when all things come together. There will be no heat checking.

Friability in grinding wheels is critical if you intend to have good success. It's important that the wheel break down when the grain gets dull. Too hard and it doesn't happen, too soft and you get no life from your wheel.

Loading is another problem. If you find your wheel gets real dark around the cutting edge and you get heat traces in the grind, you may be loading the wheel. A more open bond may be required so the swarf can enter and leave the wheel as necessary. Loading of a wheel leads to wheel hop and very poor finishes. It's hard to distinguish between wheel loading and a dull wheel. Both present similar problems and can be identified by small bits of the material being ground imbedded in the grinding wheel. These bits raise hell on the finish.

I have one more tip, and this one is a strong personal prejudice. Do not grind dry, do not grind with spray mist. Tremendous amounts of heat are generated in grinding. If you expect balanced performance, you have to get rid of the heat, and you have to lubricate the cut, allowing the swarf to go where it must go, that includes entering and leaving the wheel. If you don't keep the cut cool, you introduce a lot of trouble. You'll have a very difficult time controlling size if you don't control expansion, not only of the part being ground, but the wheel, too. Keep everything cool is a good policy.

I realize that some tooling grinding is done dry, and I've done more than my share of that type grinding, too, but only when running coolant is out of the question.

I strongly suggest you find and buy a copy of the manual that Norton has published for years, A Handbook On Tool Room Grinding, which will help you in selecting the proper wheels for your application. In this manual it explains how to make the proper choice for hardness and abrasive type, along with wheel density. You see these little manuals offered on ebay and they tend to go cheap.

Good luck with your grinder.


*Unanimous with Anoymous....
Posted By: Stephen Thomas

A bit down the list back at the end of last week, a number of us were contributing our suggestions as to how to surface grind, and how much should be taken off (downfeed) per pass. Mr. Anonymous posted the idea that many of us were doing it all wrong, and that a huge downfeed (well let's say .010, +/- a few) with very minimal infeed per traverse, would do a better job.

Very busy week here, and have only had intermittent time to play with this idea. But the short term summary are: It works and I'm converted; subject to further experimentation.
A couple years ago, I had welded up a number of plane irons for hand planes, with a CPM M4 cutting end on a mild steel (A36, actually) shank. These were belatedly annealed after welding, but I had not gotten around to machining and finishing (heat treating) them. A couple weeks ago, I finished one for personal use, using my old method of grinding. Wide passes, slight downfeeds between.

Well with 4 more of these irons to rough grind before machining the slots and features, and heat treat, it seemed like a "what the heck" experiment opportunity. Because of the CPM M4 thickness (1/8") and a slight bead at the butt with the A36, there is quite a bit of stock reduction to clean up and get down to the .080 - .090 range that will fit most Stanley planes (stock is about .075 - .085).

I set 4 of these irons (2.5 x 8+"up on paper on the chuck, shimmed as necessary. After kissing off to find the high spots, tried .007 downfeed, and estimated .025 infeed per traverse. It worked fine and a decent finish. Next pass I took .012 off, and backed the infeed out a "smidgeon". Counted about 75 - 80 strokes/inch, so must have been around .013 or so infeed. I contimued to grind, staying in that range of feeds. It was something of a revelation.

Admittedly, this is not a challenging situation. mild steel and annealed HS tool steel, 46H wheel, flood coolant. But the work was cooler, warped less, and developed an excellent finish. The wheel became self dressing, forming the ramp Mr. Anonymous described. There was little apparent stress induced into the work. H was a bit soft fot he materials, but with my "old" way of grinding, would still have had to dress it fairly "often". That was the best part, no dressing! I believe grinding like this, it might even have been useful to go up in hardness. Other than the ramp, the wheel stayed concentric and was completely self dressing, maybe a little too much so. It would probably have been possible to go up to a I or even J for the same situation.

This is one short experiment. Might not be for every situration. But certainly has given a whole 'nother way of thinking about grinding for many other jobs.

Don't hold me to a timeline, but when I get these machined and hardened, will give a report on finish grinding them. May even get down and dust off the 60 grit wheel he suggests, and have a go with that, too.

Thanks, Mr Anonymous!
smt

*Posted By: matt
In Response To: Re: Gettin' unanimous with Anoymous

I didn't see this thread you are talking about, but that's the way I was taught. It leaves a mirror-like finish and doesn't seem to burn as easy or warp thinner parts as bad--especially if you run coolant. The only bad thing is your wheel will not break down even, so everything is a trade off.

*Posted By: Anonymous
Matt,
You don't want your wheel to break down. That's the point of grinding the way I suggest. All of the wear of the wheel occurs at the corner, and the balance of the wheel does nothing more than spark out. By the time the corner of the wheel has moved towards the center (the ONLY break down you want), you are due for a dressing. The condition of the grind tells everything, for the finish will begin to degrade. When the proper grade of wheel is used to grind in this fashion, it is unbelievable the quality of the finish you get. As for grinding heat treated steels as compared to soft material, you'll never get as good a finish from soft as you will hard steel, but as you go to HSS grinding, the wheel does go through a little more. As I recall, the wheel won't hold up quite as well.

The idea of a grinding wheel breaking down gradually as you use it in plunge grinding is a myth. It just DOES NOT HAPPEN! The reverse is true when grinding as I suggest, for the corner of the wheel sloughs off, which you have apparently experienced.

When you plunge grind, it grinds with much more heat, leaves a terrible finish, and the wheel won't slough off, requires constant dressing to keep it performing. Wheel hop becomes a problem, and if the wheel is graded improperly it can also load. Of course, if the wheel is wrong my method is no better, either. Matching the wheel to the job at hand is important. Harder wheels for softer material, softer wheels for harder materials.

In order for you to better understand why you don't want the wheel to break down as you think it should, consider dressing your chuck. If the wheel was to break down, you would have a terrible time ever getting the chuck parallel to the travel. By the method I suggest, the wheel CORNER takes all the wear, sloughing off as it dulls, but the diameter of the wheel remains constant. My background in grinding necessitated very close tolerance work, and grinding any way but this way would have prevented achieving the desired results.

I will be very blunt when I state that I am amazed that the method of grinding that has been discussed here in this forum is so supported. I believe I mentioned that I was trained by an outstanding supervisor in grinding. This man came from the aero-space industry and knew grinding in a way we can only dream of achieving. At no time were we ever told or permitted to plunge grind on a surface grinder when virtually all types of grinding were done. Even punch and die work is better done by my suggested method.

How the plunge method has achieved such popularity is a mystery, for it is much more difficult to do and is very hard on the work at hand as compared to my suggested method. We can all thank my supervisor, now deceased, for this knowledge. I take no credit aside from being the messenger.

Thanks for confirming my suggestion, it makes my time here worth while. My earlier attempts in another forum were greeted with so much ridicule that I decided that I would try it this way.

I've learned that it's hard to tell others how to do things when they think they already know how. One has to come up with the right way to show them a better way. Thanks one and all for hearing me.
Anonymous







[This message has been edited by Admin5 (edited 06-07-2002).]
 
i just came across this discussion because it was a link in another discussion titled Tricks of the Trade that is currently in the general forum,and since it is so old i dont know if it is appropriate to comment on it here or if i should open another thread based on this one. so i will comment on it here and if something else needs to be done with it im sure someone will let me know.
i must admit im some what confused by this thread because the method that is being put forth here is the way i was taught to grind and is really the only way i finish grind on a surface grinder. im not sure what the other method that is mentioned is but if it is, as has been stated, plunge grinding maybe i can add a few thoughts that will either clarify or muddy the waters a little more or just be way off base depending on everyones own experiences and training or lack thereof.
first off plunge grinding is a very efficient method to remove massive amounts of material very quickly usually with a cnc grinder and a very coarsly dressed wheel then the wheel is finely dressed and the finish cut is taken using the method put forth above followed by one or more spark out passes. again this is usually an operation performed on cnc grinders or automatic grinders and i guess it could be done on a manual grinder but it would not be nearly as effective. this method is also used a lot in od grinding but again once most of the material is removed the wheel is dressed for finish grinding and the above method is used.
second the kind of agressive grinding you are talking about is fine and is really only limited by what your grinder can take. if your grinder has a high static stiffness then you can take a bigger cut without the grinder "opening up" and you also can only take as big a cut as the motor will stand with out slowing down. and a good flood of coolant delivered precisely to the cutting zone is crucial especially when grinding with the bigger depths of cut.
third grinding is a very abusive process and plung grinding is more abusive than the method anonymous suggests but still if you take these larger more aggressive DOC's your finish passes must be smaller and more gentile with the finish pass being with a freshly dressed wheel and only .0005" or so deep because while you talk about having a good or excellent surface finish with no damage to the part there will none the less be microscopic surface cracks that will range in depth from a few millionths to a few tenths and very light finish passes will eliminate these. again proper application of coolant is important. a number of years ago i was at the institute of advanced manufacturing sciences, which is now called tech solve i believe, and they showed us an example which basically went like this, an elevator company was having an unusally high number of failures on this particular worm gear assy that they could not figure out so they asked the institute to help them and they found out that the worm itself was the origin of the failure and that when it failed it also destroyed the gear the shafts and all the rest of the assy components the design of the worm was good the materail was good but on close examination they found that the surface had thousands of microscopic cracks and when these cracks started flaking off and fretting that failure was quick, catastrophic and unavoidable, so they started looking at manufacturing methods and found that these things were made and had .04"-.05" grind stock that was removed in three passes with the last pass being between .001" and .002" and it left alot of surface cracks so they changed the process to leave .015"-.02" grind stock that is reduced to .001" in one pass and the final .001 is removed in three passes with the finish pass being .0002". this eliminated the surface cracks and added less than 8 seconds to the total cycle time. no more failures.
fouth when we machine things to grind we like to leave less than .01" grind stock and we remove it in several passes with the last .001" coming off .0002"-.0003" at a time. sometimes we will get something that has an excessive amount of stock left on it and we will either machine it down or if we do grind it we will take heavy cuts as described and the last .001"-.0015" will come off .0002"-.0003" at a time.
one last thing we always try to have the wheel leave the part completly not only longitudinally but also transversly when surface grinding and when od or id grinding we only want 2/3-3/4 of the wheel to leave the part.
 








 
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