Grinding wheels and HSS

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Harold_V
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Grinding wheels and HSS

Post by Harold_V » Mon Jan 15, 2007 6:35 am

Successful grinding of HSS toolbits is best achieved by the use of a wheel that is readily friable and open in structure to insure free cutting and proper removal of swarf. Grain selection can be a matter of choice, but is often dictated by the availability of wheel selections. If more than one wheel is at one's disposal, a 46 grit wheel is a good choice for roughing a tool, but the finish is not as desirable as one produced by an 80 grit wheel. 80 grit wheels tend to be a poor choice for roughing, cutting slower and hotter than a 46 grit wheel, so it's not a good choice if only one wheel is available. It has long been my custom to use a 60 grit wheel, which serves both purposes adequately, and is an excellent compromise.

Hardness rating of grinding wheels is based on surface speed. The typical vitrified wheel is generally rated @ 6,000 sfpm, and behaves softer as the diameter decreases. Armed with this idea, it's wise to select wheels with the largest possible diameter, without exceeding their recommended maximum speed rating. As wheels get smaller, useable life diminishes rapidly, but it can be recovered to satisfaction if wheel speed is increased. If you're fortunate to have control of the speed of your grinder, insure that you don't exceed the 6,000 sfpm threshold, to insure that the wheel doesn't come apart. Most bench and pedestal grinders don't have the ability to change speed, but many cutter grinders do.

The typical bench or pedestal grinder is normally equipped with wheels that are too hard for such an application, and wheels are not readily available in the more desirable grades for such use. To resolve this problem, a 3600 rpm grinder can be adapted to run wheels that have a 1¼" diameter hub. Such wheels are commonly used on cutter and small surface grinders and are readily available in a broad variety of styles and selection of grits, hardness's and structure.

Type of grinder is beyond the scope of this post, but it's important that the reader understand that the vast majority of problems one encounters when grinding HSS are related to wheel choice, and while success of sorts can be achieved with almost any wheel, there are solutions to the problems that can turn a rather unpleasant task into one that is quite tolerable.

Rule of thumb for grinding is to use a hard wheel for soft materials, and a soft wheel for hard materials, and further dictated by silicon carbide's ability to dissolve in steels at high temperatures such as found when grinding. While silicon carbide far exceeds the hardness of aluminum oxide, it is not suited to grinding HSS for that reason. In keeping with these guidelines, aluminum oxide wheels become the acceptable choice for grinding HSS---with, perhaps, the odd exception, which is unlikely to be an issue. Silicon carbide wheels are rarely used in the shop unless non-ferrous materials are to be ground. They are not intended to be applied to steels or steel alloys.

Wheel hardness is not related to the hardness of the grinding media. Hard or soft wheels share the same material, but are assembled with more or less bonding material to dictates the hardness of the wheel. A wheel that requires greater effort to dislodge dull bits would be considered a hard wheel, while one that permits the bits to be dislodged easily would be considered a soft wheel. That is the friability index of a grinding wheel, and is noted as a letter of the alphabet, A being soft, Z being hard.

Another feature to be considered when making a choice is the structure index. Abrasive grains are spaced close together in a tight wheel, and spaced at greater distance in an open wheel. This index is referenced by numbers, with 0 being a closed wheel, and 12 being an open wheel. Open wheels provide a space for swarf and help in keeping the temperature of the grind at a lower level. A wheel that is not open enough will "load" and not unload, yielding a wheel that displays all the characteristics of a wheel in need of dressing because it's dull. It's clear, a balance of hardness and structure are important to successful grinding, so they should be considered when selecting a wheel.

Wheels are bonded by various methods, but the one that is of most concern to the home shop type would be a vitrified bonded wheel. Vitrified wheels are bonded with selected clays, which are high fired, and are identified by a V in the wheel designation, following the hardness and structure designation. The process builds minute lengths of glass that bond the bits to one another upon vitrification. Hard wheels have a greater amount of bonding agent than soft wheels.

Others bonding systems serve specific purposes, but are unlikely to be needed in the home shop for the most part. Resinoid parting wheels can be an exception. Other types of bond include silicate, rubber and shellac.

Abrasive designation type within a media is an indication of the process used to produce the media. One might expect to see markings such as 19A, 32A 0r 38A on a Norton wheel. All are aluminum oxide wheels, although each is better suited to different types of grinding. The 38A abrasive has excellent friability and a porous nature, making it the wheel of choice for heavy stock removal and a good choice for HSS grinding.

Assuming one was to order a Norton wheel for grinding HSS, a designation such as 38A60H8VBE would be considered an acceptable wheel. Grinding techniques vary, so some may prefer a slightly harder wheel, so experimentation would be encouraged.

Grinding wheels should always be given a "ring" test before mounting, immediately after a physical examination in which one looks for signs of cracking or pieces having been broken from the wheel. By placing the wheel on a solid object (screw driver, for example), and tapping the wheel with the likes of the handle of another screwdriver, a distinctive ring should be heard. If not, the wheel should be immediately destroyed to avoid an innocent party from mounting what is very likely a wheel that will explode when spooled up. It should be the policy of anyone running a grinder to destroy damaged wheels immediately.

Once it has been ascertained that the wheel is in good health, it should be mounted without a struggle on the arbor. If the wheel is a force fit, a risk of cracking is ever present, particularly when the flanges are tightened, so make sure the wheel fits properly, and can shoulder against the fixed flange (if so equipped) readily.

A wheel that has a bore out of perpendicular with the sides can be cracked when tightened. Should one encounter such a wheel, the bore can usually be scraped adequately to permit proper seating. It should never be horsed.

Always stand aside when spooling up a wheel, even one that is known to be good. The first few moments should serve to shake out a wheel with problems, so allow about a minute of full speed operation before standing in line with it's rotation. That's particularly important in a shop where children have access, or objects are moved about, where reasonable risk of cracking a wheel exists.

Proper dressing of grinding wheels is key to success. Wheels that don't run true and are not relatively flat are useless for tool grinding. You should be able to rest the tool being ground on a wheel and have it stay where placed, without bouncing away. The wheel should be trued immediately upon installation, then dressed on a regular basis as it's being used. One might dress the wheel several times in the process of grinding one tool, but the benefit of keeping the wheel properly dressed pays serious benefits in cooler grinding and faster stock removal. It becomes evident that a wheel needs dressing, particularly if it's too hard.

Truing a wheel can best be accomplished with a mounted diamond, or diamond cluster, but a wheel so prepared is not well suited to offhand grinding. Diamonds tend to leave a wheel quite smooth, increasing area in contact with the grind, and behaving somewhat like a bearing. Grinding with a wheel so dressed tends to be slow and hot. In order to disrupt this pattern, roughing the surface by different means is normal procedure.

The best possible surface for hand grinding is achieved by the use of a star, or impact, type dresser. They function by hammering the wheel and dislodging the old bits of abrasive, exposing new bits that are sharp and ready to grind. Problem is, this type of dresser requires a bit of skill to apply successfully, and often wastes a good amount of useful wheel as the user attempts to get the wheel running true and flat. As a result, I do not use, nor endorse such a dressing tool. Further, if you use the same method of tool grinding that I do (no tool rest), there is no place for the dressing tool to rest as it's applied.

There are available on the market items known as dressing sticks. Two varieties should be of interest for those that intend to grind HSS tools. One is a solid boron carbide stick, typically 3/16" thick, ½" wide, and 3" long. They are excellent for dressing wheels when new, but as the corners slowly round off, they begin dulling the wheel more than dressing---so I don't recommend one unless you have the ability to keep it sharp.

The next choice, and the one I recommend highly, is the use of a vitrified silicon carbide stick. They are typically 1" square and 6" long, and inexpensive (roughly $10). They are readily available from machine shop supply sources. One should last the average home shop type for a life time. Choose one that is coarse grained, anything from 16 grit up to 24, depending on availability. Avoid finer sticks. A wheel, once diamond dressed, can be fine tuned with such a stick in seconds, and done freehand without the use of a rest. It is also applied when redressing is required, keeping the wheel flat and sharp.

Happy grinding!

Harold

Edit: Redundant statement removed.
Last edited by Harold_V on Wed Apr 15, 2009 11:49 pm, edited 2 times in total.

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Marty_Escarcega
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Post by Marty_Escarcega » Mon Jan 15, 2007 7:38 am

Excellent post Harold! Loaded with information and I'll take your advice on the wheel type and dressing stick.

I have a couple more questions, have you built a working platform rest for your grinder, if so, pictures?

What sort of HP would you recommend for a bench grinder. I have one of the OLDER Sears Craftsman 6" bench grinders with the "SKF" ball bearings. :-) Though I believe it is only 1/3hp.

Secondly, what are your thoughts on belt grinders, and their place in the shop should one be fortunate enough to have one.

Very well written, thanks again Harold!
Marty
"Jack of all Trades, Master of None"

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Steve_in_Mich
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Post by Steve_in_Mich » Mon Jan 15, 2007 10:56 am

Great write-up Harold!

Will these silicon carbide sticks be useful for renewing the wheel surface?
A deal at MSC ~ $2.90 ea.;

http://www1.mscdirect.com/CGI/NNSRIT?PM ... O=16320797
Just because you don’t believe it - doesn’t mean it’s not so.

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Lew Hartswick
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Post by Lew Hartswick » Mon Jan 15, 2007 1:18 pm

Marty, I built a few rests for the grinders at school. I have the pictures
on my page at http://home.earthlink.net/~lhartswick/ the set that
is labled DSCF******* They are a little big, I hadent figured out that
I may want ot link to them when I put them there. Guess I should
"smallen" them . :-)
...lew...

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MikeC
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Post by MikeC » Mon Jan 15, 2007 1:47 pm

Another nice piece Harold.

A friend also suggested using the stick dresser to rough radical changes to the wheel shape before finish dressing a radius, angle, or similar with the diamond. He was also kind enough to provide a couple of suggested sticks, one for diamond and one for AL wheels:

Norton
37C24-52VK (AL)
5468779 89A1206 4AVZ (Dia)

Are these familiar to you? He says the first one lasts nearly forever and the second (for diamond) goes REAL fast, as it is extremely soft.

Also, I just bought some wheels off Eaby for the cutter grinder. These are 6x3/16" wheels I intend to use for gashing endmills. 38A60-K8VG. I wanted something hard enough to hold a corner when splitting points on two and four flute center cutting endmills. Does this sound about right?

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BadDog
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Post by BadDog » Mon Jan 15, 2007 6:01 pm

I find that I currently prefer to "grind" HSS tools on my belt grinders. Easily swapped belts in a variety of grains and always flat, great where that works. I generally only resort to my grinder wheels for corner work or to use the 40 grit wheel for roughing the bit shape.

So, what is the thought on use of belts?

This will likely change when my surface grinder is running...

Edit: Just realized, Marty asked a similar question. :D
Russ
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Marty_Escarcega
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Post by Marty_Escarcega » Mon Jan 15, 2007 9:00 pm

LewHartswick wrote:Marty, I built a few rests for the grinders at school. I have the pictures
on my page at http://home.earthlink.net/~lhartswick/ the set that
is labled DSCF******* They are a little big, I hadent figured out that
I may want ot link to them when I put them there. Guess I should
"smallen" them . :-)
...lew...
Excellent, thanks for sharing the link of your rest! I seem to remember seeing something similar somewhere but someone also scribed angles on the plate for easy reference?
Marty
"Jack of all Trades, Master of None"

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Post by Harold_V » Mon Jan 15, 2007 9:01 pm

Marty_Escarcega wrote: I have a couple more questions, have you built a working platform rest for your grinder, if so, pictures?
As I'll explain in a future post, no, I do not have a table, and would refuse to use it if I did. I'll try to go into more detail in the future post, but, for now, suffice it to say that a table is nothing more than training wheels, and does little more than make one dependant, at the cost of poor results.
What sort of HP would you recommend for a bench grinder. I have one of the OLDER Sears Craftsman 6" bench grinders with the "SKF" ball bearings. :-) Though I believe it is only 1/3hp.
Grinding toolbits, unless very large, say, above 1/2", generally does not require a lot of hp. I've used a ½ horse Craftsman motor that belonged to my father as long as I've had my own shop, and have yet to exceed it's ability. What you'll come to understand when you select the proper wheel is that the wheel cuts readily, and requires very little pressure. If pressure is required, you have the wrong combination.

Please keep in mind that I do not use my grinder for heavy grinding, such as grinding welded components, nor should anyone. Wheels for grinding HSS should be more or less reserved for that purpose alone. They aren't really suited for much else. A second grinder with harder wheels is very important for those with needs beyond grinding HSS.

The problem you're likely to encounter with your 6" grinder is the inability to buy a wheel that's suited to grinding HSS--- which I have already addressed. If, by chance, the grinder runs @ 3,600 rpm, and you're willing to make adapting arbor(s), it could prove to be a very nice machine. In your case, it may require building new wheel guards as well, but they can be easily made from short lengths of steel pipe.
Secondly, what are your thoughts on belt grinders, and their place in the shop should one be fortunate enough to have one.
My comments are sure to anger a few, but please hear what I have to say and try to understand that there is a serious amount of logic in my position.

Depending on the nature of the task at hand, I normally speak against the use of belts for sharpening cutting tools. There are exceptions.

The problem with belt use is that belts do not run flat. If you'll pay attention to what a belt does just above the point of contact, you'll notice that it buckles outward. That is the worst possible scenario I can imagine. Unlike grinding with the radius of the typical type 1 wheel, which provides for a slight hollow grind, a belt tends to round off the cutting edge ever so slightly. It is akin to starting with a dull cutting tool, and often is the source of horrible finishes and difficulty in holding size.

Do I use a belt for some grinding? Yes----I do-----but I usually regret the decision, often because of the diminished performance of the tools so ground.

My suggestion for those that prefer a belt (they tend to move material fairly rapidly), is to use one for roughing, but switch to the proper wheel for finish grinding. Properly applied, a wheel will beat a belt every time.

One other issue with belts as opposed to wheels: You are drastically limited by the use of a belt, very unlike a wheel. That is likely not evident to the novice, who has great difficulty in grinding cutting tools. However, as one's skill level improves, it quickly becomes evident that not only belts, but tables become a limiting factor.

I encourage every person to learn to grind free hand----without the use of a rest. It's harder to do at first, but easier when mastered. Again, I'll address that in a future post.

Harold

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Post by Harold_V » Mon Jan 15, 2007 11:28 pm

Steve_in_Mich wrote: Will these silicon carbide sticks be useful for renewing the wheel surface?
A deal at MSC ~ $2.90 ea.;

http://www1.mscdirect.com/CGI/NNSRIT?PM ... O=16320797
From all indications, an excellent choice!

I normally use nothing more than a dressing stick.

Harold

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tornitore45
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Post by tornitore45 » Mon Jan 15, 2007 11:31 pm

Good post, thanks.
Harold, you mention that Si Carbide is not suitable for ferrous because the SiC dissolve in the steel at high temperature.

What happen then? What about using Sic for a final touch up where hardly any heat is generated.

I usually rough my HSS with a SiO weel and then give a mirror finish with the SiC weel. Is that so bad?

Thanks
Mauro
Mauro Gaetano
in Austin TX

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Post by Harold_V » Tue Jan 16, 2007 12:17 am

MikeC wrote: A friend also suggested using the stick dresser to rough radical changes to the wheel shape before finish dressing a radius, angle, or similar with the diamond.
That has always been common practice. Fact is, unless one has a profile dresser, wheels are commonly hand formed without a diamond. It's just not as hard to do as it may sound, particularly when the vitrified dressing sticks are used. Depending on the intended purpose of the wheel, it is often desirable to not diamond dress at all.
He was also kind enough to provide a couple of suggested sticks, one for diamond and one for AL wheels.

Are these familiar to you? He says the first one lasts nearly forever and the second (for diamond) goes REAL fast, as it is extremely soft.

Norton
37C24-52VK (AL)
That is the exact stick I recommend----if not one of the more economically priced imports.
5468779 89A1206 4AVZ (Dia)
Indeed----but to be applied with a large serving of common sense. One does not "dress" a diamond wheel in the common sense of the word. Diamond wheels tend to load over time, particularly if they're run with improper or insufficient coolant. Also, a diamond wheel that has been subjected to steel benefits greatly from the application of such a stone. They are soft and friable, to break down quickly under the diamond. The purpose is more to clean the matrix of the diamond wheel, never to reshape it. A touch, long enough to see a change in the appearance of the surface of the diamond wheel (cleaning the crud) often restores the wheel to excellent performance.
Also, I just bought some wheels off Eaby for the cutter grinder. These are 6x3/16" wheels I intend to use for gashing endmills. 38A60-K8VG. I wanted something hard enough to hold a corner when splitting points on two and four flute center cutting endmills. Does this sound about right?
While they would serve the purpose, you may find you'd have better success using a parting wheel for gashing. Parting wheels tend to cut much faster and cooler, although there may be exceptions.

As is referenced in grinding handbooks, heat checking of grinding heat treated objects is an ongoing problem. It has customarily been addressed by hand dressing wheels, so the moment of contact with the wheel is brief, and cutting is fast and cool. Rapid traversing of the object past the wheel is also instrumental in reducing the problem.

Your choice of wheel has reasonable designations (were it 7" diameter, I'd suggest it as a wheel for grinding HSS toolbits), but one that is diamond dressed would tend to cut quite hot. The wheels you chose would still need to be reduced considerably in width for your intended purpose, so the benefit of using such a wheel can be lost. Grinding wheels that are fine enough respond fairly well to having their cross-section reduced, but they tend to be quite fragile. By sharp contrast, parting wheels, which are not vitrified, but resinoid bonded, withstand much greater speeds, and are, in fact, very resilient compared to vitrified bonded wheels.

Harold

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Post by Harold_V » Tue Jan 16, 2007 12:21 am

BadDog wrote: So, what is the thought on use of belts?

This will likely change when my surface grinder is running...

Edit: Just realized, Marty asked a similar question. :D
Russ,

Please refer to my comments on Marty's post.

A surface grinder can be a wonderful addition to one's arsenal for grinding tools, but it will have serious limitations and not serve you well for many of the tools you may desire to grind. You will be very best served to learn to grind tools by hand-----and without a tool rest. If you're interested in the concept, please watch for a future post on the subject.

Harold

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