- Joined
- Mar 28, 2011
- Messages
- 14
Hi All
Nelson has asked for some input on basic stuff; I've offered to write a series of short tutorials on grinding for the new guy. My ideal is to do a series, with installments every couple of weeks.. we'll see how long I hold up.
Kent "Cyclotronguy" English
GRINDING
Installment of the 1st
GRINDING:
A process by which material is removed from an object most frequently, by the abrasive action of a rotating wheel or a moving belt that contains abrasive grains.
At it’s most basic a grinding wheel can be made by mixing a bonding material, typically clay, with abrasive grains of such substances as silicon carbide or aluminum oxide. The mixture is then compressed into a wheel and suitably hardened and shaped prior to use. By varying type of abrasive, grit of abrasive, bond and the space between the bond makers can address a tremendous spread of grinding applications
Grinding is used in many manufacturing processes to produce a fine surface finish on an object and to bring the size of an object to within very fine tolerances. For many products grinding is only one step in a finishing process that involves additional operations such as honing, lapping, polishing, buffing and burnishing.
That being said, grinding is likewise capable of removing significant amounts of material quickly, provided the abrasive is matched to the job at hand….. we’re going to consider the process from the choice of abrasive, with an eye to making grinding more efficient for the hobby machinist.
Why are we doing this? There have been numerous comments from forum members how they set out to shape a tool bit and the grinding process takes some large fraction of an hour to bring that tool to rough shape. Presuming we’re not grinding one of the exotic cast steels for a big planer, this is just not reasonable. A couple minutes is reasonable for all but bits which require material removal of 10’s of mm’s.
Under ideal conditions it is generally accepted that the grinding of any tool steel, cast or forged should have a removal rate of 0.1 in^3 minute / 1hp or better.
Lets start with the basics. The rules that apply to single point cutting tools also applies to grinding. The difference of course is that in lieu of presenting a single cutting point to the work, we’re presenting thousands, tens of thousands….. hundreds of thousands of cutting points with a grinding wheel. And it is those hundreds of thousands of cutting points that we gain huge advantage. Each point is presented briefly to the work and then allowed to cool and recover, until next time. This is in stark (dare I say “sharp”) contrast to single point machining where we present a cutting tool and work it literally to death..
In single point machining we optimize conditions, depth of cut, feed rate and SFM such that we force the cutting tool to operate just below a critical point where the tool itself starts to break down too quickly……. And so it is with abrasive machining. We want the greatest depth of cut, rate of feed and highest surface feet per minute that the tool and work will allow.
To achieve these ends the type of abrasive, the grit and abrasive bond that hold it all together are crucial elements.
Let me premise what is to follow with a couple thoughts. As hobby machinists we frequently operate at a technology level that is decades old… and so it goes with grinding. From this point forward, I’ll treat this treatise as being “behind the curve”.
For the sake of discussion only, lets assume you went down to your local “Tools-R-Us” and bought a bench grinder, and now you want to put it to use. Your grab a HSS tool blank and try and grind something for use in your lathe…. Maybe a fly cutter for your mill.
After a very short, “rub and scrub” with your new bench grinder, that tool is too hot to hold and you’ve removed “not much” and you’re discouraged. What’s up with that? Well chances are that your new grinder came with a dark gray or black wheel.
If you dismounted that wheel and read the code hidden under the wheel flanges it would most likely say something like A36 followed by the wheel size. (wheel coding is not universal from maker to maker in a strict sense, number and letters represent a range of values rather than exact value)
This is an abbreviated designation that leaves out much, were it complete it would likely read something like Type 1, A36Q5V followed by the actual size. OD, width and ID.
That tells us that it’s a plain wheel, Aluminum Oxide of 36 Grit with a very hard bond and closed structure of fired clay.
The grinding specialist looks at those numbers and thinks, this is the wheel I use for off-hand grinding soft steel rivets out of a shovel to change the handle. This is the wheel to use for removing the mushroomed end of a cold chisel, this is the wheel I use to shorten a hardware store machine screw by hand. This wheel is perfect for odd jobs a maintenance mechanic or homeowner might wish. It is however a very poor choice for tool grinding…. Why?
The A designator tells us that the abrasive is a blocky if not slightly spheroid devoid of aggressive points, it’s tough it doesn’t break down, it’s not self sharpening. The Q after the abrasive grit tells us that this is a very hard, tenacious bond that will not give up it’s abrasive without a fight. The abrasive is also packed densely, with very little space between abrasives. In an ideal world this wheel plows it’s way through relatively soft material at about 3000 sfm, generating sufficient heat in the process to reduce the plowed out material to ash. Ideal for it’s intended use as an “all round” grinding wheel.
To grind tool steels efficiently we require something quite different in the way of a grinding wheel. We want a very sharp abrasive that self sharpens by fracturing; we want a soft bond that readily lets go of dull or small abrasive. We wish a very open bond that helps minimize heat build up and allows a substantial “depth of cut” and lastly we want at least 5000 sfm and 6000 sfm wouldn’t hurt our feelings in the least.
What’s my ideal roughing wheel look like for off hand grinding of tool steel? Well, something for instance that says WRA-24/30-j11 vbe on the wheel blotter, would suite me fine.
Deciphering all those number
WRA, A mixture of white and ruby Aloxide abrasive; a very sharp frangible and agressvie abrasive.
24/36 a mixture of coarse abrasives ranging from 24 to 36 grit.
J, a very soft bond that readily gives up worn abrasive
11 a very open structure wheel that allows great depth of cut for each grain of abrasive
v, Vitrified (clay) Bond
be, A bond strength suited to tool steel.
A CAVEAT: This wheel is specific to the task at hand, just as the A36Q5V is specific to the maintenance mechanic. Were one to attempt to grind a chunk of mild steel with the WRA24/36-j11 vbe the results would be quite unexpected. The wheel would go away in a cloud of dust, and the steel would come out looking furry, deeply imbedded with what was just moments ago a complete grinding wheel.
That example behind us, next time "feeds and speeds"
Cheers
Cyclotronguy
Nelson has asked for some input on basic stuff; I've offered to write a series of short tutorials on grinding for the new guy. My ideal is to do a series, with installments every couple of weeks.. we'll see how long I hold up.
Kent "Cyclotronguy" English
GRINDING
Installment of the 1st
GRINDING:
A process by which material is removed from an object most frequently, by the abrasive action of a rotating wheel or a moving belt that contains abrasive grains.
At it’s most basic a grinding wheel can be made by mixing a bonding material, typically clay, with abrasive grains of such substances as silicon carbide or aluminum oxide. The mixture is then compressed into a wheel and suitably hardened and shaped prior to use. By varying type of abrasive, grit of abrasive, bond and the space between the bond makers can address a tremendous spread of grinding applications
Grinding is used in many manufacturing processes to produce a fine surface finish on an object and to bring the size of an object to within very fine tolerances. For many products grinding is only one step in a finishing process that involves additional operations such as honing, lapping, polishing, buffing and burnishing.
That being said, grinding is likewise capable of removing significant amounts of material quickly, provided the abrasive is matched to the job at hand….. we’re going to consider the process from the choice of abrasive, with an eye to making grinding more efficient for the hobby machinist.
Why are we doing this? There have been numerous comments from forum members how they set out to shape a tool bit and the grinding process takes some large fraction of an hour to bring that tool to rough shape. Presuming we’re not grinding one of the exotic cast steels for a big planer, this is just not reasonable. A couple minutes is reasonable for all but bits which require material removal of 10’s of mm’s.
Under ideal conditions it is generally accepted that the grinding of any tool steel, cast or forged should have a removal rate of 0.1 in^3 minute / 1hp or better.
Lets start with the basics. The rules that apply to single point cutting tools also applies to grinding. The difference of course is that in lieu of presenting a single cutting point to the work, we’re presenting thousands, tens of thousands….. hundreds of thousands of cutting points with a grinding wheel. And it is those hundreds of thousands of cutting points that we gain huge advantage. Each point is presented briefly to the work and then allowed to cool and recover, until next time. This is in stark (dare I say “sharp”) contrast to single point machining where we present a cutting tool and work it literally to death..
In single point machining we optimize conditions, depth of cut, feed rate and SFM such that we force the cutting tool to operate just below a critical point where the tool itself starts to break down too quickly……. And so it is with abrasive machining. We want the greatest depth of cut, rate of feed and highest surface feet per minute that the tool and work will allow.
To achieve these ends the type of abrasive, the grit and abrasive bond that hold it all together are crucial elements.
Let me premise what is to follow with a couple thoughts. As hobby machinists we frequently operate at a technology level that is decades old… and so it goes with grinding. From this point forward, I’ll treat this treatise as being “behind the curve”.
For the sake of discussion only, lets assume you went down to your local “Tools-R-Us” and bought a bench grinder, and now you want to put it to use. Your grab a HSS tool blank and try and grind something for use in your lathe…. Maybe a fly cutter for your mill.
After a very short, “rub and scrub” with your new bench grinder, that tool is too hot to hold and you’ve removed “not much” and you’re discouraged. What’s up with that? Well chances are that your new grinder came with a dark gray or black wheel.
If you dismounted that wheel and read the code hidden under the wheel flanges it would most likely say something like A36 followed by the wheel size. (wheel coding is not universal from maker to maker in a strict sense, number and letters represent a range of values rather than exact value)
This is an abbreviated designation that leaves out much, were it complete it would likely read something like Type 1, A36Q5V followed by the actual size. OD, width and ID.
That tells us that it’s a plain wheel, Aluminum Oxide of 36 Grit with a very hard bond and closed structure of fired clay.
The grinding specialist looks at those numbers and thinks, this is the wheel I use for off-hand grinding soft steel rivets out of a shovel to change the handle. This is the wheel to use for removing the mushroomed end of a cold chisel, this is the wheel I use to shorten a hardware store machine screw by hand. This wheel is perfect for odd jobs a maintenance mechanic or homeowner might wish. It is however a very poor choice for tool grinding…. Why?
The A designator tells us that the abrasive is a blocky if not slightly spheroid devoid of aggressive points, it’s tough it doesn’t break down, it’s not self sharpening. The Q after the abrasive grit tells us that this is a very hard, tenacious bond that will not give up it’s abrasive without a fight. The abrasive is also packed densely, with very little space between abrasives. In an ideal world this wheel plows it’s way through relatively soft material at about 3000 sfm, generating sufficient heat in the process to reduce the plowed out material to ash. Ideal for it’s intended use as an “all round” grinding wheel.
To grind tool steels efficiently we require something quite different in the way of a grinding wheel. We want a very sharp abrasive that self sharpens by fracturing; we want a soft bond that readily lets go of dull or small abrasive. We wish a very open bond that helps minimize heat build up and allows a substantial “depth of cut” and lastly we want at least 5000 sfm and 6000 sfm wouldn’t hurt our feelings in the least.
What’s my ideal roughing wheel look like for off hand grinding of tool steel? Well, something for instance that says WRA-24/30-j11 vbe on the wheel blotter, would suite me fine.
Deciphering all those number
WRA, A mixture of white and ruby Aloxide abrasive; a very sharp frangible and agressvie abrasive.
24/36 a mixture of coarse abrasives ranging from 24 to 36 grit.
J, a very soft bond that readily gives up worn abrasive
11 a very open structure wheel that allows great depth of cut for each grain of abrasive
v, Vitrified (clay) Bond
be, A bond strength suited to tool steel.
A CAVEAT: This wheel is specific to the task at hand, just as the A36Q5V is specific to the maintenance mechanic. Were one to attempt to grind a chunk of mild steel with the WRA24/36-j11 vbe the results would be quite unexpected. The wheel would go away in a cloud of dust, and the steel would come out looking furry, deeply imbedded with what was just moments ago a complete grinding wheel.
That example behind us, next time "feeds and speeds"
Cheers
Cyclotronguy
