What you don’t know - or don’t care about - can hurt you!
Text, photos and video by Tom Hintz
Posted – 1-26-2011
Shortly after I launched NewMetalworker.com I received an email from a fellow who was recovering from what he termed a “new metalworker injury.” The accident occurred while he was using a common 8” grinder. This fellow had virtually no experience with metalworking tools but had recently begun putting a shop together. He was touching up a chipped screwdriver on his brand new grinder things went very bad very quickly.
While re-shaping the end of the screwdriver he turned the tip downwards so that it was facing in the same direction as the rotation of the wheel. In that process he accidentally ran his knuckle into the grinding wheel. As he flinched he lost control of the screwdriver and it was sucked driven downwards between the grinding wheel and the tool rest, exploding the grinding wheel.
The shattering wheel drove a roughly pie-slice-shaped piece into his thigh and sending him on an ambulance ride to the hospital ER. The good news is that he will be laid up for just a few days. The scary news from his doctor was that the piece of grinding wheel came within a couple inches of a femoral artery that could have had mortal consequences if I had been damaged.
This email left me thinking about the first times I had used a new machine and how many close calls I had along the way simply because I was not familiar with the machine or the safe procedures involved in using it. Certainly a grinder is a simple machine but as this accident shows, using it wrong can have serious consequences.
Guards & Shields
It is far too easy to find photos and video of grinders that have been stripped of their wheel covers and debris shields. I have heard all sorts of justification for their removal but none that make sense. When things go wrong with a grinding wheel it happens in all directions, not just towards the operator. The heavy wheel enclosures are designed to contain those events to protect the operator and anyone else in the vicinity.
The see through guards are also frequently removed but that also exposes the operator to unnecessary dangers. The act of grinding metal on stone should raise warning flags. This process simply has to generate debris that is flung outwards by the spinning wheel. Wearing safety glasses is important but so is keeping that clear shield between you and the grinding wheel.
I have heard reasons for removing grinder guards saying that the piece being ground could not be held to the grinding wheel with the guards in place. Or the piece could not be held at the correct angle to grind it properly. Both of these excuses suggest that they were using the wrong machine for the job than justifying the removal of safety features. Think about what you are trying to do and consider alternative hand or mounted grinders that could to the job more safely.
No Repair Stones
The first thing we need to understand about grinding wheels is that once they have any kind of a crack or a chip that can’t be dressed out, they are junk. There is no viable repair procedure that anyone, much less an amateur should be trying. If a grinding wheel becomes damaged cut your losses, wrap it in rags and finish killing it with a couple swift blows of by the biggest hammer you have. The last thing you need is for someone else to find it and try using it.
All grinding wheels are designed to be used within a specific RPM range. That target RPM will be listed on the grinding wheels label and the speed of your grinder is also listed on a label or metal plaque. Make sure that you are running the stone wheel at its target RPM.
Turning a grinding wheel faster or even slower can have consequences. If a grinding wheel is spun faster than its rated speed it can shatter from vibrations alone or from a small accident that introduces a seemingly harmless impact type shock. When a grinding wheel blows up you don’t want to be anywhere in the vicinity! The weight of even small pieces of a stone grinding wheel can do surprising amounts of damage to things much harder than the operator!
Grinding wheels are designed to be effective at a specific SFPM or Surface Feet Per Minute. That means how much of the grinding surface is passing under the object being ground. Go faster than the rated SFPM and in addition to stability problems you can build much more heat than is necessary and the effectiveness goes down. Too slow and the effectiveness can suffer as well. Read the label and any paperwork that came with the grinding wheel or attachments being used to be sure that you are using the right RPM for both the grinder and the tools being used.
If a grinder that previously did not vibrate much now does, stop the wheels and look for a reason. Modern grinding wheels (the better ones anyway) are rarely out of balance enough to cause a serious vibration. In most cases if a grinding wheel is out of balance you will notice that the first time it is run. When a grinder develops a vibration something happened to cause it. Look for loose nuts on the spindle or something wrong with the stone itself. Surprisingly small chips can cause a vibration at 1750 or 3500 RPM. If you recently had a big event like dropping a tool that jammed the grinder check the spindles to see if one side might be bent.
Keep in mind that while simple out of round conditions can often be fixed by careful dressing of the wheel little else can be repaired. Things like cracks or relatively large chips can also have you replacing the wheel. None of us enjoy spending money but buying a new grinding wheel rather than hoping a damaged one does not explode is a much smarter plan.
There are devices for balancing new grinding wheels like the one in a Tool Review I did a few years ago. (Click Here to see that review) I have found that buying better quality stone grinding wheels virtually eliminates built-in vibrations and the ones developed with these wheels is generally very small.
One thing you can do to reduce or eliminate a small vibration is to turn one of the wheels so it is 180-degrees to the other wheel. Just draw a line across the edge of both wheels at the tool rest and then loosen one and turn it so that line is at the back of the grinder while the other wheels mark is still at the tool rest. That simple fix can cancel out a small vibration and extend the life of the wheels in the process.
There is a reason all grinders spin the stone wheels towards you, over the top and down towards the tool rest. As with routers and saws, we want to be presenting the object being ground against, not with the direction that the wheel is turning. That way we have the consistent forces of the wheel that allow us to control the object being ground. If the piece was presented in the same direction as the wheel is turning it would try to grab the piece and throw it. This is what happened to the person that sent me the email that got this whole story started. When he touched his knuckle against the wheel he lost control of the screwdriver that was pointed in the same direction as the wheels were turning and that drove it between the wheel itself and the tool rest. That creates an instant load that shattered the wheel and caused the injuries.
We want to use the tool rest as a base, supporting the object being ground so that we can control how hard the object is pressed against the grinding wheel. Using the rest also makes it easier to grind a specific shape and to be more accurate no matter what shape or grinding job we are doing.
Some tool rests have grooves in their surface that help when sharpening drill bits or other tools. Some tool rests have a flat surface, some can be adjusted for grinding different angles. The key is to use the rest that you have as it was meant to be used. There are aftermarket tool rests and jigs that can make some complicated grinding jobs easier and safer.
In most cases the tool rest should be very close to the grinding wheel. As more tool length hangs over the tool rest the chances of developing a vibration in the tool itself go way up. Those vibrations can cause a chatter or even a dangerous catch not to mention producing a terrible finish in the ground surface. Keep the tool rest as close to the grinding wheel as possible without it actually rubbing on the wheel.
I also see people grinding on the sides of a wheel and that is a no-no according to just about everyone that makes grinders. The stone grinding wheels are not designed to have side loads put on them. Lightly rubbing a tool or something on the side of stone wheel might not cause it to blow up but if anything goes wrong while using the side of the wheel you are almost assured of an explosion. If you have a job that only works on the side of a grinding wheel, you are using the wrong machine/tool.
Light pressure is always better for both the object being ground and the grinding wheel itself. Forcing a piece into the stone puts huge loads on it and can compromise the surface particles that actually do the grinding. Using a light pressure against the grinding wheel lets it do its work and produces a better finish on the object being ground. If the rate of material removal is too slow change to a more aggressive grit rather than increasing the pressure. Your grinding wheel will last much longer and you will generate less heat in the object being ground. Plus you can be far more accurate when using a lighter, more easily controlled pressure.
Grinding is a must-have capability in nearly any metalworking shop but that does not over rule being safe while doing it. Taking the time to use grinding machines correctly includes adjusting and using the safety equipment properly. Risking any level of injury is dumb but considering the energy stored in a spinning grinding wheel, taking risks on grinders rises to a new level. The good news is that using a grinder correctly is neither hard nor time consuming. It does require that the operator pays attention to what they are doing, something that is unfortunately far less automatic than it should be.
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