Grinder Trouble

[Russ Prior]

First, my apologies if this has been covered earlier; I've looked and can't find the answer to my question. Is there a problem using a 1/2 horse (5.0 amp) grinder with two CBN wheels? My grinder (a Craftsman, variable speed, 8", with 5/8" arbors) is less than a year old. It really groans when starting up and recently blew a fuse on the circuit board. The darn thing is a mini fuse but is 250V 10Amp. I can't find a replacement anywhere. I tried a 10A, 125 V fuse and it burned up immediately. I was thinking that 125V would work becasue it's only a 125V tool but I now think that the fuse must be associated with the capacitive motor start system, which could be in excess of 125 volts. Is this too whimpy a grinder for 2 CBN (8") wheels? The grinder looks a lot like the Rikon sold by Woodcraft except it is variable speed.

 

[Dennis J Gooding]

Half horsepower should be fine for woodturning tool applications. Start problem sounds like either a capacitor problem or dirt on the contacts of the start-up switch but could be a winding problem. Is it still under warranty?

 

[john lucas]

it takes some power to run CBN wheels. I have a 3/4hp slow speed grinder and it groans when starting up with just one CBN wheel. I don't think a 1/2hp motor will run it. You can try spinning the wheels by hand before you flip the switch. That may help

 

[Bill Boehme]

First, my apologies if this has been covered earlier; I've looked and can't find the answer to my question. Is there a problem using a 1/2 horse (5.0 amp) grinder with two CBN wheels? My grinder (a Craftsman, variable speed, 8", with 5/8" arbors) is less than a year old. It really groans when starting up and recently blew a fuse on the circuit board. The darn thing is a mini fuse but is 250V 10Amp. I can't find a replacement anywhere. I tried a 10A, 125 V fuse and it burned up immediately. I was thinking that 125V would work becasue it's only a 125V tool but I now think that the fuse must be associated with the capacitive motor start system, which could be in excess of 125 volts. Is this too whimpy a grinder for 2 CBN (8") wheels? The grinder looks a lot like the Rikon sold by Woodcraft except it is variable speed.

A half horse motor is just fine. A variable speed motor is not the best choice because it operates by high speed switching between two and four pole operation (3600 and 1800 RPM). The inertia of the heavier wheels may be causing heavier than normal current in the windings and the speed control might be causing some vibration.

No doubt, something else on the circuit board gave up the ghost and then took the fuse along with it. The fuse only serves to keep the motor from burning up as well.

Rather than spending money on repairing that grinder, I would suggest the Rikon grinder that Woodcraft periodically has on sale for around a hundred bucks. It is a great 1750 RPM half horse grinder and has a five year warranty.

 

[BobCoates]

Ditto what John said " try spinning the wheels by hand before you flip the switch" I do this with mine.
Bob

 

[diverjoe]

grinder

I have the Rikon grinder with two CBN wheels and it is a slow starter also.

 

[Bill Boehme]

It is worth mentioning that start capacitors usually fail gradually rather than suddenly. A symptom of a capacitor wearing out is slow starting. As long as the start up is tolerable there is no need to replace the capacitor.

 

[robo hippy]

There can be big differences in actual horse power from motors with the same ratings. If you put 2 CBN wheels on a 1/2 hp grinder, you are at the lower end of the capability of the motor to spin that much weight. If I compare my old blue no name grinder that Woodcraft used to carry to my Baldor grinder, both are rated at 3/4 hp, but the actual power difference is about the same as the weight difference, about 2 to 1 in favor of the Baldor. I can easily stall the blue grinder, and it is almost impossible to stall the Baldor. From chatting with Dave Schweitzer at D Way, the Rikon grinder seems to hold up fairly well. Still, since it is only a 1/2 hp grinder, I would hand spin the wheel to get it started. When you are sharpening with a lower power grinder, if you are stalling the wheels/grinder, then you are putting a lot of strain on the motor. A gentle kiss is all you really want unless you are trying to reshape a scraper or gouge. We all tend to way over sharpen.

robo hippy

 

[Russ Prior]

Thanks to all who replied. A few things come to mind: I do spin the wheels before starting up and this seems to help. The capacitor looks fine from the outside; I'm hesitant to take it apart to look inside since there appears to be no problem. I don't know that I have a particularly light touch but I don't ever cause the grinder to slow down when sharpening. My grinder is rated at 1/2 horse and 5.0 amps. I notice that the Rikon is rated at same hp but is just 3.0 amps. Does that mean that the Rikon is capable of producing less power? I'm fuzzy on the physics of amps, horsepower, work, etc. Russ

 

[Dennis J Gooding]

I would never give much credence to HP claims for Craftsman products. On the other hand, I have found them liberal in warranty situations. There is no fixed relationship between starting torque and max developed HP in these motors. Max HP is developed at near max run speed and at a current draw usually much smaller than the starting current if there is a fixed or inertial load on the motor. Does anyone know what is the moment of inertia of a CBN wheel compared to a conventional wheel? As I recall, the weights are not all that different, but much of the mass of a CBN wheel is concentrated in the rim.

 

[robo hippy]

I haven't weighed the D Way wheels, but the Cuttermaster from Canada is over 8 pounds and 1 inch wide. I would guess the D Way to be 5 plus. I tried stepping on the bathroom scale with it and without it, and got about 5, but it feels heavier than that. For sure, they do feel heavier than standard wheels.

robo hippy

 

[Duane Meadows]

First, my apologies if this has been covered earlier; I've looked and can't find the answer to my question. Is there a problem using a 1/2 horse (5.0 amp) grinder with two CBN wheels? My grinder (a Craftsman, variable speed, 8", with 5/8" arbors) is less than a year old. It really groans when starting up and recently blew a fuse on the circuit board. The darn thing is a mini fuse but is 250V 10Amp. I can't find a replacement anywhere. I tried a 10A, 125 V fuse and it burned up immediately. I was thinking that 125V would work becasue it's only a 125V tool but I now think that the fuse must be associated with the capacitive motor start system, which could be in excess of 125 volts. Is this too whimpy a grinder for 2 CBN (8") wheels? The grinder looks a lot like the Rikon sold by Woodcraft except it is variable speed.

The voltage rating of a fuse is about it not arcing over after it blows! The second fuse did not blow because it was a 125 volt rated fuse. As others stated, likely due to a component failure on the circuit board. Yes, slow startup may have been a contributing factor.

 

[stu senator]

slow blow fuse?

There are "slow blow" fuses, they take longer to blow at the same current and there are "fast blow" fuses that blow almost instantly. Most are this kind.

I don't know what is the spec on the fuse that failed but due to the larger inertia of the steel with its weight at the rim the starting current will probably last longer then with a regular wheel and blow a standard fuse.

Stu

 

[Bill Boehme]

There are "slow blow" fuses, they take longer to blow at the same current and there are "fast blow" fuses that blow almost instantly. Most are this kind.

I don't know what is the spec on the fuse that failed but due to the larger inertia of the steel with its weight at the rim the starting current will probably last longer then with a regular wheel and blow a standard fuse.

Stu

From my experience it seems like the fuse on the circuit board is mainly to protect the driver electronics that controls the motor. The lab technicians where I worked would joking say that transistors were put on circuit boards to protect the board mounted fuses. That could very well be true because transistors have reached the point of being very inexpensive while simple devices like fuses are more expensive than ever. It is a well established fact that no matter how fast acting a fuse might be, and at ten amps especially, it won't be nearly as fast acting as any semiconductor that will give up the ghost in microseconds (maybe even femtoseconds). If it hasn't been mentioned yet, repair parts like the circuit board are often priced too high to be worth repairing.

 

[stu senator]

Motor's take time to get to speed. The more inertia the longer the time that will be measured in seconds, not fractions of seconds. If a semiconductor or fuse can't take it, it blows.

I don't know how the motor electronics work, just the basic motor.

Stu