Rikon 8" slow grinder on sale $99 at Woodcraft

[Gretch Flo]

Normally $139. woodcraft@reply.bronto.com. 1/2 horse, 1750 rpm, 2 oxide wheels. Just bought mine, Gretch

 

[robo hippy]

I am waiting to hear back from them. I wrote in and asked if their 1/2 hp motor would handle the heavy CBN wheels. They were thinking the Woodcraft Woodriver wheel which is diamond matrix on an aluminum hub. I sent in links to the CBN wheels that are on the market, and that has been sent in to headquarters where they are going to look into it. They did mention that they want to expand their grinder line.

robo hippy

 

[Bill Boehme]

I am waiting to hear back from them. I wrote in and asked if their 1/2 hp motor would handle the heavy CBN wheels. They were thinking the Woodcraft Woodriver wheel which is diamond matrix on an aluminum hub. I sent in links to the CBN wheels that are on the market, and that has been sent in to headquarters where they are going to look into it. They did mention that they want to expand their grinder line.

robo hippy

The weight of the wheels has no effect on the horsepower needed (as in zip, nada, etc). The only thing affected is initial acceleration and that difference in acceleration between an empty shaft and one with a couple steel flywheels (which is in effect what the CBN wheels are as far as the motor is concerned) is inconsequential. The moment of inertia of the motor armature is far more than the moment of inertia of the D-Way CBN wheels. The great majority of 8 inch grinding wheels are 1/2 HP. Even my big Delta 23-725 bench grinder only has slightly more power at 3/4 HP. I think that the large Baldor is one HP. The one advantage of larger grinders is smoother running because of the more massive armature, but that really isn't a factor when using the well balanced CBN wheels.

No telling what they are liable to come up with at Woodcraft HQ. If you're lucky, they'll pass the question to the trash man[SUP]1[/SUP], but more likely it will go to some bottom feeding marketing person. 😀

[SUP]1[/SUP] If you are a Dilbert fan, you'll know about the world's smartest trash man who is actually a brilliant scientist.

 

[nicktodd]

It looks amazingly like Jet Model that they use to sale. I am on my second set of wheels with no issues.
To add bills note; I think you would want to have a similar wieghted wheel on either side.

 

[James Cheever]

I am waiting to hear back from them. I wrote in and asked if their 1/2 hp motor would handle the heavy CBN wheels. They were thinking the Woodcraft Woodriver wheel which is diamond matrix on an aluminum hub. I sent in links to the CBN wheels that are on the market, and that has been sent in to headquarters where they are going to look into it. They did mention that they want to expand their grinder line.

robo hippy

Robo Hibby -

I have the exact same grinder and run the radius edge 80 and 180 grit CBN wheels from D-Way. It runs very, very smoothly and grinds excellently. There are no issues with the wheels.

 

[robo hippy]

The horse power does matter, and a lot. Mostly it is the stress of starting up, add to that the pressure of grinding. The motor needs to drive the wheels when resistance/tool is pressed against the wheel. A few times may not hurt, but a lot of use can. There is a lot of difference in motors as well. My old blue no name grinder from Woodcraft has a 3/4 hp motor as does my 8 inch baldor . The power difference is as big as the weight difference, which is almost double. I can easily stall my blue one, but can barely slow down my baldor. If my grinder had CBN wheels on it, and I was using my 'little' grinder, I would hand start the wheel before switching it on. The heaviest CBN wheel is the Cuttermaster from Canada, at 8 pounds. I think the D Way are about 6. I don't know what standard matrix wheels weigh, but I would think the CBN is at least double the weight as well.

robo hippy

 

[Odie]

I checked the Woodcraft site, and don't see where this Rikon grinder has a capacitor start motor. If it does, start-up won't be a problem, but heavy grinding with 1/2hp might be. Once the shape of a turning tool is established, 1/2hp is good enough power to keep that particular grind trued up.

ooc

 

[Gerald Lawrence]

It looks amazingly like Jet Model that they use to sale. I am on my second set of wheels with no issues.
To add bills note; I think you would want to have a similar wieghted wheel on either side.

I of was thinking of getting a CBN to go on my slow speed Woodcraft. Are you saying that the grinder would be damaged w/o same weight on other end? i could see getting pressure on the berings at the opposite end, but this would be no more than the pressure on the end with CBN wheel.

 

[Bill Boehme]

The horse power does matter, and a lot. Mostly it is the stress of starting up, add to that the pressure of grinding. The motor needs to drive the wheels when resistance/tool is pressed against the wheel. A few times may not hurt, but a lot of use can. There is a lot of difference in motors as well. My old blue no name grinder from Woodcraft has a 3/4 hp motor as does my 8 inch baldor . The power difference is as big as the weight difference, which is almost double. I can easily stall my blue one, but can barely slow down my baldor. If my grinder had CBN wheels on it, and I was using my 'little' grinder, I would hand start the wheel before switching it on. The heaviest CBN wheel is the Cuttermaster from Canada, at 8 pounds. I think the D Way are about 6. I don't know what standard matrix wheels weigh, but I would think the CBN is at least double the weight as well.

robo hippy

If it will make you feel better to believe that then get a grinder with a large motor -- it will make some dealer happy and you will have a warm fuzzy, too. If you feel that the half horse motor is so marginal and that power matters "a lot", then consider that 3/4 horsepower isn't much cushion if power is on the ragged edge with a half horsepower. It sounds like you have already decided on the answer to your question to Woodcraft based on conjecture.

I can't image a situation where I would be applying enough pressure to overload a grinder motor since sharpening turning tools only requires a feather light touch. A half horse motor produces typically 1.5 LB-FT torque at 1725 RPM, which would be 4.5 pounds tangential force on an 8 inch wheel. Now, if you were sharpening a piece of steel, that doesn't mean pressing with 4.5 pounds into the wheel -- it would take a far greater radial force to produce a tangential component of 4.5 pounds. In fact, if your grinder isn't bolted down to the workbench, you would push it off the bench before reaching that point.

If your "little" grinder has to be hand propped to get it going then it isn't the grinding wheels -- in all likelihood, the capacitor is just about worn out. You can find the capacitor in the base of the grinder if you turn it upside down. Usually there will be a cover plate on the bottom. If it isn't the capacitor, then it would most likely be the centrifugal switch. You would need to remove the motor end plates to get to it. The other possibility is the start winding is open (not very likely, but it can happen), but if that is the case then toss it and get another motor because it isn't worth the cost of repair.

I have some unused 8 inch grinding wheels so I checked their weight. The silicon carbide wheel was less than an ounce under 4 pounds and the aluminum oxide wheel was three ounces under 4 pounds. The 8 inch D-Way CBN steel wheels weigh 6½ pounds. As I said previously, weight is not nearly as important as moment of inertia which is determined by geometry. The matrix wheels are a simple flat disk while the CBN wheels have most of their weight close to the rim which results in a greater moment of inertia than the matrix wheels, but still not a factor in anything other than acceleration to running speed from standing still. Acceleration time is less than one second for matrix wheels and I can't tell any difference in acceleration time using the CBN wheels on a half horsepower motor at a friend's shop. The important point is that wheel weight has absolutely nothing to do with running power once up to speed. The acceleration torque load of getting up to speed with a pair of 6.5 pound CBN wheels is a nit. Besides, the moment of inertia of the armature swamps out the small moment of inertia of the wheels whether matrix or CBN.

Baldor grinders probably have a service factor of 1.15 since it is designed for continuous industrial use. Most consumer grade grinder motors probably have a service factor of 0.85 since they run intermittently under load. Horsepower is needed to overcome friction losses. Inside the motor this would include hysteresis drag, iron loss, copper loss, bearing friction, and wind friction around the spinning armature. Those losses result in heating, but are not counted against shaft output power. In other words, a one horsepower motor has a maximum shaft output of one horsepower plus power to overcome internal motor losses. Externally, there isn't much: Power to accelerate the motor to full speed and once at full speed, that term goes to zero. Other than that, the only losses due to friction are air drag of the spinning grinding wheel which isn't much since it is aerodynamically clean and the friction loss due to grinding.

While it is possible to stop a grinder by jamming a tool into the wheel, normal grinding forces are just a fraction of that. Don't be mislead into thinking that the grinder is overload if the motor slows down. That is normal for an AC induction motor. The generated output torque exactly balances the load torque and the amount of torque needed to do that is indicated by the amount that the motor slows down. It is known as "slipping sync" and the amount of sync slip determines the motor current. Torque produced by the motor is directly proportional to current. An unloaded motor typically has a slip frequency of about 2.5 Hz and the slip frequency increases (i.e., the motor slows down) as load is increased, thus increasing motor current to exactly balance load torque with generated torque.

BTW, if you are considering getting CBN wheels, go to the D-Way website and view Dave's video on installing the 8 inch wheels on a half horsepower bench grinder. It would be a lot better than asking Woodcraft corporate office. They're great folks, but they are retailers, not machine designers.

 

[Andy Chen]

I of was thinking of getting a CBN to go on my slow speed Woodcraft. Are you saying that the grinder would be damaged w/o same weight on other end? i could see getting pressure on the berings at the opposite end, but this would be no more than the pressure on the end with CBN wheel.

As long as each wheel is balanced, the different weight on the two sides of the grinder would have absolutely nothing to do with how the grinder will run. Do you ever worry about putting a counter weight on the outboard side of your head stock while you are turning on the lathe, even a heavy blank? There is no difference between the two situations.

 

[Gerald Lawrence]

As long as each wheel is balanced, the different weight on the two sides of the grinder would have absolutely nothing to do with how the grinder will run. Do you ever worry about putting a counter weight on the outboard side of your head stock while you are turning on the lathe, even a heavy blank? There is no difference between the two situations.

Thanks I had not thought of it like that .

 

[robo hippy]

The reason I asked Rikon about it was because I don't know. I do have a tendency to prefer more heavy duty tools because I use them in a heavy duty method. My thinking was similar to thinking about turning 12 inch bowls on a 1/2 hp motor and a 1 hp motor. There is a huge difference. It will be interesting to hear what they have to say. I would prefer to err on the side of caution. When I am relieving the heel on my gouges or scrapers (yes, I do it on scrapers as well) I am using more pressure. When grinding the bevels, I am much lighter. When grinding a burr onto my scrapers, I use medium pressure. I suppose in an ideal shop, I would have one set up for shaping and heavy metal removal, and the other for fine touch ups. I really wish Stuart Batty would get his Omni Grinder on the market.

As for having a heavy wheel on one side and a lighter one on the other side, the only difference I can see that making is if one or both of the wheels are unbalanced. This will wear the bearings.

robo hippy

 

[Mark Mandell]

Mine's Littler Than Yours

I've used an itty-bitty ol' 6" General grinder for at least 10 years for my turning tools. I have an 80 grit AlO wheel on one side for shaping and a D-way 180 CBN wheel for sharpening only. I stress "sharpening" as the CBN is unsuitable for shaping; I merely kiss the steel on it to bring back the edge, never with enough pressure to perceptively slow things down. In fact, even when shaping (or reshaping) a tool on the AlO stone side (or on my other bench grinder), I'm not so impatient as to put so much pressure on the wheel that it slows the motor down much at all. Were I to do that, I'd be over-heating the steel, clogging the stone with friction-melted metal, and thus be reducing the grind stone's effectiveness.

Just a note here that only M-2 and above alloys should be touched to CBN wheels (diamond wheels being for carbide) as softer alloys will smear and ruin the CBN wheel which can't be dressed or cleaned like "stone" wheels.

 

[James Cheever]

BTW, if you are considering getting CBN wheels, go to the D-Way website and view Dave's video on installing the 8 inch wheels on a half horsepower bench grinder. It would be a lot better than asking Woodcraft corporate office. They're great folks, but they are retailers, not machine designers.

Concur with Bill. Email Dave at D-Way.

I did so before I bought the Rikon grinder from WC. He told me that alot of his customers had this same grainder running his CBN wheels without an issue.

His info helped make the decision easy.

 

[Steve Worcester]

While the HP will make a difference in startup, can it spin the heavy wheels, I can say that even with "standard" wheels on my old Woodcraft slow speed, I could slow it down. With my Baldor and CBN wheels, ain't gonna' happen.

More likely how they rate HP as they clearly aren't the same.

 

[Bill Boehme]

While it might not surprise me if an imported motor was a little shy of stated power, the main issue here is start up acceleration. For applications that do not require fast start up acceleration such as bench grinders, manufacturing cost can be reduced by using lighter wire in the start winding along with a smaller start capacitor. The start winding is only connected in the circuit during start up acceleration and is then disconnected by the centrifugal switch. The start up circuit has no connection at all to the motor's power rating -- other than the obvious one that it ought to be appropriately sized for the application. Also, there is benefits to a slower start up acceleration -- less mechanical stress on the motor and less stress on the wiring.