Anyone burn up their grinder with CBN wheels?

[Marty Tippin]

The Rikon slow-speed grinder is on sale this month at woodcraft for $99. Sounds like it's a sale made just for you!

[David Delo]

The Rikon slow-speed grinder is on sale this month at woodcraft for $99. Sounds like it's a sale made just for you!

Already ordered Marty but thanks for the heads up. Since putting the CBN wheels on about a month ago I had already been scouting around for a second grinder to use for all other materials. Got 2 on the way so temporary inconvenience for permanent improvement.

[Bill Boehme]

Here is a list of potential causes of the problem. Lots of things can cause it to stop running, but it rarely means that the motor is completely dead.

• If the motor doesn't do anything when the switch is turned on, does this include no faint humming sound either? If you have very good hearing and are in a very quiet location, turn the switch on and listen for a hum. If you do or think that you might if your hearing is like mine then grab one of the wheels and give it a very quick forceful spin. Does it continue to turn and slowly build up speed? If so, the problem might be the start capacitor or the centrifugal switch assembly or the start winding.
• Start capacitors are off the shelf items. I've never heard of one not being available. Never buy one from the tool manufacturer or else you will pay triple for it.
• The centrifugal switch is located at the rear of the motor. It consists of a couple springs, a couple weights, linkage, and electrical contacts. It is a simple mechanism and should be easy to tell if something isn't quite right.
• If it's the start winding then the cost of repair is more than the cost of a new motor.
• This grinder motor includes some electronics for variable speed control by implementing high-speed switching between two and four pole modes. There is a high probability that the electronics have failed. If Delta no longer supports this grinder with replacement electronics or if the cost is too outrageous then a new grinder might be the best option.
• Can the higher moment of inertia of CBN wheels be responsible for the problem? Definitely YES if the motor is on the wimpy side and low efficiency (a number of low cost imports fall into this category).
• How does the heavier wheels matter? The instant that a motor is turned on, it draws LRC (locked rotor current) which may be up to ten times FLC (full load current). As the motor accelerates, the current drops until it reaches its steady state speed at which time the current would be the no-load value which is typically 20 to 25% of FLC. Generally the acceleration time is a fraction of a second and the motor windings are able to handle this brief over current condition. If it takes several seconds to come up to speed then the windings may be slowly deteriorating because of excessive heating until one day the motor decides not to run.

[David C. Roseman]

[snip]

• Can the higher moment of inertia of CBN wheels be responsible for the problem? Definitely YES if the motor is on the wimpy side and low efficiency (a number of low cost imports fall into this category).
• How does the heavier wheels matter? The instant that a motor is turned on, it draws LRC (locked rotor current) which may be up to ten times FLC (full load current). As the motor accelerates, the current drops until it reaches its steady state speed at which time the current would be the no-load value which is typically 20 to 25% of FLC. Generally the acceleration time is a fraction of a second and the motor windings are able to handle this brief over current condition. If it takes several seconds to come up to speed then the windings may be slowly deteriorating because of excessive heating until one day the motor decides not to run.

Interesting, Bill. Thanks for posting this.

David D., can you estimate about how many times you'd started the grinder, from the time you installed the heavier wheels until it failed? I was assuming it was very few, since you say in post #1 that you added them recently. I'm curious about that for the future. I've had a heavy 8" D-Way CBN wheel on a Woodcraft slow-speed grinder for a long while with no problems. Also, we did a group-buy of a bunch of the 8" HTC Hurricane CBN wheels a few months back in one of our local clubs, and zero problems of any kind have been reported so far on any of the various grinders. Both the D-Way and the HTC weigh in at eight pounds each, so moment of inertia at start-up is indeed much greater than with the AO wheels.

David

[David Delo]

Interesting, Bill. Thanks for posting this. David D., can you estimate about how many times you'd started the grinder, from the time you installed the heavier wheels until it failed? I was assuming it was very few, since you say in post #1 that you added them recently. I'm curious about that for the future. I've had a heavy 8" D-Way CBN wheel on a Woodcraft slow-speed grinder for a long while with no problems. Also, we did a group-buy of a bunch of the 8" HTC Hurricane CBN wheels a few months back in one of our local clubs, and zero problems of any kind have been reported so far on any of the various grinders. Both the D-Way and the HTC weigh in at eight pounds each, so moment of inertia at start-up is indeed much greater than with the AO wheels.David

Yes Bill, that's some interesting information,Dave,I don't really know how many starts but probably less than 100 on the Delta GR-450. Took delivery this morning of a new Delta 23-197 and a new Rikon 80-805. Put the CBN on the Delta for now because if fit just a little bit better on the platform I had for the old grinder. The Rikon was going to take a little bit more fab time and I'm far enough behind as it is. This new Delta has a tad shorter shaft so I needed to leave off the inside alignment washer to get enough thread on the arbor nut. I'm using the wheels from Ken at Wood Turners Wonders. New Delta moans pretty good at start up and seems to take a bit more time to get to full speed but it does seem to run quieter at full speed. I think I start doing like Scott H. does and give the wheel a pull before turning the switch.