# Slowing down a 8" Delta Grinder



## ibpdew (Nov 18, 2012)

I have a 8" Delta 3550 RPM bench grinder that I would like to slow down to around 1,750 RPM for sharpening tools. Has anyone found/used a resonably cost way to use something like a reostat to do this? My other option would be to purchase a 8" Porter Cable grinder on sale now at Lowes with the VS all ready built in. I think it was only $100 on sale. thx


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## Fred Hargis (Apr 28, 2012)

It would be very tough to do, so much so I would consider changing grinders. They have universal motors, and about the only to change the speed with be with a frequency controller (VFD) and they only work on certain single phase motors. Woodcraft also has a slow speed 8" grinder for around $140, it comes with the white wheels if those are missing on the PC. They used to go on sale every once in a while, it may pay to wait.


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## SteveEl (Sep 7, 2010)

I have no idea what I'm talking about, but I noticed this on the HF webpage and recalled you post. It claims to "Works with any universal AC/DC brush type motor, 15 amps and under"


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## woodnthings (Jan 24, 2009)

*Can't get there from here*

That's an Induction motor unlike a brush type Universal which CAN be speed controlled. Just get the 1725 RPM if you can. Another solution for great sharpening results is a vertical stationary belt grinder. That's what I use. It's easy to change the belts/grits to suit your sharpening needs..... 60, 100, 120, 220...etc. 
The greater length of the belt allows for a cooler grinding process.


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## cabinetman (Jul 5, 2007)

I was thinking to get a bench mount arbor mandrel like this, and put on a large pulley as a drive pulley. Use your bench grinder and put a small pulley as a drive pulley. Then you have your grinder driving two mandrels. 








 








.


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## Maylar (Sep 3, 2013)

cabinetman said:


> I was thinking to get a bench mount arbor mandrel like this, and put on a large pulley as a drive pulley. Use your bench grinder and put a small pulley as a drive pulley. Then you have your grinder driving two mandrels.


My grinder is like that. I bought a mandrel and mounted it between pillow block bearings. The motor hangs below and the belt is tensioned by the motor's weight on a hinged platform. Pulleys chosen as a compromise between grinding and buffing. Has worked for me for over 25 years.


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## FrankC (Aug 24, 2012)

If you can get one for $100 go for it, there is no easy way to vary the speed on your Delta, and any jury-rigging would probably cost that much in the end anyway.


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## Fred Hargis (Apr 28, 2012)

Fred Hargis said:


> It would be very tough to do, so much so I would consider changing grinders. They have universal motors, and about the only to change the speed with be with a frequency controller (VFD) and they only work on certain single phase motors. Woodcraft also has a slow speed 8" grinder for around $140, it comes with the white wheels if those are missing on the PC. They used to go on sale every once in a while, it may pay to wait.



Sorry for the brain fart I had when I typed the above, I should have said they have induction motors (no brushes).


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## duncsuss (Aug 21, 2009)

ibpdew said:


> I have a 8" Delta 3550 RPM bench grinder that I would like to slow down to around 1,750 RPM for sharpening tools.


Just curious ... do you really need to slow it down?

From what I read, there's no danger to High Speed Steel tools even if your grinder is running fast (especially so if you've got the white wheels fitted). You just have to use a light touch to avoid grinding the entire tool away when all you meant to do was sharpen it.


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## john lucas (Sep 18, 2007)

Ditto what was just said. If your using High Speed steel tools you can't damage them by overheating using the high speed grinder. In fact in the spec sheets the proper surface feet per minute that they recommend for sharpening HSS is very close to what you get with an 8" wheel at 3450. You do need to learn to have a light touch for better sharpening but that's a good thing.


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## Bill Boehme (Feb 9, 2014)

Fred Hargis said:


> It would be very tough to do, so much so I would consider changing grinders. They have universal motors, and about the only to change the speed with be with a frequency controller (VFD) and they only work on certain single phase motors. Woodcraft also has a slow speed 8" grinder for around $140, it comes with the white wheels if those are missing on the PC. They used to go on sale every once in a while, it may pay to wait.


They're AC Induction motors (not universal which can run on AC or DC). Universal motors have brushes that provide the commutation to determine rotation direction.

The speed of a universal motor is governed by the applied voltage. Universal motors are used in small hand tools like drills, sanders, shop vac, and routers. The electronics to control the speed is very simple -- basically a thyristor that chops the AC input power on and off for varying degrees of the cycle to provide the equivalent of varying the input voltage. The downside is that the mechanical output power takes a really big hit (power is proportional to square of the voltage) that have both a wound field and rotor. Universal motors are also noisy.

The speed of AC Induction motors, on the other hand, are controlled by the frequency of the AC power which is 60 Hz here in the US. Reducing the voltage to an AC Induction motor would only cause it to overheat because the current would increase to provide the torque needed to minimize slip frequency. Speed controllers, called variable frequency drives, for AC Induction motors are complex and expensive. They also require the use of a three phase AC Induction (single phase won't work). The way that they work is to take the line voltage and convert it to DC and then use a microprocessor controlled circuit to generate a pulse width modulated output that simulates the effect of three phase AC power to the motor at the desired frequency. The output power actually bears no resemblace to three phase sinusoidal AC, but as far as the motor is concerned it produces the same effect. There are some drawbacks to using variable frequency drives that necessitate the use of three phase motors that are rated for inverter duty -- any old three phase motor won't do. Motors rated for inverter duty have heavier duty insulation to handle the high voltage transients created by the pulse width modulation process and also they have provision to operate safely at higher temperatures because of the reduced cooling when running slow.


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## Fred Hargis (Apr 28, 2012)

Bill, I know you're an EE and know far more about this stuff than I. But it seems to me that when I had a day job we had a few applications where we used a single phase on a VFD; Though I can't remember the details. I do remember that they were special motors made for the purpose (or had some attribute that allows the use of the VFD). Do I remember this incorrectly? (BTW, The "universal" part was me typing thew wrong word, this aging thing really sucks).


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## john lucas (Sep 18, 2007)

Fred I'm not Bill but a Variable Frequency Drive changes single phase to 3 phase so it will run a 3 phase motor and can also give it variable speed.


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## Fred Hargis (Apr 28, 2012)

Here you go, guys: single phase input and single phase output. Note that it's for use with a permanent split capacitor or shaded pole single phase motor. I'm less sure what a shaded pole motor is but do remember we had the permanent split capacitor types in our applications.


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## Bill Boehme (Feb 9, 2014)

Fred Hargis said:


> Bill, I know you're an EE and know far more about this stuff than I. But it seems to me that when I had a day job we had a few applications where we used a single phase on a VFD; Though I can't remember the details. I do remember that they were special motors made for the purpose (or had some attribute that allows the use of the VFD). Do I remember this incorrectly? (BTW, The "universal" part was me typing thew wrong word, this aging thing really sucks).


Yes, I was aware of that, but for the type of use that concerns woodturners (driving a lathe) I didn't want to muddy the water with a bunch of extraneous stuff. The single phase motor/VFD combination are mainly for specialized applications such as air handlers where starting torque is essentially zero and load torque is constant. In general purpose use, three phase motors are less costly than single phase and there is no good way to implement a capacitor start motor in a variable speed application that requires high starting torque.


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## BassBlaster (Nov 21, 2010)

Just an FYI, the Rikon slow speed grinder that was posted from WC frequently goes on sale for 90 bucks.:thumbsup:


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## Bill Boehme (Feb 9, 2014)

Not that it matters much, but the Rikon grinder horsepower rating is overstated a bit. In actuality, the real mechanical output power is more like1/3 to 2/5 horsepower rather than 1/2 as stated in the advertising. The electrical input is stated to be 115 VAC at 3A which is 345 watts. However, a motor's horsepower rating is determined by the mechanical output power which is the speed times torque measured by a dynamometer. A high quality single phase capacitor-start induction motor has an efficiency in the range of 75 to 80 percent. Even if the motor had an efficiency of 100% (which is physically impossible) it still would not be possible for it to output 1/2 horsepower mechanical power given the electrical input power which is less that a half horse. Assuming the best case scenario of 80% efficiency which is being very generous for a low cost motor, mechanical output power comes to about 0.37 horsepower. Most likely the real output power is closer to 1/3 horsepower. How important is this? Not a big deal, but I would like to see truthful advertising. A 1/4 horsepower motor would actually be adequate for sharpening turning tools. Now if you were sharpening turning tools the way that Doug Thompson does then mechanical output power would matter -- a lot.


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## BassBlaster (Nov 21, 2010)

I have the WC slow speed grinder. I dunno if its the same as the Rikon one or not but I know the Rikon replaced the WC version. I assume they are one in the same but I really don't know. If they are the same, it is more than adequate for lathe tool sharpening. I dunno that it would be good for general grinding or not but that's not its intended purpose.

That said, I agree, if its really 1/3 HP, then say so.:thumbsup:


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## firehawkmph (Apr 26, 2008)

So I guess I shouldn't believe my Sears shop vac is 6 1/2 horsepower? Damn!
Mike Hawkins


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## Bill Boehme (Feb 9, 2014)

BassBlaster said:


> I have the WC slow speed grinder. I dunno if its the same as the Rikon one or not but I know the Rikon replaced the WC version. I assume they are one in the same ....


About a year ago they started having quality problems with the supplier. When the supplier couldn't get their act together, they ditched them and changed ro Rikon.



firehawkmph said:


> So I guess I shouldn't believe my Sears shop vac is 6 1/2 horsepower? Damn!
> Mike Hawkins


It is amazing that they can put that much horsepower in a motor no bigger than a beer can -- and draw the same electrical power as a 1/8 HP motor.


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## ibpdew (Nov 18, 2012)

My thoughts would be to go with Ricon from WC. Do you need to keep checking site for sale price or do they have some type of notification system, ie email? thx


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## Dave Paine (May 30, 2012)

ibpdew said:


> My thoughts would be to go with Ricon from WC. Do you need to keep checking site for sale price or do they have some type of notification system, ie email? thx


If you purchase a few times from a local Woodcraft store, you will get the flyer in the mail.

If you go to the Woodcraft site, you see the latest flyer and can sign up to get the flyer in the mail.

http://www.woodcraft.com/Tab/001.aspx


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## ibpdew (Nov 18, 2012)

Got it. Thx Dave


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