Disk grinder

Ty Adams

For those of you with disk grinders. What are you using? I'm looking at the 1 horse 9" from tru-grit. Thanks
I just checked that one out. It's says it's 3Ph 220 volt. Do you have three phase service in your shop? I'd like to get a disc grinder but the oven is at the top of my list right now.
Ok, thanks. I haven't looked into phase converters in quite a while. So the vfd on the grinder converts it to single phase 110 volts?
Ty, I've been waiting for somebody with real disk grinder experience to chime in, but since they've not yet I'll toss in my 2 cents.

First to Opaul, - yes, most all the variable speed grinders using a VFD are ran on 1ph, but using a VFD to convert to 3 ph for the motor. The 1hp will run just fine on 120vac, while for 2hp you really need 240vac because of the current demands of a 2hp motor at full load on a 120vac breaker which is "typically" 20 amp.

The 1hp motor should plenty of power for a disk grinder. It's my understanding they are not used for "Hogging" metal, but for finer grits and finish. I've used my a tad for putting final finish on bevels and it works really good for that. Using 220 grit I'm usually around 1,000 rpm. The 120vac grinder you are speaking of - this one? https://goo.gl/kjKfA4

I'd be happier if it were on a 1750 rpm motor rather than 3450 rpm motor, but either will work just fine. A VFD speed control is VERY important for a disk grinder. I made one using a 1725 rpm 1ph motor, and tried to control speed with a foot switch, but that didn't work so good.

That's the one thanks Ken. Somewhere I read that you can double the speed of the motor with a vfd? Flatting out scales and the flats on blades is the reason I am looking at one.
Technically a VFD can output just about any frequency but the motor will have limits. Too slow and the fan will not cool the motor. Too fast and the bearings will fail prematurely.
I have the 9 inch disc 1* bevel from KMG mounted on a cheapie 3ph Ironhorse motor. I run it off the the same VFD as my belt grinder. A VFD on a disc grinder is extremely useful, not only for speed control but also for the ability to reverse direction. A disc builds up a lot of heat so running it slowly helps a lot.

I'm hoping Calvin Robinson will chime in. I believe he only uses a disc, start to finish.
I have pretty much the same setup as John, but I have the tru-grit base, 2hp IronHorse motor running off of the same vfd as my grinder. I made some dryer cord outlet coming off of my vfd and plug whatever grinder I want to use in. Also my kbac vfd is only giving me 1.5hp of the 2hp motor. That is the limitation of 110v versus 220v
I watched a video from Nick Wheeler the way he finishes his Bowies is on a disc grinder. From what I understand the Harbor Freight Disc sander is not a bad unit? You can get single phase VFD and make that variable for a lot less money than the Tru-Grit one! I'm sure the TG one will smoke the HF grinder though.
From what I understand the Harbor Freight Disc sander is not a bad unit? You can get single phase VFD and make that variable

I don't understand how you're going to make the HF unit variable speed? Isn't the HF unit a 120 vac single phase motor? Do you have a link to the HF disk grinder? The only disk grinders I'm aware of are the belt/disk grinder combos - such as the 6"X48" belt with 9" disk I've got.

I'm not aware of a "single phase VFD". You might be thinking of the normal VFD used that does take 1ph and converts to 3 phase in order to make a 3 ph motor variable speed?
Ok, thanks. I haven't looked into phase converters in quite a while. So the vfd on the grinder converts it to single phase 110 volts?
a phase converter/VFD changes single phase 110 to 3 phase 240. I have 8" and 6" discs that mount on VFD controlled motors. 8" and 6" are widely used commercially so there are many PSA(self adhesive) discs and disc heads to choose from. if you already have a VFD to supply 3 phase, just get a footed motor and a disc head. I mounted mine on the side of the bench. you can leave it at that or spend more for adjustable work surface. my porter-cable 4x36 has an 8" disc, but the speed is way to high for steel.
before buying a big disc sander, over 9", you need to look at cost per disc. with 8" or 9", you can cut your own disc from sandpaper sheets and apply your own glue.
I will amend my previous post. Yes they do make Single phase\Single phase VFD's. However it will not work with the HF disc grinder because it is a Capacitor start motor according to their operators manual on their website.
OK, I see they do make 1 ph variable speed drives - just wonder how they work. I never did find that - "Though they lack the higher efficiencies of their three-phase siblings, single-phase motors – correctly sized and rated – can last a lifetime with little maintenance”"

I see on the same linked page they say sanders are NOT suitable for 1ph drives. It seems only things like fans (or other low torque starts) are suitable.

Here is a 1hp 220vac 1ph drive with either 1ph or 3 ph output. https://goo.gl/m5XnTs This is only an open frame which would need some type of enclosure and still priced at $300! Here's a full price list, and it looks like a 1/2 hp is max for 115vac 1 phase..... and it list price of $488 for the open frame. Installed into a 4X enclosure it's $888!

From the manual: "Many 3-phase inverter manufacturers claim that they can run single-phase motors effectively. This is normally accomplished by wiring only 2 phases; however, this method may cause instabilities due to the lack of feedback from one of the motor connections. Furthermore, motor torque will be reduced considerably because the phases are 120° apart. Although the FHP uses this method of connection, its fundamental design enables it to operate efficiently under these conditions. "

From the manual they clearly state certain 1ph motors can be used:
"In addition to standard 3-phase induction motors, the following motor types may be used with FHP Series drives: • Permanent split capacitor (PSC) • Shaded pole • AC synchronous"

I have learned something today. That means it's a good day!

Ken H>
Depending on what you want you could build a disc sander with a VFD pretty easy. Thier is a gentleman that goes by Nathan the Machinist that makes 9" discs for motors. 1-11/2 HP motor a VFD and his plate. I watched this off of You-Tube this is the setup that Nick Wheeler uses. I'll see if I can find a link to his site.
If you are getting a disc grinder set up, look into 3M feathering adhesive or other medium tack adhesive and trimming 9"x11" sheets of abrasive instead of using PSA discs. Way cheaper to switch grits fast and easy. I use a 1/16th rubber backing between the disc and abrasive. It doesn't compress in use unless you really lay into it to get some stubborn scratches out. A raw metal disc with small grit abrasive (cut 9"x11" sheets) can be frustrating as any particles between the abrasive and disc telegraphs into the piece you are sanding. The rubber "liner" takes most of that out.