Dedicated Blade Sharpener

BobInMN · May 29, 2004

I've been looking at some of the mower blade sharpeners on the market like the Magna Matic. It looks like they do a great job, but as I told you once before I'm cheap, I can't justify spending $500 - $1000 for one. I use my bench grinder now. I don't have a steady enough hand to do the job I want. I've been thinking of a couple of alternatives. One is to buy a cheap bench grinder and adapting it by removing the guards and grinding from the rear. This way I could build a flat work table below the wheel to slide the blade back and forth. The other thought I had was to buy a cheap 4" hand grinder and mounting it to a stand and building a sliding table under it to slide the blade on. Kind of like the concept when you use a valve grinder. Just nipping at the blade a little at a time to avoid building up heat and taking the temper out of the blade. Any thoughts on this? I know the RPMs on the bench grinder is something like 3300 RPMs, I'm not sure of the RPMs on the 4" handheld. Do you think it might be to fast?

ZJ_HR · May 29, 2004

Handheld 4" grinders have 9-12000 rpm. It is fast, but not a problem for sharpening, if you don't press grind disk to the blade, i.e. make very gentle "cut" with it.
This is a way how we sharpen our knives on sickle bar mower and teeth on circular saw (handheld). /forums/images/graemlins/wink.gif

Mrwurm · May 29, 2004

I searched far and wide and found a 4in hand grinder with variable speed. I believe it runs anywhere from 2000 rpm to 10,000. I bought it just for sharpening mower blades. Price was around $150. I can get make and model if requested.

Jim_Fisher · May 29, 2004

Bob,

( I can't justify spending $500 - $1000 for one. )

FYI, I'm seeing Oregon's on eBay for less than $300.00. Still pretty salty, though.

Lawnmower Blade Grinder

I like your idea of making a sliding table for a bench grinder, though. I've used a pair of vice grips as a guide to help me maintain a consistenty angle but it's still a tedious job.

( I know the RPMs on the bench grinder is something like 3300 RPMs, I'm not sure of the RPMs on the 4" handheld. Do you think it might be to fast? )

Not sure that the difference in wheel diameter between the two wouldn't at least partially compensate for the difference in motor speed. I sometimes use a pneumatic die grinder (high speed) with either a carbide burr or a small stone on mower blades. I haven't noticed any more problems with overheating than when I sharpen them on a bench grinder.

Jim Fisher

beenthere · May 29, 2004

That (""flat work table below the wheel to slide the blade back and forth"") is how I grind my mower blades, with a table below the wheel. The blade lays flat and the angle stays the same. Works great. I will try to get a pic posted. Need to use it soon and will take some shots. Just a grinder wheel on the shaft of a 1750 rpm motor.

BobInMN · May 30, 2004

Thanks for the input guys. One other thing I was wondering about, does it make any difference which way the grinder is spinning? I mean does it make any difference if you grind from the cutting edge of the blade back or from the back of the blade to the cutting edge?

rswyan · May 30, 2004

( One other thing I was wondering about, does it make any difference which way the grinder is spinning? I mean does it make any difference if you grind from the cutting edge of the blade back or from the back of the blade to the cutting edge?
)

It may - I think if you grind from the back to the blade edge it will probably create a burr on the bottom of the blade. I always file from the edge to the back of the blade.

If you don't want to go $500 but are willing to go about half that (for all the trimmings) you could pick yourself up a Delta 23-710 Sharpening Center:

http://www.deltawoodworking.com/index.asp?e=136&p=843

This is combo wet/dry grinder with two stones. The $280 price (at amazon.com, including free shipping) will get you the Sharpening Center, the Knife Grinding Attachment (does planer blades up to 15 inches), and heavy duty stand. I ordered all three items and they shipped separately - so far only the stand has come in. I assembled it tonight - very nice and pretty heavy-duty. The only downside on this setup that I could see was that 8" wet stone that it ships with is 1000 grit (I'll probably pick up something coarser for doing mower blades) - that and I'll probably have to fab up a little bit of a jig to properly hold a mower blade.

FWIW, the reason I'm going this route is that I have been hand filing my blades - it's a great way to put an edge on a blade (ya could shave with them) - but it requires a good deal of time and alot of effort (especially if the blades have nicks or chips - which mine often do) and I've finally gotten tired of filing.

DavidB · May 30, 2004

A cheap six or eight inch grinding wheel on a bench grinder should work just fine for sharpening mower blades. It's the only thing that many mower shops use...

You should have a coarse wheel and it should be sharp. Sharp you say?

After using a grinding wheel for a while the sharp bits of the abrasive become worn and the wheel is dull. It also gets worn unevenly and out of balance. You should have a diamond dresser (the bit in my "custom" wood block) and a dressing wheel. The diamond dresser (I bought mine used for a couple of bucks at a surplus store 20 years ago) is run back and forth across the face of the wheel until it is uniform and the wheel is back in balance. The wooden block holds the tool a uniform distance from the wheel while resting on the tool rest. Then the dressing wheel is run across the face to tear out grit and expose new sharp cutting surfaces.

With some practice you should be able to put a perfectly sharp edge on your mower blades in a few minutes. Don't ever take any material off the bottom of the blade. Only sharpen from the top or the blade will not cut well. You should also have a blade balancing tool to balance the blades after sharpening. They may also need straightening. I put them in the vise and bend them straight again with an 18" crescent wrench. The cutting surfaces should lay flat from one end to the other on a straight edge after you are done.

Here are some tips on bench grinder safety:
http://www.elmacovijay.com/safetyrules.htm

Try asking your mower shop if you can watch them sharpen one of your blades for you. That may be enough for you to figure out how to do it for your self. A special grinder isn't necessary...

Good luck! /forums/images/graemlins/smile.gif

have_blue · May 30, 2004

( They may also need straightening. I put them in the vise and bend them straight again with an 18" crescent wrench. )

The blades I buy are crooked as a rainbow, brand new. Well, that's exaggerating, because they just have a nice little curve to them so they'll be straighter at running speed. I was delighted to see this attention to detail.

I used to sharpen my blades on the mower because they were too hard to remove. Then I built a special pallet fork to lift the mower up high to cleanly get at the blades for removal. That's the best 2 hours I have invested in a long time.

PineRidge · May 31, 2004

I used to sharpen my blades on the mower because they were too hard to remove. Then I built a special pallet fork to lift the mower up high to cleanly get at the blades for removal. That's the best 2 hours I have invested in a long time.



Have_Blue with my last tractor it was necessary to use the boom pole to lift my RFM deck high enough to get under it to service the blades. With my TC-40D I let the 3 point hitch do the lifting. Then with a air impact tool I can have all 3 blades off the mower spindles in a matter of minutes. Straightening, sharpening, and balancing takes much more time. I keep an extra set sharp and hanging on the wall for a quick change over.