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Sharpening jointer/planer knives on surface grinder

JohnMartin

Hot Rolled
Joined
Jul 8, 2006
Location
Cumberland, Maine
I've always sent woodworking jointer and planer knives out for sharpening. But, now with a surface grinder, I'm wondering if I can't do it myself. Couple of questions, though.

1. I'd be grinding without coolant. Knives are HSS, .125" thick. Can I get away with dry grinding (light passes, freshly dressed wheel) without burning the edges?

2. Any recommendations as to best wheel grade to use?

3. Better to hold the blade flat and use a wheel dressed at the proper angle, or make up a jig to hold the blade at an angle and use a square dressed wheel? 12" knives would be too long for a sine plate, so I was thinking about making up a jig with a long pivoted bar to which I would clamp the knife. Pivoted at the ends, or maybe resting on dowel pins as with a sine plate. Set screws as jacks to adjust angle, and top face milled back to support and locate the heel of the blade. Any other ideas?

Thanks, John
 
if you know the angles you want make a dedicated fixture and just clamp the blades to it.
also if you have room angle the blade so you dont have contact for the whole pass just a series of contacts this will relieve the tendency of the wheel to generate heat.

as far as wheels if i recall correctly you may want a I J or K wheel somewhere in the grit range of a 46 or so a vritified wheel is good for hardened steel.

if you can rig some system for using coolant you will get a better finish and not burn anything, small edges can collect heat very quickly and can become toast if you suffer a moments inattention...jim
 
Jim hit all the important points. I'll add that these knives reference from the bottom (nonsharpened) edge - all knives should end up at exactly the same dimension after sharpening. It's much quicker to set the knives to correct height if they are all the same.

The best sharpening services grind a primary rake angle & grind a secondary 'micro-bevel'. You don't have to do yours this way, but the edge is less delicate & much longer lasting. There's a good description of this in the Fine Woodworking book Power Saws and Planers
(ISBN: 0942391837), available online for under $10.

----------------------
Barry Milton
 
Most machine knives have some waviness or distortion. Their thickness is parallel but not straight or flat. Build a fixture that accepts them all and clamps them in position so they are forced flat. Then you can grind all knives at once.

Use a mist coolant at the very minimum. And grind gently. Don't push the wheel dress. When it stops cutting agressively (not long in HSS, only a few minutes grinding time) re-dress the wheel.
 
There's a fair amount in the archives on the subject. If you use search, and keywords "planer blade sharpening", among the hits is this one where a number of people put pictures and drawings of the way they do it.

http://www.practicalmachinist.com/ubb/ultimatebb.php/topic/1/15296.html#000004

Don't use a K wheel, too hard. But 46 I or J is ok. 60 will work, but hotter, risky with your set up. I don't think you will have much success without at least mist coolant. You won't really hurt the steel, but the knife will crawl around from the heat and be difficult if not impossible to grind straight.

If you have good coolant, you can grind straight down, a tenth or so at a each table reversa. If not, and safer in any case is to set the table travels for a reasonably fast rate, and the infeed about .005 or so. Take about .001 or .002 downfeed between passes, untill you get a sense of how it's going to work for you. if you do have coolant and a good fixture, it is possible to get a little more agressive. But as soon as the dress is gon n the wheel and the blade heats it will lift .005 in the middle, you'll have a hollow blade, and have to start again. Also, it is possible to heat a blade to where it does not returnd to straight on the back edge. Again, this is most likely to occur when you are grinding along smoothly, straight donw, withour coolant, and suddenly something (wheel slightly dulls, etc) happens and the knife heats and rises up into the grinder, which accelerates the runaway heat process.

Be patient!

smt
 
I have done planer blades and you can do it. Use a fixture to hold the blade at the angle you want. Make an adjustable stop on the fixture so you can project as much of the blade as needed to match the blade width up on all of them. Start with the narrowest and make them all the same width. As John Martin said, angle the fixture on the table to make the wheel contact short. A mist is good. If your table is a 6x12 you may have a problem.
 
My father was talking about doing this about a year ago when he bought a nice vintage Rockwell planar. He planned to take flat (machined flat) plate of steel and mill angled flats down the length. He could then drop the blade blanks in the flats and toss on the surface grinder. Since the angled flats are all the same depth, all the blades would be ground the same height which is important. He also claims that with a thin enough plate, the right depth of milled flat and orientation on the surface grinder, the blanks would be held tight just by the magnetic plate. No other hold down needed. In the end, he found a nice but inexpensive set of blades on Ebay so he hasn't gone this route yet. I imagine the next time he finds a old nail the hard way, he might go ahead spend the time making his jig.
 
I built a fixture about 2 years ago to sharpen the blades on my 4" jointer. I haven't regreted it once. Blades alway come out the same height and parallel. The 46 grit wheel is right for grit but I can't tell you hardness, don't have a wheel in front of me. I'll look tomorrow at work, to see what I have used. Mark
 
John, Used to work at a sharpening shop and the small knives that wouldn't fit on the knife grinder (less than 6") we would do on a tool grinder. We had a little fixture made up to set the cutting angle and the relief angles. We used a BCN cup wheel and ground them dry.. Worked really well on M2 steels. Used a diamond wheel on carbide and Tangtung knives. Dave
 
Several years ago I managed to fabricate a 48"
planer knife sharpener.
It would do (3)12" blades at one time. Longer than this I had to do them one at a time. This little machine had a stationary bar for attaching the knives. Behind this bar was another of the same size that held the cutter assembly.
The cutter assem. was attached to a 50" threaded rod coming from a offset gear reducer and motor.
The cutter motor was a 3600rpm while the rapids was controllable with a 1/4hp and on a varister which controled the feed rate.
I used a cup type stone tho flat rounds would also attach.
TIR in 12" was less than .001 with razor sharp edges.
Not bad for makeshift assem.
It is perhaps ugly as hell but it sure does a good job.

Patch
 
lca078, the blades will not be held down on the fixture by the magnetic table. For example if you lay two parallels on top of each other flat on a magnetic table and turn it on, the one on top can be lifted off easily or scooted off by the grinder wheel if you try to grind it. I takes a plate made with strips of steel and brass lamanated together to pass the magnetism to something on top. The lamanations will be standing verticle or perpendicular to the table. You can buy accesories for the surface grinder to do just that.
 
Carl, if the fixture is thin enough, the force may be adequate if he puts blocks the front of the blade, but you are right, it is darn risky business. On my quick & dirty fixture in the link (the one with the DoALL grinder) I was surprised that the fixture will hold a knife "adequately" without the setscrews on the electromagnetic chuck. Maybe because the knife is surrounded by the slot? But i don't risk it. One hot spot and that thing would be flying through the shield on the end of the table . :eek: But i do just about barely finger tighten them so they don't warp the knife or make a scalloped edge from the pressure points.

smt
 
I've tried something similar. What happens, I think, is the magnetism is absorbed by the metal below the groove in contact with the mag table. If you stand parallels on edge and push them together they can be ground and it would be the same with the blades as long as you had a block on each side to support the sides. but having them tilted would cause trouble even if they were on the table. I can tell you from experience that everything has to be on the mag table or clamped in a fixture or use the lamanated blocks. Even if it did not fly out, the vibration or chatter would get the blade and wheel. I think if your grooves are snug and the screws are finger tight on your fixture it would be ok but I would still use just a little more tightness. I have run surface grinders and Blanchards and when something comes off the table it gets real interesting FAST. We had a fixture to sharpen blades at an angle on the Blanchard and if the stones ever caught the blades it would throw that sucker all over and tear up the stones. I just hit the red button and backed away. Blanchards are well guarded, surface grinders are not and they are VERY dangerous. Man, that stuff goes everywhere and fast. on surface grinders I just duck and hug the base of the machine and let 'er rip. We had some corner supports that contained a flat plate and the plate rested on a post at each corner for the Blanchard. When you grind the plate you can hear the chatter and the plate gets hot and warps if not carefull.
 
"lca078, the blades will not be held down on the fixture by the magnetic table"

If my dad ever gets around to making his blade jig, I'm sure he'll play with it and ensure it's a tight set-up before turning on the grinder. He's done a lot of wacky stuff as he spent over 30yrs as an instrument maker for the AF. I have to admit, most of the things he's told me that I doubted, he's proved to me the next day in the shop.
 
Well heres my .02 cents .I took two pieces of tool steel apox. 3/8" thick x 1 1/2" widex12" long and milled the angle of the planner blade on each.I simply used these two pieces as blocking ,laying all three planner blades against the first blocking plate and finishing with the second blocking plate. so actually thet are only held down by their corners and the blocking keeps them in position.IT would not hurt to block the ends also.I have done this on 6"-24" planner blades on a Blanchard and never blew one off the mag. I have also done on it with my 12" blades on my Sanford surface grinder . I would skew the whole set up on the mag to help reduce heat. chris
 
Diamond and CBN wheels are just the thing, however they are expensiv. ASD 75% concentration is the most economical diamond type for all pupose dry grinding. Although CBN is recomended for hard tool steel (bond type BB for dry grinding). These wheels cost about $100 - $250 or more depending on size and shape, etc. On the other hand the wheel jim spoke of is about $45.

Whatever you decide be sure and have a good rigid fixture. Check the archives. Grinding wheels spin very fast.
 
I have 3 fixtures, a stationary 6 inch. a fully adjustable 7 inch (0-90 Deg) and a adjustable 15 inch.
The 15 is made using the method of your post.
I put Dowel pins (3) in the bottom grove and make a record of the pins for the blades being sharpened. these run from .750 to 1.000 usually.
On the big one, the clamps have a Set screw at the back and a fulcrum screw in the middle. I set the 15 at a angle on a 6 X 18 grinder to do full edge at same setup.
If you gring without coolant, I would suggest a copper sheet under the blade to remove heat quick.
a small spray bottle may help to.
Rich
 








 
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