Burning while dry grinding planer knives
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I recently bought a dedicated planer knife sharpening machine from grizzly that looked substantial, and was priced right. It can handle up to 20″ blades (which i dream about having to sharpen) But the thing came with a rather coarse 120 grit “friable grit” stone (white oxide?). They sold finer stones that fit it, so i decided i wanted a nicer edge so i got the 320 grit stone. Well it seems to really burn my chisels while sharpening, something my pink stones on my grinder don’t do. My question is does a finer stone generate more heat than a coarse, and is that maybe why they sell the machine with a coarse stone? The other aspect is that it turns at 3250 rpms, i would think 1750 would be better for dry grinding. Would you think i could a) wet the surface anyways with a spray bottle, or b) slow the motor somehow? (its a direct drive induction motor.) c) just stick with a coarse stone, and finish up with a hone d) return the sharpener as unfit to sharpen planer knives?
Replies
Have you tried using the coarse stone for general sharpening/shaping the bevel, and the finer stone with a very light touch to remove the scratches left by the coarse stone? Or is it too much trouble to change stones? Any dry stone will burn or overheat the thin edges of steel with enough pressure and time, esp at that spd. 3450 does seem awfully fast. Maybe it's just an adjustment on the machine or knife holding jig to take off less steel with each pass. Also, FWIW, the grit numbers on wheel stones aren't equivalent to grit numbers for sandpaper or benchtop sharpening stones, because of the RPMs.
Don't know if any of this helps - just an opinion. Best of luck,
First, yes a finer grit will generate moe heat, a lot more. Second, I would trust that the engineers at Grizzly know what they are doing from testing, and have provided a system that will work just fine. Take light cuts, .005 or less, and move the grinder fairly fast, about the speed you would move a handsaw. When you get to your final depth of grind then keep moving the grinder, (or knife) at this setting untill no or very few sparks are coming off. This should leave an edge that is ready to hone. It only takes a few swipes with a honing stone to remove the burr. At our shop we only hone the flat side of the knife, not the bevel, and they cut so smooth that a board will stick to the jointer table from suction like gauge blocks. Keep practicing and you will learn the process very quickly. If you can get some old worn out knives to practice on it will save some frustration.
Rich
The Professional Termite
I have used the Grizzly sharpener. This is not a "one pass" sharpening job. The blade should barely kiss the stone as you slide the blade carrier quickly from right to left and return. If the leading tip of the blade begins to become rounded then back off. You are taking too deep a cut. If the steel burns, you are sliding the knife carrier too slowly.
Before you begin, turn the motor turned off and check to see that the blade barely touches the stone when you slide the blade carrier. Now back off a bit and turn the motor on and and make a test slide to see if wheel is grinding. Adjust with very small increments until it begins to grind. Do not let the blade stop on the wheel. Patience is the major virtue. When you have ground the knife suficiently, continue sliding the kinfe carrier back and fourth until grinding wheel barely touches the knife and you will find a remarkable smoothly ground surface that does not need a finer stone.
If my instructions are too officious, I apologize. It is because I wished that I had the instruction when I first used the sharpener. My planer knives lost a lot more steel than necessary while I learned.
BJ
thanks for taking the time to post your experience. This should prove to be helpful. I'll go try it now.
KP
You are right. 1750 RPM or thereabouts is a sharpening speed. 3250 RPM is a grinding speed. I am not for sure if there isa third party variable speed device that will work well with your motor to slow it down. Someone with more knowledge or experience would need to advise you. If you keep the grinder, the above threads should work for you for sharpening. This is how I do mine too, except that I dip the blades in water after each pass to cool them to help keep the hardness. I have been in communication with Baldador Motors to ask them to make such a variable speed sharpener/grinder, though no word yet that they will. Sounds exciting. Happy woodworking!
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