Help needed with SGA

Gab-e

Gab-e

Joined
Sep 1, 2012
Messages
86
Hi everyone,

I recently bought a surface gringing attachment (OBM) to prep blades before ht (removing deep scratches in material) and to finish the flats post ht. Yesterday I took the grinder out and cleaned all of my AEB-L blanks on it before going back inside and realizing I had messed up :oops:. I used a 120 grit belt and light pressure, it wasn't throwing sparks and advanced the chuck only when the blade almost stopped touching the belt. The measured thickness from front to back is pretty decent but where it hurts is the variation on the other axis (blade height). For a handle +/- 1" high, I measured a high spot about +0.003" in the center. With the sides convexed there will definitely be an issue with handle scale gaps on the top and bottom.

In order to try and fix it, today I tested the following without any success:
-Swapped the contact wheel for another one I had and reground the mag chuck
-Tried operating at half speed instead of 100% (3500rpm motor with 6" drive wheel)
-Used different pressures, from lighter than yesterday to 'I gotta be careful, because it might grab the blade'
-Tried a 60 grit ceramic belt and a 220 grit structured one (much thicker)
-Adjusted the arm in order to get a bit more tension on the belt

Is there something I missed or am I asking too much from it ?

If you have any suggestions or tips, they're all welcomed and if the concensus is that nothing better can be achieved with it, is it realistic to believe that the blades could be salvaged by one of the pro ht'ers here that uses a full size surface grinder ?

Thanks,
Gab
 
Two things come to mind:
1: Have you tried a brand new belt for a finish pass? If your belt is worn more in the middle then it could be contributing to the problem.
2: Have you resurfaced your mag chuck with a fresh belt recently? If it's slightly out of flat it could be transferring that convexity over to your blade.
 
I resurfaced the chuck after changing the wheel and checked it with a ruler and 123 block; it looked pretty flat, no rocking when pressing from side to side.
But I have to admit, I'm cheap when it comes to belts and didn't change for a brand new one. I'll definitely try that tomorrow :thumbsup:
 
If I understand you correctly the blades are getting runout (or thinner) at the edges (start and finish of pass specifically)?

This is a common problem with SGA's and rubber wheels... I've had JTknives JTknives surface grind some blades for me and he uses a belt conversion on his surface grinder but he uses it with a steel wheel (no rubber) and it was perfectly flat with no runout.
 
razor-edge-knives said:
If I understand you correctly the blades are getting runout (or thinner) at the edges (start and finish of pass specifically)?

This is a common problem with SGA's and rubber wheels... I've had JTknives JTknives surface grind some blades for me and he uses a belt conversion on his surface grinder but he uses it with a steel wheel (no rubber) and it was perfectly flat with no runout.
Click to expand...

It is thinner at the edge+spine on the entire length of the blade. After searching and posting this here, I kept looking for an answer and found a post on another forum that mentionned the wheel hardness as being a possible cause...Had I known this, I would have saved a lot and sent my blade to someone with a proper SG instead :p
 
Ok a few things, a belt grinder conversion is not a surface grinder and honestly .003 over the width is really good out if a system like that. When surface grinding on a real surface grinder you CAN NOT use a rubber wheel. The wheel will flex and roll the edges over. The only saving grace a surface grinder with a belt conversion has is moves across the X and Y axis so any inconsistencies in belt thickness evens out as you do a few spark out passes. If your attachment could move in the x and y direction like a real surface grinder then you might have a shot at getting a more consistent thickness. I would also look for a solid aluminum wheel and ditch the rubber wheel. If you think about it the sides of the wheels will be softer then the center as the sides have room to expand outwards.
 
Gab-e said:
It is thinner at the edge+spine on the entire length of the blade. After searching and posting this here, I kept looking for an answer and found a post on another forum that mentionned the wheel hardness as being a possible cause...Had I known this, I would have saved a lot and sent my blade to someone with a proper SG instead :p
Click to expand...

we charge $5 a blade lol
 
JTknives said:
Ok a few things, a belt grinder conversion is not a surface grinder and honestly .003 over the width is really good out if a system like that. When surface grinding on a real surface grinder you CAN NOT use a rubber wheel. The wheel will flex and roll the edges over. The only saving grace a surface grinder with a belt conversion has is moves across the X and Y axis so any inconsistencies in belt thickness evens out as you do a few spark out passes. If your attachment could move in the x and y direction like a real surface grinder then you might have a shot at getting a more consistent thickness. I would also look for a solid aluminum wheel and ditch the rubber wheel. If you think about it the sides of the wheels will be softer then the center as the sides have room to expand outwards.
Click to expand...

So you would recommend an aluminum wheel? You don't experience wear with that? I am debating getting aluminum or steel for mine (when I get one lol) so any input is appreciated.
 
razor-edge-knives said:
So you would recommend an aluminum wheel? You don't experience wear with that? I am debating getting aluminum or steel for mine (when I get one lol) so any input is appreciated.
Click to expand...
steel is great and that's what I use but thy are heavy. I would think you would want to go with as small of a wheel as possible on a hand operated surface grinder. This would reduce surface area contact and torque required to drive it. would also be easier to operate by hand. I have never used one of these but that was what I was thinking.
 
JTknives said:
steel is great and that's what I use but thy are heavy. I would think you would want to go with as small of a wheel as possible on a hand operated surface grinder. This would reduce surface area contact and torque required to drive it. would also be easier to operate by hand. I have never used one of these but that was what I was thinking.
Click to expand...
Thanks, yeah I was thinking more for an SGA for my grinder (reeder makes a nice double arm SGA for more rigidity).
 
The Reeder runs best on a 5" contact wheel with a 1.5HP motor on the grinder. The horizontal movement is really smooth to operate. The double tool arm setup is best for rigidity.
 
you mentioned you recently got the attachment. did you flatten the table before using it? make sure the magnetic table is fully flat. get all the dust off the magnetic table. color all the metal strips faces ( that the knife usually sticks to) with a sharpie magic marker. then take a fresh 220 grit belt and only touch the table surface enough to remove the marker. you will need to move the belt tracking up and down to make sure you are getting the entire width of the table, the sharpie will tell you. keep an even speed all the way across the table. doing this flattening helps to be sure your wheel face and table are calibrated flat to each other. then i rub the edges of each metal piece a few times with a folded piece of 220 grit at a 45 degree angle , the remove any burr that might have formed. you dont want that folding over later and messing up your flatness.
 
Stacy E. Apelt - Bladesmith said:
The Reeder runs best on a 5" contact wheel with a 1.5HP motor on the grinder. The horizontal movement is really smooth to operate. The double tool arm setup is best for rigidity.
Click to expand...

Thanks Stacy! Have you measured at the very edge of a part (say .005" in from the edge) with a micrometer? Curious what the runout is compared to the center of the piece on a 90 duro wheel
 
john april said:
you mentioned you recently got the attachment. did you flatten the table before using it? make sure the magnetic table is fully flat. get all the dust off the magnetic table. color all the metal strips faces ( that the knife usually sticks to) with a sharpie magic marker. then take a fresh 220 grit belt and only touch the table surface enough to remove the marker. you will need to move the belt tracking up and down to make sure you are getting the entire width of the table, the sharpie will tell you. keep an even speed all the way across the table. doing this flattening helps to be sure your wheel face and table are calibrated flat to each other. then i rub the edges of each metal piece a few times with a folded piece of 220 grit at a 45 degree angle , the remove any burr that might have formed. you dont want that folding over later and messing up your flatness.
Click to expand...

Yep, did all of that, twice in fact since I changed the wheel at some point to see if it would help. The mag chuck/table is flat, when I check with a straight edge, there's no visible gap. But with a blade narrower than 2in, it seems as others have mentionned that the wheel deforms a bit...
 
I'll be honest. I don't see why folks get all tied up in knots over runout and such on a knife surface grinder. It isn't like we are making rocket parts. The grinding is to give a smooth and basically flat surface. You will finish it from there on the grinder and by hand. People trying to get +/- .001 and doing distal as well as bevel taper on a SG are just too picky.
I might flatten a blank on the SG, or clean up a forging, but the rest I do on the grinder and with stones and paper.
 
Stacy E. Apelt - Bladesmith said:
I'll be honest. I don't see why folks get all tied up in knots over runout and such on a knife surface grinder. It isn't like we are making rocket parts. The grinding is to give a smooth and basically flat surface. You will finish it from there on the grinder and by hand. People trying to get +/- .001 and doing distal as well as bevel taper on a SG are just too picky.
I might flatten a blank on the SG, or clean up a forging, but the rest I do on the grinder and with stones and paper.
Click to expand...
Yeah, I'll admit I'm anal about my knives haha, guilty as charged! I just want to minimize any extra time/steps as much as possible :)
 
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