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Help with mounting grinding discs on Jung HF40 surface grinder spindle

Batavia

Plastic
Joined
Apr 17, 2024
Location
Leiden, The Netherlands
At our Makerspace we have a surface grinder (a Jung HF40 I believe), and a couple of aluminium oxide and carbide grinding wheels for it. I have only used it a couple of times since I joined the Makerspace, and it gave wildly varying results each time; very wavy the 1st time, perfect mirror the 2nd, nice and flat but no proper mirror shine the 3rd.
Unfortunately no one at the Makerspace seems to have a lot of experience with surface grinders, so their guess is as good as mine.

I know surface grinders have a disc with 3 balancing weights that can be fixed in place through a screw or hex key'ed bolt and ours has it as well, however I was of the understanding that said disc is normally supposed to be fixed to the grinding wheel in such a way that you can slide the whole assembly over an arbor and put it on a balancing setup to balance the whole thing with those 3 weights, and after that mount the whole assembly back on the surface grinder's spindle. However that doesn't seem to be the case with our surface grinder, the disc with those weights can't be fixed to the grinding wheel in any way, it's just slid over the spindle after sliding the grinding wheel over it first and then locked in place with a large nut, which kind of defeats the purpose of the whole balancing disc. The surface grinder did come with a balancing arbor, but its diameter is too small for either the grinding wheel, the balancing disc or anything else to be slid over it, not to mention it would be pointless because as mentioned there doesn't seem to be a way to fix the balancing disc to the grinding wheel.

After some searching online it seems there's a thing called a "standard taper balancing wheel", which is indeed a whole assembly containing the balancing weigh disc and grinding wheel that can be removed from the surface grinder's spindle and balanced on a balancing setup, then mounted back onto the spindle of the surface grinder. And in fact I see a fair bit of resemblance with our own surface grinder, as it too seems to have some kind of arbor or outer spindle mounted over a thinner spindle when I look at it closely. However I have no idea how I could remove said outer spindle, we have a tool with 4 equally spaced fingers along its circumference which fit exactly in 4 equally spaced notches along the circumference of the outer spindle. The problem is I have no idea how I could remove the outer spindle, because assuming it is indeed slid over the outer spindle and locked in place with the 4 notched nut, I have no way of unscrewing said nut as I don't know how to lock the spindle in place so I can apply force with the tool for unscrewing what seems to be the spindle nut. Does anyone know how I can lock the spindle or otherwise remove the outer spindle, or whether there's something else I'm missing here? I would really appreciate any help I can find, so much thanks in advance!
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And another problem, or what seems to me might be an issue but I lack the knowledge and experience to know seem to be the white rubber spacers with which the grinding wheels are mounted on the spindle. They definitely have some play when the grinding wheel is mounted on the spindle with it, both between the spacers and the grinding wheel but especially between the spacers and the spindle, when I mount the grinding wheel on the spindle with said spacers there's some noticeable play of the wheel on the spindle. Is that normal or does anyone think they might also play a role in the unpredictable grinding results I had? If so, what would you recommend I do/use instead?
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This Video shows how to change remove the grinding wheel from the spindle from an Jung G60 (perhaps similar to Jung HF40):
You are right: The grinding wheel is generally mounted on a flange. To balance the complete unit "grinding wheel" plus "grinding wheel flange" on an external balancing device, the arbor should fit into the flange. After balancing, the grinding wheel and the flange are no longer taken apart.

In your photos, the flange itself has been disassembled to remove the grinding wheel from the flange. But the main part of the flange has not been removed from the grinding spindle, because the locknut at the spindle that locks the flange hasn't been removed first. So the flange and the grinding wheel aren't one unit any longer, need to be assembled and balanced again.

Try to remove only the small locknut on the spindle nose (without loosening the large nut) and then pull the flange.

Cheers,
Karl
 
I have no way of unscrewing said nut as I don't know how to lock the spindle in place so I can apply force with the tool for unscrewing what seems to be the spindle nut
Grab the wheel securely by hand and then undo the castellated nut. If necessary get someone to tap the nut with a piece of copper as it may not have been removed in a long time. Looks like a standard RH thread unlike the flange which is LH. This should expose a internal nut which requires a thread normally of fine pitch which when screwed in will press off the wheel assembly.
And another problem, or what seems to me might be an issue but I lack the knowledge and experience to know seem to be the white rubber spacers with which the grinding wheels are mounted on the spindle
These are adapters so that you can fit a larger wheel bore on the flange, I would replace the current wheels with wheels that have the correct bore.
I have only used it a couple of times since I joined the Makerspace, and it gave wildly varying results each time; very wavy the 1st time, perfect mirror the 2nd, nice and flat but no proper mirror shine the 3rd.
The wheel balance is critical to get a really good finish. You will need to find or buy the taper and balancing device to balance the wheels properly. Remember that the wheel needs to be trued before balancing. Ignore the device in the video this is a over complicated solution looking for a problem. The wheel balancing devices supplied with the machine are either a blade type which is a U shaped trough with 3 leveling screws or a similar looking trough but with 4 knife edge bearings. The taper has a taper in the center that matches the wheel taper slightly longer than the max wheel width and a parallel section of the same dimension on either side.

Basic balancing procedure.
  1. Tap test wheel to test for cracks, wheel should ring. Dull sound indicates a cracked wheel.
  2. Assemble wheel and flanges without balancing weights.
  3. Mount on grinder.
  4. Dress wheel till there are no high spots.
  5. Remove wheel and fit taper.
  6. Place on balancing fixture.
  7. When wheel settles mark highest point with marking pen.
  8. Place one weight opposite highest point.
  9. Place other two weights 120 degrees from first weight on either side.
  10. Adjust two weights closest to the mark in equal amounts till the wheel no longer rolls around to the heaviest spot.
  11. Mount wheel on grinder and dress lightly.
 
I can provide you with spare flanges depending on the size
I have plenty of flanges from ex-Philips machines
The biggest diam of the taper of the flange is about 25mm
The threaded hole for ejecting the spindle is M26x1.5mm
Perhaps a few other sizes as well
Jung has 3 different sizes AFAIK
I am in the Netherlands so shipping should not be a problem
https://veltmanmachines.com/
 
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At our Makerspace we have a surface grinder (a Jung HF40 I believe), and a couple of aluminium oxide and carbide grinding wheels for it.
Carbide is ground with diamond wheels
The green wheels you probably refere to ,used for manually grinding carbide, are useless on a surfacegrinder IMHO
Peter
 
*A makerspace grinder should have an OFF-direction arrow on the spindle nut and on the wheel mount's flange mount nut, so users don't crank/turn the nut the wrong direction.

The dressing video shows the diamond straight up toward the spindle center and the rounds off the diamond so to make a crusher not a dresser, the diamond should angle to the wheel 15* or so leaning left, and set a tad left of of center. the diamond turned often so to a new sharp facet.
The balancer put on and then locked on the mount with the flange nut put tight. And then the adjustment is made moving the weights.
With not having a balancer then just put the weight even and call that OK.
Mirror finish is decorative but not needed / flatness is a good grind. Flatness is made with keeping the part cool, perhaps with a hand bottle sprayer. If the part gets even a little warm the then flatness is lost.
Blocking in a tall part, or clamping (two clamps for grinding) a part to a 123 block (or something) makes it safer to hold on the mag chuck. With a block-in, the block-in should touch the part high up in the part…in the go direction (the direction it would go flying if the mag was off.
 
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The wheel hole should fit close on the mount .001 /.002 or so in order that it is not way out every time you re mount it. wheel should have a mount-up line so every time you mount Gravity will make it near true to the last dressing.

Wheel should have a ring test every time they are mounted. wheels on a peg board are less likely to get dropped. wheels could have a clear curtain to cover the from room dust and oil.. some wheel are coolant soaked on one side.and so out of balance on one side...that is from not shutting let coolant first so the wheel spin out coolant. and having coolant spray on a parked wheel.
Qt; (very wavy the 1st time,)
Wavy in the long direction is often poor balance, lose wheel, or not dressing
the flange nut shold be as toght as you can make with a hand on the whee and a hand on the wrench with about a 6" long wrench for the average guy/gal..
 
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QT: (noticeable play of the wheel on the spindle). on the wheel mount,
One can fill that space with a shim to make it better balanced. One might not the wheel blotter >.010" - .25mm (Or what) shim
 
If you cannot get the nut loose the way seen on the video give it a swing with the wrench in place and let the wrench bump on a piece of steel on the magnet
That does the trick in general

Peter
 
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