surface grinding question

J

JMJones

Joined
Jul 14, 2010
Messages
945
I just got my surface grinder up and running. The first thing I wanted to do was flatten a tang on a kitchen knife. Anyway after grinding one side, flipping it over and after grinding the other side, it is kind of bowed. I noticed that the magnetic chuck flexed the tang when I turned it on. Any idea of how to fix, or avoid this?
 
Yup a little off each side. You can fly across the metal if going with shallow passes. Plus it keeps the heat down from beginner movement errors. You will get the hang of it. For now shallow cuts, and you can get deeper with more practice.

Do a couple passes then feel the metal. It gets hot quick.
 
John,

First, congrats on the surface grinder.

When grinding thin sections on a magnetic chuck, some electric chucks have a control on them which allows you to adjust the strength of magnetic pull, without that feature, I use shims to build-up under the parts to keep them straight when the chuck is turned on. I have shim packs with shims from .001" to .035" in .001" increaments. You'll have to hold the parts flat on the chuck how you want it with one hand and try fitting shims with the other till you get it shimmed out, if you are grinding the tang you may also have to shim the tip end to keep it from bending when the chuck is on.

Two things are going to happen when you do this, you are going to loose grip and cooling. Even though the shims are steel they will not transfer the magnetic field thru them 100%, the fix for this is to get some bar stock slightly thinner than what you are grinding and block the sides and the end the grinder is trying to throw the piece off the chuck ( three sides ). This technique is not for those with a weak bladder, the surface grinder can be more dangerous than a buffer when it comes to throwing pointed objects and I haven't seen too many buffing wheels explode while doing so. The magnetic chuck acts as a hugh heat sink, when you insert the shims between the chuck and the blade you loose that cooling effect, you will need to use coolant or flip often as Don mentioned above.

All the above is only for grinding with the blade long-ways on the chuck and assumes you can judge how much you can take per pass safely depending on your set-up, if you want to turn the blade sideways and grind a step at the guard shoulder, you will need to make a fixture to clamp the blade to which supports the tang and is held on the grinder by the magnetic chuck.

Steve Seib
 
sseib Nailed it.

If there is bow in your material before you start with a surface grinder, it will be there when you finish too.

Shims are the key.

I also use hot melt glue or plaster of paris on occasion.

I'm liking hot melt more and more lately.

I put the part on the table, with the magnet off.
Hot melt the perimeter, and grind carefully just until I have a flat surface, and no more.

Flip the part, fire up the magnet, and grind as usual.

Start with the side on the table that gives you the most contact with the table...
 
Ha hha ha ha hahhhh!!!! I actually hate surface grinders...
 
Carl_First_Timer said:
I didn't expect that either. I would not want to use one for thin stuff. But I love 'em for tooling.
Click to expand...

Sorry, I guess I'm just in rare form tonight. I actually typed out a reasonable response about keeping it cool and flipping regularly, but after reading the other responses it wasn't really adding much. But I find I have strong feeling about surface grinders. I actually hate them. This goes back to a couple crappy specimens I had to use in school and continues to this day with a soon-to-be-boat-anchor surface grinder in my shop today. A nice flood coolant CNC surface grinder would be cool (ha ha, play on words) but stone wheel surface grinders are generally just an evil necessity.

Don't ask me about tool post grinders...
 
Thanks for the info guys. I was planning to convert to a belt but this one is automatic and i am pretty impressed with how fast I can remove material at .002 per pass. I will definately try flipping often and may try to the hot glue as well. For the the piece in question, it was actually so close that a quick trip to the flat platten and it was fixed. I know it is kind of backwards to go from a surface grinder to a belt on a platen.
 
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Typically warping a bar that doesn't already have a warp; is a result of heat and too much magnetic pull for the material thickness. Fixing the magnetic pull is easy, adjust the chuck for a good hold and apply 1/16" steel plate all around the material being ground to help keep it in place.

Heat is caused by improper grinding technique, a loaded wheel, and/or using the wrong wheel.

A loaded wheel is an easy fix, just dress the wheel. Most of the time though a loaded wheel will be the result of using the wrong type of wheel.

For grinding any type of tool steel your going to want to use a Norton 32A46-hvbe wheel. There are other options but at $70 (or $240 for 5) they are a great deal. The wheel is the best suited (at a reasonable price) for hogging off annealed or hardened knifemaking steels while also leaving a nice finish.

Now the last and most common mistake I see is improper grinding technique. Think of the grinding wheel as a end mill or lathe bit, you only want to use the very edge to do the cutting. Personally for grinding damascus billets I take 0.010" off with each pass but you can take up to 0.040" with each pass with this technique and the right stone. The key is to move your y axis 0.010"-0.020" each time you make a pass. This uses the front edge of the stone to cut into the material and then uses the rest of the wheel to spark out the material as you continue cutting. Not only is this method faster but it is also far safer. The surface grinder is a scary tool if you don't use it correctly.

-Dan
 
Been doin some playin with a prototype of the surface grinder attachment I will be releasing at BLADE this year, and .010 per pass on 2" width has been no problem.
 
My surface grinder is converted to a 2" X 72" belt. Misting the billet with a water mister keeps all nice and cool. Before using cooling, I was getting occasional warpage. When using belts, be sure they are sharp.
 
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