Internal grinding on a lathe

[mikey553]

I've had several sets of small slitting saws for a number of years, but never used them. I did not have an arbor. So I made 2 of them, one for 6 mm bore and the other - for 1/4" bore. All is good, but 5 saws had an undersized bore, about 4-5 thou short of the standard size. Instead of making more arbors or undersize the existing ones I decided to make all the bores standard.

How would you do that? 4-5 thou is a lot of material for lapping or sand paper (on the arbor). I have tried sand paper with an arbor in a cordless drill, but it would take forever to do all 5 saws.

It would be nice if I could grind the bores to size. I already had my little grinder setup for the lathe, the problem was how to hold the saw blade concentric to the spindle. I have tried the donut shaped magnet held in the lathe chuck. The magnet was too weak and did not hold the blade reliably. Well, I tried it anyway.

I used the tailstock center to position the blade on the magnet and just started grinding. Every time I checked the bore size with a dowel pin the blade moved and I had to center it again. But after I hit the correct size and zeroed the cross slide the other blades were a piece of cake. It took me about 5 minutes per blade including centering the blade.

I protected the ways during the wheel dressing with a diamond. During grinding itself there were almost no materials deposited on the lathe. Not a single spark was observed and my wheel was rotating at 30000 RPM.

This operation was quite safe despite of the magnet weakness. If a blade decided to fly, it would be contained by the grinding wheel arbor. None of the blades did that, but all of them moved a little in the process.

[Randy]

I would have used aluminum soft jaws, and bored a shallow recess to hold the saw, starting with the smallest, then stepping out to larger sizes. But my chuck has removable jaw tops. With a chuck like yours I would have made a collet-like fixture to hold in the chuck.

[mikey553]

I would have used aluminum soft jaws, and bored a shallow recess to hold the saw, starting with the smallest, then stepping out to larger sizes. But my chuck has removable jaw tops. With a chuck like yours I would have made a collet-like fixture to hold in the chuck.

It would not work this way. I had only .004-.005" to grind on a diameter and the bores were not very concentric to OD. The whole post was about the magnet for workholding. I did not expect it to work, but it did and did it very well and it was universal regardless of the blade OD. Plus no tooling had to be made.

[Randy]

I did see that you made the magnets work, and should have added a congratulatory comment on a clever solution. Well done. But you asked how I/we would do it, and that's the question I answered. As for off-center holes, I didn't catch that. My goal would be to be to keep concentricity.

But again, thanks for sharing a clever setup.

[BCRider]

The magnet was a slick idea.

And that makes me wonder if your ring magnet is the sort which is one pole on one face and the other out the other face. If that is so you could "focus" the power of your flat ring magnet with a similar soft iron or mild steel focusing cup for a better grip in the future. Of course that would be for next time. And since the job is done there may not be a next time. But if there is the effort to make up such a ring might give you a tighter grip.

[mikey553]

The magnet was a slick idea.

And that makes me wonder if your ring magnet is the sort which is one pole on one face and the other out the other face. If that is so you could "focus" the power of your flat ring magnet with a similar soft iron or mild steel focusing cup for a better grip in the future. Of course that would be for next time. And since the job is done there may not be a next time. But if there is the effort to make up such a ring might give you a tighter grip.

Yes, the magnet had one pole on one face and the other pole on the other face. It is a weak, low grade magnet, but that was all I had on hand with the correct dimensions. Its OD=18 mm, ID=6.7 mm, thickness=5.9 mm.

I may get a stronger magnet for the next time. Neodymium magnets are very strong. I have one not much bigger than the one used for grinding (27 mm OD) and it can lift my milling vise (76 lb) and some more..
I do not quite understand your idea of steel focusing cup. Can you elaborate?

[BCRider]

For the cups idea? Have a look at this Lee Valley link. Ignore the washer. Or rather the "washer" would be the part being held. The parts you're wanting are the cup and the magnet. When the magnet is dropped into the cup and a thin card or paper separator is used to ensure an even gap around the edge the cup conducts the other pole from the back side around through the cup and you get a VERY strong focused N-S field at the edge of the magnet and cup. It holds about 1.5x or more strongly than just the magnet by itself.

We see that in a speaker. If you've ever pulled apart an old speaker the magnet in the center is set down into a soft iron cup with the gap where the speaker coil fits. It's done that way to have the coil working in that strong focused field in the gap.

The thinking is that you could increase your grip power quite a bit by making up a mild steel cup similar to the idea shown. The edge of the cup should be flush as you can make it with the front face of the magnet.

[mikey553]

The cup is an interesting idea. Actually my strong neodymium magnets are set in cups and magnets have a countersunk hole for mounting. But I was under impression the cup is to protect the magnet and to provide a structural base for attachments. Most magnets are very fragile and need to be handled with care.

I may try this idea on the ferrite magnet I already have. So you say it should be a "paper" gap at the magnet OD. My neodymium magnet does not have any visible gaps and it is so strong that I don't think I can separate it from the cup for measurements. I can try to make a cup with no gap and then make some gap to compare. Is low carbon steel the best material for the cup?

[J Tiers]

.......................... Is low carbon steel the best material for the cup?

The lowest carbon steel would be good. But, if the "cup" is thicker, it will not matter much. The point is to carry the magnetic field around to the "front" with low loss, and most any available ferrous material will do that if reasonably thick.

It would be good to leave a small gap around the OD, to avoid "short circuiting" the magnetic field. You want the most field strength where the work will be. The gap can be filled with epoxy etc.

[Randy]

I have task lights mounted on 2" cup magnets from the hardware store. They're remarkably strong for ordinary ceramic magnets. McMaster-Carr (and Amazon for that matter) list cup-mounted neodymium magnets. They must be insanely strong.

[darryl]

Today I'm having to bore out threads on a flange so it will fit nicely on a shaft. To keep the balance I'll be machining a recess into a piece of mdf mounted on a faceplate. The flange will tuck into that recess snugly, and is automatically centered. Wood screws hold the flange to the mdf.

In this case, the OD of the flange is held concentric. If I wanted to hold the ID concentric, I might do the opposite of this, that is to machine a short stub in the center of the mdf to hold the part in perfect alignment. Screws or various types of clamps could be used to hold the part down. Then you end up machining away the stub to get at the ID of the part.

With either method, you're machining the 'centering device' in place, so the concentricity is automatic. If you remove and reinstall the thing from the lathe, then the accuracy goes out the window.

[mikey553]

My neodymium magnet is 1.24" diameter by .23" high with the cup. I've got it from Amazon. It can lift about 90 lb with a straight up pull. McMaster have very good magnet selection with ratings, but I don't see anything as strong in a similar size. The cup is made from a .090" thick material if it matters at all. I don't think such strong magnet is good for grinding - you will have trouble removing your part from a magnet.

[BCRider]

Even if the cup touches the magnet the plating is non magnetic (I think) and that gives it SOME relief. But if you can use a turn or two of paper to hold the gap even there will be that much stronger and more even of a ring of high strength hold around the edge of the magnet. Or rather right over the gap.

A simple aluminum thin wedge would work for prying the parts away from the magnet. Because of how magnetism works over distance if you can break the joint at all then it rapidly becomes very easy.

[mikey553]

I think the plating is nickel, both magnet and cup look the same color. The hardware is stainless steel. I was able to push a this piece of paper into the gap between magnet and cup, but not all the way around. How big do you think the ideal gap should be?

Also it looks like for maximum strength the part should bridge the gap between magnet and cup. In other words the part should be bigger than the cup. Am I right?

[J Tiers]

I think the plating is nickel, both magnet and cup look the same color. The hardware is stainless steel. I was able to push a this piece of paper into the gap between magnet and cup, but not all the way around. How big do you think the ideal gap should be?

Does not have to be huge. Maybe 25% of the magnet thickness for thinner ring or button type magnets. You want to avoid having much magnetic flux go from the exposed face to the cup directly. That way, when there is close contact of the magnet face and the part, and also of the cup to the part, that the maximum flux goes from the back side magnet pole through the cup, and from the cup edge to the face (other pole) of the magnet through the part.

ANY air gap, which includes any gap filled with non-magnetic material, is better than close contact, around the rim of the magnet from magnet to cup.

For carrying magnetic flux, almost any close contact is not as good as one continuous piece of material. The flux will pass though available magnetic material rather than air, since it finds the path of "least resistance", the "lowest energy state". So a gap around will direct the field into the part being held.

The cup needs to be fairly thick, because ferrous material can only carry a certain amount of magnetic flux before it is saturated. And, also, the path through the cup should be as short as possible without reducing the gap, because a long path does not carry flux as well as a shorter path.

Also it looks like for maximum strength the part should bridge the gap between magnet and cup. In other words the part should be bigger than the cup. Am I right?

Yes, that is the point of having the cup. You get the magnetic field from the back side brought around, and the "circuit" is closed from "north" to "south" poles through the cup and the part. The magnetic flux has to pass through the part or there is no holding, and more is better for holding.