Question on facing dissimilar metals

[Boswell]

I want to make a disk 4" in diameter and 1/2" thick out of aluminum. Then I want to press 10 1/4" diameter drill rod into equally spaced holes. In order to ensure the smoothest surface, I can think of a few possibilities and would like any suggestions
1. Cut the drill rod slightly long by 5 or 10 thousands then put chuck on lathe and face both sides
a. Should I worry that the cutting tool will be cutting both aluminum and steel? it would be an interrupted cut until the drill rod was flush with the aluminum

2. Same as above but use the belt grinder to bring the drill rod down to the surface of the disk.
a. would have to follow up with some hand sanding to get a decent surface finish

3. use the lathe to part off each of the 10 drill rod pieces so they will be flush on both sides when pressed in.
a. Still would likely need to sand each face a little to ensure perfectly smooth.

Open to other ideas or suggestions.


THIS is what I am making but using aluminum instead of 3d printing in PLA

 

[twhite]

You will definitely get some disruption on the surface. That can be smoothed out. If it is critical to have the steel perfectly flush you might have to go with grinding.
If a few tenths of a thousandth is ok. I might approach it by making the pins as close to net length as possible then take super light cuts to get smooth.

The main thing is not having to remove much material.


Cutting oil is my blood.

 

[Boswell]

The only need for "Perfectly flush" is cosmetic. In fact the two faces of where the pins will show will be somewhat obscured by the two rotors with magnets. Still I would like to be smooth to 1 or 2 thousands. I don't have a surface grinder so no precision grinding is available.

 

[twhite]

Then in that case nice fine finish cuts. Take several. That will get you to where you need. I would definitely use carbide with a very minimal corner radius.


Cutting oil is my blood.

 

[RJSakowski]

The rotating disk with the magnets will induce eddy currents in your aluminum design. Is that acceptable? A plastic like Delrin might be a better choice for the center disk.

 

[Boswell]

RJ, I was wondering about the use of aluminum. The only examples on youtube that I saw all used plastic. As Aluminum is non-Ferrous I was thinking It would probably work. I do have some Delrin. Would 360 Brass do the same? I have some Delrin but I don't think it would look as good and will be harder to attach the magnets to the rotors. the spin speeds will be very slow, this is just a "desk" Toy to be spun by hand. Do you think the eddy currents will cause an issue? If fact, what effect do you think the eddy currents would have?

 

[RJSakowski]

If you remember the videos of dropping a neodymium magnet down a copper or aluminum tube, you will see the effects of eddy currents. At the very least, you would experience drag on your gear train. Any conductive material will do the same with the higher conductivity materials being worse.

It might be worthwhile experimenting though. It looks like a fun toy.

 

[ChazzC]

RJ, I was wondering about the use of aluminum. The only examples on youtube that I saw all used plastic. As Aluminum is non-Ferrous I was thinking It would probably work. I do have some Delrin. Would 360 Brass do the same? I have some Delrin but I don't think it would look as good and will be harder to attach the magnets to the rotors. the spin speeds will be very slow, this is just a "desk" Toy to be spun by hand. Do you think the eddy currents will cause an issue? If fact, what effect do you think the eddy currents would have?

A lot of home exercise equipment uses an Aluminum disk between magnets to provide adjustable drag, so the effect can be significant depending on the strength of the magnets and the gap.

I second @RJSakowski ’s suggestion you look at the videos of magnet drops (Mr. Pete has one, and I’d include a link except finding videos on the YouTube app is a pain).

 

[Inferno]

Put the magnets in the aluminum disk instead of the steel rods.
You will have to measure the thickness of the magnets to get them perfect since you don't want to remove the surface of the magnets.
If you get your dims right, you can press the magnets in and they'll never move. Maybe add some super glue to help slide the magnets in and hold them tighter.

Don't quote me on this but I think ALL metals will have eddy currents.

On my lathe head I just super glued the magnets to the pulley and used a hall effect sensor.

I know that's not what you're trying to do but it gives you an idea.

 

[Boswell]

Thanks for the ideas. I did a little research and there might be too much reluctance due to eddy currents but the orientation of the magnets to the aluminum is different from what people use when demonstrating the effect so when my magnets arrive, I'll do a little testing to see if I can get a feel for the magnitude of the effect. If I need to, I can do what RJ suggested and use Acetal. I was planning on a diameter of 4" for the rotors and a foot of 4" diameter Acetal is running around $90 us. I also don't think it would look as good but, it doesn't mater what it looks like if it does not work. By work, I mean that the rotary motion is fairly fluid.

As for assembly, from what I have been reading, even though these are nickel platted they are still fragile and best to not use "press fit". I was thinking that a 2-3 thousands gap and some Loctite 638 should work. However this would be for the aluminum. For Acetal, I would have to come up with a different adhesive. I don't think much sticks to Acetal but I am sure something is out there. Worst case I would machine a pocket from the backside of the rotors leaving a thin bottom of the pocket that would face the interior or even sandwich the magnets between two "Half" disks.

I'll share a drawing of what I am thinking once I get the magnets and get closer to deciding how to attach them.