Help with picking the best/ most magnetic material to attract a magnet

DavidScott · Feb 18, 2018

I am using magnets to hold some parts together in a new product I am developing. 1/2" diameter neodimium magnets on one side and a 1/2" disc of a magnetic material that does not corrode when it get wet. What would be the best material for this? It can be up to .125" thick, easy enough to machine or punch, no more than 75 cents material cost per piece, and hold with the most force to the magnet. So far I am thinking ferritic stainless steel pucks, perhaps 430. I started doing some searches and after reading a fair amount of info I am more confused than ever, wow, it gets complicated!

How do you compare the magnetic pull performance of different materials? Is it the permeability of the material? How do I know how thick the material needs to be?

bigjon61 · Feb 18, 2018

Without knowing more specifics of the application I would suggest some cheap A-36 sheet punched out and then barrel zinced. Cost per each would be pennies or less if of any quantity.

adama · Feb 18, 2018

Any reason not to just use a second magnet? Most low corrosion cheap materials are going to be noticably less magnetic than steel.

DavidScott · Feb 18, 2018

I want to be able to hold in place with loctite, or similar, and quantites will be around 2,000-5,000 a year. Puck must be burr free and +/-.002 thickness and diameter.

Wild West · Feb 18, 2018

DavidScott said:
I want to be able to hold in place with loctite, or similar, and quantites will be around 2,000-5,000 a year. Puck must be burr free and +/-.002 thickness and diameter.

Boker's, Inc. | Stamping and Washer Specialists

jrmach · Feb 18, 2018

bigjon61 said:
Without knowing more specifics of the application I would suggest some cheap A-36 sheet punched out and then barrel zinced. Cost per each would be pennies or less if of any quantity.

What he said

DavidScott · Feb 18, 2018

adama said:
Any reason not to just use a second magnet? Most low corrosion cheap materials are going to be noticably less magnetic than steel.

I have prototyped that but I don't want to have to get the direction right when assembling, too big a risk for scrapping a part. Parts have a decent value by the time the magnets and pucks are installed.

My customer liked my magnet on magnet prototype though. Maybe it should be on the table, just use magnetic assembly fixtures so if the magnets are put in the parts wrong they repel and pop out of the pockets before the loctite sets. Guess I will get a pole detector when I get more magnets tomorrow. Less than a years supply would be 24 cents a magnet so it is probably the most economical solution.

adama · Feb 18, 2018

DavidScott said:
I have prototyped that but I don't want to have to get the direction right when assembling, too big a risk for scrapping a part. Parts have a decent value by the time the magnets and pucks are installed.

My customer liked my magnet on magnet prototype though. Maybe it should be on the table, just use magnetic assembly fixtures so if the magnets are put in the parts wrong they repel and pop out of the pockets before the loctite sets. Guess I will get a pole detector when I get more magnets tomorrow. Less than a years supply would be 24 cents a magnet so it is probably the most economical solution.

Yeah i did kinda leave that bit out, but its the trade off and like you say, a high fuck up potential. Pole detectors, easiest is just a humble button compass. Its also pretty easy to slide a stack of magnets diagonal and swipe a sharpie mark down them once ID'd

9100 · Feb 18, 2018

To avoid getting them wrong, stack up a bunch of them. They have to be N to S to stay in a stack. slide one off as you need it. You could put a bunch in a brass or whatever tube and slide the end of the tube over the hole in the part, wiping one off each time.

Bill

Milland · Feb 18, 2018

Keep in mind that most rare earth magnets have very low corrosion resistance, even when nickel plated. If either of the magnet sets are exposed to moisture you should have a robust fully sealed and coated process in place to prevent degradation. Especially if this is something that will be exposed to salt water, you'll want to do accelerated weathering testing to prevent recalls due to magnet damage.

9100 · Feb 19, 2018

Milland said:
Keep in mind that most rare earth magnets have very low corrosion resistance, even when nickel plated. If either of the magnet sets are exposed to moisture you should have a robust fully sealed and coated process in place to prevent degradation. Especially if this is something that will be exposed to salt water, you'll want to do accelerated weathering testing to prevent recalls due to magnet damage.

True, you might look into ceramic magnets.

Bill

DDoug · Feb 19, 2018

9100 said:
True, you might look into ceramic magnets.

Bill

Is anything gonna cost under the OP's limit of 75 cents ?

Conrad Hoffman · Feb 19, 2018

AFAIK, the most magnetic materials plus corrosion resistance are hard to achieve. I'd try a disk of 400 series stainless. Also, I assume you're using something like Loctite Speedbonder, which is pretty good. Still, I'd worry about eventual adhesive failure if the device is in tension. +1 on the cup design magnets, way more effective.

kb0thn · Feb 19, 2018

digger doug said:
Is anything gonna cost under the OP's limit of 75 cents ?

Ceramic magnets are really really cheap. We buy a bunch from an outlet in Colorado called Magnet Source. Little disc magnets are pennies.

We use two little ceramic disc magnets in an assembly. They have to be opposite polarities. We take the stack of magnets from the shipping carton and pop them onto a 10 ga sheet. All the magnets come off the stacks in the same polarity. Then we dust the top of the magnets with spray paint to make N. Then we wave a cheap magnetic polarity detector over the magnets as we take them off the sheet and re-stack. 1000 magnets takes less than an hour. Two magnets per assembly. $15 value on the assembly. 1 hour labor / 500 assemblies to color code the magnets make sense for my operation.

Here is the detector we use:
McMaster-Carr
10 years on and it is still on original battery.

DavidScott · Feb 19, 2018

Ceramic magnets wouldn't work. Anemic performance and not the shape I need, I like the smooth rounded edges of the neodymium magnets. I am installing the magnets so they are proud .062" to locate in a pocket on the mating part. I have been using these neodymium magnets in a wet environment for several years in another application with no ill effects so I am not worried about them corroding. Yes I am aware of this problem with them.

I have played with the magnet on magnet parts and one other advantage is the parts self-align when trying to assemble them, get them close and they snap together. The magent on steel disc parts you have to get just right for them to snap together. There is another downside to using magnets on the removable part, the two magnets when assembled create a much stronger field that bleeds through the aluminum parts. This stonger magnetic field will probably be a problem for the finished assembly on some of the parts, not all of them. I am leaning toward using magnets where I can, I will just make magnetic assembly fixtures to make sure they are assembled correctly, but need something else for the other parts.

Car2- Does putting the magnet in a cup increase the pull more than sandwitching it between 2 steel plates?

TDegenhart · Feb 19, 2018

You haven't said whether the assembly is intended to be permanent or not.

Tom

DavidScott · Feb 19, 2018

TDegenhart said:
You haven't said whether the assembly is intended to be permanent or not.

Tom

Not permanent. There is a main body with other things attached to it with removeable ends that will be held with magnets and other locating features. The ends must be easy and fast to swap but when assembled they can not come apart easily. Sorry to be so vague but it is a new product and if word gets out it would hurt the current sales of existing products.

TDegenhart · Feb 19, 2018

PM sent

Tom

Help with picking the best/ most magnetic material to attract a magnet

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