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268312 "Ed O'" <edo@e...> 2019‑04‑21 I need some engineering type help with switched magnets for jigging
Kind of on the periphery of the charter so please do a direct reply if you have
info and think it is too far afield.

I am attempting to create a work holding device or jig to hold some stuff while
I work on it with hand tools.

I think the basic question is:  do any properties of what a magnet attaches to
affect the strength of the bond?

Short question that gives a feel for what I want to do:
Magnetic bases for dial indicators are switched so you can engage or disengage
the magnetism.  Is the hold strength of the magnet affected by the mass or
thickness of the iron that it will attach to?  In other words if I use a piece
that is 2 x 3 x 1/8 will it result in less grip than say using a piece that is 2
x 3 x3/8?

Longer question with more specifics:
When using a hand screw the reach is limited by the length of the jaw from the
threaded rods.  I am attempting to create something that would not have this
throat limitation.

I am using a Magswitch (https://www.amazon.com/Magswitch-8110005-MagJig
-150/dp/B001B8HMKG/ref=sr_1_2_sspa?keywords=magswitch&qid=1555873085&s=gateway&s
r=8-2-spons&psc=1">https://www.amazon.com/Magswitch-8110005-MagJig-150/dp/B001B8
HMKG/ref=sr_1_2_sspa?keywords=magswitch&qid=1555873085&s=gateway&sr=8-2-spons&ps
c=1) in one jaw and a piece of mild steel in the other jaw.  I am currently
using 1/8 thick steel.  If I try and hold two pieces of veneer between the 2
jaws it holds well, 2 pieces of 1/8 thick stock and it holds, but not well.
Does altering the thickness of the mild steel side effect anything?

Here is a picture of the prototype holding some 1/4 inch stock (it actually will
be 2 x 1/8+ pieces that I want to work, that I need to maintain the orientation
of and cannot use glue or carpet tape to hold together)

https://www.edoii.com/auction/magswitch/img_1981.jpg
https://www.edoii.com/auction/magswitch/img_1982.jpg
https://www.edoii.com/auction/magswitch/img_1983.jpg
https://www.edoii.com/auction/magswitch/img_1984.jpg

I bought a piece of 3/8 flat bar stock from HD and the hold appears to be
slightly  better than with the 1/8 bar stock I already used.  I don't want to
cut it up and make another prototype just to find out the hold is going to be
just nominally better.

Any engineering types out there that can point me in a direction?

Thanks,

Ed O'
268313 curt seeliger <seeligerc@g...> 2019‑04‑21 Re: I need some engineering type help with switched magnets for jigging
>
>

Ed has a cool project in mind, but I don't quite know where my undergrad
texts are right now...

> I am attempting to create a work holding device or jig to hold some stuff
> while I work on it with hand tools.
>

One thing you seem to be running into is magetisms inverse-squared
relationship between force and distance. So the force between the jaws
holding something 1/8 the of an inch thick is 4x weaker than holding
something 1/16th of an inch thick.
You might think of it from the other direction-- how much force do you want
for holding your work, and how thick will it be? That'll tell you how
strong your magnets need to be. A typical clamp generates about 400 lbs or
more of force, and if your piece is 1" thick and your vise faces are 3x5
inches, you'll need a metric boatload of Gauss to make the vise work.
Might give these folks site a try:
https://www.kjmagnetics.com/blog.asp?p=how-much-will-a-magnet-hold

They have a calculator that shows a 3x5x1 inch magnet of their strongest
material seems to get you to the ballpark
https://www.kjmagnetics.com/calculator.asp?calcType=block

Thanks for sharing this, bucko. Seems like a fun adventure to be on
268314 Erik Levin 2019‑04‑21 Re: I need some engineering type help with switched magnets for jigging
Ed asked a series of questions regarding switched magnets (no electrons used,
and with a history to the beginning of the 20th Cty)starting with:
> I think the basic question is: do any properties of what a magnet attaches to>
affect the strength of the bond?
Yes. Many properties.

> Magnetic bases for dial indicators are switched so you can engage or disengage
> the magnetism.  Is the hold strength of the magnet affected by the mass or 
> thickness of the iron that it will attach to?  In other words if I use a piece
> that is 2 x 3 x 1/8 will it result in less grip than say using a piece that is
2
> x 3 x3/8?


Yes. Thinner material will have less hold, but beyond a certain thickness, the
difference is minimal. What thickness depends on the particular magnet. I have
some that will hold at near maximum force on 1/8" material, and others that have
only a small percentage (maybe 25%) on on 1/8", but hold near maximum on 1/4" to
3/8". The strength of the permanent magnet determines the actual force, but the
distance between the poles and their size are the primary factors in how it
reduces with the surface you put the magnet on and what gap (magnetic) there is
between the magnet poles and the steel.


> If I try and hold two pieces of veneer between the 2 
> jaws it holds well, 2 pieces of 1/8 thick stock and it holds, but not well. 
> Does altering the thickness of the mild steel side effect anything


The force drops off rapidly as the distance between the magnet and the steel
increases. Thicker steel wont help much beyond a certain point, and you are
probably already there.

You may do better with a switchable that used a different pole configuration, or
another magnet on the other part rather than steel, if you can find  a
switchable (or modify one such that it has) with the opposite pole
configuration. A switchable with a different pole setup might do better as well.





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268315 Christopher Dunn <christopherdunn123@g...> 2019‑04‑22 Re: I need some engineering type help with switched magnets for jigging
Ed

I agree with Curt and Erik. The force exerted by a magnet will
increase with the thickness of the metal it's attracted to, up to a
point. Beyond that more metal doesn't help. Making the metal bar
longer might help to reduce the distance that the returning magnetic
field lines need to travel, or it might not do anything depending on
how the magnets are configured.

We have some magnets at work (McMaster part number 5848K93 i believe).
By accident, two of them came together and it took two of us pulling
as hard as we could to get them apart. We made this mistake only once
and now store them separate wooden boxes. If you put one in your fist,
small pieces of metal will leap off a table a few inches away, but it
takes little force to remove the metal from the back of your hand. The
point being, it doesn't take much of an air gap to reduce the force a
magnet exerts.

An electromagnet allows for very high levels of force with the ability
to turn them on or off, but they aint cheap.

Thanks,
Chris
268318 Kirk Eppler 2019‑04‑22 Re: I need some engineering type help with switched magnets for jigging
Lee Valley has a little bit on the use of cups around their magnets to
increase their strength.  Click on the "tech" highlight in the ordering
section.

http://www.leevalley.com/US/wood/page.aspx?p=58750&cat=1,42363,42348

These may help, if you embed these in the far jaw of your vise, away from
the magswitch.   Also, what about 2 Magswitches, or are they opposed and
push each other away?  Could you reconfigure them to be a pair?  Or two on
the same side to pull your plate?

Here is a page on magnetic field strength that may give more clues.
https://www.kjmagnetics.com/blog.asp?p=surface-fields-101

We did a thing back in HS on field shape, with metal shavings on top of a
thin piece of ?plastic? and could see the shape of the field change wiht
the magnets.  We didn't have supermagnets back then, so I don't know if it
would be as dramatic as now, or more so.  I had an industrial magnet that
the teacher wouldn't let near his setup.

Finally, when trying to separate two magnets, go sideways, not away.
Rather than lifting them like LPs on a spindle, slide them and scratch the
LP.  Much easier.

KE




On Sun, Apr 21, 2019 at 12:56 PM Ed O'  wrote:

> Kind of on the periphery of the charter so please do a direct reply if you
> have info and think it is too far afield.
>
> I am attempting to create a work holding device or jig to hold some stuff
> while I work on it with hand tools.
>
> I think the basic question is:  do any properties of what a magnet
> attaches to affect the strength of the bond?
>
> Short question that gives a feel for what I want to do:
> Magnetic bases for dial indicators are switched so you can engage or
> disengage the magnetism.  Is the hold strength of the magnet affected by
> the mass or thickness of the iron that it will attach to?  In other words
> if I use a piece that is 2 x 3 x 1/8 will it result in less grip than say
> using a piece that is 2 x 3 x3/8?
>
> Longer question with more specifics:
> When using a hand screw the reach is limited by the length of the jaw from
> the threaded rods.  I am attempting to create something that would not have
> this throat limitation.
>
> I am using a Magswitch (
> https://www.amazon.com/Magswitch-8110005-MagJig-150/dp/B001B8HMKG/ref
=sr_1_2_sspa?keywords=magswitch&qid=1555873085&s=gateway&sr=8-2-spons&psc=1">htt
ps://www.amazon.com/Magswitch-8110005-MagJig-150/dp/B001B8HMKG/ref=sr_1_2_sspa?k
eywords=magswitch&qid=1555873085&s=gateway&sr=8-2-spons&psc=1)
> in one jaw and a piece of mild steel in the other jaw.  I am currently
> using 1/8 thick steel.  If I try and hold two pieces of veneer between the
> 2 jaws it holds well, 2 pieces of 1/8 thick stock and it holds, but not
> well.  Does altering the thickness of the mild steel side effect anything?
>
> Here is a picture of the prototype holding some 1/4 inch stock (it
> actually will be 2 x 1/8+ pieces that I want to work, that I need to
> maintain the orientation of and cannot use glue or carpet tape to hold
> together)
>
> https://www.edoii.com/auction/magswitch/img_1981.jpg
> https://www.edoii.com/auction/magswitch/img_1982.jpg
> https://www.edoii.com/auction/magswitch/img_1983.jpg
> https://www.edoii.com/auction/magswitch/img_1984.jpg
>
> I bought a piece of 3/8 flat bar stock from HD and the hold appears to be
> slightly  better than with the 1/8 bar stock I already used.  I don't want
> to cut it up and make another prototype just to find out the hold is going
> to be just nominally better.
>
> Any engineering types out there that can point me in a direction?
>
>
>

-- 
Kirk Eppler in HMB, CA
268322 Don Schwartz <dks@t...> 2019‑04‑23 Re: I need some engineering type help with switched magnets for jigging
On 2019-04-22 5:41 p.m., Kirk Eppler via OldTools wrote:
> Finally, when trying to separate two magnets, go sideways, not away.
> Rather than lifting them like LPs on a spindle, slide them and scratch the
> LP.  Much easier.

LV suggests a little jig for this. Drill a hole the thickness and 
diameter of one of the magnets, drop a stuck pair in it, and slide the 
top one away. Works every time - and no blood blisters!

Don

-- 
“It is better to be killed by a woman with a knife than by a man with a gun.”
Che Guevara

A thermometer is not liberal or conservative. - Katharine Hayhoe

Of all the tyrannies that affect mankind, tyranny in religion is the worst.
Thomas Paine

Being offended doesn't make you right.
268324 Ed Minch <ruby1638@a...> 2019‑04‑23 Re: I need some engineering type help with switched magnets for jigging
Luthiers (I play one on television) use magnets a lot for clamping small pieces.
Sliding the two apart is the trick they use.  The ones I have are about 7/16”
diameter and 1/8” thick and the sliding is very easy for this size.

Ed Minch

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