Knife Rack #fail

[lighthearted]

So we needed a knife rack for our new store and instead of just throwing up an ugly metal one I decided to make a wooden wooden.  I own a small benchcrafted one in my shop.

 

I bought a magnetic tool bar, used a beautiful piece of scrap wood, routered out a channel and fit the the bar into it. The thickness of the wood between the knives and the magnet was about a 1/16" -3/32".  I tried it in my shop with a rasp and it worked great.  I bought it to the store, hung a knife on it and crash!  it slid off.  . Took it back to my shop.  Planed it down until I accidentally planed thru to the magnet bar.  

 

So i attempted to make another.  With even an improved way of mounting the bar.

 

Again. Fail.  Knife slid off.

 

Seems so simple.  What am I doing wrong?  Are these magnetic tool bars really not that powerful?  Do I need stronger earth magnets? Even tried two different species of wood.  1st one was super dry.

 

Thanks for the help

[T-astragal]

May be the knife not the magnet

[Beechwood Chip]

Magnetic force decreases very quickly with distance.  I wouldn't put anything between the knives and the magnets, except maybe a magnetic metal.  Maybe it worked with the rasp because the rough surface kept it from slipping.

[lighthearted]

May be the knife not the magnet

I doubt it--steel knife

[wdwerker]

Rougher surface , thinner wood over magnet, stronger magnet ... Some combination of these should work.

http://www.kjmagnetics.com/

K&J have a huge selection of magnets, but not nessacarily knife bars. They do have very powerful rectangular magnets.

Do a test run of a rougher , thinner strip over the magnet before you commit to a combination .

I have used a diagonal cross hatch of belt sander 60 grit scratches to provide some traction.

Some stainless steel has so much nickel that weak magnets don't grab it well.

[lighthearted]

Magnetic force decreases very quickly with distance.  I wouldn't put anything between the knives and the magnets, except maybe a magnetic metal.  Maybe it worked with the rasp because the rough surface kept it from slipping.

Works with the Mag-blok  http://benchcrafted.com/Magblok.html

[wdwerker]

If you use very powerful N52 grade magnets the rougher surface is not necessary . If magnet is powerful enough even high nickel stainless will hold somewhat, but the knives in the Ad picture are high carbon steel( Damascus I think)

[wdwerker]

I bet the magnets in the mag block are inserted from the back and lie about 1/16 or less under the surface.

K&J has great technical data about how much the force diminishes and distance increases. It's easy to drill holes with a forstner bit and install disc shaped magnets with a dab of epoxy if you want the solid wood look.

A 1/2" dia. X 3/16 thick $1.71 (N52 grade) magnet that will hold 11 lbs is only capable of 2 -3 lbs if it's under 1/16 of wood. So distance is a serious factor.

[T-astragal]

I doubt it--steel knife

Some stainless doesn't attract well to magnets especially when it's thin.

[Tpt life]

Just to clarify; you are attempting a DIY Magbloc? First guess would be weak magnets.

[wtnhighlander]

I doubt it--steel knife

Sure it isn't stainless steel? Most stainless steels contain little or no ferrous metal, and aren't attracted to a magnet.

Oops, sorry for doubling up. Wrote that before I saw the whole thread.

Regarding the strength of pull, as I recall, magnetic force follows the inverse square law, so the pull strength falls off in relation to the square of the distance from the surface. So a wood spacer of 1/16" will allow four times less magnetic pull than a spacer of 1/32".

[RPCV_Woodworker]

have you considered covering the magnet bar with a super thin commercial veneer instead of solid wood? Not sure how much thinner you could get it, but the companies that make veneer business cards have some seriously thin options

[ponderingturtle]

Sure it isn't stainless steel? Most stainless steels contain little or no ferrous metal, and aren't attracted to a magnet.

Oops, sorry for doubling up. Wrote that before I saw the whole thread.

Regarding the strength of pull, as I recall, magnetic force follows the inverse square law, so the pull strength falls off in relation to the square of the distance from the surface. So a wood spacer of 1/16" will allow four times less magnetic pull than a spacer of 1/32".

Broadly it depends on what alloy of stainless steel is used.  My Wustoff stainless steel kitchen knives are a magnetic stainless, also you need magnetic stainless steels for induction cook tops.

 

And I am not sure it is a inverse square law as it is not a monopole.  On checking it actually goes as r^3, so it falls off even faster than that.  Doubleing distance reduces force by a factor of 8 not 4.  So halving the distance between the knives and magnets would increase the force by a factor of 8 not 4.

 

So thinner wall, and a stronger magnet would be needed.

[Beechwood Chip]

And I am not sure it is a inverse square law as it is not a monopole.  On checking it actually goes as r^3, so it falls off even faster than that.  Doubleing distance reduces force by a factor of 8 not 4.  So halving the distance between the knives and magnets would increase the force by a factor of 8 not 4.

 

Thanks!  I remembered that it was distance cubed, but then doubted myself because most things are distance squared.  Thanks for for confirming and explaining.

[ponderingturtle]

It can be even more complicated than that, especially if you have multiple magnets.  Remember you can't easily collapse a dipole to a point source so using multiple small magnets can cancel each other out.  Technically the flexible magnets are quite strong but the force falls off real fast as they are narrow bands of alternating magnets.

 

Also the direction of the polarization of the magnets is also important.  You want a pole pointing toward the knives, not perpendicular to them.

[ponderingturtle]

To the OP what magnets did you use?

 

I would think about trying something like these

[woodbutcher74]

I used to work at a place the made fertilizer equipment. Most fertilizer eats the crap out of regular steel, so everything was made out of stainless steel. When customers came to look at equipment to buy, the first thing they did was throw a magnet at it. If it stuck they walked out. Nobody probably cares but thought I would throw this out.

[Marco Divetta]

We had a similar problem in our shop kitchen ... We had a set of solid steel knives where the handle and blade was all one piece of steel. The small knives were ok but the large heavy knife would slide down and fall off because the transition from blade to handle was a soft concave tapper which would pull the knife off the magnet when it slipped down. There was never a problem with similar sized knives with composite handles because there was a flat area at the base of the handle that rested on the magnet.