Mag Chuck

I would not even consider making one as the problems of obtaining materials, machining and magnetising would make the whole thing prohibitive. Peter

Reasons

1 I do not intend making one 48" x 12" 100mm x 50 would be enough for a first off. 2 High power Magnets are not expensive and easily available. 3 It is a project that would stretch my brain cells. 4 I need (would like) something for the grinder that I am getting going.

Richard

Would you like a 24"x8" to get started with? Needs the operating cam re-making due to wear.

£60 and it's yours

Mark Rand RTFM

I don't know of published information but the magnetic design is not dificult and the construction is straightforward engineering. It's become much easier since rare earth magnets have became readily available. These have the enormous advantage that their coercive force is so high that they don't need magnetising in situ.

two I have made. The foot rule shows the size. The right hand one is a magnetic chuck, mechanically switched, using ferroxdure magnets. The left hand chuck is a magnetic sine table using Alcomax magnets which are pulsed on or pulsed off by a separate control unit. To forestall the immediate question - there are NO drawings! - I rarely draw anything and much of the design tends to be dominated by the size and shape of bits I happen to find in my junk box '

However this is one of the rare cases where the old irish proverb is directly applicable* because both chucks were made before rare earth magnets were available and this is now be the preferred magnetic material..

The right hand chuck shows the basic mechanically switched design. The top surface is. alternate bars of light alloy and mild steel (the darker grey). Below this there is a movable sandwich of ferroxdure and mild steel bars alternating at the same pitch as the top surface. The top set of bars are roughly square section. The lower sandwiched bars are rectangular section

- much deeper - about 4:1 ratio to accomodate large area magnets. The magnets are magnetised in the long direction of the chuck.

When the bottom steel bars are lined up with the top steel bars the steel at each side of the ferroxdure magnet behaves like a horshoe magnet conducting the flux directly through the top steel bars to the work piece. When the sandwich is moved one half pole pitch in either direction each top steel bar magnetically short circuits the corresponding lower pair of steel bars diverting the flux away from the workpiece.

Large force is needed to move the sandwich as it has to slide against the friction generated by the combined force of all the magnets in the stack. A largish spanner on the hex nut is needed to rotate the eccentric that provides the motion .

Magnetic attraction depends on the square of the flux density so the top steel bars should be working near their saturation flux density. The working flux density of rare earth magnets varies quite a lot but this is not critical.. As long as the pole area of your magnet(s) is at least 2 or 3 times the top area of its corresponding top bar it will be OK.

What used to be a problem is the minimum magnetic length (thickness) of magnet needed to ensure that it doesn't become partially demagnetised when the chuck is switched on without a workpiece. That is one of the reasons for the thick light alloy bars on the ferroxdure chuck- to accommodate the magnetic length (ie thickness) of the necessary ferroxdure magnets. With rare earth magnets this is no longer a problem - even the thinnest magnets commonly available are more than thick enough.

It's best to find a suitable set of magnets and design the chuck around them. It's only the pole piece area that counts so you can use more than one magnet per pole piece area. Old hard disk magnets are useful for odd experiments.. First separate them from the hardware them by bending away the soft iron mounts. Then break them in half. This is because they are normally magnetised as a two section NS SN magnet. You want single NS or SN.

Hope this helps

Jim

*"If you want to go there I wouldn't start from here!"

Mrk

Thanks for the offer, unfortunately my table is only 12" x 5" so it is a tad too big

Richard

I am currently looking at some patent information to get some ideas.

It sounds pretty much the way that I work! The accumulated material from the scrappy etc is analysed then the design accomodates what is available. I do drawings for finicky bits where I need to determine clearances, angles etc but generally I work as you do. Thanks for the pictures they inspire me

I am looking at

for magnets.

This sort of "number" information helps a great deal.

I have multiple sketches on my desk considering the options!

Good info!

My thoughts exactly

A collection of ex Hard Drive magnets lies on the bench at the moment. These together with some 1/8th inch thick parallels are giving me more insight into the requirements. Your comments on being magnetised as two sections is interesting, I will check this out.

It has!

Would you object to answering some queries off list once I get my thoughts together?

Thanks again for the post.

Richard

Hi Richard, There is a video on Youtube on this very subject, and it is about the right size for your project.

The voice over is not > I am thinking of making a Magnetic Chuck for a surface grinder. Does

Interesting...looks pretty simple to build too.

Regards, Tony

Thanks for that, as Tony says "interesting". Maybe my eyes are bigger than my belly but I really had in mind something more on the lines of Jims photo's.

Richard

I have had a play with some designs and decided that I needed to know what sort of attractive force is appropriate in a mag chuck. I took a look on the web and found a Vertex chuck from Chronos 175 x 100 mm attractive force 18Kgf which works out at (18 x 10)/(17.5 x 10) = 1.03 N/cm^2 Eclipse quote their Standard at 80N/cm^2

Bit of a difference but maybe my calcs are wrong?

My initial 50 x 100mm design seemed to work out at about 11.3 N/cm^2

And that is with 26 Euros worth of magnets!

Or maybe I am wrong!

Richard

This is a simple design, very low cost and easy to build but it can only produce a fraction of the holdown force that is available in sliding pole piece designs. This may not matter for light duty work particularly as Tryall has recognised this limitation and provided a set of adjustable stop pieces to prevent sideways movement.

The reason for the reduced force is that, even with optimum design, the maximum flux density available at the surface of a rare earth magnet is not much more than half the flux density that is possible when the flux is guided with soft iron/mild steel. pole pieces.

The magnet arrangement used can't achieve this because it doesn't take into account the magnetisation pattern used for hard disk magnets.

shows the problem. This is a hard disk magnet overlaid with a piece of magnetic film. This clearly shows the way in which it is magnetised as a pair of magnets - the left hand dark area is NS thickness magnetised S pole uppermost. - the right hand area is reversed polarity with N pole uppermost.

A magnet can only achieve it's maximum flux density in a closed magnetic circuit . This means that the N poles must connect to S poles through a continuous ferromagnetic (soft iron/mild steel) path interrupted by the minimum practicable air gap.

When used in this sort of magnetic chuck the workpiece forms part of the ferromagnetic path but, as in the originall HDD application, the reverse side of the magnet needs to be covered with a mild steel plate to connect the rear N to the rear S poles. This plate doesn't appear to be present in the Tryall design. If this plate is present (by simply using magnets still attached to their original backing piece) the total airgap is so much smaller that there is little to be gained by series stacking more than one magnet. The key thing is to minimise the thickness of the non magnetic top surface of the chuck.

Jim

Hi Jim, Does that mean you don't like it or just that he could do better? Ned Ludd

Sounds like the latter to me.

Regards, Tony

I have a powered magnetic chuck / table (presumably electromagnetic though it might be electrically operated) in my 'interesting junk' pile at the moment. It was made by Humphreys. Can anyone tell me something about it ?

-adrian

For a mickey mouse magnetic chuck for small undemanding work it's a pretty fair effort. He hasn't made best use of the magnets

- the changes discussed in my last paragraph would simplify the construction and slightly improve the performance.

However, there's no way that it could get anywhere near the performance of a professional magnetic chuck. That needs a properly dimensioned polepiece arrangement to both concentrate the flux and permit a near zero airgap closed magnetic circuit.

Jim

skrev i en meddelelse news: snipped-for-privacy@4ax.com...

Will this magnetic configuration (Halbach array) help on anything?

This is a most unusual and intriguing configuration. The pictorial flux diagram shows that, for the same total magnet volume, it would produce greater single sided flux than that produced by the equivalent unbacked (no rear plate) magnet pair of the ex HDD magnets.

However this useful flux is mainly generated by segments 2 and

1. The orthoganol fields of segments 1,3 and 5 are mainly concerned with cancelling and re-enforcing the return fields of

2 and 4 and so are only partially useful because they do not wholly add to the total useful flux.

In contrast, when a low reluctance backing plate is added to to a HDD magnet pair, because the return flux is short circuited, the total volume of both magnet segments produces useful single sided flux.

This is an opinion based on looking at the pictorial flux diagram. I have not used one of these arrays - there may be other advantages.

Jim

There's a good source of rare earth magnets here:

Regards, Tony