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Magnetism in Stainless steel

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Sam Longley 101/04/2020 09:08:03
742 forum posts
26 photos

I want to make a set of gimbals for my Sestral compass.The original ones were made in brass, but the main ring is a thin "Z" shaped ( approx 6,15,6 ) I do not know how to form a ring that section. Plus the second part is a semicircular cast piece as a "T", tipped on its side, with a bracket all cast in at mid point, for mounting on a dovetail to the bulkhead.

So I intend to make the first full ring in 304 stainless steel strip rolled into a circle. The second part will be similar but heavier gauge with a 50 *30 *8 mm brass piece either riveted, or silver soldered, to it to form the bracket. Brass as easier to machine a tapered dovetail.

I know that SS304 is supposed to have minimal magnetism, and I expect that on its own I might be OK. However, has anyone actually checked the magnetism of 304 AFTER it has been worked. It seems from tech data that the magnetism can increase. If so that would make the whole exercise pointless.

So does the magnetism increase noticeably or not?

I do not really want to buy heavy brass sections as this is only a spare compass & really a project for the layoff. Anyway, I do not know how to form a ring in thin brass as a "Z" section for stiffness or how to form the heavier "T" section so both pieces would have to be flat strip on edge but bigger than SS & prohibitively expensive

Thanks

Journeyman01/04/2020 09:39:22
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779 forum posts
134 photos

Is aluminium a possibility, there are plenty of grades that are seawater corrosion resistant. Being light, a relatively heavy section could be used and most grades are cheap(ish). On completion could be anodised for further protection.

John

SillyOldDuffer01/04/2020 10:39:20
5631 forum posts
1157 photos
Posted by Sam Longley 1 on 01/04/2020 09:08:03:

I want to make a set of gimbals for my Sestral compass.

...

I know that SS304 is supposed to have minimal magnetism, and I expect that on its own I might be OK. However, has anyone actually checked the magnetism of 304 AFTER it has been worked. It seems from tech data that the magnetism can increase. If so that would make the whole exercise pointless.

So does the magnetism increase noticeably or not?

...

Thanks

The answer would appear to be a straight yes, magnetism can increase. On the face of it Stainless 304 (and 316) are unsuitable for this application.

First, any uncompensated magnetism near a compass will deflect it, and 304 is slightly magnetic.

Second, of 304 and 316: 'Any process which can change the crystal structure of stainless steel can cause austenite to be converted to the ferromagnetic martensite or ferrite forms of iron. These processes include cold working and welding. It is also possible for austenite to spontaneously convert to martensite at low temperatures. To complicate matters further, the magnetic properties of these alloys depend on the alloy composition. Within the allowed ranges of variation of Ni and Cr, significant differences in magnetic properties may be observed for a given alloy.'

In other words, results are unpredictable. Not sure I'd want to rely on a compass where North might move in cold weather!

Just wondering though, is the compass for navigation, or just yacht bling? Or something in between? An accurate compass, chronometer, sextant and magnetic deviation tables were all important when blue-water navigation depended on dead reckoning, plane sailing, and star sights. However, maybe a slightly untrustworthy compass can be good enough today, especially for rough coastal work, even if it's several degrees wrong because the boat is navigated by eyesight backed by GPS, radar, & radio beacons etc?

If compass accuracy doesn't matter, then stainless would be fine. Otherwise I'd want to do the job properly.

Dave

Jouke van der Veen01/04/2020 11:06:17
42 forum posts
6 photos

Sam,

Why not doing some tests. Roll a piece of steel strip with the degree of deformation as you intend to use and test magnetic properties. Subsequent heat treatment may reduce ferromagnetic properties. And 316L is less sensitive to increasing magnetism by cold work than 304. There is a lot of info on the internet.

Regards,

Jouke

Circlip01/04/2020 11:30:35
1088 forum posts

" Vee clamps to be made from low magnetic permeability " was spec required for Plastic Minesweepers. We used 304L and 316L.

Regards Ian.

Andrew Johnston01/04/2020 11:48:13
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5410 forum posts
627 photos

Austenitic stainless steels are paramagnetic, ie, weakly attracted to a magnet but cannot be magnetised. Cold working austenitic steels can induce martensite, due to deformation, which is ferromagnetic. The effect can be reduced, or eliminated, by using a high nickel steel and by full annealing after manufacture.

Personally I wouldn't use stainless steel, but if one must use it then 316 will be better than 304 as the nickel content is higher.

Andrew

Clive Hartland01/04/2020 11:58:04
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2556 forum posts
40 photos

Having had to re-design Optical Theodolites for being non magnetic we discarded S/Steel and went for Bronze.

All the grub screws were changed as some were steel. They had s/steel pins to guide the focus tube so they had to come out and be made of bronze.. spacing rings in the optics as well.

A point here, as the gimbals swing the velosicty will be higher with Alu due to less weight but a non ferrous metal will be slower due to the mass. I have seen cast brass/ bronze gimbals.

You certainly do not want a woildly swinging compass on light gimbals, it should process gently and evenly and not bang about..Fabreicate it from Brass/bronze materiels.

I have seen compasses used on warships as masters that are about 2 to 3 feet in diameter and as deep.

Edited By Clive Hartland on 01/04/2020 12:00:07

Sam Longley 101/04/2020 21:52:38
742 forum posts
26 photos

Thanks everyone. I do want to be able use it for navigation. Whilst I have chart plotters & GPS I love navigating by simple compass & log, so may well use it as a steering compass at some point, if I cannot use the existing one.

Looks like it is going to be all brass ---& a bit of polishing

Sam L

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