Metal marking

See

, and / or Rather expensive, for just marking some odd cutters, but... :-) As to making one yourself, I'd go for the spare electrodes of SPI, and maybe also the pen handle. The rest is essentially a transformer + rectifier. Cidron

The carbon rod from a normal zinc/carbon battery used with a car battery or other suitable power supply is quite good. You might need a reasonably high wattage bulb in series with the rod to limit current if using a car battery.

I've got one of these devices - they're pretty crudely made. The guts is a 6" length of 10mm mild steel cut off at

45 deg at the business end and wound with 2 layers totalling about 70 turns of 1.5mm enameled copper wire. There's no attempt at a closed magnetic circuit - the vibrating armature is just a small plate of 3mm mild steel parallel with the 45 deg end and screwed to a short strip of spring steel strip about 8mm wide and 1mm thick. The strip mounts on an insulating collar mounted just at the end of the winding. The whole lot slides inside an outer bakelite tube. It didn't come with a power pack but it works quite happily on 6V raw AC. An electrode made from a short stub of electric fire element works for very occasional work but wear is pretty rapid. Much better is a short stub of 1/8" tungsten TIG welding electrode sharpened to a point.

Jim

Thanks for reply. sound pretty easily made, any chance of posting a picture or drawing ???

Regards

Ron

Self demagnetization depends on the ratio of length to diameter. With a length- to-diameter ratio of 15:1, there is little to be gained by completing the magnetic circuit in this case.

Absolutely true when applied to permanent magnets which have very low permeability and normally work on a recoil loop. A bit different for an electromagnet with a soft iron circuit where H is provided by the ampere turns and B is determined by total circuit reluctance. Since the iron permeability is typically several thousand an L/D ratio of

15:1 is nothing like enough to bring the air return reluctance down to a level comparable with the iron circuit reluctance.

Apart from its attractive simplicity, the open magnetic design may be deliberate. The increased power consumption simply results in more arc power.

Jim

I've posted some pictures in the drop box headed Arc Pen Tool. There's an error in my earlier post

- the TIG electrode is 1/16" not 1/8".

The first picture is the Actograp device. The long screw is used to adjust the gap.

The second picture is a home brew device that I made before I picked up the Actograp tool at a car boot sale. It sparks a finer, less deep, line than the Actograp. In practice I use both depending on the size of the job.

The picture is pretty self explanatory. It's based two bits of 1/4" square steel screwed together. Strictly speaking, the lower bit should be spring temper silver steel but I'm pretty sure that I used mild steel for both pieces. The springy part is 0.07" thick by 0.7" long. Can't remember the number of turns but it's 20SWG (0.036") and measures about 1/4 ohm - gets pretty hot if it's used for a long time. Air gap is 0.015" set by a shim at the fixed end.

Jim

The problem is the same whether the magnet is permanent or electro. The bulk of the reluctance of the solenoid is in the core. The effect of a return circuit is primarily to reduce the self-demangnetization of that core. With a l/d of 15, the improvement due to a return circuit would not be worth the added cost/weight/bulk.

Sorry to show my ignorance but where is this "drop box"

Russell.

Jim

I'm sorry but you need to re-read your text books. Permanent magnets and electromagnets do not behave in the same way. Because the recoil permeability of permanent magnet material is less than 5 the reluctance of a permanent magnet core is significant. The permeabilty of the soft iron core of an electromagnet is so much higher than that of permanent magnet materials that the bulk of the reluctance is NOT in the core but in the air return path.

The best way of demonstrating this is a direct measurement. A JPG of the setup is posted in the drop box under the heading "Arc Pen Magnetics"

This uses the Actograp 15: 1 l/d ratio 70 turn core unit with the iron circuit almost fully closed by a jury rigged mild steel return path. An 0.04" air gap was left at the 45 deg working face to permit insertion and removal of the fluxmeter 50 turn pancake search coil.

With 1 amp DC through the winding, withdrawal of the search coil resulted in a deflection of 26 divisions on the fluxmeter .

The mild steel return path was then removed and the experiment repeated at the same DC current but this time with the search coil withdrawn from direct contact with the exposed 45 deg working face.

With this open magnetic circuit the fluxmeter deflection was 3 divisions - only 12% of the flux level achieved with a closed magnetic circuit.

This clearly demonstrates that well over 80% of the total reluctance is in the air return path.

Jim

No, that simply demonstrates that the core reluctance is higher without a return circuit.

The question concerns the design of the marker. Since inductance cannot absorb power, that power is lost only in the core (hysteresis and eddy current), the winding (resistance and proximity loss) and doing the work (vibrating the scriber). Adding a return circuit will increase core loss (for the same flux density) but reduce some of the winding loss. To achieve a real improvement, the core and return circuit would need to be laminated or otherwise divided (ferrite would probably not have sufficient saturation flux density). I suspect that the existing design is about as good a trade-off as could be achieved at the cost.

As for the text book, I wrote some of it.

Tim.

My comments referred to the performance of an 15:1 aspect ratio electromagnet - whether or not the addition of a ferromagnetic return path makes a substantial difference to the operating conditions.

Direct measurement has shown that the addition of a partially closed return path increases the flux produced by

1 amp of excitation by a factor of 8.

Whether or not this is useful is a different matter. Reduced excitation current reduces the arc power which in turn reduces the marking capability of the pen.

This thread has strayed a long way from the core interests of this Newsgroup and seems to be of pretty limited interest to other members. Since I think both our viewpoints have been adequately aired I will not be posting further on this subject.

Jim