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Magnaflux part size current density help needed

welderboyjk

Cast Iron
Joined
Nov 22, 2006
Location
south west michigan
We are looking to get a Magnaflux machine here at work. New units are quite expensive but have about 5000 amps (AC). I've found a decent looking used/refurbished machine with a lot more attractive price but it only has 1500 amps AC power.
Anyone familiar with these enough to be able to give advice on how big of a part could be readily checked with 1500 amps?
We are looking to mainly check shafts about 1.25 in Dia. x 22" ling with a set of gears on one end of the shaft.
By way of comparison, what would it take to do an automotive(v-8) crank?
Thanks.
 
After some consideration I'll couch my answer by saying the 1500 amp machine is a likely candidate, if the parts your checking will never be any larger. I ran a 3000 amp machine for several years and it would do cranks up to 6 feet long. Using a 3 turn coil on the highest setting the amp gauge would bounce up to 2400 amps and then settle back to 1500 amps. Automotive cranks I'd charge at half power, but I don't recall what the amp gauge would read, probably 750-1000 amps, the shaft you mentioned unless the gear is over say 8" would need less power. For reference 3000 amp machines are small in the automotive world, most are 5000 to 8000 amp. The folks at Magnaflux would tell you for sure if the 1500 amp machine would do the job.
Couple of other thoughts:
With the contraction in the engine rebuilding industry the last several years there are quite a few affordable used machines in the hands of the automotive equipment dealers. Unless you go for the auto-demag feature the machines aren't any more complicated than a buz box welder, if the windings are intact they work. IIRC the coil can be made smaller diameter (may have to make a new one from heavy welding cable) and induce the magnetic field in the work more efficiently, letting you use the smaller machine. When rating machines there is amps capacity and amp turns capacity. In my example above 1500 amps flowing through 3 turns of coil would be 4500 amp turns coming out of a 3000 amp machine. The 3000 amp capacity would come into play when doing an end shot on the part, passing all the current through the part end to end or point to point with clamps or probes.
 
Update

I found the boss a nice looking little machine on Fleabay. It was 1500 amp, 10" coil, and 40" between centers. Well after he talked to the seller we ended up with one that is I think 2500 amp with a 12" coil, and like 60" between centers. It should arrive this week.

BTW I found something on he magnaflux website in the doc. library. Basically by old standards (pre early '70s) the rule of thumb was 1000 amp per inch of diameter. After that the standards started being reduced. Currently at 300 - 500 amp per inch of dia. Max part size IIRC should be half of coil dia. for a coil shot. There are many other factors like ac only being good for surface cracks but this is a start.
 
The rule of thumb for circular magnetization (head shot) is to use 700-1200 amps per inch of article diameter or diagonal measurement. Using your article diameter of 1.25", that would work out to 875-1500 amps. Obviously the end with the gears on it would be closer to the upper end of that range.

To find the current needed for longitudinal magnetization with a coil, you would use the following equation:

K/(L/D Ratio) = Amp turns

L = length
D = diameter or thickness
K = 45000 (a constant)

So yours would be 45000/(22/1.25) = 2556 amp turns

Now divide that number by the number of turns in your coil and you have a rough estimate of the amps required for a coil shot.

I always double check this setting with a pie gauge on the part to verify that there is sufficient magnetism present in the part. Also keep in mind that the effect length of the magnetic field when using a coil is 6-9" on either side of the coil. Your part, at 22" long, will require 2 coil shots. Hope this helps out a bit.
 
I always double check this setting with a pie gauge on the part to verify that there is sufficient magnetism present in the part. Also keep in mind that the effect length of the magnetic field when using a coil is 6-9" on either side of the coil. Your part, at 22" long, will require 2 coil shots. Hope this helps out a bit.


Thanks for sharing the math on machine capabilities, however I disagree on the need for multiple coil shots on smaller parts. I've magged lots of 40"-60" cranks with one coil shot in the center, and as long as there is sufficient magnetism at the ends, the cracks in the crank show up right to the ends. Cracks like the ones that come out the corners of the front damper keyway that start running lengthwise and then turn and run across the quill. I've also found the multiple coil shot jobs on long shafts can be a royal pain to de-mag, running around with the coil chasing residual high spots.
Just my 2 cents.
 
Thanks for sharing the math on machine capabilities, however I disagree on the need for multiple coil shots on smaller parts.

Just going off what I learned in my NDT Level II inspector class. This information came from the text and level III inspector who signs off our inspection procedures.
 
Well they are pulling wires to the thing right now, but we don't have any consumables (carrier/particles) yet :( so i guess it's still going to be a few days before we get to play with it.
 
Powder "settling" out

Well I've played with the machine a bit now.
We're using carrier II and 14a powder.
The machine does not get used everyday.
The question I have is, how often should it be turned on to "stir" the powder. It wasn't used for a couple of weeks and the powder all settled out of the oil and didn't come back when I turned it on. I resorted to adding a little more and using a long brush to reach in and break it loose. The problem is it seemed to be clumpy. Not near as fine as before.
 
Magna-fluxing is easily accomplished with standard shop equipment. The original process was demonstrated and prooved with a Hobart "pig" welder. Unless your're outfitting a production facility the pro gear may be overkill.

I used to do a lot of magna flux type work using a DC welder. I used a coil of about 10 turns of welding lead with about 8" ID. On a 300 Amp machine that works out to 3000 Amp turns, a pretty healthy field that if closed by a core would mean a hell of a pull.

The cool thing about the welder (if it has pro features) is it can be controlled with a foot pedal, you have variable current control, and if you switch to AC you can de-mag the part petering out the current gradually. The only thing you have to be careful of is duty cycle.

A good welder of this talent would cost about $1500 and has other uses in the shop if you don't already have one. A purpose made magna flux machine is a white elephant unless you use it full time.

Some materials and process require you to pass heavy current though the part. Use a spot welder. Here you have to be careful how you make electrial contact with the work. A point contact may locally heat the work and cause metallurgical damage. I've seen 4/0 welding cable stripped back about 1/2" and the fine wires fuzzed out or stacked braided ground strap used for this purpose.
 
Forrest, I had never thought of using a CC power source(tig/stick) as a means of feeding a magnaflux unit. But I suppose it is basically the same thing.
Being a white elephant is no problem here. When the boss wants it we get it.
I don't think the "home brew" machine would work here. I can't imagine someone from the nascar winston/nextel/sprint/next big money cup world coming in and seeing us magnafluxing parts with a welder.

As a follow up, Forrest I enjoy reading your posts. The information is quite often well worth the read, but I think you forgot to read/skimmed a couple of the posts in this thread.
The machine is here and we have been using it for a while now.
poke, poke, nudge, nudge.
 
Blink blink, flinch flinch.

Yeah OK. Guilty of not fully reading the OP. It's my eager helpful nature but thanks for the kind words

If you're serving a customer base like NASA you better know your stuff. Even worse is NASCAR because you have to have new-looking flashy goods. NASA understands well wrought and documentable expedients and clever economical work arounds but the motor sports people only understand hot rod mag and cereal box/comic book level logic, technical bling, and magic box technology. You can show them pages of material certifications but if it aint "billet" it's no good. 99% of the motor sports crowd don't know the difference between "torque" and "power."

I jobbed motor sports stuff for years until the collective ignorance and presumption of expertise drove me nuts. The serious pros were fine, it's the wanna-bes that drove me to distraction. Problem was for every serious pro there were a thousand wanna-bes. Lotta money there but the hassle was not worth it to me.

So, yeah. Gotta have the brand name goodies. Ask a highly technical question of a GP website like this and you have to filter out glib nonsense from the ignorant to get to the stuff actually addressing the question.

Magna-Flux properly wrought is serious NDT covered by specs somewhere. I used to farm my NDT out so I don't know much about the nuts and bolts. Best place for technical info you can take to the bank is from the equipment manufacturer or the specs controllig the NDT process for your application. In the case of high performance automotive I suggest commercial aircraft specs. They have the most comprehensive documentation. Aircraft piston engines are very similar to auto engines and the landing gear and its attachment to the fusilage is analogous to the suspension.

Here's a place to start: http://www.magnaflux.com/DocumentLibrary/tabid/180/Default.aspx or better, contact them directly
 
Per ASTM E-709

Field strength for longitudinal magnetization using cable wrap and encircling coils, use direct or rectified current at 35,000 ampere turns divided by the sum of 2 plus the length over diameter ratio of the test part. For example, a part 10” long X 2” diameter has an L/D ratio of 5. Therefore, 35,000/(2+5)= 5,000 ampere turns; if a 5 turn coil is used, the amperage required is 5,000/5, or 1,000 amps. This applies to parts with an L/D ratio equal to or greater than 4.
JR
 
After being in the motorsports industry for 13 1/2 years I have developed a saying.
"There are very few people who should own a race car, there are even less who should work on them."
 
Per ASTM E-709

Field strength for longitudinal magnetization using cable wrap and encircling coils, use direct or rectified current at 35,000 ampere turns divided by the sum of 2 plus the length over diameter ratio of the test part. For example, a part 10” long X 2” diameter has an L/D ratio of 5. Therefore, 35,000/(2+5)= 5,000 ampere turns; if a 5 turn coil is used, the amperage required is 5,000/5, or 1,000 amps. This applies to parts with an L/D ratio equal to or greater than 4.
JR

So there you go. A relevent spec and a pertinent citation.
 








 
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