Polarities of Blanchard grinder magnetic table poles

This is a Blanchard grinder with a 60" rotary table. It was overhauled a few years ago but has never held work like it should. The surface has 25 segments but only 5 coils. I checked the polarities of the magnet segments with a magnetic compass and found that the two outer ones read N, then the next 7 were S. This pattern repeated across the table. The S poles were over the coils and the N poles were above the spacers between them. In other words, all the coils are connected in the same polarity and the return poles are the narrow segments between them. It seems to me that the coils should be connected alternately. Lifting the table out and changing the connections would be a major operation, so I want to be absolutely certain before I tell the customer to do that.

If anyone has a similar grinder, just holding a magnetic compass turned vertical over the table with a little bit of current on and moving it from the center to the OD while watching the needle should answer the question.

Bill

I guess the first questions is are you having problems holding parts on the table??

Lets say you take your 6" scale, as you go from outside to inside you scales should bounce back and forth thru the poles.

bottom line-lest say you put a 1" thick x 12" x 12" plates of steel on the table. if you put it in several spots and you cant move it-you magnet is working.

the power to the Blanchard magnet comes from the collector ring in the middle. so I would start at the middle and work out.

it would be possible an outside coil is shot but the insides work.

All the coils are working. This table was rebuilt along with the rest of the grinder several years ago. The person who did it is one of the best mechanics I have known, but he is not a physicist. The table has not held parts well since and several have been thrown off. I suspect he hooked all the coils the same way instead of alternating from coil to coil. The pictures show a Darley compass at various positions. The two outside rings are north poles, then there are a lot of south poles, followed by only a couple of north ones. Lifting a 5' table, turning it over and removing the bottom plates to get to the wires is a non-trivial task and I want to be completely sure it is needed.

Bill

When you rebuild a magnet chuck there are a lot of calculations that need to be done to make sure the correct amount of windings are made and properly done.

I am not questioning the ability of you mechanic but this kind of job is best left to people who do it for a living.

There are several shops in Michigan and one in Rockford that rebuild chucks.

For the reason of safety I think you should have an expert look at it.

If it is not holding parts this is a red flag.

There's a lot of choices on eBay for Gauss meters: gauss meter in Industrial Test Equipment | eBay

If there's a similar electromagnet for reference available to you, you can take readings of the magnetic strength at different parts of the chuck, and through comparison decide whether yours isn't developing the correct power. Should be more definitive than using the compass, and you can then use it for strength tests as part of a preventive maintenance program.

I have an instrument for measuring field strength. The compass was a simple way to illustrate the pole pattern. The coils are the original ones and are developing good fields, I just think they are hooked up in the wrong pattern. To hold a part securely, you want it bridging between north and south poles to make a complete magnetic circuit. A part that isn't large enough to reach across the wide south pole band will be making the return of the magnetic circuit through the air, which greatly weakens it and that is exactly what the operator tells me. Parts near the center more easily cover the smaller pole circles and hold well but ones farther out tend to move.

I am virtually certain that it is just a matter of changing connections, but lifting a 5 ft table and turning it over is a large enough task that I want definite confirmation before doing it.

We are trying to arrange a visit to another shop that has a similar grinder to check the pole pattern.

Bill

A 60" Blanchard chuck is 1620 watts @ 230 VDC the ohm reading of the magnet should be 30 ohms. Can be checked in the cabinet of the chuck control. Maybe label C1 and C2. Make sure you unhook the wires when you check the ohm reading. Are both legs equal? It is very important that each leg is equal. If not the coils may not be wired correct. Or the coils may not be correct.

Magnet Maker said:

A 60" Blanchard chuck is 1620 watts @ 230 VDC the ohm reading of the magnet should be 30 ohms. Can be checked in the cabinet of the chuck control. Maybe label C1 and C2. Make sure you unhook the wires when you check the ohm reading. Are both legs equal? It is very important that each leg is equal. If not the coils may not be wired correct. Or the coils may not be correct.

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There are only two wires from the chuck slip rings. How can there be two legs? Is the center grounded? The controller is a standard Neutrofier.

I see that you tried to send me a PM message. My PM box is always full, even though I try to clean it out. Please send me an email at [email protected] .

As I keep saying, I think the coils are good but should be connected to have a N S N S pattern instead of the all S the way they are now. The only north poles are the narrow dividers between the coils which I doubt can carry enough flux.

Bill

All the permanent magnet tables i have encountered alternate, Whilst the UK national curriculum has its critics, from a young age its covered like poles repel unlike attract its kinda common accepted knowledge is it not? What more proof do you need? A magnetic chuck grips parts by the fact the parts complete the poles path - act as keepers, The more alternating poles a part connects the stronger the grip.

You want proof, get a straight bar magnet, put a steel part a few inches long on one pole, now try your compass, if you stuck it on the south pole of the magnet the steel parts top face is also going to be south.

On a electromagnet, if its setup so they fight there not just not gripping, but actually reducing the strength of the grip in the correct areas, as one coil ends up fighting another!

Look at it anouther way, the OEM would not have bothered to put a magnetic separator between like poles, it would achieve no purpose (with the exception of a fine pole chuck, here rings - seperatos are added bettwen the feild poles so the feild can make multiple jumps up and down as the thin parts can't carry all the flux well between widely spaced poles, it also boosts the grip as its only were the feilds exit and enter parts surfaces that grip is provided.

IMHO you might do better convincing your self turning your compass 90 degrees to the pic, as you slide it towards the middle of your table it should rotate - show how the field jumps from one pole to the next alternately going north - south all the way to the middle. Any like poles are not giving you any grip what so ever.

Everything has been said - no cheating physics in this world.....

You want them to alternate, otherwise smaller or unlucky placed pieces have to close the magnetic circuit through air, which is bad.

Just make sure that your measurements are right, but even then, if the coils are not wired wrong, they must be faulty themselves, gotta pull that table anyways.

adama said:

IMHO you might do better convincing your self turning your compass 90 degrees to the pic, as you slide it towards the middle of your table it should rotate - show how the field jumps from one pole to the next alternately going north - south all the way to the middle. Any like poles are not giving you any grip what so ever.

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I have done that and checked the field strengths. The pictures were set up in a way that I could take them with the camera hand held. I simply need to find out what the proper polarities are. If this was a 10" table, I would flip it over, change the connections, and see if it worked better. I am not going to recommend the customer do that with a 60" one without being certain. Guesses don't fly here.

Bill

I checked mine but they are 20's and only have three coils so of course they alternate with each.
Not much help.
There is this from the Blanchard manual. Again for a 11-16 or 11-20.
Would the manual for yours have better info?
Bob

Whats the deal with flipping a 60" table, im kinda assuming with machines that size they have lifting gear on site? If so its really not a big deal. Hell once you have the lifting gear attached, chances are you have it out and flipped with less than 10 pushes of buttons on the typical cranes pendant.

Whats there to guess? Like poles don't attract its a basic fact, with the same level of certainty that gravity makes that table to heavy too lift by hand :-)

What is it your waiting to find out thats going to convince you? Man up, pull the table, rewire it, testing it with a simple low voltage supply off the machine (watch for back EMF it hurts like fuck on coils this big!!) Slap it back on and send in the invoice, be the hero and get paid for it :-)

adama said:

Whats the deal with flipping a 60" table, im kinda assuming with machines that size they have lifting gear on site? If so its really not a big deal. Hell once you have the lifting gear attached, chances are you have it out and flipped with less than 10 pushes of buttons on the typical cranes pendant.

Whats there to guess? Like poles don't attract its a basic fact, with the same level of certainty that gravity makes that table to heavy too lift by hand :-)

What is it your waiting to find out thats going to convince you? Man up, pull the table, rewire it, testing it with a simple low voltage supply off the machine (watch for back EMF it hurts like fuck on coils this big!!) Slap it back on and send in the invoice, be the hero and get paid for it :-)

Click to expand...

You don't pull a table you pull a chuck that fits into bores, edges, and the drive gear. It binds, catches and pisses you off.
You also have to remove the guarding that on most Blanchards has not been unbolted for a while.
Putting it back you pray the spring loaded brushes don't get damaged or bound.
Even on a little 20 you do everything else possible first.
I wonder what a 60 Blanchard chuck weighs?
They are not like a SG or HMC table which is much easier.

I helped install the table. We hung it from a forklift and one person had to crouch inside the machine to guide it, the kind of place I would not go unless the disconnect had my padlock on it and the only key in my pocket.

If I had to work with someone who had the "Drop that sucker in and get it done" attitude, I would leave the job. I was driving the forklift and the aforesaid mechanic was the inside man. I consider it a major compliment that he would do that. He commented at the time that there were a lot of people he wouldn't trust in my role. If needed, I would have switched places and let him be the driver. There are damned few people I can say that about.

I am very familiar with back EMF. You turn the supply off and let its bleeder resistor bring it down slowly. One time I met the fellow who controlled the magnet in the Fermilab nuclear accelerator. He said it took hours to bring the magnet up or shut it off.

Bob, The test in your manual is what I did with the compass. The results were a bit nebulous, but I think a couple of the coils need to be reversed as described.

Bill

FYI, did not realise it was that hard, i don't mean to be nasty or come across as just drop that sucker in there and get it done, but you have the fault diagnosed, carbide bobs image of his manual proves it as much as you can, unless you can find the correct manual for this machine, but i really doubt your going too get a diffrent answer from that, its now time to do it. By all accounts its bloody dangerous in the current state, i have thrown parts with my 1hp jones and shipman, will pass on being within a thousand yards of a Blanchard with a in correctly wired chuck :-S

As you need to mess with the coil connections underneath it might make more sense to sit it down on a pair of heavy trestles if you have them available rather than flip? Yeah you have to crawl under to mess with the wires, but at least like this you can easily then try it if flipping its as hard as it sounds?

As to bleed resistor, all well and good, just bare in mind its probably not on the coils, but in the controller, so if you do just hook up a small low voltage lab supply go careful!

It is kind of funny how many people are afraid of Blanchards.
Perhaps it is the sound when they start up and you know there is some serious mass moving.
But even when you screw up and forget to turn on the chuck, while the wreck is substantial, and the whole big machine may jump off its mounting pads...........
The parts and wheel pieces tend to stay inside the guards.
Mostly it just scares the shit out of you and the whole shop comes running to see what happened.
3-6 months on one and for flat grinds you will never want to use a surface grinder again.
Bob

This customer did have a guard damaged. There is an awesome amount of inertial energy stored in the motor and you don't want it getting loose. Next to the Blanchard they have a Camut grinder with the same stone setup that sweeps a 24" wide table. The table is linear, like a normal surface grinder with no cross feed, it covers the whole width in one pass. The motor is only 60 hp, but that is still plenty of energy. It was being run by an old timer who has been around the block more times than he can count. In the middle of a job, he noticed that one of the switches on the chuck control was intermittent, so I was called to replace it. While I worked on it, I sort of apologized for holding him up. His reply was "Take your time." We both knew exactly what he meant.

The supply I will use is a simple Variac- transformer- rectifier- filter one, nothing fancy like regulators. It has its own bleeder.

I will keep you posted.

Bill