Thoughts On When To Dress Magnetic Chuck

[JoeLee]

Lately I have been noticing some small discrepancies on parts that I have ground. Don't really notice anything on small parts under 2" wide or 2" in length but do see some thickness variation on larger parts over 3" wide and 6" long. Last time I dressed this chuck was probably 20 years ago or so. I've always kept it clean and never slid parts across it. When I did dress it I did it with the magnet engaged as directed by the mfg. So the other day I decided to indicate the surface of the chuck. What I found wasn't an alarming difference but what bothered me the most was that when I engaged the magnet the center of the chuck would rise about .0003. Usually it's the other way around I believe. Here is a diagram of my readings.
I indicated it with the magnet engaged and with the magnet off for comparison purposes. If I remember correctly the center of the chuck would pull down a couple tenths when engaged not rise.
Internal problems?? not sure. The lever moves with the same amount of force as it always did. MO means magnet off in the diagram.

Should I dust the chuck again or do I have other issues? The chuck is an older B&S 6" x 18" with the lead segments. It's a PIA to grind it.

JL.................

[JoeLee]

Here is a picture of the chuck.



JL................

[wierdscience]

If it's been that long since being surfaced I would think it's time. Trick I picked up from an old toolmaker is to soak down the surface of the magnet with WD-40 between passes. The WD keeps the lead and brass damping from loading the wheel up and also reduces heat input.
It also helps seeing where the highs and lows are while grinding. I do that by coating the surface with a fat black marker before the spark pass.

I've got a 6x12 B&S magnet very similar to that one, the magnets in it are nearly completely dead. The one I have just uses steel bar magnets.

[Mcgyver]

When I install mine I first check the fit of the table to the chuck with blue then carefully ground it in with flood coolant and a long labourious spark out. There after when I just have to be sure, I grind a parallel in place. That to can trip you up....i after several poor results trying to get something flat I discovered I had a garbage made in India parallel that I could actually twist in my hand!

Anyway, assure a full contact fit to the table, spark out with flood and grind in a parallel when it really matters are the take aways.

[Doozer]

How much rust is under the chuck?

-D

[JoeLee]

If it's been that long since being surfaced I would think it's time. Trick I picked up from an old toolmaker is to soak down the surface of the magnet with WD-40 between passes. The WD keeps the lead and brass damping from loading the wheel up and also reduces heat input.
It also helps seeing where the highs and lows are while grinding. I do that by coating the surface with a fat black marker before the spark pass.

I've got a 6x12 B&S magnet very similar to that one, the magnets in it are nearly completely dead. The one I have just uses steel bar magnets.

That's what I did when I dressed it in. Now I have a mister that I think I will use this time. I wouldn't ink the entire surface because as thin as that ink is too much of it will load the wheel. Maybe draw some circles around the areas where the difference is and it can be watched that way.

JL..................

[JoeLee]

When I install mine I first check the fit of the table to the chuck with blue then carefully ground it in with flood coolant and a long labourious spark out. There after when I just have to be sure, I grind a parallel in place. That to can trip you up....i after several poor results trying to get something flat I discovered I had a garbage made in India parallel that I could actually twist in my hand!

Anyway, assure a full contact fit to the table, spark out with flood and grind in a parallel when it really matters are the take aways.

After I restored the machine I did indicate the table before mounting the chuck. There was no rust on the table. Coolant was never used on this machine. It was dead flat all over. I had the chuck ground at a local shop on a big grinder. They ground top and bottom sides, the top had some imperfections in it that I wanted removed. When I mounted it on the machine I dusted the top. Didn't need to take much off, maybe a couple tenths overall.
It's been good ever since, until recently I noticed these discrepancies.

JL...................

[Rich Carlstedt]

Joe
Many years ago when I used manual grinders with lead chucks, I was taught to wipe the table first with paraffin wax to prevent
the wheel from loading when resurfacing.
be careful of indicator movement/deviations caused by the magnet being on
Rich

[Doozer]

Just a point of reference, because it was mentioned here....
Every Brown and Sharpe mag chuck I have used was very weak.
Either the magnets were never that strong or the lost some jam over time.
I have an O.S. Walker mag chuck, an older one. The pre-ceramics branding.
Anyhow, I am always impressed how strong it is for an older mag chuck.
Back to the grinding question...
A friend of mine was having a heck of a time grinding in a Brown and Sharpe
chuck. The solder around the poles kept loading the wheel before he could
get across the chuck. He talked to a Norton rep and he sent my buddy a
7" grinding wheel, black in color. Not sure if it was AO or SC, the word is that
it worked well for the job.

--Doozer

[JoeLee]

Joe
Many years ago when I used manual grinders with lead chucks, I was taught to wipe the table first with paraffin wax to prevent
the wheel from loading when resurfacing.
be careful of indicator movement/deviations caused by the magnet being on
Rich

I'll keep that in mind when I dress this, but the idea of using wax is now to me.

JL................

[mikey553]

Joe,

If your grinder is that old, maybe it would make sense to check the ways. I have a 6 x 12 grinder, which was used in a toolroom for many years. When I checked the ways, they were worn and no longer straight. If you dress the chuck with worn ways, it will check very good with an indicator, but your parts will not be flat. You will have a mini-copy of your ways in the part. The bigger the part, the more effect it will have on accuracy.

My ways are worn to the point I have to mill them first before scraping. The highest point is in the middle, the ends drop down from that point. I believe it is a natural wear on this inexpensive machine with poor lubrication and gravity trying to change the shape of the ways. Years ago I was making a telescope concave mirror using the same technique. When you are rubbing two surfaces against each other, the top surface becomes concave and the lower - convex.
Mike

[JoeLee]

Just a point of reference, because it was mentioned here....
Every Brown and Sharpe mag chuck I have used was very weak.
Either the magnets were never that strong or the lost some jam over time.
I have an O.S. Walker mag chuck, an older one. The pre-ceramics branding.
Anyhow, I am always impressed how strong it is for an older mag chuck.
Back to the grinding question...
A friend of mine was having a heck of a time grinding in a Brown and Sharpe
chuck. The solder around the poles kept loading the wheel before he could
get across the chuck. He talked to a Norton rep and he sent my buddy a
7" grinding wheel, black in color. Not sure if it was AO or SC, the word is that
it worked well for the job.

--Doozer

I wanted to replace this chuck after I restored the machine, but the Walker chucks were around 2K at the time if I recall. I also thought of Suburban. Don't want anything with leaded segments. Brass would be better and stainless better yet, at least for dressing issues.
The B&S chuck has plenty of gripping power for stuff over 2" long or so, as long as you span a couple segments it'll hold. It's small parts that tend to get pushed forward sometime.
That's where a fine poll is nice, and that's what I would get.
The other small problem I have with this is residual magnetism after I turn it off. It's hard to take large stuff off of it, like my tool makers vise for example. I have a couple 3/8" sq. aluminum bars that I use to gently pry it off the surface. It's always been that way as far as I can remember.

JL:...............

[wierdscience]

That's what I did when I dressed it in. Now I have a mister that I think I will use this time. I wouldn't ink the entire surface because as thin as that ink is too much of it will load the wheel. Maybe draw some circles around the areas where the difference is and it can be watched that way.

JL..................

What grit wheels are you running?

Rich also has a good point about the magnet influencing indicators. My Starrett Last word (mostly steel) indicators show influence, but my Teclock stainless ones don't

[JoeLee]

I indicated the chuck with my Interapid DTI. Not sure if magnetism has any influence on it. Maybe someone else can answer that.

This is the instructions from KO Lee on dressing and resurfacing the chuck. I believe I followed this when I first dressed it 20 years ago. https://leblondusa.com/wp-content/up...esurfacing.pdf

JL...................

[Rich Carlstedt]

Just a point of reference, because it was mentioned here....
Every Brown and Sharpe mag chuck I have used was very weak.
Either the magnets were never that strong or the lost some jam over time.
.........................................--Doozer

Doozer,---- My experience as well !
Rich