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Scraping in the underside of a mag chuck

implmex

Diamond
Joined
Jun 23, 2002
Location
Vancouver BC Canada
Hi All:
So I got the J&S 540 grinder but I had to take the mag chuck off and take it apart to fix the activating mechanism.

When I got it back together, the underside is not flat...it's off by a mile.

So my question for you all;
Do I re-scrape the ways flat, scrape the table flat and then scrape in the underside of the chuck, or do I just match scrape the chuck to mate with the table as it is.

The grinder measures very flat...within a tenth or so as best I can measure with granite straight edges and a tenths clock.
The ways still look good to me, the original scraping is visible and consistent for the entire surfaces I can see without taking the table off.

I'm awfully tempted to just match scrape the bottom of the chuck and call it good.
I can't see doing any harm to the grinder and if it's not satisfactory, I can still pull it apart and do the full meal deal on it.

Am I just a lazy pig who should know better, or would you agree I can do this just fine for basic surface grinding on injection molds and the like?
I do have to precision grind at times, but all my parts are so small these days I will probably never have to grind an 18" long piece again in my lifetime.
Precision grinding for me is hitting size, flatness and parallelism within a tenth on a piece 2" x 3".

A buddy of mine recommended just potting the magnet with Moglice and save myself all that scraping, but I don't get a warm and fuzzy feeling doing something like that.
Theoretically it sounds fine but I'd hate to see what it looks like if coolant leaks under it and rots there for a decade.

What say ye all.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
 
Qt {I got it back together, the underside is not flat...it's off by a mile.}

How much is off by a mile? concave or convex? I wonder if someone bent it with a chuck grinding burn.

You might flip it on the set pad and grind the bottom to <.0005 / .0002 or so, but I would not Mag it to the set pad. I would not want my table to be Maged up and have no way to de -mag. Then scrape it if you want it closer.

Yes, your set pad need be flat and true. Some guys regrind the set pad after a re-scraping the ways.

Guess I might consider 1/64 gasket material with it being greased, no I have never tried this.
Amazon.com: Custom Accessories Shop Craft 37711 9" x 36" x 1/64" Fiber Gasket Material: Automotive

But yes I don't know if the stuff would swell in tima so would not trust it. I have just greased the bottom, used 30wt oill, and used way oil under chucks.
 
A hard grinding burn would make it convex. stressing in contraction... is the top also out of flat?//but it may have been reground. Be sure to use all caution when wet grinding the chuck Top. Do you have a decent wheel? Yes. I have dry ground chucks but do not recommend doing that. Hand feel the parked wheel in a number of places to know about where the first hit will be so you do not crash from the get-go.

J&S 540 is a nice grinder. the big shop bought me one so I had it brand new. It was an 8-24 and was programmable, I often used it manual because the program wouldn't do everything I wanted to do.
 
Hi again michiganbuck:
I didn't check the flatness of the chuck before I took it apart.
However once I took it apart it's quite possible the cast iron box that forms the base of the chuck stress released once the bolts were loosened.
So it didn't return to a flat state when I put it all back together.
Now it has that 0.002" hump in it and that twist of about 0.0005".
That's not a lot over 18" but it's enough that I'm convinced it'll fuck up the grinder if I don't address it before I bolt the chuck back on the machine.
So I want it to mate to the pad as perfectly as I can, and then just nip it with the clamps.
If it mates really well with the pad, I can kiss the top a tenth at a time until the top is flat too, and hope it doesn't move again.
Yeah, I'm gonna wet grind it with a 46 J wheel with a nice brisk dress.
I'll map the chuck surface first so I know where the high spot is, and I like to kiss the chuck a tenth or two at a time and just take the time it takes to bring it into flat.
I've never been a fan of hogging while trying to flatten a mag chuck...it's always been faster and better for me not to burn the shit out of it by getting impatient.

I'm doing the rude and dirty version by just assuming the grinder itself is still essentially perfect, based only on how the table moves when I put a tenths indicator on it.
I THINK it's good but I don't KNOW it's good and I don't know what "best practice" is and how "acceptable practice" differs from it.
A proper machinery rebuilder would probably pull the table and check the saddle with a straightedge, but I don't have one and I don't want to turn this machine into a lifetime project.

If nobody I respect squawks bloody murder about my plan, I will just do the abbreviated version first, secure in my hope that I won't hurt the grinder.

As I said before if I can't get it to grind to the standard I need, I can always pull the table and scrape everything myself or alternatively, send the machine out for a full rebuild.
At least a J&S is worth it...the old Kent that it replaced was a pretty crummy machine that I'd never put rebuild money into.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
 
If the top pad on the table is pretty flat, I'd just re-grind the top and bottom of the mag chuck. I recently re-did the chuck on my MicroMaster after disassembling it to clean, reoil and reseal just like you did. Mine also went way out after disassembly/reassembly. I ground to 80% cleanup on the bottom, 80% on the top, then 100% on the bottom, 100% on the top, keep flipping until it ground consistently flat by watching sparks (no clamps on for any of this, only stops at the ends of the chuck). Then kissed the bottom flat, set it down on some thin plastic shim to isolate it electrically from the table, (attempting to kill any galvanic reaction/corrosion) put the clamps on and kissed the top flat. This works very well for me. If the chuck is within a couple tenths of flat when sitting free I don't see any need to scrape it. If it's got a high area in the middle with no contact underneath then I might. A chuck with less than a thousandth of an inch convex bottom should pull itself tight when clamped with no issues.
 
A non-flat pad and a non-flat chuck bottom will bend the ways when clamped up.
So everything is a matter of degree.

It is most practical if the pad is parallel to the ways. And flat
It is most practical if the chuck bottom is flat.

So what i have done several times on a couple grinders is grind the pad minimum possible, then scrape it to a small surface plate.

Then put the chuck on upside down, do not use the magnet, because if the top is not flat it will merely distort the table.
Block it with something in the T-slots both ends, but non-influencing. Then grind the chuck bottom. The 3 i've worked on were not full surface - lots of relieved area, so very little extra effort to carry them (one is a 10 x 24) to a surface plate and spend another hour scraping it closer.

Then do your prefered magic rites with holy oils, oil paper or not, etc, and such, bless it three times to keep the rust demons at bay, and clamp as per general instructions one end snug, the other not so much.

Then grind the chuck.

It's actually not that bad, and you don't have to always wonder.

Except whether the rust demons are creeping under....

smt

PS - grinding the pad on a surface grinder is like surfacing the table on a shaper or planer.
Not to be done often, and to be fully considered when undertaken. However, it used to be a recommended procedure when changing a chuck. Think i first referenced it in an old B & S #2 manual but could have been in a different one.

PPS, probably not obvious from above, the 618 got a full rescrape except the column.
The 1030 got a quick and dirty touch up mostly on Z (in/out) before doing the mentioned chuck fittings the first time. Other times was due either to fitting a new/different chuck, or once due to rust.
 
Especially if there is the risk that the observed warp is due to stress on the top of the chuck, I think that eKretz's strategy is the best. Otherwise, you risk that, once you touch the top, the bottom becomes concave.

Since you're scraping it, I strongly recommend you to do a couple of blind scraping passes to the top and observe how the warping changes at the bottom.

Paolo
 
Keep in mind that when you are grinding the full chuck face it becomes a very large part so one pass makes heat. It can be an aid to overtravel, so off a bit at the ends to give some heat rest. I have given big parts a pause at the ends off the part.

I think grinding the set pad should not be done if there are any doubts of the condition of the ways being true. Plus if an unskilled grinder hand tried doing that and put a severe burn in the set pard the table could be stressed out of flat.

To avoid often grinding a mag chuck is good because the chuck top can get thin. This is yet another reason to move parts about the chuck so it gets wear all about not just in one place. Plus grinding out a wheel nick or the like is poor practice.
 
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Condition of the ways is everything.
no sense grinding away pads or chucks if the grinder is not true.

But that is one reason i prefer to scrape pad, and chuck base, as final op between mating.
It takes way condition, grinder hand practice, & even most temperature issues out of the equation.

From there down or up, it's the ways, but at least the chuck-to-table mount is non-distorting.

smt
 
I think I understand scraping rotary magnets using table to master (slightly spherical base won’t make a difference?), but don’t understand same with long rectangular magnets on conventional surface grinders. Wouldn’t it be better to test/rescrape the base first using known quality surface plate?

Not criticizing, just trying to understand.

L7
 
It would be good in my opinion to know the set pad is flat and true but one could not know that indicating from the wheel head. You would need to use a precision parallel, small plate or something that is near dead flat, and perhaps a machinist level.

Still, I have mounted chucks on set-pads that were not perfect with just figure 8 oil honing the highs to near-flat...yes using a 6 or 8" Norton stone...and did the same to the chuck bottom.

Grinding a chuck bottom is tricky because you grind with chuck mag turned on but don't want to mag-up your table/machine.

I see guys using small stone 2 ,3 or 4" on a chuck top on youtube, I never do that for making holes/lows, I use a chuck-use-only 6 or 8" Norton (or better stone) smooth side and hone the whole chuck.

I have ground chuck tops with a mile of wear and grind such an abused chuck with the mag turned off until I get to .003/.002 or so near clean up and then turn on the mag. it seems to me that a chuck grinds a little hotter with the mag turned on.

Back in the day when the USA had more work one might have made a business of a service call to grind chucks.
 
They do get warmer but there is the rub. Will the chuck heat evenly? Does the heat all sink out the pad at the bottom? Will coolant use keep the top of the chuck cool? I think good chuck makers take all of these into account as much as possible. I have seen some mag chucks that distort more than others. Some almost none.
 
Sounds like you are an experienced grinder, so I wouldn't spend time scraping. Instead:

(1) kiss the table pad very lightly, just to make sure that it's flat.

(2) turn off magnet

(3) set magnet upside down on 3 points, block it in, make sure that it is as level as possible, grind magnet bottom with care

(4) flip magnet over, grind top with care

(5) check that top and bottom are flat and parallel, if not repeat (3) and (4) with a very light cut

(6) You're done: clamp magnet to table pad
 
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I think I understand scraping rotary magnets using table to master (slightly spherical base won’t make a difference?), but don’t understand same with long rectangular magnets on conventional surface grinders. Wouldn’t it be better to test/rescrape the base first using known quality surface plate?

Not criticizing, just trying to understand.

L7

You are correct, in reality my method of the final scraping of the magnet to match the table will not do much for accuracy.

The main reason I do this is to make sure we have a perfect seal between the magnet and the table so that coolant does not get in between them and start to rust and then create tolerance issues.
 
Hi Cash and others:
My motivation is to keep the clamping of the chuck from pulling the table and the ways out of alignment.
If I get a good seal between chuck and pad that keeps the rust at bay, I will happily take that as a bonus.

Since the underside of the magnet is convex (well it was...now it's just about mated to the table), and I could slide feeler gauges under the ends, I decided to bite the bitter pill and commit to getting rid of the gap so the mag chuck clamps can no longer influence the flatness of the ways.
It will ultimately be no better than the condition of the ways allows, but at least I won't make it worse.

If the grinder is still as good as it seems to be, I will have a winner.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
 
I have never scraped or ground in the bottom of a chuck.
At the most I flat file it.
I will put goo, plastic or gasket material under it anyways.

Now the questions:
Given that the bolts on the end put local tension into the table should a table be scraped in with the chuck in place? How many here do this?
How tight should these bolts be? Does mounting a chuck change things? Just over finger tight or gorilla tight so that it will never move in a big wreck?
Mine are mostly smaller grinders so the tables love to twist and bend everywhere but even on a 10x54 B-port type machine this can become a problem.

For sure know flat on the attachment can only be good.
Bob
 
I have never scraped or ground in the bottom of a chuck.
At the most I flat file it.
I will put goo, plastic or gasket material under it anyways.

Now the questions:
Given that the bolts on the end put local tension into the table should a table be scraped in with the chuck in place? How many here do this?
How tight should these bolts be? Does mounting a chuck change things? Just over finger tight or gorilla tight so that it will never move in a big wreck?
Mine are mostly smaller grinders so the tables love to twist and bend everywhere but even on a 10x54 B-port type machine this can become a problem.

For sure know flat on the attachment can only be good.
Bob

I've never bothered scraping, mine have always been flat enough not to need it after grinding. I've only had to grind a couple tables, but I've re-ground plenty of mag chucks. Normally the bottom is fine with a quick once over using a hone, but I have had to regrind bottoms after disassembly/reassembly of the mag chuck (all the way to the innards for cleaning/relubrication/repair) pretty much every time. I've never noticed much of a deviation from flat with the resultant parts when checking on a surface plate compared to what I started with so I guess the tables were stiff enough to resist deformation.

I don't tighten the chucks very tight at all, just enough to keep them from moving in normal use. I tighten the clamps with one finger on the wrench, slightly tighter on the left than the right. In a crash the table might possibly slide a little, but as far as I'm aware I've never had a hard enough hit to do that.
 








 
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