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Surface Grinder Table Flatness

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electrosteam

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#1
I have a Kent KGS 200 manual surface grinder 1984, about 6 x 12 inches, with unknown history.

The magnetic chuck was clamped extremely tightly, and is now off the table.
The table was removed, the rollers and retainers cleaned, a light lubricant applied and the table re-fitted.
Rolls very nicely.

My metrology is a bit challenged at the moment, but the table surface does appear to be high at the ends.

What would be an acceptable standard of flatness, concave or convex, for the table on a grinder like this ?

Thanks in advance for any advice,
John
 

Bob Korves

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#2
You want the table to be flat and parallel, both top chuck mount and bottom ways. Make sure the table has not been sprung from the over tightening. First, assure yourself that the table surfaces are flat and parallel top and bottom, with equally flat top carriage ways. You can use a very thin coating of high spot blue to see the fit of carriage to table. After you make sure that is correct, or correct it as needed, you can grind the chuck mounting surface to get it flat and parallel with the ways below. You only need to grind until you have bearing at all the corners of the chuck and maybe 50% of the table area underneath. Don't grind the hell out of it, you only need to create a flat chuck mounting, not an overall mirror surface. Grind the bottom of the chuck as well to make it flat. Then put some anti rust oil or other coating on the table and t-slots and the bottom of the chuck. Make sure there is no grit between table and chuck. Do not over tighten the chuck, there is little tendency for it to fly off, and as you have found out it can warp the table, temporarily or permanently. Be gentle with the wrench. Finally, you can grind the top of the mag chuck.

To really get it right requires starting with the four upright member slides (Y axis ways), getting them square and parallel, then the wheel head housing and rear bracket plate, then the bed casting ways, the carriage bottom then top, and finally the table, bottom then top. Starting with the table can upset the geometry of the machine to where it is far more work intensive to get it all correct later. But of course this is renovating the entire machine...

See the book "Machine Tool Reconditioning" in the files section of this site.
http://www.hobby-machinist.com/threads/connelly-on-machine-tool-reconditioning.41802/

Warning, I am a newbie at surface grinders and have little actual experience at doing any of this. I just happen to also need to regrind the chuck mounting surface on my surface grinder soon and have done some studying.

Surface grinders are (or should be) very high accuracy machines, more so than ordinary lathes and mills. Try to do your best possible work on it, and try not to cause additional harm in the process.

Oh, and finally, welcome to Hobby Machinist!
 
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Bob Korves

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#3
Here are a couple good related videos. Stan is a very good machinist and millwright. His work is always done correctly. These are early videos with his first surface grinder, but he knows what he is doing. His channel is full of very good videos about surface grinding and surface grinder care. I recommend his channel highly.

 

4gsr

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#4
Along with Bob has said, get you a good oil stone that is reasonably flat. Not one that is gouged out from many years of sharpening your knife on. Soak it in mineral spirits then hone the top of the table with the oil stone. Don't do it real heavy just enough to remove any high spots of positive metal from the table top. And as Bob said flip the magnetic chuck upside down and surface grind the bottom for 100% clean up. Ken
 

TakeDeadAim

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#5
If the chuck was over tightened to the table it may be tweaked, kind of to the shape of a Pringles potato chip. I would start out by establishing the table is flat to the travel. If you look at the first shot in Stan's video where he has the test indicator on the z axis and moves the table back and forth to check the mounting surface, (where the magnet mounts), on the table you should be able to tell if it has been tweaked or not. If it has that surface needs to be re-cut, either by machining or scraping to get it flat. Once the top of the table is flat and parallel to the travel of the table you can move to the chuck.

The bottom of the chuck should be flat as well, a surface plate and some Dykem or Permatex bluing paste, (not layout dye), can help you find the high spots. When the bottom of the chuck is flat, mount it to the previously flattened table top and follow the steps Stan uses in the above video to grind the top of the magnetic chuck flat.

If you think of this type of machine alignment as a sandwich you have to built it in layers making sure each layer is flat before you add another layer/ingredient. Have fun and be sure and let us know how it comes out. For sure ask more questions if there is something you don't understand.
 

Bob Korves

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#6
If the top of the table is sprung like a potato chip, the underneath of it will be as well. If you do not address that you will not have straight and level motion of the table. The table will climb and fall as it traverses. You must fix that before you grind the chuck mounting surface flat.
 

TakeDeadAim

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#7
I doubt the two fasteners that hold the chuck on a grinder would be enough to pull the entire table out. The more likely scenario would be pulling the area around the bolt holes up creating a raised area around the bolt holes or t-slots depending on the age of the Kent machine. But if the indicator shows otherwise then more work is involved. Pushing down on the end opposite that being examined by the indicator would be a great place to start without straightedges etc.
 

electrosteam

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#8
Thanks for all the comments and advice.
I have now given the table a vigorous rub-down with plain paper wrapped around a ground block with Lanox (lanolin based spray) concentrating on the areas outside the stain left by the chuck.
The table now tests (flashlight only) much better against my 18" camelback straight edge, good enough to put into service.
The problem appears to have been surface contamination that was not obvious.

A good lesson on not being too hasty in assumed concerns.
The really good thing is that all the responses have given me plenty of ideas, and informative links, for investigations now and in the future.

I will get my camelback checked/certified somehow and then review the situation.
John
 

Bob Korves

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#9
Do you have a surface plate in current calibration as large as the straightedge is long? If so, you can test your camelback and scrape it into calibration yourself if you have the skills. You will probably need to ship the camelback for calibration otherwise, and it will be quite expensive (probably more than you paid for it if you bought it used) and might come back broken from damage in shipment. Checking a precision machine while using a precision straightedge without having it in current calibration is effectively like doing the work with the lights off... A flashlight is not the way to test it, either. Get some high spot blue or some scraping ink and use the (calibrated) camelback directly on the grinder table to test it for flatness. From my reading, it is easy to spring a surface grinder table by over tightening the mag chuck to the table.

Actually, checking the table for flatness does not really achieve anything. You will be grinding that surface flat and parallel using the built in grinder anyway. Or will you? What you really want to know is if the bottom of the table has been sprung, and address it if necessary. Then the way to check it for flatness is with a straightedge, cut the way to check it for constant height is with a high accuracy test indicator mounted to the spindle housing as the table is traversed, transverse as well, over the entire surface. Then again, note that with a perfect potato chip table to carriage interface, the indicator might stay at zero while the ends of the table go up and down as they traverse. The work won't be flat. Or, if it is flat, the indicator reading will be rising and falling. See, the plot continues to thicken. Do some mental gymnastics on how the machine works, and the ways it can be incorrect, and then use a method of testing that finds all the errors and determines ultimately what is causing them. The reference I posted above yesterday is the bible of machine tool reconditioning.

Now, if you are at all like me, you just want to get that grinder running and have some fun with it right now. Great! Do it! If you have problems with accuracy or geometry you can always go back and readdress the issues with a well thought out plan.
 
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electrosteam

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#10
Bob,
Thanks for the comments and advice, all good.
The plan:
- check the table for lift/sag with a 1/10th thou DTI on spindle/base at ends/centre as table is traversed,
- blue print the table against the straight edge,
- clean and blue print chuck base against the straight edge,
- regardless of above results, mount chuck and use machine,
- then certify the straight edge and repeat investigation if necessary.
John
 

electrosteam

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#11
I needed to get the SG up and running to get some simple jobs done.
I figure I will learn a lot just using it, perfection can come later.
The table was tested as Ok with the DTI, the chuck cleaned like the table, and the chuck mounted not too tightly.
The wheel is 38A60-J, 162x11 mm, used without coolant at 2950 RPM.
Small sample pieces of MS and CI gave a reasonable finish resulting in <0.01 over 80x30 mm.
Very pleased with progress so far.
I am sure the only reasonable finish is due to my lack of experience.
Coolant is next to be introduced, and experiments with a coarser wheel and various speeds (VFD).

Testing surface plates against the straight edge should start in a couple of weeks.
John
 

Christian Poulsen

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#12
Lot's (if not all) of good advice above...I "rebuilt" several surface grinders in the past including grinding and/or scraping in the ways (vertical; wheel head, and horizontal: table (whether rollers or not)...one thang' that seems to be missing above is after the mag chuck is ground and mounting is finished (usually by way of using another surface grinder) is the wheel used in the final grinding of the chuck...ifn a diamond wheel (about 220 grit) is not available, better make sure the "rock" wheel is proper and doesn't loose much when grinding (about .0002 a pass at finish then down to practically nothing (.00005) (a half tenth LOL) as yoose' get to ends (X) and the back then front of Y...Back and fourth front to back over and over until that indicator mounted (mag) on the wheel head housing or wherever, over the entire surface of the chuck reads .0001 or less!...Note: Sometimes grinding wet gets better results ("flatter") (and remember to let the wheel "normalize" with the coolant running on it a while) and sometimes dry (depending on the wheel)....as does sometimes and usually grinding that chuck with magnet on...Another note: When done and its nice, move the work pcs around on that chuck when yoose' can to avoid the dreaded gradual dip in the center that developes (this used to drive me crazy in a shop when it seemed everybody automatically set their parts smack in the middle of the mag. chucks day in and day out.
 

Christian Poulsen

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#13
...I was just thinking (Hey! I heard that!)...another trick after dressing "flat" a good, hard (I liked some of the Norton pink) "rock" wheels is to lock or move those table stops together (for so no X table movement!) and plunge grind into a pc. of mild steel a little (think!, careful!, easy!, only the back side of the wheel into the left side/edge of the soft steel!!)...this takes off the loose "pebbles" and dust that are going to come off during the 1st pass or so on the table (or work pc.)
 
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