Surface grinder does Cylindrical Work

[Abratool]

That is not an actual valid test for measuring a grinder as the table is ground in-situ...

I think you have to get five small pieces of steel, put one in each corner and one in the middle and grind them, then measure the difference in size....

Or something like that.... I have no surface grinder so am only going by what I have read...

Kicking my self seriously I did not grab a 3ft churchill that was on ebay 15 months ago that was sort of local.... Well more local then Sydney or melbourne where most of that stuff is located...

RC
Been thinking overnight about the test you propose, & perhaps I have missed something ?
The indicator test I did yesterday was to mount the indicator under the grinding head in a fixed position, & then traverse the horizontal table under it, & as mentioned all accurate.
However this morning I set up the 5 blocks as you outlined & ground them all in position, with the same result, all spot on
Have I missed something here?
It would seem that once the table is gound in situ & is accurate & all systems are rigid then the self generating action that follows produces the required results?
regards
Bruce

[RayG]

RC
Been thinking overnight about the test you propose, & perhaps I have missed something ?
The indicator test I did yesterday was to mount the indicator under the grinding head in a fixed position, & then traverse the horizontal table under it, & as mentioned all accurate.
However this morning I set up the 5 blocks as you outlined & ground them all in position, with the same result, all spot on
Have I missed something here?
It would seem that once the table is gound in situ & is accurate & all systems are rigid then the self generating action that follows produces the required results?
regards
Bruce

Hi Bruce,

That test doesn't work, imagine if the ways were twisted or slightly bowed, you would still get the exact same height on each test piece, because the surface of the chuck would follow the ways, but it still wouldn't necessarily be flat.

I think the only quick check is to grind something about the size of the chuck, that's rigid enough not to be distorted by the magnetic field (or shimmed) and then check for flatness against a reference surface like a surface plate. To blue it against a surface plate, you would have to scrape a break-up pass across the ground surface to be able to get a good read on the blue. Blueing a ground surface is hard to read since it just smears everywhere.

There are probably other better methods, might be worth while to check Connelly to see if he describes a method...

Hopefully, someone with more machine survey knowledge can weigh in on the discussion.

Regards
Ray

[Abratool]

Ray
A very good point Ray thanks.
I just a few mins ago, went & did a check on a 6" by 6" cast iron angle plate I ground on this same machine a while back.
Used bearing blue on a known reference scraped surface plate.
The longitudinal grind looks to be in order, but the cross feed on the machine has produced a convex surface.
Now, by convex we may be talking tenths of nothing, I am not sure
The next step I suppose, is to find out the answer, to that question ?
However, I have booked in to the scraping course, & can see after that, could be going crackers, pulling down the machine to hand scrape the slides.
Its all interesting.
However, my wife reckons I am already going crackers. She does not understand "Scraping" & it is difficult to descrbe that process to someone who has no idea of it.
Maybe there might be another support group for scrapers !
regards
Bruce

[harty69]

hi Bruce

I think the best test once you have done your scraping training is to scrape the base of the mag chuck flat then grind the grind the top of the chuck then take it off and blue it on a surface plate to see where your at and while on the surface plate turn the chuck the correct way up run a dial indicator over the top to see what the error is (you will learn this in the scraping course checking for parallel) then make a decision can you live with it or is it a scraping job

cheers
Harty

[Pete F]

Harty I'm trying to understand why not just grind the base instead of scraping it if the top is going to be ground in situ anyway? I read that if the magnetic chuck is taken off it should be reground when it goes back on (presumably since it never goes back absolutely precisely the same way). One (of the few) things I've learnt about grinding is that the tolerances are just stupidly small, and things that I could just take for granted with other machining are a whole world of pain at the grinder.

My understanding is that if a chuck is ground on a "perfect" machine it will be equally good (assuming the grind is good, it took me seemingly forever to get mine to the point I was happy with it). If the work isn't coming out flat (again assuming no operator error) the fault will be with the machine and not the chuck.

Pete

[Abratool]

Pete
Exactly.
Thanks
Bruce

[RayG]

Hi Bruce,

When you take a lick off the chuck, it will just follow the ways, so that doesn't help.

From your test piece it sound like the ways on the cross feed might be slightly worn in the middle section, but if it's only a few tenths, then that's hardly a problem. The way Harty described using a dial indicator setup over a surface plate would be the go, to check for parallel and that would give some measurement as to how convex it is.

Scraping the ways is a bit too advanced for me, RC, GQ or Phil would be better placed to describe how you would go about it using a spotting master.

Regards
Ray

[Pete F]

Hi Bruce,

When you take a lick off the chuck, it will just follow the ways, so that doesn't help.

Regards
Ray

That's right Ray. However rather than taking the chuck off wouldn't it be better to simply grind something and test it? It seems to me you'd otherwise be constantly chasing your own tail.

Pete

[RayG]

That's right Ray. However rather than taking the chuck off wouldn't it be better to simply grind something and test it? It seems to me you'd otherwise be constantly chasing your own tail.

Pete

Hi Pete,

Correct, and that's just what Bruce did earlier today... a few posts back I think...

Regards
Ray

[Abratool]

Yes

[Abratool]

Hi Bruce,

When you take a lick off the chuck, it will just follow the ways, so that doesn't help.

From your test piece it sound like the ways on the cross feed might be slightly worn in the middle section, but if it's only a few tenths, then that's hardly a problem. The way Harty described using a dial indicator setup over a surface plate would be the go, to check for parallel and that would give some measurement as to how convex it is.

Scraping the ways is a bit too advanced for me, RC, GQ or Phil would be better placed to describe how you would go about it using a spotting master.

Regards
Ray

Ray
I think this is the logical method.
I will run a dial indicator over my test ground piece using a dial indicator off my known reference flat scraped surface plates (got 2 of them) to determine the error.
The Churchill surface grinder is about 70 yrs old so I do expect some error.
regards
Bruce

[Pete F]

Hi Pete,

Correct, and that's just what Bruce did earlier today... a few posts back I think...

Regards
Ray

Ah ok, I obviously misunderstood when you said this "The way Harty described using a dial indicator setup over a surface plate would be the go..." I'll plead the jetlag case (and I'm only in PER )

I was wondering why you've found it necessary to regrind your chuck now a few times Ray?

Bruce my grinder is just a small one and doesn't use coolant. I see yours does, but one thing I always need to be super careful of is heat distorting the work, and at this level even the slightest amount will do it. Obviously I'm not trying to tell you how to suck eggs, but thought I'd throw a reminder out there FWIW when you're checking it to this level of accuracy. Even at times I wouldn't have expected it, like one time I was grinding some parallels I'd made up myself that had some holes in them. I recall how much the metal distorted over the top of the holes compared to the rest of the parallels, even though the holes were a good distance from the edge. As I mentioned above, my chuck took a loooong time to get to where I was really happy with it. Sweet FA heat was being put into it, yet it was enough to distort it across it.

I swear grinders are the work of the devil

Pete

[harty69]

my method was based on what we have on hand
yes if the base was ground on a known good grinder and had not distorted over time or from handeling then it would be as good as a scraped base
but we dont know if Bruces has been if it has not then when you bolt down the chuck you can deform the table thus introducing an error this also applies to the table its self if we want to be really picky how long since the chuck has been taken off now we have coolant on is there corrosion forming under the chuck we dont know about and introducing error
yes you will need to regrind the chuck once you put it back on after doing this
the reason for using the chuck its self is if you just use a test piece you need to make sure that it cant be deformed to the shape of the chuck when the magnet is turned on or we get a bit of grit under the piece so if we remove that possibility for this error we are in a better position to make an accurate measurement splitting 10th's I know but we are talking about grinders

cheers
Harty

edit this is only to asses the condition of the machine you would not do this to check how your scraping of the machine is going if it needed it thats a Phil question

[RayG]

I was wondering why you've found it necessary to regrind your chuck now a few times Ray?

Hi Pete,

Generally just a skim whenever there is a special job, that warrants it.. or the chuck starts to look a bit ordinary.

Regards
Ray

[Pete F]

Harty, yeah I guess I see what you're saying, and agree you'd want the bottom to be pristine. How I did mine was to turn the chuck upside down, turn it on so no distortion was caused to the machine's table. If I wanted to get really anal I would have ground the table first, but it looked pretty good, and as long as it wasn't so far out as to cause distortion when the chuck was bolted, I think it will be fine. I then installed the chuck and VERY carefully ground it while it was turned on. I'm very happy with the way it turned out.

I agree with what you're saying about the test piece distorting by being pulled down. It would definitely want to be suitable as a true "test" piece. Personally once the chuck goes back on I would like to then consider that as now a permanent part of the machine and not taken on and off. But maybe my logic is flawed.

Ray I'm curious, do you find your chuck "moves" enough to warrant regrinding? Holy Cow, on a manual machine like mine I sure as heck hope I don't have to regrind mine again in a hurry

Pete