What is a good test run for evaluating a used surface grinder for purchasing ?

[max.levesque]

I'm new to surface grinding (and a hobby machinist), and I'd like to get a used surface grinder.

If I bring a steel rectangle, grind it on the machine on both faces, and test for flatness with a straight edge, then test for thickness variations,
would that make a good test to ensure the machine is decent ?

Are there better tests ?

 

[Paolo_MD]

Personally, I would bring a precision level (no more sensitive than 0.0005"/10") and two sets of feeler gages (or one set and two blocks of similar thickness): sit the level on the feeler gages/blocks and shim till it reads zero.
Move the table (preferably slowly) and observe what the bubble does: it will tell a couple of things. First, if the ways are worn unevenly over the travel and by how much. Second, especially on machines where the table is driven by rack and pinion, if the wear is at a point that causes too much gear engagement, resulting in ripples in the table movement (resulting in the bubble vibrating).

Clearly, you won't find a machine that will read perfectly flat and, if your level is too sensitive or the machine is not properly resting on the floor, you might read mostly the rocking of the machine or the movement of the floor under the shifting of your weight and/or the shifting of balance of the machine.

Before you go, you need to answer yourself what kind of precision over what lengths you want to achieve with your grinder and if the wear of the machine you are inspecting is tolerable or not for your needs.

Paolo

 

[Luke Rickert]

It might be a better idea to have multiple smallish parts (all the same height and big enough to stick without risk to moving) and grind them at once with one in the center and then four corners of the chuck. I suppose you really need balance and dress the wheel and regrind and stone the chuck before doing any tests)

It all does depend on what you want to with the machine. If you can test grind the sort of part you want to make that would be ideal

Also there are so many variables in getting a good grind on a part beyond the condition of the machine a test grind isn't necessarily going to tell you what is going on. Using individual parts avoids the issues with long parts bending if they aren't completely flat to start with (or needing to shim etc)

As mentioned above a level can be informative as well although be aware how long it can take the bubble to settle on a sensitive level.

 

[john.k]

Take a solid piece of cast iron with some holes in it ...say 1",1&1/2 " ......and see if the wheel "drops " into the holes ,this is easily seen by a sort of a track on the leaving side .....this side will also be low compared to the flat surface............if the owner will lift the table ,you can then see what the slides are like.......or it may have rollers or balls.

 

[Luke Rickert]

The other option is find a cheap grinder and plan on rebuilding it (just needs to run to start with) It is totally crazy if your time is worth anything but you know what you have at the end of the process. Rescraping my Mikromat was an adventure to be sure

 

[Richard King]

I like what MD said. You should also check a couple of other of things not related to the accuracy.. Is it hydraulic? If it is, run it and listen to the pump. Is it quite and have a steady sound? Run it for 10 minutes and look at the oil in the tank. If it's white, the pump is sucking air. 2) Go to the local auto parts store and buy a stethoscope and run the spindle and listen to the spindle bearings once they are warm after running 10 minutes, put the probe on the spindle just behind the wheel cover and shut off the spindle, worn bearings will rattle as they coast to a stop.

Run the table and saddle out as far as the travel and use a flashlight and look at the ways, see if you can see scraping marks. As far as grinding. Make sure to use coolant, don't grind anything without it. Lay a flat bar on the chuck and turn it on and try to lift it off, do this all over the chuck. I have an appointment and got to go. I have a few other suggestions ill write later. . .

 

[Luke Rickert]

While coolant or at least grinding oil are great, there is no reason accurate work can't be done dry. There are tons of grinder hands who do toolroom type grinding dry and there are many small, high-end grinders that are designed to run without coolant. Particularly for a test, the presence or lack of coolant isn't going to make the difference between a grinder appearing badly worn or acceptable unless the person running it is rather careless.

The other thing about scrape marks, the ends of the underside of the moving table really isn't interesting (unless it is fully supported by the lower ways) that surface generally sees little to no use. If you want to judge wear in the way you need to take the table off (or slide it past its normal travel) and check the ends of the lower ways where the cantilevered weight of the table bears. Unless it is a very small machine that might not be super easy to do.

best of luck

L

 

[stephen thomas]

It might be a better idea to have multiple smallish parts (all the same height and big enough to stick without risk to moving) and grind them at once with one in the center and then four corners of the chuck. I suppose you really need balance and dress the wheel and regrind and stone the chuck before doing any tests)

That's how used tool salesmen used to sell bad grinders to unsuspecting good people.

With a freshly ground chuck, the small parts will all measure fine, though the ways be banana shaped, and the chuck swaybacked like a saddle.
Indicating the chuck is almost as revealing for the same reason - it mostly maps the ways as they were ground, and will read -0- except where the rocking is really bad.

Do what Paolo said, or if you have a long true parallel straight edge wide enough to lay on the chuck without falling over, indicate on that (chuck off) because it will take a flat surface independent of the ways, and compare it to the ways as the table travels. I like a 9 x 12 or 12 x 18 granite surface plate to shim on the table or chuck to read -0- in 4 corners, if that is possible (It might not be as ways wear). Now you can map all directions. It looks like there are now even import 6 x 18 granite plates that would be more convenient, though 2" is rather thin for a claimed grade A that length.

https://www.amazon.com/Granite-Surface-Plate-18-Grade/dp/B006JYKIVC?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=A19VW1BL9ZXZVA

BTW, if you want to indicate directly onto the chuck surface with an indicator sufficiently displaced from the wheel contact point (sticking out sideways from the wheel CL at least 6") you can get a quick check if the ways are really bad. But the question becomes, was the chuck freshly ground, and if so, is the surface shape flat when it is off? IOW the chuck is ground in the on situation and might not be flat when off. But if you indicate it directly when on, the magnetism will influence most indicators and might make it look worse than it is.

 

[max.levesque]

Thanks for all the great suggestions, I'm always amazed at the quality of the information shared on this site.

The machine is a Proth PSGS 2550 Hydraulic Surface Grinder

As for my needs in terms of accuracy, it will probably sound irrational to many, given that I'm a hobbyist.

My hobby projects are prototyping stirling engines, stirling duplex heat pumps, vuilleumier cycle heat pumps, and tight tolerances on critical dimensions is one of the variables that affect their performance.
I must also say finding ways to improve accuracy of tools, is part of the fun, building myself a DiY cylindrical grinding fixture will be my first project on the surface grinder, this is why I'm looking at machine with X travel longer than what is commonly found.

Some will say that my money would be better spent on picking up less expensive hobbies, like music, or mushroom picking (or spending it on a psychiatrist !), but the reality is that "economics" don't apply here, at least not in the traditional sense. Some will pay to go on a hunting trip, others will pay to bring a machine that produce clouds of dust in their garage !

I have read in many places that decent surface grinders can "easily" produce 0.0001" flatness and straightness over their travel rectangle.

Not sure to what extent this is is a realistic expectation for a used machine (or it only true of ultra high end machines), but if they are rare, I have no problem waiting a year for it, or having to do some restoration, it's actually part of the fun.

So far there seems to be consensus on the following evaluation procedure:

1. check height variations on the table with a test indicator mounting the mag base on the spindle column.
2. listen for bearing noise
3. check for "bumps" in the travel of both axis (revealed by occasional "excitation" of the bubble on a level)
4. check spindle run out with a test indicator

As for grinding a test piece, it seems there is no consensus on flattening a "big rectangle" or small pieces at multiple place on the table.

The "big rectangle test" could show "false problems", while the "multiple small pieces test scattered on the mag table" would fail to reveal poor straightness in an axis.

Could flattening 2 thin rectangles, one for each axis, placed lengthwise on each axis (length matching each axis, width of apox 1 inch), be a compromise ?

 

[???]

Thanks for all the great suggestions, I'm always amazed at the quality of the information shared on this site.

The machine is a Proth PSGS 2550 Hydraulic Surface Grinder

As for my needs in terms of accuracy, it will probably sound irrational to many, given that I'm a hobbyist.

My hobby projects are prototyping stirling engines, stirling duplex heat pumps, vuilleumier cycle heat pumps, and tight tolerances on critical dimensions is one of the variables that affect their performance.
I must also say finding ways to improve accuracy of tools, is part of the fun, building myself a DiY cylindrical grinding fixture will be my first project on the surface grinder, this is why I'm looking at machine with X travel longer than what is commonly found.

Some will say that my money would be better spent on picking up less expensive hobbies, like music, or mushroom picking (or spending it on a psychiatrist !), but the reality is that "economics" don't apply here, at least not in the traditional sense. Some will pay to go on a hunting trip, others will pay to bring a machine that produce clouds of dust in their garage !

I have read in many places that decent surface grinders can "easily" produce 0.0001" flatness and straightness over their travel rectangle.

Not sure to what extent this is is a realistic expectation for a used machine (or it only true of ultra high end machines), but if they are rare, I have no problem waiting a year for it, or having to do some restoration, it's actually part of the fun.

So far there seems to be consensus on the following evaluation procedure:

1. check height variations on the table with a test indicator mounting the mag base on the spindle column.
2. listen for bearing noise
3. check for "bumps" in the travel of both axis (revealed by occasional "excitation" of the bubble on a level)
4. check spindle run out with a test indicator

As for grinding a test piece, it seems there is no consensus on flattening a "big rectangle" or small pieces at multiple place on the table.

The "big rectangle test" could show "false problems", while the "multiple small pieces test scattered on the mag table" would fail to reveal poor straightness in an axis.

Could flattening 2 thin rectangles, one for each axis, placed lengthwise on each axis (length matching each axis, width of apox 1 inch), be a compromise ?

I've used Proth in toolrooms and they were okay, not a top of the line machine. The precision level test and visual, feeling for vibration, audio and general cleanliness checks will get you a good idea. Sometimes you just need to take a chance, obviously not if it's a lot of money. I bought a Svend Jakobsen grinder yesterday for AUD2500 using a Wyller bubble level with .0005" in 10" graduations, I levelled the bubble in the centre of the table at wound it 300mm left and right. Moved half the width of a graduation so about .00005" over 600mm, more than accurate enough over a 50mm workpiece. I also checked for wear steps in the ways and general cleanliness.

If a machine has grit under the table between the ways etc I walk away, it usually indicates someone using a airline to clean.

 

[stephen thomas]

1. check height variations on the table with a test indicator mounting the mag base on the spindle column.

Some of us specifically advised against such foolery.
Unless you put something known/qualified flat on the table or chuck & shim it to an equal known value at each end.

If you indicate a chuck or table directly, you are likely to either fool yourself out of an acceptable machine, or to accept a machine that is actually below your intended standards.

Also, without a lot of experience, your expectations of what a "typical", "average" quality, machine like your link can attain over the full extremes of travel might be a little optimistic especially if the part is not extremely rigid and thermally stable. Small pieces, sure. But get one, use it, enjoy. That's the only way to really start learning how to get closer to your ideals. They are wonderfully useful machines.

smt

 

[ballen]

Do what Paolo said, or if you have a long true parallel straight edge wide enough to lay on the chuck without falling over, indicate on that (chuck off) because it will take a flat surface independent of the ways, and compare it to the ways as the table travels. I like a 9 x 12 or 12 x 18 granite surface plate to shim on the table or chuck to read -0- in 4 corners, if that is possible (It might not be as ways wear). Now you can map all directions. It looks like there are now even import 6 x 18 granite plates that would be more convenient, though 2" is rather thin for a claimed grade A that length.

When I did this I found it hard to shim the surface plate closely enough. So what I did was to measure the height on an X/Y grid, and then use a spreadsheet to do a least-squares fit to find the closest plane and the residual deviations from that closest plane. For my grinder the errors were at the +-2 micron level over the entire surface. See post below and the following posts for some pictures and details of this process.

The horror under the chuck

Hi Buck, Because the geometry of the machine is very close to perfect.... I agree to that, the blue in shows enough hits IMHO to be filled with a coat of oil or grease and so not be a problem for a good number of years .. and then a next chuck top grind and so another under cleaning may be...

www.practicalmachinist.com

 

[stephen thomas]

Yes, with a known flat, you don't have to have it perfectly parallel as long as the extremes are established and defined.
For example, if the travel is exactly 15" in X and the surface plate is 0 at one end and +.006 at the other, you know the indications should change at a rate of .0004/inch.

Regarding the link to previous post - sharpy lines on a surface plate have thickness.
Sharpy or grease pencil marks have been used as shims when grinding close.

Worked with a scraping class with Forrest once, i had brought a granite knee for people to work perpendicular to.
Someone made a (admittedly fresh = somewhat thick) sharpy mark to butt the knee to for reference.
When asked why, they explained so the reference does not change as in might if randomly using other areas of the plate. Which is logical - even surface plates are not perfect.

As work progressed, readings were taken in cycles, the blue was re-smoothed, sometimes tilting the knee for convenience smoothing, it got moved back and forth. A couple students got careless moving the knee, the readings suddenly became unstable. They either had been scraping with the one edge of the knee resting on the sharpy line, or had been more careful avoiding it, but failed to notice for the most recent spotting cycle that it was now resting over the line & changing the spotting pattern (reading).

smt

 

[ballen]

Sharpie marks are about 30 microns thick:

But the indicator tip cuts right through them: the Heidenhain MT30 probe I am using resolves 1 micron and I didn't see any jumps as I ran over the sharpie lines.

 

[Richard King]

When I taught a class in Austria 4 years ago, this old dog learned something (ME) from the 2 brothers who hosted the class. They also were mechanical engineers who learned to scrape like me, from their Dad. Ballen took a scraping class there, so he can verify the brothers. They had a Elb Surface and they were going to grind a small cast iron 1" x 8" x 12'' surface plate. I figured they would scrape it flat first on a granite plate not many PPI, just touching all over with 40% - 60 percent of contact. Franz put 3 machinist jacks (the ones with a pointed top) on the mag chuck at at the airy points (approx. 30% of the length of the plate)) . Then they set the plate on the jacks and indicated the top surface to within .0005" or 0.0127mm and blocked around the plate to hold it sides, front and back. He turned on the magnet and kissed the part. then flipped it over on the jacks and kissed the other side, indicating the ground side perfect.

That way eliminated any issues with a worn chuck. Then we set the plate on a granite plate and it was within .0001" co-planer. The grinder was in really good shape too. They did the test as one of the students from the UK bought it.

I taught a class last spring at the shop in IL and one of the young students had rescraped aby hand a Okamoto surface grinder and he like the OP said the grinder was dead nuts as he mounted a mag base so the .0001" indicator directly under the wheel contact point. I told him that was a bad test and proved it to him buy putting a scraped straight edge on top of the chuck and it hinged in the middle. We can discuss the best methods to test it, the simplest way for a rookie who doesn't own a scraped straightedge is to use the level method that Paolo mentioned. Forget about indicating the chuck!

 

[ballen]

Hi Rich,

the technique that you describe, grinding something which is supported at three points, avoids any worn chuck issues. But it won't compensate for a grinder where the ways are worn and no longer straight. Is that right?

The technique that Stephen and I describe, where you support a surface plate on three points on the grinder table and then measure it from the wheelhead, is fairly inexpensive. Everyone here already has access to a computer, and those usually have a spreadsheet program. A brand new 450 x 300mm (18" x 12") surface plate from China costs about 100 bucks and is big enough for a 6" x 18" grinder. The only other thing needed is a 1 micron or 0.0001" dial indicator or test indicator on a magnetic base. Again, most people here probably already have that.

Changing topics: how is your post-op recovery going? I have the impression that things are getting back to normal for you.

Cheers,
Bruce

 

[Richard King]

yes if the ways are worn the method I described would not grind flat..coplaner. I wanted to share that to teach...before I croak.