Surface Grinder Accuracy

No, it does not. No more than running an indicator along a mill table confirms the squareness of the mill head. Look at it this way. If your wheel is fixed (which it is) and the table rises and falls, the surface is ground in a corresponding pattern, therefore yielding what appears to be a surface that is flat. When you run an indicator on the table, all it's telling you is how faithfully the wheel has followed the pattern, the ways. It may or may not be flat, and most likely is not, especially if you're using an older machine. . You can't check flatness that way--it should be done by comparing on a surface plate of known flatness. Only then can you discern the rise and fall of the surface, which then translates into flatness.

Harold

But only when the values are *not* established by checking from the ways as has been suggested. All that tells you is how precisely the wheel has followed the ways, whicy may or may not be straight and flat.

Harold

Oh yes! You even can't clamp an indicator near the wheel and messure the surface. You need something independent from the grinder in all. Either a touching plate, a ruler etc.

I had this setup in my mind, but didn't write it down.

Nick

Unless the longitudinal feed distance was always the same (long production run) there is likely to be more wear near the center of the ways, from frequent smaller jobs. The result is that the ways wear from straight to slightly circular, the same way hand grinding turns a flat blank into a spherical telescope mirror. If someone reground the table it would have the same curvature as the ways. The thickness from the ways to the table top is then constant, so an indicator on the spindle head would show a constant reading, but the table top is NOT flat. I think this is what the bicycle wheel analogy meant.

The cure is to check the ways with a straightedge or surface plate and have them scraped to whatever accuracy you need and can afford.

My old $100 surface grinder is a good example. The ways are worn about .015" deep from a long production run. I put shims under the ends of the mag chuck to avoid bending it

jw

Before you grind in a chuck, make sure the table is free of burrs and clean it meticulously, then put the chuck on UPSIDE DOWN and take a cleanup pass over the bottom, and THEN turn it right side up and loosely bolt it down. Then get out your dial indicator and set the long fence on the mag chuck so it sticks above the rear edge of the chuck, and indicate on the fence and adjust the position until the chuck is oriented correctly and where you want it, and only then should you grind in the top face.

Quoting from memory from a KO Lee book I have somewhere -- GWE

Guess I'll have to dig up the MTR book and see what other useful info it has on surface grinders.

Interesting suggestion about torquing down the chuck.

The feed rate you suggest correlates with my very limited experience - faster rates seemed to give a better finish.

BTW, no one's mentioned the 5-block test yet so far as I can see. Place 5 blocks on the chuck, 4 at the corners and one at the center, grind the top and bottoms of them, and then measure each. I think that K. O. Lee recommends that in some literature I have but the user community seems to have varying opinions of the usefulness of the test.

Mike

In this case the used grinder came to me with the chuck already installed and it looks to have been that way for many years. I'm inclined to chance just dressing the top as is. Maybe that's hour wise and days foolish, though, from a time of use standpoint.

I'm still interesting as to why you are so concerned about the grinder's capability.

From a toolmaking perspective, .0002" flatness is virtually a joke in all but the most critical applications. As you found out, it doesn't take much to move .0002" if there are any forces acting on your work piece.

Further, you haven't even gotten in to grinding your work piece yet. It's not hard to grind a part to .0001", assuming it's fairly stable.

Unless you're grinding very large work pieces (unlikely given the size of your machine), you're probably not going to have trouble with the grinder. More likely, in my experience, you'll have trouble with your work piece flexing. While a good table is important, there are many other areas you could have trouble with that will take far more time and be far more aggrevating to iron out.

Just my $0.02 worth... It's like worring about which oil to buy for your car before learning how to drive.

Regards,

Robin

Gads..for once I actually agree with the tyke.

Gunner

Liberals - Cosmopolitan critics, men who are the friends of every country save their own. Benjamin Disraeli

I don't know where the logical place in this thread is to put this post so I will just respond to myself. I want to thank Grant, Nick, Harold, Bruce, Trains4, jw, Mike and anyone else I missed for showing me the error of my "ways." No pun intended. I now better understand the issue of indicating for flatness. The bulk of the responses concerned the flatness issue, but I had other questions about grinding technique. If anyone would like to "way" :) in on these, it would be appreciated.

I had another thought about determining flatness that I will run by this august group. I am not sure about the practicality of the setup so don't comment on it. What I would like to know is if anyone sees any fault in the logic.

Say that I set a surface plate next to the surface grinder table. The surface plate would be mounted on three toolmaker jacks placed in as large a triangle as possible. I then mark three dots on the surface grinder table, again placed in as large a triangle as possible. I attach an indicator to a surface gage type base. The base of the indicator would rest on the surface plate with the indicator point alternately placed on the three dots on the surface grinder table. The jacks under the surface plate would be adjusted until the indicator reading is the same for all three dots. I believe this should make the plane in which the three dots lie parallel to the plane of the surface plate. I could then slide the surface gage with indicator around the surface plate to take readings over the entire surface of the surface grinder table. I believe that this would show the relative "flatness" of the surface grinder table in respect to the plane of the three base dots.

Comments?

Using the surface plate is definitely the way to go. You need an external reference.

I would work the dial indicator from both surfaces. Start with the dial base on the grinder and indicate to the plate in X and Y directions: level the surface plate as needed so it reads zero at the ends of the travels. This gets the surface close to the average plane of the table motion.

Now you can put the dial base on the surface plate to indicate the whole surface of the table. Check the table when it is in several locations. Actually, you could remove the table and check the ways as well.

Another test of the ways is to place a precision level on the table and watch the bubble as the table is moved around. Check for cross-level (twisting) as well. In this case, gravity is your external reference.

By the way Machine Tool Reconditioning has a whole chapter on grinders. If you want to see ultra-precise, find a copy of Moore's "Foundations of Mechanical Accuracy" where he describes how Moore jig grinders are built and checked.

Bruce

Ron, your zeal is commendable, and as Harold puts it, you just might be finicky enough to make an excellent precision grinder operator. :-)

I bought a little KO Lee grinder once, and ground in the chuck as best I could. I get parts accurate to .0002" over 4" and that's just excellent by me. I suggest you actually use your grinder for a bit before you go chasing gnat's asses ..

GWE

craftsman snipped-for-privacy@comcast.net wrote:

That would work. The best thing about this idea is that you are seeing how flatness works. Once you get the basic idea it makes it much easier to visualize ways to check your work. I like to draw exaggerated pictures to show myself how things can go wrong. It's always easy for me to draw something and assume that everything is perfect. It's the error though thats most important. ERS

Just use a precision level. Shim it up so the bubble is in the center and run the table back and forth. IF its going back and forth over the mountain of unworn metal of the ways in the middle of the travel you will see the bubble go back and forth as you run the table left and right. A cheap and dirty way to tell is with a laser pointer and a mirror. Put the mirror on the middle of the table mounted upright and aiming along the table, Clamp the laser pointer so it hits the mirror and bounces off to a far wall. Now all you got to know is a little trig.

John

Cross feed per pass, fraction of wheel width is 1/10 for a hardness of greater than 52 Rc and 1/4 for a hardness of 52 Rc or less, and a table speed of 50 to 100 fpm. Rough cut 0.003, finish cut 0.0005 max.

Table 2. Basic Process Data for Peripheral Surface Grinding on Reciprocating Table Surface Grinders page 1190 Machinery's Handbook #26

The above data is almost guarantee to cause a train wreck with a small SG. - GWE

The laser pointer method can be extraordinarily sensitive but use TWO mirrors setup to be almost parallel.

Fire the fixed laser pointer beam so that it zigzags several times between the two mirrors finally finishing off the end to land on any convenient white card scale.

One mirror is fixed to the test surface and the second mirror anchored to a fixed location. The angular sensitivity doubles at each reflection so it doesn't need many reflections to reach scary sensitivity!

Surface silvered mirrors give the best results but ordinary vanity mirrors are usable if you are content with a few reflections. Old hard disk platters are also usable but don't try to cut them - it destroys the flatness.

The mirrors need to be a long way apart so that the zig zag angle is small and the beams do not move beyond the limits of the mirror surface as the table is traversed. There will a small movement in spot position in the direction of the zigzag angle as the table is traversed so the measurement must be made at right angles to this direction i.e for up and down angular measurement the zig zag path should be horizontal.

Jim