Surface Grinder Accuracy

My KO Lee book specifies flood cooling, not mist. Temperature control is critical, of course.

What kind of flatness are you getting?

GWE

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

Just as a point to think about: If your ways are bent (preferably in an arc) you can grind the table how long you want, it will just have the same shape as the ways.

Nick

I'm wondering what you typical grinding job is and what accuracy you are looking for and what you need and are they two different things...all those zeros scare me! One of my techs was taking a piece to the surface grinder and when asked why..."To make it shiny!"

(TOP POSTED THE RIGHT WAY!!!)

craftsman snipped-for-privacy@comcast.net wrote: ... In my initial grinding I have

I assume this was measured from the grinding head to the surface. This doesn't mean the surface is flat, just true to the wheel. The straighness of the ways has to be checked separately, which is probably the meaning of the quoted tolerance of 0.0002"/ft.

I let the wheel spark out. Unless you have a very stiff machine the wheel will deflect, even when cutting 0.0001". Slight differences in the hardness of the table surface is a factor that sparking out removes.

Bruce

As I indicated in my original posting the variance that I am currently getting is .00018" in the longitudinal direction and .00046" in the transverse direction. These are the worst case variances. Some areas are significantly better. In regard to coolant the K.O. Lee Rep advised that I should get satisfactory results with mist coolant. Some books I have read state that in many instances it is easier to obtain proper cooling from a mist setup than it is from a normal flood setup. Grinding wheels are porous and act as a fan. The wheels produce a thin layer of air around the periphery of the wheel that forces liquid coolant away from the wheel and starves the grinding contact point. Special flood coolant nozzles have been designed specifically for grinding to essentially scavage this air layer and allow the coolant to get to the contact point. Most grinders do not have this special nozzle. Because a mist system is pressurized it is fairly simple to direct the spray directly at the contact point. The bulk of heat is obviously generated at the contact point. Mist systems provide very effective evaporative cooling which from what little physics I can remember is a very effective heat transfer mechanism. The downside of mist systems is that they provide little lubrication. It is my understanding that this lack of lubrication is a surface finish issue, not a cooling issue. Because in many grinding operations surface finish is important, flood systems are popular.

I'm pretty new to surface grinding, but isn't a cross feed of .0050 on the light side?

John Martin

I don't have a typical grinding job, but since I am grinding the surface grinder table I want it to be as close to specifications as possible. Following the table I will grind the magnet. Anything I grind after these two pieces will never be more accurate than the variance I obtain on them.

The indicator base was placed on the column in order to eliminate any possible movement of the wheelhead. The measurement does therefore confirm the flatness of the table which translates into the straightness of the ways. The variances that I provided are specifically for the table surface. Not coincidentally the variance allowed for the ways is identical to that allowed for the table. Checking the ways is a substantially more difficult process that requires some significantly sized inspection equipment.

Although I assumed that I properly sparked out the surface, I probably should take additional passes.

Do you have a manual or hydraulic feed machine? If it is manual, do you take a larger crossfeed when sparking out?

It is the value that the K.O. Lee Rep recommended. Personally I think that I got better results with a larger cross feed. One book advises that you should feed about 1/4 of the wheel width but no more. This would be .125 for a 1/2" wheel. I like .100 because it is one full revolution on my handwheel.

I understand your point, but since I have no facility to check the ways I can only indicate the table. This measurement then translates to the condition of the ways. If I get the table into spec, the values of which is my main question, then the ways are in spec.

Actually, it doesn't. It just means that the distance is repeatable at that point. You're still using the ways to generate your measurement, so if your ways deviate you're not seeing it.

I can't suggest a better method, mind you, unless it's to use a surface plate calibration tool. I'm not sure if that's a more useful measurement or not, but it would be a better way to measure what you're _claiming_ to measure. Personally, I think whether or not it grinds to a consistent depth would be more useful... and this is what you're measuring.

Perhaps someone else can improve upon my insight here...

--Glenn Lyford

Consider if you removed the entire table and way assembly and instead mounted a bicycle wheel below your indicator. Then turned your bicycle wheel and adjusted it until the meter read zero deflection. Would you then assert the bicycle rim is flat?

GWE

Is it really necessary to grind the table perfectly flat before tackling the mag chuck?

I also have a KO Lee grinder (S714) and have only been planning to dress the top surface of the chuck. Gotta resolve a scalloping problem first, though, and that's long overdue.

BTW, can you convert "quick" longitudinal feed rate to something like inches per minute, at least roughly? I've been half-heartedly trying to get a handle on what sort of feed rate should be used.

Mike

Did they recommend 0.005" cross feed or 0.050"? At 0.005" cross feed you will be making over 1200 passes to get across the table!!!

Mike

My mistake, I missed a decimal point in my post. The K.O. Lee recommendation was .050".

Maybe I am missing something, but I don't see the analogy. When I indicated the table surface I zeroed the indicator at one point on the table and then traversed the table back and forth and in and out to get the readings over the complete surface of the table. I believe that this indicates the flatness relative to the column. To use your bicycle wheel analogy, if I held a bicycle wheel under a dial indicator and moved it right and left and in and out, I certainly would know it was not flat. Maybe we are saying the same thing in different ways. All I am saying is that if you get an identical indicator reading over the entire surface, then the table is flat and is traveling in the same plane both longitudinally and crosswise. If the indicator readings are not the same the problem can either be the table is not flat or the ways are not correctly oriented.

I did not invent this test. It is how the K.O. Lee Rep told me they make their check, and it is clearly spelled out in Machine Tool Reconditioning.

In Machine Tool Reconditioning the process is to grind the table and then the chuck. I would perform my to date controversial test and see if your table is flat. My table definitely had some low spots. I think that anything you clamp to a out of flat surface moves some minute amount. I was amazed when I got an indicator that measures to .00001" how little force it takes on a thick table top to get significant movement. Beyond this I am totally unqualified to tell you what to do.

I did get some good information from K.O. Lee about mounting a magnetic chuck that may be useful to you. I think most people including me clamp them down too tightly. You incrementally tighten both ends up to

In regard to "quickly" that is the instruction I got from K.O. Lee. I recognize its relative uselessness. I think all you can gain from this is that slow is not good. One of my books has a procedure on chuck grinding and it states to use "a fairly rapid table speed (50 to 100 sfpm)." To make it easy use 60 sfpm. To achieve this you should cover a 12" magnetic chuck in 1 second. If you are hand cranking this is really moving. Hope this helps. I am finding that grinding seems to be as much art as confirmed procedure.

craftsman, I had the same thoughts when I bought a used Bridgeport. If I machined a flat plate and measured from the mill head to the plate. I saw no deviation, but placing a straight edge on it, I could see light under the edge when placed across the plate, and when placed lengthways, it rocked in the middle. Upon examination, I could also see wear on the ways( the frosting was missing at the ends of the saddle). What I did to understand what was going on was to place a surface plate on the table and measure the distance between the top of the plate and the head on 1" squares, ie measure each inch along the length of the plate, move it 1" crossfeed and repeat the 1" measurements along that length. What I found on plotting the data was that because of the wear on the ways, the longitudinal ways were worn curved, with the ends of the saddle worn down more than in the center. On the crossfeed, the ways were worn dished. Using a 12" x 18" surface plate, from the center of the plate to either end, 9", the ends of the plate were 0.004" closer to the head than in the middle because of the wear. Crossfeed showed 0.002" further from head on front and rear of plate than in middle. One thing you might do is take a surface plate and place it upside down on the ground surface of your table and see if it rocks. If it does not, put some marking blue on the plate and run the plate on the ground surface and see where the blue transfers to the table, in the center, or on the 4 corners, or?? That should give you an idea as to the true flatness of the table surface.

You'll come to understand that all too well as you fail at projects that are critical. Few experienced machinists make *good* precision grinders.

Harold