Burn streaks on the work

[Sunset Machine]

I needed to pull the 6x12 magnetic chuck off for cleaning and the manual says to regrind it with a 60H wheel and coolant, .050" stepover, .0003" downfeed.

It burnt badly (dark streaks) so I tried a 60K with a noticeable improvement, but still leaving streaks. I tried .0001" at a .025 stepover, same results. Last night, a 46K. Still streaking.

Any words of wisdom would sure be appreciated.

[EdK]

Can you post a picture of the streaks?

Ed

[TomG]

It's been a while since I've run a grinder, but let's see what I can dig up...

First of all, you are going the wrong way with the hardness. Burning streaks mean the wheel is loading up rather than breaking down and needs to be softer, not harder. Try an F or G 60 grit wheel.

I would also move the wheel MUCH more than .050". Creeping over a little at a time like than heats the chuck locally and causes distortion. Moving over 1/2 to 3/4 the width of the wheel per pass with a .0001" depth should give better results. Follow up with a similar pass at zero depth and let it "spark out". The chuck should be barely warm when you are done.

Tom

[Sunset Machine]

The chuck seems to be cast iron w/steel (?) strips embedded in it. I'll try a softer wheel and a wider step-over. With my chuck looking like this, even I wouldn't want me working on my stuff with it... But Ed wants me to show it to EVERYBODY, so here it is.

Oh, I'm using coolant. At .0001", there are no sparks and the chuck doesn't heat. Should I be doing this dry?

Another grinding question: Finding the top surface. I want to make a finish pass of a few tenths, maybe just .0001". That kinda rules out crashing the wheel into the work until it sparks. So, after truing the wheel with a diamond nib, I turn the power off and slide a piece of paper under the wheel while bringing it down to get close. I then use a felt tip marker and make a black spot the size of a postage stamp near a corner. Wheel behind the chuck but centered over the black spot, I turn the power on. Then with the cross feed, I move the table in and out while slowly bringing the wheel down until the ink is removed and finally proceeding with a zillion passes back and forth.

Is there a better/easier method of finding the top of the surface to be ground?

[TomG]

Most of the grinders I've used were dry, but if you have coolant then you should definitely use it. I believe the chucks are made of iron plates with some sort of non ferrous spacer, either aluminum or brass.

I think you are putting too much thought into picking up the surface. Just move the wheel close by eye and drop it a little at a time while feeding the table until it touches. It's one of those skills that comes with experience, but once you figure it out, it'll be second nature.

Another thing I forgot to mention is you should put a coarse dress on the wheel. Cranking the dresser across the wheel rapidly will create a surface on the wheel that is less apt to load up.

Tom

[EdK]

(03-16-2014, 10:30 AM)Sunset Machine Wrote: ... But Ed wants me to show it to EVERYBODY, so here it is.
...

My intention wasn't to embarrass you but to add some clues to the mystery. It's the old "a picture is worth a thousand words" concept.

Ed

[PixMan]

I agree with Tom. The most probable reason for the burns is that the wheel is DULL. You need a good, sharp diamond and a coarse dressing speed to "open up" the abrasive grain structure on the wheel. I sincerely doubt that the wheel is loading up with steel at such low depth of cut and stepover unless it's far too smooth.

A couple of questions after looking at the pattern a little more:

Is your grinder a hydraulic reciprocation or manual? If a manual feed, it is a gear driven table or cable drive? I notice that the burn marks are very uniform across the short dimension of the chuck. This could indicate that the table is bouncing around if a gear driven one, as I think it would be hard to get that effect if you simply had a severely out-of-balance wheel.

[Sunset Machine]

Hey Ed, all in fun. You're right - I should've posted a pic right off. That's a "finish pass" of .0002 with a 38A47K wheel. The catalog says that's too shallow for that grit, but I thought I'd try it anyway. Catalog says for under .004 use a 60K, .100 cross over. That didn't work either; it just made wider burn marks.

I bet those *are* aluminum strips (going north and south in the pic) and they're being embedded with dark debris from the wheel. It just dawned on me that the aluminum might be loading the wheel. I'm dressing the wheel gently at about .002 and then a quick pass over the nib at .0005" as fast as I can crank the wheel. It's a manual, rack & pinion.

For some reason I had better results going harder, from a 60H to a 60K, meaning I could make it about halfway across before burning, but 3"x12" of it looked good. Definitely loading and/or dulling, a dark streak forms on the wheel. Looks like I need to get a couple more wheels and experiment.

[EdK]

(03-16-2014, 09:49 PM)PixMan Wrote: ... This could indicate that the table is bouncing around if a gear driven one, as I think it would be hard to get that effect if you simply had a severely out-of-balance wheel.

I'm no expert, or even a novice, with surface grinders but I agree with Ken. My first thought after I saw the picture was that something is loose.

Ed

[roundrocktom]

There are a couple of great video's up on You Tube, in addition to Tom's. :)

"That lazy Machinist" is Marc Lecuyer on Utube.
http://www.youtube.com/watch?v=zCUoQIoCsdQ

I'd still like to add a heat treat oven and surface grinder to my shop, so it is on the to-do list.

Local shop had an open house Saturday, with Tormach demo's. Their PSG (Personal Surface Grinder) is a 612 machine, and looks in line for a small shop. They will be adding a "belt sander attachment" for the knife making guys, but what I liked was being able to set it and let it run (x & y are automatic, you adjust the Z down.... of course I'm thinking milling machine, but you get the idea). Often I set my mill and lathe on fine power feed for smooth finishes, so a semi-automatic grinder fits into that scheme of being in the shop, but not having to spin wheels non-strop.