grinding versus snagging

[Tmoose]

We have provided wear plates cast from wear resistant alloys, maybe approaching Nihard etc for our coal crushing machines for decades. A typical design is 15 inches or so in an irregular pie shape with 3 short (1/8 inch tall) bosses 1.5 inch diameter cast in a widely spaced pattern on the "back." A clearance hole passes thru the plate in the center of each boss. There is a counterbored hole about 3/4 inch deep above each boss to protect the bolt that secures the plate to the machine housing.

Old hand drawn drawings called for a variety of nominal thickness and counterbore depths from the cast working face. The 3 boss faces have the ISO callout for flatness within 0.005" and a surface finish of 250 micro inch rms.
I believe the intent is to just clean the boss faces and reduce the risk of cracking the brittle plates when the bolts are tightened.
I have heard previously they were finished by setting them face-down in a "tub grinder" but these days that name seems to be a machine for grinding/chipping wood waste. I'm thinking the machine may have been a big wet belt sander like this.

http://img.directindustry.com/images_di/photo-g/horizontal-belt-sanders-29633-2408315.jpg

We are about to embark on finishing the castings in house, but the surface grinder operators are rightfully complaining about how long it will take having no useful datums to reference and a bunch of thickness dimensions referencing cast surfaces.

I think what we need is glorified snagging operation on the 3 boss faces using a machine with a platen.

What is the modern machine best able to process parts made of wear resistant materials quickly and using their own surfaces as the "datum?"

 

[MikeHalloran]

I disagree that snagging would be sufficient; I think the bosses need coplanarity.

The first machine that comes to mind for that job is a Blanchard grinder. With a nice coarse wheel it should be able to achieve an acceptable cycle time.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

Hi Mike,

the first issue (and maybe the only issue) is fixturing using our surface grinder. There are no cast datum features on the part except the bosses themselves.
The former machine used is a large (several feet in diameter) rotating stone of some sort in a large tub. The parts were placed boss down on the rotating stone free range and they rode up against a fence. The part weight was sufficient to get the job done unattended.

As you say the important result is flatness/coplanarity (to prevent cracking when the bolts are tightened.) The bottom surface of the counterbore is left as-cast, so undoubtably the bolts don't seat flat all the time.

I used the word snagging just to emphasize the surface finish is not very important.

 

[MikeHalloran]

The machine you describe has the kinematics of a lapping machine, but they are normally used with very fine wet abrasives on a rotating ceramic table maybe two feet in diameter, and typically remove microns at a time. If they are used properly, the workpieces come out very flat, and by some process not easily distinguished from magic, the table also stays super flat.

A Blanchard grinder normally has a rotating or linear worktable, not unlike that of a surface grinder. The abrasive is contained in bricks clamped around a polygon rotating on a vertical axis. That vertical axis rotates and descends as the workpieces go around or back and forth on the lower table. It leaves a characteristic pattern on the ground surface, not unlike that from a rotary sander, except for a rather large radius. They are capable of very fast stock removal, and generate a nice flat surface on the workpiece.

They are more complicated and probably more expensive than the tub grinder you describe; I have never seen one, but Blanchard type grinders should be easy to find. You would just clamp your plates flat on the worktable, and grind the pads until they clean up; just like you would do on a surface grinder, but much faster.






Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

What I'm hearing is the issue with the "clamping down" part. The other surface of the wear plate would need to be prepared to have the bosses start at a similar height. And the bosses would have to be trued up first to permit cutting the cast side true their real nature.

 

[MikeHalloran]

I can imagine some sort of magnetic or wedge-jacked fixture under the workpiece that jams it up against an overhead bridge sort of thing, then radial clamps engaged, then the bridge swung out of the way before grinding, so the bosses end up with a mostly uniform depth. ... but it quickly gets complicated.

Maybe you can buy a used tub grinder of appropriate size.
... perhaps from the outfit that is losing your business.


Mike Halloran
Pembroke Pines, FL, USA