Cheap mill abuse...don't choke

Is that step at the end of the table showing how much you had to grind off? It looks like almost .150"
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Shotgun said:
My mill table was shaped like a bowl. I think it was shipped that way from the factory. It's a round column mill, and the pattern is what you would get from letting a block get too hot will grinding. The center swells from the heat more than the sides, the grinder cuts it flat in that condition, and then the middle sinks like a souffle after it cools back down. The sides were .006 higher than the middle, with several swells and dips. Well, I don't have the money to keep buying accessories AND send the table out to be reground, sooo.....

Got an 8" grinding wheel made for an angle grinder. It has a 7/8"-11 arbor, so I cut a 2" piece of a bolt off my homemade steadyrest, and turned one end to fit in my ER32 collet. Lowered the head all the way down, and mounted the disk. It didn't tram, so I had to knock off the top of one side of the disk's arbor. The nut in the disk registers against the bottom of the collet, and removing a little material from the nut forces it to cock in the correct direction.

Cranked the speed up to the highest it will go, brought the disk down to make sparks on the high spot, and slowly start moving the table around. When it stops making sparks, I lower it about half a thousandth. So far I have a mirror finish, with just a few small places that are still making sparks. You can also tell a spot is high by listening for the wheel contacting the table. Go over the louder portions a few times, even with no sparks, and they will quiet down.

So far, I have a mirror finish on the table top, and grinding dust everywhere else. The next step will be to disassemble the table and power wash it.
Click to expand...
I look forward to seeing what you find at the end of the process.
 
If the wheel will cover the whole surface in one setup it should work. As said no different than surfacing a mag chuck on the surface grinder. I also surfaced the table on the shaper, using the shaper.

Greg
 
f350ca said:
If the wheel will cover the whole surface in one setup it should work. As said no different than surfacing a mag chuck on the surface grinder. I also surfaced the table on the shaper, using the shaper.
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Greg - I get it that a wheel that covers the whole surface in one go is best, like when the top of an engine block is reground, but what is the likely result of using a smaller wheel in two passes?

You should see the two pattern marks, but would there really be a significant "step" where the second pass overlaps the first?
 
I meant doing the whole table end to end without swinging the head to reach the ends. Don't think my Bridgeport clone has enough table travel to do it.
When I did my shaper table I had to use a carbide cutter to avoid wear going across, but could stroke front to back without resetting the ram position.

Greg
 
f350ca said:
I meant doing the whole table end to end without swinging the head to reach the ends. Don't think my Bridgeport clone has enough table travel to do it.
When I did my shaper table I had to use a carbide cutter to avoid wear going across, but could stroke front to back without resetting the ram position.

Greg
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The big issue with using one large wheel to cover the whole table is getting the tram right. First of all, that ain't easy on a round column mill to begin with. And then, at least initially, what are you tramming TO? Which part of the bowl :) If that big wheel is cocked sideways, I'm going to be grinding another bowl into the table.

What I've done so far is to take it down so that I have -.001 in one or two small spots, .002 across most of the table, and .003 on one high spot at the front of the table. All measurements are relative. That is much better than it was, so I trammed the mill again, taking the height differences into account. I took and recorded measurements at four corners ( right front, left front, right rear, left rear), with a circle that approximates the bolt circle holding the colum. Then I left the indicator, and moved the table under it to each spot. That gave me the relative level that each tramming position was indicating off of. A little math, and I had the number that each bolt position on the column base had to move up or down. Since the position of my measurements matches the position of the bolts, the calculated values are direct readings. I use a feeler gauge set for shim. If the position had a .022" shim in there, and I need to go up .002", I swap it out for a .024". I use a pickup magnet to pull the previous shim out. That way, I don't have to leave the shim sticking out. Each leaf of the gauge set is enough for three shims.

With the newly trammed head, I'm running the grinder again.
 
How I mapped out the table. Running it around under an indicator. The fractional parts of the numbers are somewhat of a guess. "It wasn't on a mark, but closer to one than the other" sort of guess.
 

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addertooth said:
If the ends are high, there may have been a non-grinding answer for that problem. If the Gibs are loose, AND the table is cranked far (left/right), the weight of the table hanging out with no base support can "lift" the end under the head. At first glance, it would look like the table was ground low in the middle, and high on the ends. If you tighten your gibs, and the center becomes the "new high spot", then loose gibs were your *original* problem.
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The gibs are tight, but I'm going to crank them down till it is difficult to move the table and then run a test.
The thing that would argue against it would be the "waves". Mapping out the whole table, there were high spots in the middle, and it was not symmetric from side to side.
Really, the table was a basket case.
 
graham-xrf said:
Greg - I get it that a wheel that covers the whole surface in one go is best, like when the top of an engine block is reground, but what is the likely result of using a smaller wheel in two passes?

You should see the two pattern marks, but would there really be a significant "step" where the second pass overlaps the first?
Click to expand...

You'll be able to see the two passes if the angle of the light is just right, but you won't be able to feel or even measure it. The final passes are made slowly, with just enough pressure to barely hear the wheel touching. If you're one of the lucky who still have some, you can comb your hair in the finish.
 
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