- Joined
- Aug 13, 2020
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- 1,342
Lessons Learned:
Maybe I should have named this thread "I had to destroy my mill to save it"?
Warning: Don't try this unless you have time for your mill to be down a week. This is a SLOW process. But, maybe this will speed things up for others that might attempt it.
First of all, do NOT use the grinding device that I showed previously. This device is meant for a hand angle grinder, and is thin. The one pictured is 1/4". It flexes. Imperceptibly so, but if you use one and then switch to a cup type grinder, you'll see that the "pad" will leave crowns on the table. With enough pressure for it to remove material, it will flex down into the slots, and then back up. We're looking for tenths accuracy across the table, to the thousandths that it is flexing is a big deal.
That said, my table started out with a wave pattern that had crests on both ends and the middle, and from front to back. The max differences were about .0006. I now have it down to .001 across the table (except for one bad spot where I screwed up.) I'm taking one more pass today, because I think I can get closer. Follow this process to get it done ASAP (which is still a really long time).
1) Mount a dial or drop indicator to the quill. The indicator should have a resolution of at least .0005.
2) Run the indicator across the table, using a sharpy to mark the reading every few inches or every time it centers on a mark.
3) Connect matching readings. You'll end up with a rudimentary topological map. Color in the high areas with the sharpy. I just filled it in with slashes. Fill the next highest areas with a different color sharpy.
4) With the cup grinder in the quill, bring it down to the cup just makes the barest of a scraping sound as you move the high spot under it. Do not bring the grinder down a stationary table. Do not get greedy and try to cut more at once. Listen very carefully, and stop dialing the quil down as soon as you hear the first scrape.
5) Now very slowly move the entire table under the grinder. If you did step 4 correctly, you will see that it skims off the markings on the high area, and makes the next highest fade a bit. With the flexible pad I started with, everything would get erased. With the cup, you can see where the high spots are scalped off. The meaurements in the low spots won't be touched.
6) Take a break. I mean, go to bed. Leave it alone. Don't touch it. Go talk to your significant other for a change.
7) Wake up, have a cup of coffee, then repeat steps 1-6. You're remeasuring again, because while the cup is an order of magnitude stiffer than the pad, it still flexes. You'll see that the high spots are reduced, but they're not completely flat. You're taking a break, a LONG break, because that table heats up as you're grinding, completely changing the measurements. I've noted up to .001 difference between measurements taken immediately after grinding, vs the next morning. What's worse, it expands at different places depending on how much grinding work that spot got. The high spots will read higher than they would if the table temperature has a chance to equalize.
This localized heating can work to your advantage in the initial stages. The table will rise up at the high spots to meet the grinder, because the grinder is heating. But, it is a feedback loop. If it rises up too fast, the grinder really goes to work on it, forcing it to rise up faster, causing the grinder to work harder, then it starts throwing sparks all over the place, followed by you panicinc to switch the machine off, and when you get up the next morning you have a new low spot, and the world has new curse words that you just invented. I now know a lot about low spots, run-on sentences, and curse word creation.
Anyway, you might safely be more aggressive in the beginning, but as you start approaching truly flat, you'll want to be barely skimming the surface and giving the table longer breaks to cool. Five tenths will be a deep cut. Also, as it gets flat, pay particular attention to the feel of the cranks as your turning. I noticed that the force necessary to turn the crank was noticeably different in the high areas. It wasn't "difficult", but I could definitely tell that it took move force. I let that help guide me to places where I'd make multiple passes. The force doesn't seem to drop during those multiple passes, possibly because the warming table is coming up to meet the grinder. Use the turning force as as a hint, but don't try to grind everything to an equal force in one session. The cup sound is also informative. You can definitely hear when it is working, up to it making almost a bongo drum like sound when it is really digging in. Reduce the DOC if you hear that.
I will post a picture of the final table results later today or maybe tomorrow. I'm waiting for the table to cool.
Maybe I should have named this thread "I had to destroy my mill to save it"?
Warning: Don't try this unless you have time for your mill to be down a week. This is a SLOW process. But, maybe this will speed things up for others that might attempt it.
First of all, do NOT use the grinding device that I showed previously. This device is meant for a hand angle grinder, and is thin. The one pictured is 1/4". It flexes. Imperceptibly so, but if you use one and then switch to a cup type grinder, you'll see that the "pad" will leave crowns on the table. With enough pressure for it to remove material, it will flex down into the slots, and then back up. We're looking for tenths accuracy across the table, to the thousandths that it is flexing is a big deal.
That said, my table started out with a wave pattern that had crests on both ends and the middle, and from front to back. The max differences were about .0006. I now have it down to .001 across the table (except for one bad spot where I screwed up.) I'm taking one more pass today, because I think I can get closer. Follow this process to get it done ASAP (which is still a really long time).
1) Mount a dial or drop indicator to the quill. The indicator should have a resolution of at least .0005.
2) Run the indicator across the table, using a sharpy to mark the reading every few inches or every time it centers on a mark.
3) Connect matching readings. You'll end up with a rudimentary topological map. Color in the high areas with the sharpy. I just filled it in with slashes. Fill the next highest areas with a different color sharpy.
4) With the cup grinder in the quill, bring it down to the cup just makes the barest of a scraping sound as you move the high spot under it. Do not bring the grinder down a stationary table. Do not get greedy and try to cut more at once. Listen very carefully, and stop dialing the quil down as soon as you hear the first scrape.
5) Now very slowly move the entire table under the grinder. If you did step 4 correctly, you will see that it skims off the markings on the high area, and makes the next highest fade a bit. With the flexible pad I started with, everything would get erased. With the cup, you can see where the high spots are scalped off. The meaurements in the low spots won't be touched.
6) Take a break. I mean, go to bed. Leave it alone. Don't touch it. Go talk to your significant other for a change.
7) Wake up, have a cup of coffee, then repeat steps 1-6. You're remeasuring again, because while the cup is an order of magnitude stiffer than the pad, it still flexes. You'll see that the high spots are reduced, but they're not completely flat. You're taking a break, a LONG break, because that table heats up as you're grinding, completely changing the measurements. I've noted up to .001 difference between measurements taken immediately after grinding, vs the next morning. What's worse, it expands at different places depending on how much grinding work that spot got. The high spots will read higher than they would if the table temperature has a chance to equalize.
This localized heating can work to your advantage in the initial stages. The table will rise up at the high spots to meet the grinder, because the grinder is heating. But, it is a feedback loop. If it rises up too fast, the grinder really goes to work on it, forcing it to rise up faster, causing the grinder to work harder, then it starts throwing sparks all over the place, followed by you panicinc to switch the machine off, and when you get up the next morning you have a new low spot, and the world has new curse words that you just invented. I now know a lot about low spots, run-on sentences, and curse word creation.
Anyway, you might safely be more aggressive in the beginning, but as you start approaching truly flat, you'll want to be barely skimming the surface and giving the table longer breaks to cool. Five tenths will be a deep cut. Also, as it gets flat, pay particular attention to the feel of the cranks as your turning. I noticed that the force necessary to turn the crank was noticeably different in the high areas. It wasn't "difficult", but I could definitely tell that it took move force. I let that help guide me to places where I'd make multiple passes. The force doesn't seem to drop during those multiple passes, possibly because the warming table is coming up to meet the grinder. Use the turning force as as a hint, but don't try to grind everything to an equal force in one session. The cup sound is also informative. You can definitely hear when it is working, up to it making almost a bongo drum like sound when it is really digging in. Reduce the DOC if you hear that.
I will post a picture of the final table results later today or maybe tomorrow. I'm waiting for the table to cool.
She who keep the wallet (among other things, apparently) would not be happy with a couple hundred. Though, that certainly is a good idea.