- Joined
- Nov 16, 2012
- Messages
- 5,596
Hi...
Here's a quick photo sequence of some surface grinding. The piece is a home made turret block attachment made from 4140 and heat treated to Rockwell 55C. -Pretty darn hard...
First thing with this piece and any other is to remove all the scale and file down any/all edge burrs. Do a good job of that as a small burr of a thou or two will prevent the piece from sitting flat on the mag table. That will not only throw the geometry of the part off, it will lead to a lot of extra time standing in front of the grinder.
Before putting the piece in the grinder, take a good machinist square and check all the sides for squareness. It's possible you have three good sides and one that's a little out of square. In that case, set the part face down on a good side and true-up the bad side first. If you set it down on a bad side first, the error will propagate as you flip sides -and you'll get to spend more time standing in front of the grinder. Use your best judgement when selecting the starting side. It's always wise to check first and save yourself some time. If you have a raw piece of stock, it's usually wise to mill it flat first.
To setup the piece, with the machine powered off, I use my thumb to rotate the wheel while gradually lowering the head until it just barely makes contact. Once that is established, without the motor turning, I run the part in all directions under the piece to see if any areas are sticking up high. You may need to back-off the head if there are high spots.
Next, the machine is turned on and the head is lowered in this case about 0.0003" -which is a fairly deep grind. When first starting, you must push the piece into the direction of rotation. Failure to do so could loosen and unscrew the wheel and it will happen in a heartbeat. A complete pass is made first "left to right" then "right to left" back to the starting position, then, the part is moved forward by 50 thou (or so). The part is started at the back...
... and moved-in about 50 thou at a time until it's completely covered. Although not shown here, the part is moved completely under the wheel. You don't stop when the front edge of the wheel reaches the edge of the part. You keep going until the backside of the wheel clears the ending edge of the part.
On this part, it took about 8 cycles starting from top to bottom each time lowering the head about 0.0002" until the entire surface started to flatten out. Here's the half-way point in the cycle. As you can see, there's a little slope in the part.
And finally, here's the finished surface. This was a 46 grit wheel and it looks OK if you ask me.
I flipped the part over and did the other side. It went much faster than the first side and the whole surface took an even grind unlike the first side which you can see, had some slope to it.
The best part of this is taking a micrometer and measuring afterward. This part does not have critical external dimensions yet, I intended for it to be 2.200" wide. When it was milled, I made it several thousandths oversize. After cleaning-up both sides, it came out to 2.2012" and the variance was well under a tenth everywhere you check. If so inclined, I could make successive passes and bring it down by 0.0012" -but I am not so inclined.
Another quick note... In the summer time, parts heat up quickly and I usually use Kool Mist. Today, the shop is nice and cool (pleasantly so) and I only ran light compressed air at the surface with no liquid evaporative. The part stayed nice and cool the whole time. If it did get hot, the variance in width would have easily blown 3 ten-thous.
Finally, it took about 40 minutes to do both sides of the part.
Ray
Here's a quick photo sequence of some surface grinding. The piece is a home made turret block attachment made from 4140 and heat treated to Rockwell 55C. -Pretty darn hard...
First thing with this piece and any other is to remove all the scale and file down any/all edge burrs. Do a good job of that as a small burr of a thou or two will prevent the piece from sitting flat on the mag table. That will not only throw the geometry of the part off, it will lead to a lot of extra time standing in front of the grinder.
Before putting the piece in the grinder, take a good machinist square and check all the sides for squareness. It's possible you have three good sides and one that's a little out of square. In that case, set the part face down on a good side and true-up the bad side first. If you set it down on a bad side first, the error will propagate as you flip sides -and you'll get to spend more time standing in front of the grinder. Use your best judgement when selecting the starting side. It's always wise to check first and save yourself some time. If you have a raw piece of stock, it's usually wise to mill it flat first.
To setup the piece, with the machine powered off, I use my thumb to rotate the wheel while gradually lowering the head until it just barely makes contact. Once that is established, without the motor turning, I run the part in all directions under the piece to see if any areas are sticking up high. You may need to back-off the head if there are high spots.
Next, the machine is turned on and the head is lowered in this case about 0.0003" -which is a fairly deep grind. When first starting, you must push the piece into the direction of rotation. Failure to do so could loosen and unscrew the wheel and it will happen in a heartbeat. A complete pass is made first "left to right" then "right to left" back to the starting position, then, the part is moved forward by 50 thou (or so). The part is started at the back...
... and moved-in about 50 thou at a time until it's completely covered. Although not shown here, the part is moved completely under the wheel. You don't stop when the front edge of the wheel reaches the edge of the part. You keep going until the backside of the wheel clears the ending edge of the part.
On this part, it took about 8 cycles starting from top to bottom each time lowering the head about 0.0002" until the entire surface started to flatten out. Here's the half-way point in the cycle. As you can see, there's a little slope in the part.
And finally, here's the finished surface. This was a 46 grit wheel and it looks OK if you ask me.
I flipped the part over and did the other side. It went much faster than the first side and the whole surface took an even grind unlike the first side which you can see, had some slope to it.
The best part of this is taking a micrometer and measuring afterward. This part does not have critical external dimensions yet, I intended for it to be 2.200" wide. When it was milled, I made it several thousandths oversize. After cleaning-up both sides, it came out to 2.2012" and the variance was well under a tenth everywhere you check. If so inclined, I could make successive passes and bring it down by 0.0012" -but I am not so inclined.
Another quick note... In the summer time, parts heat up quickly and I usually use Kool Mist. Today, the shop is nice and cool (pleasantly so) and I only ran light compressed air at the surface with no liquid evaporative. The part stayed nice and cool the whole time. If it did get hot, the variance in width would have easily blown 3 ten-thous.
Finally, it took about 40 minutes to do both sides of the part.
Ray