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QSIMDO
06-07-2013, 11:30 AM
I have to remove material from one end of a cylindrical object and keep both end planes parallel.

Much too large to fit in my lathe but it will fit the mill.
At first I considered just fastening to the table and making the necessary passes. However, I'm not all that confident on the
tram of the head in Y axis and, since it's a dovetail column mill, it'll be a full blown project to get it corrected.

In the interim, what if I were to machine the cylinder using my rotary table?

Seems to me that would keep the end planes parallel and, since the cylinder is tubular as i move inward on the Y axis I wouldn't end up with a dished center.

Did I miss anything?

TGTool
06-07-2013, 12:33 PM
I think the geometry works out right. Now dished or domed will be determined by whether the rotary table is flat on the mill table. If tipped up on the side you're moving in on the cylinder will be raised in the center, if high on the opposite side it will be low in the center. You can test, and and adjust (shim) as necessary to be correct and having the rotary table base larger than the cylinder will be an advantage. Go for it.

Paul Alciatore
06-07-2013, 12:52 PM
If you want to do it that way, rotating the table and feeding in from the circumference until you reach the middle, AND you are not sure of the tram, then you MUST start that in feed from the side of the cutter that is lowest or closest to the table. That will insure that that low point on the cutter will be the point that finishes all parts of the piston face. If you start from a side of the cutter's outside circle that is higher, then that will be the cutting height at the center when you reach there, but other parts of that cutting circle will be lower and they will be over the outer portions of the piston and will cut off more material there. So, you will wind up with a convex face.

This low point on that cutting circle is not definitely fixed at one of the four primary directions (+X, -X, +Y, or -Y). If the head is out of tram in both X and Y directions, it could be somewhere in between. This would necessitate a feed from OD to the center along a diagonal line. It would be better if you could at least insure tram in the X direction before starting so the feed would be in the Y direction only. Of course, the best situation would be to insure tram in both X and Y directions.

You also need to insure that the RT is dead parallel to the feed direction of the table. Notice that I did NOT say table top. Think of it as if you were finishing it on a large lathe with a cup style grinding wheel in the tool post grinder. Think of all that can go wrong with that set up. And then turn that picture on end (tail stock end of the lathe facing UP).

Paul Alciatore
06-07-2013, 01:19 PM
Tramming a vertical mill in the Y direction is not that difficult. Most vertical mills have a column that is bolted to the base, usually with four bolts. You can loosen them just a bit, perhaps a single turn or less. Then loosen just two of them on ONE side by one or two more turns. They are still well engaged in their holes, but loose enough so that the column can be tilted away from that side. This tilt can be maintained with shoulder pressure, keeping both hands free for cleaning and shimming. Now, with the column tilted, clean out the space between the column and the base. A putty knife is helpful. It can be covered with a rag wet with oil or WD to remove the final scraps of paint or whatever. Now re-tighten those two to the barely loose point and repeat the procedure on the other side.

After thoroughly cleaning both sides, ONE AT A TIME remove each bolt and clean and grease the threads before torquing it. Use a small brush or other swab to clean the internal threads in the hole also. Now tighten the bolts back down using a torque wrench for uniformity and check the tram. It is important to use the same torque value every time you make adjustments from this point forward.

Add shims near or around the bolts to correct the tram. Shim kits for this purpose are available from machine supply houses. Re-torque all four bolts to the same, uniform value and check the tram again. Repeat the adding or removal of shims, torquing, and checking until the tram is correct.

Most shim kits have values only in whole thousandths. You can make thinner shims from household aluminum foil that is about 0.0007" thick. These may help, especially if you use two of these for 0.0014" which is close to 0.0015". Combining this with a 0.001" change on the other side will give you a half thousandth increment. Or, one or two thicknesses of the foil will give you thirds of a thousandth.

At the very end of the shimming process, you can try a bit more torque on two of the bolts to lean it over that way just a bit.

Using the above technique is safe if you always have at lease three bolts in their holes and within two or three turns of tight. NEVER have more than one bolt out of the hole or more than three turns loose at one time. NEVER! It can be done by one person. It can get the tram within better limits than the rest of the mill is likely to meet. And it can be done in an hour or two by someone who has never done it before.

QSIMDO
06-07-2013, 01:43 PM
If you want to do it that way, rotating the table and feeding in from the circumference until you reach the middle, AND you are not sure of the tram, then you MUST start that in feed from the side of the cutter that is lowest or closest to the table. That will insure that that low point on the cutter will be the point that finishes all parts of the piston face. If you start from a side of the cutter's outside circle that is higher, then that will be the cutting height at the center when you reach there, but other parts of that cutting circle will be lower and they will be over the outer portions of the piston and will cut off more material there. So, you will wind up with a convex face.

This low point on that cutting circle is not definitely fixed at one of the four primary directions (+X, -X, +Y, or -Y). If the head is out of tram in both X and Y directions, it could be somewhere in between. This would necessitate a feed from OD to the center along a diagonal line. It would be better if you could at least insure tram in the X direction before starting so the feed would be in the Y direction only. Of course, the best situation would be to insure tram in both X and Y directions.

You also need to insure that the RT is dead parallel to the feed direction of the table. Notice that I did NOT say table top. Think of it as if you were finishing it on a large lathe with a cup style grinding wheel in the tool post grinder. Think of all that can go wrong with that set up. And then turn that picture on end (tail stock end of the lathe facing UP).

Thanks Paul, but it's actually a very thick walled cylinder so, by the time I reach the middle, it'll be quite clear of the cutter and happily the lowest part of the cutter will be the only part to do any work.

And I've got the tramming process down OK, just not the time involved for that and all the other things I want to do to the mill to attack it at this point.
A year from now I'll be retired and up to my elbows in oil & swarf and shim stock!

MrFluffy
06-07-2013, 06:21 PM
Do you mean the tram of the head, or that your knee is out in y in reference to the z axis?
if its the head only, it strikes me that the head being out of tram to the z in the y wont affect the job as long as both sides of the cutter clear the job as the lowest point will cut regardless. Knee drooping in y is of course a different kettle of monkeys.

gvasale
06-08-2013, 11:34 AM
Where in CT are you?