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ZINOM
10-19-2003, 10:07 AM
I always thought about backlash in the context of how it affects the micrometer dials on my machines and thier measurements.

Recently I was watching a video where the narrator talked about backlash in relation to cutter rotation.....I guess my question is: should I put the work in it's final position along a particular axis, removing the b'lash in a certain direction depending on the rotation or direction of movement?

I probably typed that in a confusing manner, but I always put work under the spindle the same way no matter what....removing the b'lash by turning the handwheels the same direction all the time so that I know if I need to make a small movement I know which way to turn.

I'm just wondering if I should remove the b'lash by turning the handwheel a certain way if I'll be cutting left to right, as opposed to from right to left...?..?


I could say this a lot easier than I can type it....sorry.

John

debequem
10-19-2003, 10:29 AM
Good question!

I am just learning about different cuts, for instance climb. I think that means that the cutting rotation is such that the flute strikes the work piece as to swim across like a paddle boat does in the water. I might be backwards, so I hope someone can clarify this.

However, if we take my above example, when this happens the mill tends to force the work and the table back. Sort of like feeding wood into a table saw.

If you make the cut in the other direction, so that the flute strikes the opposite direction, the work is pulled in the same direction as the table is being moved.

I think that problems can happen when you reverse direction on the mill and the backlash in the table can result in a little table float if the cut is deep enough.

Oso
10-19-2003, 10:44 AM
The backlash direction depends on the cut, yes.

First, do not do climb milling if you have any backlash...the cutter will "walk in" to the work the backlash distance and take too big a bite (Ok excess chipload per tooth). That will probably break or mess uip something...don't do it.

If you are cutting so that you move the table AWAY from you, take out backlash by backing the table TOWARDS you and then moving it forward to your desired position.

If your cut is with the table moving TOWARDS you, reverse the above, moving the table AWAY and then coming back towards you to the desired position.

The idea is that cutting pressure will take out the backlash if you do not. That will mis-position the table from the indicated location by the amount of the backlash. You cancel that by the above.

beckley23
10-19-2003, 11:31 AM
I think what you are talking about is the difference between climb and conventional milling. In climb milling the cutter wants to pull into the work, thus it is taking the largest bite first and as it rotates thru the cut the bite gets less. UNLESS YOUR MILL IS SET UP FOR CLIMB MILLING, DON'T DO IT. The potential for bad things happening is great. In a mill that is set is set up for climb milling, the X axis nut is of a two piece design and are counter rotating so that the nut halves can be turned and locked to
eliminate the backlash with the feed screw. When the nuts are thus set, it is very, very difficult to turn the screw by hand.
One of the advantages of climb milling is that the finish is better vs conventional milling.
In conventional milling the cutter starts out with a smaller bite that increases in depth as it is fed into the work. It is also trying to push away from the cut, and lift the workpiece.
Cincinnati Milling Machine published a book, Treatise on Milling, that goes into a thorough explanation. I don't think it is in Lindsays 1918 reproduction, but it is in the 1951 version, which you find every now and then on ebay.
Harry

Evan
10-19-2003, 12:13 PM
It's really no different than using a radial arm saw for wood. Do you push the cut or pull it? It partly depends on how rigid your machine is.

wierdscience
10-19-2003, 12:56 PM
Don't forget your feedrate!

I have and do make climb cuts on mills so sloppy they shake in the breeze,I get away with it because I use a slow feedrate and a lite cut,remeber you have to generate enough force to move the entire workpiece/vice/and table.This won't happen on a 1/4"endmill on a b-port,but it will on a mini-mill.

Cutter diameter will also have a bearing on climb cutting,the larger the diameter the greater the negative effects.

However it is important to remember that climb cutting even on a new virgin machine with tight ways is still the most accurate method because of cutter defection.

spope14
10-19-2003, 01:30 PM
I teach climb milling as the only way to do a finish cut on the Bridgeport mill or for that matter, on the (very few jobs we do) on the Horiz mill for finish.

The key word here is finish cuts. .005 max, maybe two cuts at that depth.

Climb milling as a habit to rough material on a B-Port is eventually fatal to some part and cutter.

debequem
10-19-2003, 02:16 PM
When the tool or part breaks... Isn't that called Clyme disease?


Sorry. That just slipped out.

darryl
10-19-2003, 04:39 PM
The cut should be taken in the direction that doesn't put pressure against the gib, but on the fixed side of the slide, on either axis. This should also be the direction that the table is traversed towards when making a cut. The dial markings are most useful when moving the table in those directions. The dial markings are not useful as an absolute measurement system, but only as a tool to help in positioning the workpiece. If you want to add and subtract the measured play in the leadscrews from the wanted dimentions, and keep rechecking the ammount of play to know how much to continue adding and subtracting, go ahead. It's too much to keep in mind, and causes errors and frustration.
I don't generally climb mill, but often enough the cutter will pull the material towards itself anyway, even with conventional milling. You could be milling in the x axis, and the y axix will be sucked towards the cutter. I will often snug the gibs enough to prevent easy moving of the table, and take light cuts, especially when milling to final size. There's more stress and wear on the leadscrews this way, but I don't leave the gibs snug all the time, just when finishing, and when accuracy is needed. I don't like the idea of eliminating backlash with just the leadscrew, there's a potential for much increased wear on it and the nut, and the play in the gibs will still be there. I'm serioulsy considering the hanging weight idea again, as it could be set up to take out sliding play, and leadscrew play, at the same time. This looks like more of a problem for the long axis, because of the distance the table travels. I'm going down to the shop right now to get an idea of how much hassle this would be to implement.

darryl
10-19-2003, 05:08 PM
Ok, well forget the weight idea. I can see it would introduce it's own problems. While looking at this possibility, I discovered that the ball thrust bearing on my x axis has about .025 play, here there should be none. The y axis has .015 total, and is not a ball thrust, just a bushing. Hmmm. Time to clean, lube, adjust, repair? Cheap machine