Poor finish

[LLDesigns]

When milling a profile, it seems that I am getting the better finish on the wrong side of the cut. If the spindle goes CW and the cut is going CCW, why is it that the outside has a better smoother finish than the edge of the part I intend keeping?

[WoodSpinner]

When milling a profile, it seems that I am getting the better finish on the wrong side of the cut. If the spindle goes CW and the cut is going CCW, why is it that the outside has a better smoother finish than the edge of the part I intend keeping?

try it with a climb cut instead.

John

[LLDesigns]

I understand that i can change the direction which becomes a climb cut, but is there a reason why it does this?

[Robin Hewitt]

When milling a profile, it seems that I am getting the better finish on the wrong side of the cut. If the spindle goes CW and the cut is going CCW, why is it that the outside has a better smoother finish than the edge of the part I intend keeping?

CCW outline is climb milling, the cutting forces bend the tool into the workpiece. Cut downhill and the tool shies away from the workpiece so there is metal left for the finishing pass.

Pic is two similar parts, top climb mill bottom conventional

[LLDesigns]

Thanks for the explanation. Makes more sense why it happens

[Robin Hewitt]

There is a snag...

Downhill cutting is more likely to "grab" if the tool is not cutting it's full diameter. ie: not slottting. You really need to eliminate any backlash before using it.

[Robin Hewitt]

Maybe I should expand on that a bit...

If you are climb milling the nut driving the axis pushes the tool against the workpiece. The tool pushes back against the nut. Everything is hunky dory.

If you are conventional milling the nut pushes the tool against the workpiece and the slide is instantly drawn on to the tool through the backlash. Clunk. The nut catches up and it repeats. Your finish is a series of vertical lines.

[LongRat]

I think you have those in the wrong order. Climb milling is where the tool tries to climb over the cut - it pulls the workpiece in the direction of feed. It gives a better finish because there is less chip re-cutting and because the chip starts off thick and ends up at zero thickness as the tooth completes the cut.
With conventional milling, the chips are dumped in front of the tool and can be recycled into the cut again. Also the tool will tend to pull laterally into the cut too. When milling with a machine with backlash, generally the advice is to use conventional milling to rough out the part and then skim off a thin finishing allowance with climb milling. As long as the allowance is small, there will not be sufficient force to pull the table through the backlash and you won't get any nasty surprises. With CNC with no backlash, pretty much all cuts are climb milled. It requires less power, tool life is usually better, and the finish is superior.

[fragger6662000]

and to add to the perfect text book answer is that you might consider conventional milling is there is an abrasive skin on the part which might damage the tool. by conventional milling the skin the tool breaks the material away rather than coming in contact with it.

[Robin Hewitt]

I think you have those in the wrong order.

Perfectly possible, I was taught uphill, downhill. Uphill you push against the tool, downhill it helps you on your way.

I thought climb meant uphill but I've probably got it wrong. Doesn't worry me, I'm an old man with a full order book

[mwood3]

Perfectly possible, I was taught uphill, downhill. Uphill you push against the tool, downhill it helps you on your way.

I thought climb meant uphill but I've probably got it wrong. Doesn't worry me, I'm an old man with a full order book

Easiest way to remember it...with "climb" cutting the tool tries to climb up the part!

[LLDesigns]

It seems as though in either direction, I am not getting the finish on the side that I would anticipate. The side end up not being smooth. It just seems that it is rough on the edges. What causes this?