Interrupted Cut Removes Tool Point

[mcman56]

I have a 6061 AL part about 4 3/8" diameter clamped in a 4 jaw chuck. It is sort of square with round edges. See picture. It needs a narrow tool to reach down inside the jaws. I only need to remove about 0.025" more off of the back face but each time I start, the interrupted cut just takes the sharp point off of the tool. I'm using HSS about 5/16" square with 300 rpm, trying 0.005" cuts. I realize I should have planned ahead better but now would just like to get the part completed. Are there any suggestions? Would a different rpm help? I'm no expert so could be overlooking something simple.

6061 Part

[Bentworker]

What kind of radius are you running on your tool?

[mcman56]

Very little, just a light hone to make it less sharp

[ChipMaker4130]

That material shouldn't affect (for a long time, anyway) a typical 'point' of HSS, interrupted cut or not. Is the cutter for sure being damaged, or is it just a buildup of Aluminum at the point? How much 'stickout' do you have from the holder to the cutting edge, and what is the shape of the cutting end?

[Harold_V]

I'm having a little trouble coming to terms with the idea that you're damaging the point of the tool. Aluminum can be machined almost without regard to surface speed. The point that has been made that there may be buildup is likely spot on, although I'd certainly encourage you to explore the geometry of the tool. It may not be cutting freely because it's making contact where it shouldn't. That often results in the type of finish I see, and may also be the cause of the rolled burrs on the edges. A good, sharp tool should cut without that, unless the aluminum is dead soft.

The other concern might be the nature of the aluminum. It doesn't appear to be a casting, but is it? If so, has it been heat treated, or is it in the as cast condition? It will be soft and prone to tearing, as well as prone to yielding a lousy finish.

If it's made from plate, is it heat treated? 6061 aluminum in the O condition doesn't machine all that well. If it's in a T6 condition, the finish should be much better, assuming you're not cutting dry.

Are you? If so, have you considered using a little kerosene? It will go a long way towards reducing, or eliminating, tearing, assuming that's your problem. If it is, you most likely will find evidence on the tool itself, where that is chip welding at the tip. No kerosene? Use WD-40, or even some mineral spirits with a touch of oil added. Any of those choices will improve surface finish and limit chip welding.

It helps to stone the top of your tool when machining soft aluminum, as the honed surface lowers friction, resulting is less chip welding. Don't be afraid to use positive rake, either. Zero rake typically yields poor performance in these circumstances.

H

[earlgo]

I sure hope that the 2 steel rules between the part and the chuck are secured in some way. It would be painfully unpleasant if either or both decide to leave due to centrifugal forces. If that is an 8" chuck then the circumferential speed at 300rpm is about 10 fps. Enough to put out an eye or slice an arm. Just a safety cringeworthy observation.
--earlgo

[mcman56]

It is 6061 I think t6 and machined fine up to this point. Does aluminum ever have hard spots or contamination

[mcman56]

It looks like one of the jaws sticks out farther than the other 3. The tool was contacting the jaw. I should have completed a better inspection earlier

[Harold_V]

That makes a lot more sense. It wasn't obvious in the picture, in spite of that being a consideration.

Good to read that you found the problem.

In regards to hard spots in aluminum, while I can't say that it is not possible, that has not been my experience, and I've machined one hell of a lot of aluminum in my many years in the shop.

I share earlgo's concern with those rules. Should have made mention.

How about a report on the outcome of your efforts? Did you achieve success?

H

[mcman56]

I was semi successful until making a mistake. I need to redesign to make it easier to make. However, the part was not coming out very flat and I think it was warping/ springing when machining. There are cuts on both sides and flatness is important. is there any general advice on keeping such a part flat ...minimize springing ....machine out the springing.

[spro]

It could have been attached to a face plate.

[GlennW]

is there any general advice on keeping such a part flat ...minimize springing ....machine out the springing.

Rough the entire part out within about .030"-.040" and then go back and make your finish cuts to the required dimensions.

I'd probably flip it a couple of times when roughing it out.

The best demo I had was years ago when a friend sawed into a piece of 1/4" x 6" cold rolled flat. He told me to watch the cut open up as he progressed, which it did. The light came on at that time about internal stresses in metal when machined.