3D IH Milling

[wildcat]

Tonight was the first time that I have attempted 3D milling on my IH mill. The piece below was cut from 2x2x1 aluminum using a 3/8" ball end mill held in a collet. Overall I am fairly pleased but I do see one problem. Namely, the flat area is 7 thou higher at the beginning than at the end. This creates an edge that is easily seen when holding the part in your hand and may be noticeable on the right side in the picture below. The end mill might have slipped though the cuts being made were very light. Might there be other reasons? Is it even reasonable to expect better from this mill? I have never measured any backlash in the X and Y but have measured (and attempted to correct in Mach3) .002 backlash in the Z. Though, I don't think this is a backlash problem. What else should I consider to try to isolate this error? Perhaps unrelated but I also often have an edge formed at the beginning/end of an outside contour.

[judleroy]

Try slowing down the cut or better yet a smaller tool 1/4" maybe. Also if you stopped and retracted during the cut where the line starts you may have lost position for some reason.

[wildcat]

The only retract away from the part was at the end of the finish pass. Of course while cutting the part generally all three axises were moving at once. Cuts alternated from starting at the top and then the bottom of the hemisphere... Imagine a zig-zag pattern radially arranged. If the end mill slipped, it slipped down. There seems to be at least two "lines." The obvious one that separates the beginning from the end and another at about 135 degrees which starts a region that is about 2 thou deeper. How might position might be lost on this mill? I am certain that the Gecko drives never faulted because that would have caused a major problem. How might a smaller cutter help?

[philbur]

It's my understanding that you should avoid backlash compensation if possible. I would try again with it turned off. Also your described contouring problem could, I think, indicate backlash in x and/or y, although you can get a "mark" at the start of a contour due to the longer residence time of the cutter at that location during a plunge operation.

Just some thoughts from a novice CNCer.

Regards
Phil

Originally Posted by wildcat Tonight was the first time that I have attempted 3D milling on my IH mill. The piece below was cut from 2x2x1 aluminum using a 3/8" ball end mill held in a collet. Overall I am fairly pleased but I do see one problem. Namely, the flat area is 7 thou higher at the beginning than at the end. This creates an edge that is easily seen when holding the part in your hand and may be noticeable on the right side in the picture below. The end mill might have slipped though the cuts being made were very light. Might there be other reasons? Is it even reasonable to expect better from this mill? I have never measured any backlash in the X and Y but have measured (and attempted to correct in Mach3) .002 backlash in the Z. Though, I don't think this is a backlash problem. What else should I consider to try to isolate this error? Perhaps unrelated but I also often have an edge formed at the beginning/end of an outside contour.

[srmaietta]

Is that one side meeting the other at a lower level, or is it a groove from a single pass that was too deep?

congrats on a cool 3d part! Polish her up!

~Steve

[philbur]

It looks like a Z issue. You can see the step on horizontal portions but not on the vertical section. If you study the step closely you should at least be able to determine which axis are involved in the error.

Regards
Phil

Originally Posted by wildcat Tonight was the first time that I have attempted 3D milling on my IH mill. The piece below was cut from 2x2x1 aluminum using a 3/8" ball end mill held in a collet. Overall I am fairly pleased but I do see one problem. Namely, the flat area is 7 thou higher at the beginning than at the end. This creates an edge that is easily seen when holding the part in your hand and may be noticeable on the right side in the picture below. The end mill might have slipped though the cuts being made were very light. Might there be other reasons? Is it even reasonable to expect better from this mill? I have never measured any backlash in the X and Y but have measured (and attempted to correct in Mach3) .002 backlash in the Z. Though, I don't think this is a backlash problem. What else should I consider to try to isolate this error? Perhaps unrelated but I also often have an edge formed at the beginning/end of an outside contour.

[wildcat]

Yes, the sides have different height. However, this was not intentional. Please see the attached image. The finish pass starts at the "point" of the hemisphere initially traveling along the 0 degree line and then "zig-zags" counter-clockwise up and down the hemisphere. There are two marks - one near 135 degrees (counter-clockwise) and the other at 360 degrees. The first mark delimits sides with .002 error (measured with a mic from the bottom of the piece) and the second mark delimits sides with .007 error. It is as though the Z position was slowly being lost as the mill traveled around the hemisphere. the chip loading was in general quite low while finishing especially in the area with the most error. The part was mist cooled the whole time. I'll give it another go using an endmill holder.

Originally Posted by srmaietta Is that one side meeting the other at a lower level, or is it a groove from a single pass that was too deep? congrats on a cool 3d part! Polish her up! ~Steve

[wildcat]

Yeah, I think there is an issue with the Z axis as well but I am not sure how to go about identifying the actual cause. I have measured the head height using a height gage resting on the table and with .002 backlash comp all is fine. I am pretty sure that if I disable backlash comp that passes will alternate in depth by .002 (the backlash amount). The error might have occurred if the part lifted on the right side as the tool as going around but it would had to have been pretty gradual and gosh the cuts were really light.

Originally Posted by philbur It looks like a Z issue. You can see the step on horizontal portions but not on the vertical section. If you study the step closely you should at least be able to determine which axis are involved in the error. Regards Phil

[youngfg]

Were the cuts getting deeper? If so I would say the end mill slipped down in the collet. I have had that happen to me several times.

[wildcat]

I use ETM end mill holders and Lyndex collets. This is R8 tooling. In this particular case I used a collet. Is there something better that I could be using? I have had end mills complete fall out before when taking massive side milling cuts. That's when I switched to the end mill holders. But there are those that say the end mill holders are junk because they put the tool off center. Never had an end mill slip in a holder though

[youngfg]

I have had .25" endmills slip down while slotting steel. They would slip down enough that the endmill would break from the cut getting so deep.
I have been using endmill holders so the mill can't slip.

[philbur]

I think the slipping endmill is probably a red herring if you were take very light cuts.

Regards
Phil

Originally Posted by wildcat I use ETM end mill holders and Lyndex collets. This is R8 tooling. In this particular case I used a collet. Is there something better that I could be using? I have had end mills complete fall out before when taking massive side milling cuts. That's when I switched to the end mill holders. But there are those that say the end mill holders are junk because they put the tool off center. Never had an end mill slip in a holder though