pocket tearing or burning

[EEDesigner]

There are two basic options for pocketing including Offset and Raster. I sometimes have trouble with both.

The raster zigzag pattern has the bit cutting in both conventional and climb directions. Certain woods (like red oak) tend to tear out when the bit travels in the conventional cut direction. Reducing the step-over percentage or changing the cut angle helps this problem. Using a down-cut bit also helps. However I find the tear out is too unpredictable so I instead tend to use offset patterns when cutting wood.

The offset pattern is excellent for many pocketing operations, but I find the tool lead angle and distance to be largely uncontrollable for small area patterns, especially when the X/Y aspect ratio is close to 1 for any given single plunge area. lead distances sometimes get split up into many small zigzag motions when the tool is plunged into the material as shown in the picture. The zigzag motion happens any time the lead distance is longer than the distance traveled before the first step-over in the offset pattern. This causes problems when the zigzag distance approaches the tool diameter, and it can cause severe burning with down-cut bits. My workaround is to reduce the cut depth to the point where it does not matter if the tool is plunged straight down with no lead, but that GREATLY increases the cut time. My other workaround is to pre-cut the lead-in area, but I still ruin projects when I forget to implement the first and second cuts in a coherent manner.

Pocketing works well with many materials but I see two potential problems with pocketing in wood. I either risk tear out with the raster pattern or I risk burning with the offset. Is there a setting I am overlooking to at least protect against burning my down-cut bits with the offset patterns? Is there an option to tell the offset to cut from the outer perimeter towards the center instead of center outwards? That might fix the burning issue because it would provide longer travel distances before the the first step-over, but I do not see such a setting.

[ohiolyons]

Have you tried pocketing with an offset allowance to protect the edge from tear out and then do I inside profile to remove the offset? allowance?

Trimming away the small offset allowance would be less prone to tear out.

Have you tried ramping also?

[Leo]

Whether climb cutting of conventional cutting there will always be a point where the cutter will try to cut against the grain and tear the wood. Even if you could use M4 rather than M3 to spin the cutter counterclockwise and use a left hand cutter - the problem is still there.

If you make the cuts and leave something for finishing, then soak a sealer, like shellac into the grain, then use a razor sharp cutter to make the finish cuts - maybe, just maybe, you can get a good clean finish. Maybe.

Harder and denser wood is a better option. Hard Maple works well. I don't care for Poplar. I like Cherry. Pine is aweful. Oak is just one of those open grain woods that gives you the fuzzies.

If there is a good strategy to cut softer or open grain woods on CNC I am not aware of it, but would LOVE to know.

[ohiolyons]

I almost exclusively use walnut and mahogany.

[EEDesigner]

@ohiolyons: I often do a finish pass, especially for inlays. One thing I like about the raster mode is that it (optionally) does the finish pass for you and leaves a beautiful edge. Tear out, however, can be detrimental around corners that must be preserved in pristine condition. The oak piece I recently cut had rather large pieces torn out to the point where an offset would not have helped all that much. The raster is simply too unpredictable for my liking, which is why I more often use the offset.

@Leo: I have always had pretty good luck with climb cuts. I don't recall seeing tear out when climbing, but I have trouble remembering much of anything these days. I never tried the sealer trick, but that might be a good idea. Tear out in raster mode usually happens on the first surface cut. After that first cut is complete, assuming it is deep enough, most of the tearing threats go away. If I soak the surface of the wood with a penetrating sealer then that may mitigate the first-cut tearing issue. I'll give that a try.

I do love working with wood and CNC has opened the door to a whole new world of fun in that regard! I make lots of things with all different kinds of wood. Some work better than others, but I guess my all-time favorite is bloodwood. It cuts like soft plastic without tearing or burning. I also agree with your comments about hard maple. I made a set of fancy shelves for my wife a while back with maple and bloodwood as the main ingredients. Wow, what a beautiful contrast!

For the "fuzzy" woods like oak, I find that a coarse scowering pad works well to de-fuzz it.

[Leo]

EE,

I was referring to CNC cutting on the denser woods. I also use a myriad of other woods and materials in my shop to make things.

Glad to have you join in with us.

Looking forward to seeing some of your stuff.