Climb Cutting Versus Conventional Cutting

Here's a close look at the difference between climb-cutting and conventional cutting on the CNC, with explanations of how each method affects the quality of cut. October 3, 2011

Is there a standard rule for cutting out MDF on a nested based router, clockwise or counterclockwise (with a bit rotating clockwise)? I was told to cut out counterclockwise and always had. I've recently started onionskinning parts and notice I get much better cut quality cutting out clockwise. Just wanted to see what everyone else does before I change all my programming setups.

Forum Responses
(CNC Forum)
From contributor M:
Rule of thumb for me is to conventional cut panel products and climb cut solids. Connventional cutting would be cutting counterclockwise around the outside of a part and clockwise on the inside of a cutout. Conventional cutting will give you a better surface finish on melamine and laminated products. Your bits will tend to last longer, stay cooler and put less horizontal pressure on the part. That said, if your cut quality is better with a climb cut, and the edge quality is important, throw out that junk I just said and go with what works best for your application.

From contributor J:
I was told by a rep from Onsrud to cut in the direction that gives you the best finish. She said make a cut and look at both the good part and the waste and if the waste cut looks better than the good part, reverse the direction.

From contributor T:
I’ve always heard it explained this way. The direction of cut is a function on the cutter more than the material being cut. (And yes, there will always be some exceptions.) The two primary cutting materials used are carbide and high speed steel. Carbide (also known as cemented carbide) has grains of carbide glued together with a binder, commonly cobalt, to create the cutting edge. If you were to zoom up on the edge, with an electron microscope, you would see grains of carbide held in place by the binder material. It's kind of like a chunk of concrete - gravel is held together by a gypsum binder. Carbide is the actual cutting material but it does not make up 100% of the edge. For this reason the edge of carbide is not as keen (or sharp) as a high speed steel tool.

The main advantage of carbide it that it is harder than high speed steel. Hardness adds abrasion resistance. It is the abrasion resistance that makes the tool last longer. But the down side is, because the edge isn’t as sharp if you cut with a conventional tool path, the cutter rubs a bit, beginning at the tangency point and until it “digs into” the chip. This small amount of dragging creates heat and friction, which degrades the surface quality and wears the tool.

By contrast, “climb cutting” starts at the thickest part of the chip and cuts to the thinnest part, which is at the tangency point. It’s common to see a better edge when climb cutting with a carbide tool because the tool does not rub at the tangency.

Even if you cut with the full width of the cutter, there is still a side of the cut that shows climb cut characteristics. You will need a solid machine to do climb cutting because vibration will break the cutter, due to it being brittle because of the hardness.

From contributor K:
Contributors M and T are right on and there is no arguing with contributor J's logic. Consider also: when I onionskin, I climb the first pass and conventional cut the second. The climb pulls the bit a very small amount away from the work and the conventional cut runs pretty true (since there is less material being cut away). Before adopting this method I often had a small lip at the step level due to machine/tool torsion forces or stress relief in the panel. The small cost in bit life for me is outweighed by the overall better quality of the edge. Every system is different; this just works (after a whole lot of experimentation) best for me.

From the original questioner:
Thank you all. What seemed to be sort of a boring question really generated some thought provoking answers.

Contributor K, I have been trying to resolve the same lip issue you've mentioned. I'm not sure if my software will allow for a climb-cut then conventional pass (Microvellum... will have to check it out) but two passes with a climb cut seems to manage a lot better than two with conventional.

Contributors M and T seem to conflict on which passing method generates the most heat. There seems to be logic to support either answer. I may just buy a temperature sensor to check if there is an appreciable difference.

From contributor K:
Try also a few hundredths separation between parts in the nest. This lip issue is deceptive. Took me a while to figure it out. It can occur when climb cutting nests with no separation between parts, as all the deflective force on climb can push the bit slightly into the adjacent part. In other words, the first cut is most of the way through and climb, lots of side force, away from the work piece. This pulls the bit slightly into the adjacent part. Then the conventional cut with very little force runs perfectly true. Result is a small step caused by the path from an adjacent part. If this is the case, you will see the step in the center of the nest but not on the edges of the sheet where no adjacent part exists. It also can be deceptive because it only happens for new bits. Machine compensation for sharpened bits can mask the effect by effectively creating this part separation at the machine, but not in the software. You use Microvellum, I use AlphaCam, but the idea is the same.

For a part .76 thick I climb to .73 then conventional to .765. Part separation of about 3 hundredths works well for me. Play around with these variables and see where you land.

From contributor D:
A good way to think of the forces involved and the effect on the material is to stop a tool at zero rpm and just turn it by hand. A climb cut motion enters the material on the side and slices downhill easily. It's natural to hold a stick in your hand and whittle it with a pocket knife using this same motion. Conventional cuts dig in hard and blow out more on side cuts. I work with solid wood, making doors. Grain direction affects our cuts on everything, as with small parts the tendency to throw them is much greater. It's the rule to go conventional across the grain, and climbing with it. Climb cutting a small rail with a big tenon cutter in end grain looks like a big paddle wheel hitting the end grain almost straight on, and guarantees the part will be thrown, even with the strongest suction available. Most people cutting MDF, melamine, and plywood don't have as much to worry about, in my opinion.