What cutter shape gives the best surface finish in very soft wood for deep 3d milling?
With a 2 flute, upward spiral cutter all horizontal surfaces and the ones right of the cutter are perfect but the ones left of the cutter show a hard to remove “fluffy” surface made out of half attached chips that the cutter can’t remove.
Strategy and cutting direction don’t seem to have an impact.
To expand on what @Peter_Fouche1 is implying: slot with a slightly narrower cutter, then go back and take a light, high-speed, high-rpm climb cut along both edges, and you’ll probably get a better finish. (I’m just learning about how to do this, as it happens.)
@Peter_Fouche1 cutting profile in the shape of a large, half circle. About 30x tool diameter in diameter and depth.
There are no perfectly vertical surfaces.
(Sorry, can’t show the design publicly.)
The right half of the circle having a perfect surface, the left haft showing major problems that can’t be fixed by doing another pass.
It’s not random but perfectly consistent in serveral places of the design and multiple pieces of the same wood.
With wood boards the problem is there are many things to consider. I assume you are using a natural piece of wood and not a manufactured wood product. If that is the case, you must factor in the wood grain and the direction. Cutting with the grain or against the grain will have different results regardless if you cut by hand or with a cutter. Also determine if the grain is up toward the cutter or down from the cutter. A cutter rotating into an upward grain can pull the grain apart and chip out small pieces, leaving what looks like holes. This becomes worse if the cutter is also moving into the grain at a fast rate. I realize you want to “hog out” as much material as you can in one cut, but hopefully your final “skim cut” will clean up any tear out and give you a smooth surface.
To prevent tear out when you cut against the grain, depending on the wood, your tool or cutter MUST be sharp. Make sure your cutter is ground for cutting wood, not metal. When cutting against the grain on most woods, your final cut should not take off too much material, the feed rate should not be too fast and the rotational speed of the cutter should be increased. This should give a good result.
These are just general suggestions since without seeing the problem it is difficult to say for sure. It might be good for you to visit a cabinet shop or molding shop and show a master woodworker there your problem for precise corrective measures. Wood is a wonderful product that, due to the many natural variations, look great but test your woodworking abilities to the limit. I hope this helps. It helped me during my apprenticeship as a wood Pattern Maker for foundry tooling 50 years ago.
Thanks!
My finish passes remove 0.5mm left over from the roughing.
Because I need a very long tool, I’m using a new and sharp Dremel 561 tool. Not specific for wood but multi purpose.
I’m cutting meander in the general direction of the grain. So the good and bad sides are both cut in both directions concerning the grain.
The wood is some cheap spruce/fir (DE: Fichte /Tanne) since it’s all you get in a home improvement store as a 100x80mm block.
I’ll try to reduce the feed rate but keep the high rpm for the finish pass and look for a similar cutter specifically shaped for wood. (I have some 10 flute burr grinders that give a great surface but are too short to do the deepest parts of this geometry.)
Some first tests with severely reduced feed rates did not change the surface quality but I’ll make some more throughout tests.
@Marcus_Wolschon Forgive me for not mentioning the diameter of the cutter before. A long, small diameter cutter will flex a bit when the wood grain “catches” it. A place I worked for many years ago had a general “rule of thumb” that the finish cut should never exceed 1/4 the diameter of the cutter. This was influenced by the cutter tip speed. At any given RPM, a larger cutter will produce greater linear tip speed in terms of mm traveled per RPM than a smaller diameter cutter. This “rule of thumb” was good for many instances, but was not ironclad for every thing. So, your final cut of 0.5 mm should be good if your cutter is over 2 mm diameter as a borderline case. A smaller cutter would need a smaller final cut.
You will find that the more you get into troubleshooting problems the more items you have to consider, but if you stick to the basics you will not be wrong. I guess I will be doing the same soon. I just ordered parts from Oped Builds for an OX CNC Router. I still have to order the electronics and router, but it won’t be too long.
It’s 3.2mm and not visibly flexing in these spots doing d/15 passes.
@Marcus_Wolschon This is really a challenge and I hope I am not missing something. You are using a 2 flute spiral, which should do, more flutes would be better as you pointed out, but I don’t think you can get more spiral flutes on a small diameter cutter. Your finish cut is good and you mentioned that you varied the feed rate and have the RPM at the highest setting. You also mentioned that you are using one board and the same thing happens in other areas, but in line with the other problem areas.
We don’t know if a different board would exhibit the same problems or not. The one thing you mentioned that sticks in my mind is that the cutter meanders toward the grain. To me this would mean one of the variables is not set yet. The only variables are cutter RPM, direction of cut and feed rate of cut. At this point I would drastically change the feed rate (cut it by at least 50%) to see if that is the critical factor. Any improvement means you are on the right path. You can always work your way back up. Then look at the direction of feed, which may go against your intuition, but try it. In wood the grain can change and the change in direction along with a lower feed rate can help. If this fails I am stumped and hope I did not miss an important point. Working with wood is not plug and play, each piece is different and you have to fine tune the way you work it.