Cleaning up the wiki on climb vs. conventional milling

CNC Machines Shapeoko
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I’m just getting into aluminum and find the wiki… challenging. There are little tidbits of info scattered about in various sections, including some that seem to conflict. Climb vs. conventional is one of them.

I will happily volunteer to try and clean this up if we can resolve the information. Examples:

Aluminum: finishing passes:

…using a climb cut for finishing passes produces a nice finish in aluminium (citing this comment)

Under endmill considerations, we have this… do Shapeoko users own such an “extraordinarily rigid machine” and what is “an extraordinarily rigid bit”?

Doing the finishing pass as a climb cut will give a better finish on an extraordinarily rigid machine and bit.

The section on boring holes (which doesn’t say what material):

Secondly using conventional milling produced the best results. Some with climb milling was [sic] ok as well, but the general result was that anything concerning climb milling (be it complete single cut operations or used as finis passes) gave a little worse result.

The recommendation for 6082 aluminum specifies climb, most don’t say one way or another:

3 flute, 45deg flute, carbide, 4mm diameter … 21k RPM, dry, climb cutting … Spiral downcut along a 1mm radius, .4mm per revolution (leading to a 6mm hole) … feedrate of 500mm/min with a forward step of 0.5mm (12% tool engagement)"

The dedicated wiki page for this is super confusing. We have:

Unless your machine is very rigid… you should always do conventional cutting.

This is immediately followed by:

Another recommendation is to climb mill for roughing passes and conventional mill for finishing passes.

Hopefully it’s clear that this could be challenging to make sense of. The last quote suggesting climb for roughing and conventional for finishing would lead me to think we’re taking a “gentler” cut (less stress) via climb, and applying conventional (more stressful to the machine and/or produce a better surface finish) just for the finishing pass. The quote just before that suggests that climb is more stressful (requiring a very rigid machine). Most previous quotes say climb for finishing pass…

There is also no definition of what “rigid” means. Is the SO3 rigid? When I hear this word, I think Bridgeport, and that any hobby CNC is not rigid. I expect this to be specified since it’s not a general CNC wiki, it’s for this machine.

This thread, started by @Julien is almost a replica of this, except most relevant replies are of the form “this is what I do.” I don’t see any definitive resolution (though it leans in the climb direction). The most data-driven comment was that @gmack didn’t see a big difference between climb/conventional for spindle power consumption.

From a “I just want to make stuff” perspective, my personal list of requirements to decide between these two methods would be:

  • which is faster on the Shapeoko (which I think is a proxy for “less stressful to the machine and least likely to break a bit”)?
  • which should you use if you need the best accuracy?
  • which should you use if you want the best appearance/surface finish (including leaving no “skins” at the top/bottom of a part to clean up)?
  • are there particular material characteristics (certain alloys, tendency to work harden) that sway the consideration?

So, to conclude:

  • sticking to aluminum to simplify the scope, can we make any definitive, non-debatable recommendations that I could use to clean up the wiki?
  • if the thought is that it probably doesn’t matter, I’d propose to just remove these statements altogether. They lead new aluminum cutters (like myself) to spend a lot of time thinking about this, as the comments make it seem like an important consideration… but as it stands, you can’t resolve it.

Final aside, also from a new-ish user perspective: the wiki seems frequently used like a storage space for relevant forum comments people have decided fit into various sections. I would much prefer the wiki to look like wikipedia (user validated reference info), leaving all comments and opinions for people to find if they search the forums.

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I tried my best in that section of the e-book to come up with a one-page summary of the opinions I could find, and a one-line conclusion: climb for roughing, then conventional for finishing.

But as for all attempts to model the world with oneliners, this is probably just “true enough” in many cases, and very wrong for [insert specific case/user experience here]. I’m a simple man, I need simple guidelines, and I’d rather follow a simple guideline and decide what to do when reality begs to differ, than not having any clue how to chose, and taking random decisions, or letting the CAM default settings decide for me (I’m looking at you, CC!)

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I’ll take a look at that in more detail soon! Thanks for providing it.

I would say if this holds… the wiki could definitely stand to be updated. The wiki and many posts here suggest climb for finishing (which kind of would imply conventional for roughing, otherwise why specify at all? Just say “climb cut everything”).

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Good initiative to clean up the wiki on this, I agree it’s very confusing.

Short answer: The Shapeoko is a CNC mill, climb-cut everything.

On manual milling machines, climb-cutting is difficult or even dangerous, because the cutting forces have large components in the positive feed direction, meaning the feed table is pulled into the cutter. That can be pretty difficult to control with a hand-wheel, sort of like using the wrong side on a table router (zwhoosh!).

That’s not an issue on a CNC, obviously, but may be a reason why you still hear advice to use conventional cuts. Or why they are called conventional in the first place.

In real wood, conventional cutting parallel to the grain causes sizeable splinters to come off in one piece, because the cutter acts more or less like a wedge that tries to split the grain.

In metal, starting the cut with the thicker chip (i.e. climbing) is beneficial (see the other thread). Also, carbide tools much prefer the higher compressive loads when the edge hits the stock in a climb cut, as opposed to the tensile loads that occur on exit in conventional cutting. Tungsten carbide and the ceramic friends are much stronger in compression than tension.

Of course, there’s no reason not to try conventional, but I’d vote against recommending it as default in a wiki.

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“Tool deflection during Conventional milling will tend to be parallel to the cut”

Climb all the time unless deflection is an issue. Works for me and because we don’t really have backlash issues.

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Great info, all very helpful to know. While I get it’s computer controlled, sometimes I still wonder if all statements I see re. CNC mill hold for “hobby-ish” CNCs vs. big boys. So, that’s a very helpful clarification that being motor driven always avoids what would happen on a manual mill.

Everything you said basically makes me never want to contemplate conventional vs. climb… I think the only practical evidence to the contrary was the quote about a boring test which found that conventional was better. Would dimensional accuracy be a use-case for conventional as a finishing pass?

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This also seems to support that conventional as a finishing strategy could get high accuracy, but climb for everything else, and don’t bother with conventional if you don’t need that high accuracy. At least, I’m correlating deflection -> inaccuracy. Are there any other downsides to deflection?

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Low quality finish, chatter, increased wear on tool, increased cutting forces & chipload, increased temperature.

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Well, it’s not like the cutting forces magically disappear, they just occur in a different direction. Accuracy is affected by the force component (thus deflection) perpendicular to the feed direction. Switching between climb and conventional potentially flips the sign of that deflection (coarse simplification), so that instead of cutting too shallow due to tool deflection (climb), you’d be cutting too deep instead (conventional). Of course, that’s only true for the first pass.

Me, I prefer my machine to err on the side of removing too little material (holes too small, external dimensions too large). Kinda tricky to fix the opposite…

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Thanks again, everyone. Haven’t seen any new activity and will consider this as resolved from what I’ve seen so far: namely, climb cut everything. I’m happy with that and it certainly saves me more hours of agonizing over this! Just wanted to make sure I wasn’t missing something super critical.

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See some further discussion on this at:

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Breaking Taps recently posted another interesting video where he discusses his experience milling steel with both techniques as well as high speed and high feed machining techniques.

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