Benefits of roughing before finishing in 3D

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drjonball
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Benefits of roughing before finishing in 3D

Post by drjonball »

I am new to CNC and struggling to see any time benefits from roughing and then finishing. I'm sure there must be but I am confused. Doing it this way appears to take longer and you also have the complication of a tool change halfway through.

The answer to the question "what is the benefit of a roughing toolpath before a finishing toolpath" gets the answer "it removes the bulk of the unwanted material first". Fair enough, but there is no change in the V-carve time estimates and that suggests V-Carve is making no allowance for the material removed in the roughing pass and you are therefore spending a lot of time cutting air (and by inference, you might as well just have a single 'finishing' toolpath).

Obviously there is some benefit in roughing and then finishing - it seems logical. However, how do you realise this benefit? Is there some setting like when you do a pocket toolpath and project onto the model? Shouldn't the 3d finishing pass project onto the roughed model too? Or does roughing first just allow you to use more aggressive feed and plunge rates for the finishing pass?

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Re: Benefits of roughing before finishing in 3D

Post by ger21 »

Without seeing your project, it's hard to explain what you are seeing.

Say your project requires a 1" cut depth, and you are doing the finishing with a 1/2" long, 1/16" ballnose bit. Since finishing passes are cut in one pass, this would not be possible. By removing most of the material with a roughing pass, you could then do the finish passes with the smaller tool.
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Re: Benefits of roughing before finishing in 3D

Post by martin54 »

The finish toolpath does not cut air at all, it is a single pass at full depth. The roughing toolpath on the other hand is not a single pass (unless you alter DOC ) & will take multiple passes depending on the amount of material to be removed & the DOC set in the tool database he stepover on a finish pass would generally be set at a smaller value to give a better finish :lol: :lol:

You don't always need to use a roughing pass, as Gerry says it is project dependant & without seeing what you are doing (screenshot would help) it is hard to say If you are removing a lot of material then a single finish pass might be to much for a small bit to handle :lol: :lol:

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Re: Benefits of roughing before finishing in 3D

Post by rscrawford »

I only use a roughing toolpath if I absolutely need one. Most of my finish toolpaths use a 1/2" ballnose, with a 1.75" cutting length. So I will cut 1.5" deep with it on a finish toolpath with no concerns. Usually I'm doing a 7% stepover, so I'm only removing 0.035" per pass, which puts very little strain on the bit or the machine even when cutting 1.5" depth.

Its usually just the first deep pass that is hard, because you are cutting a full 1/2" width in my case.
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Re: Benefits of roughing before finishing in 3D

Post by Rcnewcomb »

what is the benefit of a roughing toolpath before a finishing toolpath
It keeps you from breaking your finishing bit.
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Re: Benefits of roughing before finishing in 3D

Post by drjonball »

Thanks for the replies - I think things are a bit clearer now. So there doesn't seem to be any one answer. I think in my use-case, I don't need a roughing pass because my depth of cut is less than the length of the tool and I'm cutting soft material (MDF). If I break a bit I'll revise this post :D

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Re: Benefits of roughing before finishing in 3D

Post by mtylerfl »

You may want to post your file and/or a screenshot of what you are planning to cut sans a Roughing Pass.

There are a lot of missing details regarding your planned project...depth of your model(s), what the physical Bit specs are, what Feed and Plunge settings you are planning, whether you can get away with using an Offset Finish strategy to avoid a Roughing Pass, or if you’ll be using a Raster strategy that could put your bit at risk of snapping, etc.

I’m worried for you!

Oh, you’re correct that there isn’t any “one answer”. Each project has to be carefully evaluated to know when it’s safe to skip a Roughing Pass or not!
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Re: Benefits of roughing before finishing in 3D

Post by ger21 »

drjonball wrote:Thanks for the replies - I think things are a bit clearer now. So there doesn't seem to be any one answer. I think in my use-case, I don't need a roughing pass because my depth of cut is less than the length of the tool and I'm cutting soft material (MDF). If I break a bit I'll revise this post :D
That will very much depend on the tool you are using.
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Re: Benefits of roughing before finishing in 3D

Post by Leo »

drjonball wrote:Thanks for the replies - I think things are a bit clearer now. So there doesn't seem to be any one answer. I think in my use-case, I don't need a roughing pass because my depth of cut is less than the length of the tool and I'm cutting soft material (MDF). If I break a bit I'll revise this post :D
I would not really characterize MDF as a soft material. Sure is is not as hard as Oak - but it is harder than Pine.

It is a different material that sawn wood and needs to be thought of differently.

Also - like others have said there are several more considerations that have not been mentioned. There are a lot of unknowns from our side of the fence
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Re: Benefits of roughing before finishing in 3D

Post by wmgeorge »

My question is why there is not a Depth of Cut setting in the Finishing Toolpath?

To fix my screwup (see above) I am just going to make my Roughing the only pass. Leaving no material. That way I can Limit the Depth of Cut with the .125 ball nose bit.

So how will that break a bit? If your limiting the Feed Rate, Speed and Depth of Cut it would seem to Not break the tool as opposed to a non adjustable Finishing Toolpath?

Granted its going to take f o r e v e r to fix mine but it Was an important project.

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Re: Benefits of roughing before finishing in 3D

Post by Adrian »

Because the depth of cut has no relevance to the finishing toolpath. It cuts as deep as it needs to follow the contours of the model.

Breaking the bit happens by using a roughing toolpath that is much bigger than the finishing tool and/or leaving too much material after the roughing toolpath by means of the allowance setting. If you were to rough with a 0.5" bit and then finish with a 0.125" bit it would be quite likely that any "valleys" in a smaller model would be completely untouched by the roughing bit meaning that the small bit would then plunge to a depth which might be far more than it could cope with.

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Re: Benefits of roughing before finishing in 3D

Post by Leo »

wmgeorge wrote:My question is why there is not a Depth of Cut setting in the Finishing Toolpath?

To fix my screwup (see above) I am just going to make my Roughing the only pass. Leaving no material. That way I can Limit the Depth of Cut with the .125 ball nose bit.

So how will that break a bit? If your limiting the Feed Rate, Speed and Depth of Cut it would seem to Not break the tool as opposed to a non adjustable Finishing Toolpath?

Granted its going to take f o r e v e r to fix mine but it Was an important project.
You could use a .250 ball end mill for roughing and leave -say- .030 material for the finishing cut. That is how you know how much material is being cut by the finishing end mill.

Also - with the roughing cut there are a couple of different strategies that leave different surfaces for the finishing cutter. Step over in both the rough and also the finish will make a difference in the end result.

There are a few really top quality video tutorials available that explain all of this.
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Re: Benefits of roughing before finishing in 3D

Post by martin54 »

I think in my use-case, I don't need a roughing pass because my depth of cut is less than the length of the tool and I'm cutting soft material (MDF).

Can I ask what led to this belief? Just because a tool has a long cutting edge it doesn't mean that it is OK to cut at that depth in a single pass, lots of other things need to be taken into consideration like the size of the tool being used, the speed & feed settings used, he stepover, how rigid your machine is, the material you are cutting, that won't be all of them either, sure others could add others, even how good your chip extraction is can make a difference :lol: :lol:

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Re: Benefits of roughing before finishing in 3D

Post by wmgeorge »

So Experts, If I do my Roughing path (leaving NO material) as the only path and use a .125 Ball nose and limit the Depth of Cut per Pass is that not saying to the machine, follow the tool path but limit the cut depth to NO more that I have entered.... does that not happen? It just takes a lot longer to do the project. I think I have read someplace that is what a user does and he has no issues. Yes if you have limited the Depth of Cut the mill will Not cut on the side.

So again if you take two passes to do the Finishing, what difference does it make? As long as its following the Tool Path? So if the so calling Finishing takes two or more passes because You programed the Depth of Cut to be not more than .01 why would that not work??

I know you guys that do this everyday do not see an issue and up until yesterday I had no problems. I have a lot of time and money invested in this project.

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Re: Benefits of roughing before finishing in 3D

Post by Leo »

Nice thing is - you can always do whatever you want to do.

Using the .125 ball nose with a roughing pass and 0 material is certainly one way to skin a cat.

Maybe this came across the wrong way, but it seemed to me at least that you were asking for advice about using the roughing pass vs not using it.

Not everyone uses a roughing pass, depending on the project. Sometimes just a finishing pass. What you are proposing is certainly doable, yes. Will take longer - yes. But it is certainly doable.
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