If I want to do a final CNC finishing operation with a 1 mm dia. bit, what would be a “good” diameter for the bit of the previous operation?
Or more generally: what’s a “good” gap between bit diameters for subsequent CNC operations?
Is there a certain “rule of thumb” I can follow here?
Sorry if this has been asked before;
But I couldn’t find an answer here or via a quick Google search either…
Also, I’m aware the answer might depend on other factors, such as material of choice; in my case that’s primarily wood. But I’m just looking for a rough, 80/20 answer here.
It doesn’t really matter actually. At least depending on the software you are using.
If you are using something good like Fusion360, and make use of the "rest machining"options it will take care of everything for you like the cut of depth, axial engagement etc. So nothing to really worry about except choosing the most suitable toolpaths and setting your parameters right.
If you are using Luban, ít’s quite different though. Although Luban now does support multiple toolpaths with different bits, you can’t use rest machining options. (and a lot of other more advanced features are missing as well). So using multiple toolpaths in Luban usually results in setting a large stepdown, or otherwise you’ll be cutting in thin air for hours.
However, in smaller pockets or corners where your larger tool couldn’t reach, this means your fine tool will be digging deep in the material. And if you suddenly plunge a 1mm tool 10mm deep in a piece of wood or push it hard into a corner, there is a very high chance of breaking that tool.
So the main factor is which software you’re using to generate the toolpaths.
Don’t even bother trying with Luban. Have to use Fusion 360 for this.
A lot of it depends on how much material you’re taking away and how patient you are.
You could just do everything with a 1mm bit, but obviously that’s going to take a lot longer.
It also depends on if you’re trying to do organic/curved surface with a ball mill vs. a flat end mill. (Ball needs some material to cut into for best final surface)
I tend to use 3 bit sizes with most stuff I do. 1/4" for first clearing pass (3d pocket) with stock to leave set at 1mm. Then a 3.175mm bit for a second pass. Sometimes that works fine for the detail I need and I don’t leave stock. But usually I set it at .5mm and then do a final pass (3d pocket) with anywhere between a 1.5mm to .5mm bit. Sometimes I need to do a 3d pencil path (or change 3d pocket from helix to plunge) if it’s not getting to everywhere it should.
Many thanks for the quick replies @brvdboss and @sdj544 !
I use Fusion 360 with rest machining, so all good on the software side.
Maybe to re-phrase my question again:
If I do the finishing step with a 1mm bit, will it make any difference if the previous step was done with a 2mm / 2.5mm / 3mm bit?
(assuming only the bit diameter varies and all other settings remain the same in Fusion 360, so let’s say rest machining with 0.5mm stock left before the finishing step)
Sorry if this is a dumb question; just trying to understand if there’s a better / worse way of “preparing” the stock for the final finishing step…
Roughing pass: remove as much material as possible as fast as possible
Finishing pass: get a smooth clean finish
You can do the roughing pass with 2mm and finishing pass with 1mm but why? The roughing pass is going to be super slow. It doesn’t matter what you use, but if you use a small bit it’s going to be slow.
You can do stock to leave with radial set to .5 and axial set to 0. Then the clearing pass (here I’m using a 3.175mm) does all the flat areas and then the smaller details around the wings are done by the .8mm.
There’s also a trick you can do with heights to keep it from going over the same areas sometimes. Set the height to just below or above a surface by a really small amount like .0001. Then it will ignore a flat area (or raised area) and not do it, but still do the detail areas at pretty much the same height - no apparent difference in final finish.
Again, play around with toolpaths and see what happens. A lot of variations depending upon your design.
Many thanks all!
I think the point I was trying to get to – but wasn’t able to phrase properly – is that a larger bit, say 3 mm diameter, might not be able to access all areas properly, depending on the design.
And so in such a case, I should consider using a smaller diameter before the finishing step, to make sure the bit has indeed accessed all the areas it was supposed to…
But beyond this, I understand that the bit diameter isn’t critical.
