Carving and surface finish

[BillK]

Attached is a chart I worked on in jpg form.

Basically it lists the the 3 common sized ball endmills and then calculates (by using the Pythagorean theorem) the height of the theoretical peak left between stepovers. Theoretical because this assumes zero runout. Runout would tend to lower the peaks but ruin the details if excessive.

So if you are happy with the finish that you get with an 1/8 ball at a 10% step over, but feel you would like to see extra detail on depths that a 1/16 ball mill would give you, go to this chart where it shows your .00031 height for a 1/8 ball nose at 10% step. You can see that you'll get the same step height with a 1/16 ball nose at 14% but since the step is about 1/3 less it should take you about 1/3 longer to cut then your 1/8 ball nose.

I actually cut my very detailed celtic cross in cherry (also attached) at an 18% stepover with a 1/16 ball nose in 58 minutes. The step was slightly smaller than you would get with an 1/8 at 10%(.001125 vs. .0125) with a .00051 height but this also required only a light sand mopping to finish as the peaks between stepovers are easily removed.

So this chart would help anyone who wants to see what adjustments they could make to balance finish and speed. It’s daunting to look at the estimated time for machining and not know if it’s worth letting your router hum for 3 hours. This would allow a comparison for evaluation before committing to 90,000 lines of g-code.

The level of finish required is something that you have to determine based on how the carved object will be used, the detail you want to see, and how long you want to machine. It is at a point that you determine there will be no advantage to going to a lower stepover percentage.

I hope this chart can help make that decision easier for you.

[wolffie]

Hi Bill
That's great
Thank you so very much.
Wolffie

[Eagle55]

Great training information! It would also be interesting to see what those variation do with respect to carving time given a constant reference of 10 square inches of carved area for instance.

Roger

[FixitMike]

The carving time will be approximately inversely proportional to the step distance times the pass depth

i.e. if you go from a step of.045 to a step of .015 , the time will increase by a factor of .045/.015 = 3 if the area of the cut and the pass depth remain the same. If the pass depth is also halved (for a smaller tool, for example) , then the time will be 6 times as long.

For a 10 inch square with an average depth of cut of .25", a step of .030, a pass depth of .0625 and a feed rate of 60 inches per minute, the approximate time required for cutting only will be:
10 x 10 /(.030 x 60) x .25/.0625 = 222 minutes (3 hours and 42 minutes). This is very, very, approximate since it does not take into account the additional time to reposition the tool between passes. VCarve Pro uses a default factor of 2 (double) for its time estimates.

[baby15]

ya this is what i asked tony the outher day. thanks

[Ausworkshop]

That's great. My head hurts from thinking about all those calculations tho. You've put alot of time into it so thank you. I'll refer back to it when i do some carving. Can i ask you what sand mopping is. What do you use for that?

[BillK]

Ausworkshop,

Here's an example from Rockler, http://www.rockler.com/product.cfm?page ... te=ROCKLER. Many other make them with different names, I got mine from Klingspor. You chuck it up in a drill press or drill and it lightly sands any type of contour.

[Butcherman]

I am going to work with it now. This is great info. Thanks

[WoodEraser]

Bill,
Thank you for the chart and your cross looks fantastic.

[tonydude]

Bill,

Thanks for the chart and your cross looks great.

Tony