Choosing the right ball nose(3d modeling)

[marcrunner]

When cutting a 3d model, I am having trouble deciphering which ball nose bit for finishing toolpath. Is there a more mathematical approach to choosing what size ballnose to use for finishing? For rough cutting? Or is looking at the preview screen for definition the only way to choose?

Also,
When I create a boundary profile for each 3d model slice, how can I make sure that I don't cut all the way through while doing my roughing and finishing passes, in order to hold the material down until I run the profile pass. Once achieved, how do I then incorporate tabs that will not be removed with the roughing/finishing toolpaths? Thanks, Marc

[Brady Watson]

You pretty much have to trial & error with the previewer to find the quality/level of detail you desire. Do this enough times & you'll have a pretty good idea what size to choose right off the bat.

Only offset your boundary vector 1/2 the bit diameter to make sure it doesn't fall off the side. Sometimes you have to tweak the boundary vector via node editing to make sure you are keeping the ball on the work. No matter what software you use, there is ALWAYS a little bit of deviation between 2D and 3D - meaning that the boundary vector will be close...but probably not exact.

For tabs, you can do this a few different ways, from restricting the boundary vector or by adding 3D geometry that represents the tabs you want to have. Be sure to Merge it & Bake and it will be treated as part of the main 3D geometry. It will only get cut all the way through on the profile pass unless you add tabs to the profile pass that are coincident with the "3D tabs" you made, then it will still be held to the waste material.

Make sense?

-B

[marcrunner]

Thanks Brady,
Makes sense. I am afraid that if I restrict the boundary vector for tabs, that I will essentially alter the finished model by whatever thickness I pull the boundary back by. Alot still being analysed in theory on my end. Would be nice to be able to have the software create a "Modeling tab" feature. Maybe if one were to need this feature for alot of runs, they could create the modeling tabs in different heights and thicknesses in a dedicated file, calling on them when needed.

[Brady Watson]

As your abilities increase, doing these types of operations will be 2nd nature. You can easily make a "3D tab" by drawing a rectangle where you want it to be in relation to your model & then go to the Create Shape tool. Choose the bottom shape & enter a thickness for your tab. Then at the bottom choose Merge and create it. Then check it out. If it is the right height and location, Bake it into the main model. If not, tweak and try again. It literally takes less than a minute to do.

-B

[Xray]

Alot depends on size and detail of the model - If you are doing a very detailed 8x10" of a ladies face with flowing hair, you want to use a .125 bit.
If the size was more like 15x18", you might get sufficient detail with .25. So the smaller the model the smaller the bit, amount of detail is a variance that is a judgement call ,,, And all is a balance between the detail you want, and getting it done in a reasonable time frame - And there are other factors as well such as stepover which can also give you a balance between detail & time. The smaller the stepover the better the details, but it takes longer.

[marcrunner]

Brady, yes, should get faster as time goes. Not sure if anyone could actually pay me for detailed 3d work, as the time it takes is so long. Enjoyed by myself nonetheless.

Doug,
This is very true, and I may be able to find the sweet spot between a larger model that allows for a larger tool, and the fact that the larger model will require more machining time. I am going to go for a 5% stepover, on this project, and work from there.

[cowboy1296]

Just to add to what you have already been told. I do almost all of my finishing with either and 1/8 or 1/16 inch bit. First calculate your tool path with the larger 1/8 and then preview the tool path. DO NOT reset your preview and then recalculate the tool path with the 1/16. Then preview your new tool path. Watch your screen closely because the change will be quick. If there is a major change in the quality then go with the 1/16 if not so much stay with the 1/8. Hopefully that made sense.

[tmerrill]

Rick,

If you assign a dark color to the toolpath for the larger diameter bit and then preview the toolpath for the smaller diameter bit you will get a good visual indication of where the smaller bit will make the most improvement. Depending on what material you have selected for the preview, it may also help to assign a contrasting color to the toolpath for the smaller diameter bit.

Like you, I still watch the preview develop but the colors help with being able to sit and study the results.

Tim