[Shahrokh]

Hello!
I am new to cnc world. have just started a robotic workshop and would like to cnc mill a jewelry box with a lid that has a shape of a hemisphere.
I would like to hear from the experts about the strategy to make tool paths. both roughing and finishing.
I was thinking first mill and finish the inside of the hemisphere, then rotate stock, fix it with correct coordinates and then mill the outside, with some bridges left to hold the piece.

How would you go about milling a hemisphere with let's say 50 mm diameter and a thickness of 8 mm. The material is gonna be cork , so any thoughts on feeds and speeds and type of endmill and ballmill is highly appreciated.

Best regards
Shahrokh

[Melkiades]

It's two ways to manufacture this lid.
Natural - one thick piece of stock
And economy - set of flat arcs glued to stock
If you choose 1st way, you need to machine inside lid by long tools - at least 42mm
The minimal diameter of this finishing tool i seen is 6mm, but you can machine with 10-18mm diameter, depends of your spindle power.
If your Rhinocam license allows 3 axis advanced strategies, you can use horizintal finishing directly, without prior roughting.
Also you can flip stock by defining new setup with one axis rotated at 180°. Very useful for me.
And you also don't need to make bridges. Just make stock little thicker, after you process both sides, you can manually sand out thin piece of stock below

[Shahrokh]

Thank you for your reply.
Actually the form passes hemisphere which means that with the horizontal finishing some of the stock material is gonna be left out. Some type of 5 axis is needed but not normal to surface since it will crash with table. Please look at the image.

[awerby]

You could do that without a 5th axis, either by flipping it over and carving the other side, or by mounting it on a 4th axis. If you're working with the Standard (3-axis) version of RhinoCAM, set up a plate with locating pins on the central axis so your part, which should remain attached to the block it was cut out of with support tabs, will remain registered when you flip it.

[KH0UJ]

Is this what you mean on your jewelry box sir? I drew it in 3D to let you see the actual pieces on your desired measurements, that`s 50mm outside diameter with 8mm thickness, I added a chamfer lid on the top piece so that you can close it on the second piece, I also added tabs so that it wont come flying at you when you machine it on a 3 axis, when machining this on your CNC there are 2 stages, first the top, after that flip it over then machine the back portion, and so as the other piece, cut it with a hand saw after the machining process

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[Shahrokh]

Thank you for your reply and taking time to build nice models.
That sounds good. the thing I need to consider is that I want to produce 100 of them and I have cork blocks of 100X915X~50 mm. Probably I can apply the strategy you showed here to rows of tops and base pieces instead of individually. but may be the wall between two adjacent pieces will disappear.
Regards

[Shahrokh]

Thanks!
I don't have 4 axis table but actually I am going to mill with an ABB robot , so I have 5 axes in my disposal and a static table. I need to come up with a smart strategy to leverage all axes !
Regards

[KH0UJ]

If you need the codes I can give it you , but I need to know what bit design and size you want to use, zero the bit on the tip and edge of the material, then when you flip it, zero it again on the tip, and I think you need to make a clamping jig on this to clamp it on the sides then easy flip to do the second face.

[Shahrokh]

Thank you so much.
That won't be necessary. To hold the part in place I am thinking to cut an open slot in an XPS block with exact dimensions of cork and push the cork part in the slot. Friction should do a great job in holding it in place; however I haven't tested yet.
Regards

[KH0UJ]

On my side I used a 1 inch angle bar made somewhat a jig with 6mm long bolts as holders, specially on precision flipping jobs I want my Y and X to have a precised stand point, first it was a plastic 15mm clear plexyglass but after thousands of output it got brittle and by and by snap the corners, that`s why I made a steel version, more sturdy, resist oil lubricant/collant, does`nt flex if over tighten, this is also the reason I always zero on the material, XYZ manual knobs comes in handy when aligning the flipped material with the spindle OFF.

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