Long Z axis problem

[Karlf]

I'm just about ready to build my CNC router. I do have one problem. I need a really long z axis, like 18". I realize this will introduce alot of potential flex into the machine. I will be routering either foam or plastic, so I don't anticipate there being much of a problem here.

Imagine routing a plug for a mold of a kitchen sink. This is what I'm going to try to make with the machine.

The problem I"m running in to is the length of the milling bits themselves. If I need to go 18" deep then the router itself will hit my substrate.

Are there very long milling bits available? Where can I find them? Do you have any other ideas on how to accomplish such a cut?

Thanks,

Karl

[andy_ck87028]

If it really is only foam or plastic, consider the flexible drive shaft attachment that Dremel offers.

The drive shaft decouples the cutting head from the motor by about 500mm.

All you need then is adequate shafts and bearings with a lightweight / skinny head to hold the end of the flexible drive shaft.

Andy

[MonoNeuron]

Wow, that's going to be a really big problem. I am a machinist by trade and we don't like to let anything hang out more than 7 times it's own diameter or there will be way too much whip and flex and if you are going to be routing at 20K revs with an 18" long cutter involved then I wouldn't want to be anywhere near the thing about 2 seconds after it starts up.
I will have a good think about this one as it's a very real problem that should have an answer.
I think molds that are this deep are cut in slices and fitted together after by gluing or some other process. DeskProto4 does slices (I think).
there maybe someone out there that does this for a living and so may have more of an idea than me. Machining has more branches than a Fig Tree so not everyone has all the ideas.
Rich.

[Hemispheres]

If you need to go 18" deep then at a guess this would mean a 36" z axis! Still what you need is not a long cutter but a boring bar with tips, though depending on the diameter of the hole even with no load this might flex.

[jimcisme]

Think .. never mind , early moring brain fart..lol

[Buzz9075]

18" wow have you ever put an 18" drill bit in a drill and turned it on... starts to whip... one of the reasons we can use long lead screws on the machines.

[Hemispheres]

Maybe I have the design in mind wrong but if the plug is 18" deep and the sink is 12/18" deep too, then... puzzled though because the sink in my mind would only have about 2" of plug outlet moulded on.

[bladders1960]

Unfortunately the best way to do this sort of work is to build a 6 axis x,y,z, plus rotation through x and y, plus rotation around z, therefore you can use say a 20mm ball bit rotated to horizontal in the x and/or y, use the z to run down the side, rotate through the x or y to vertical at whatever radius you need the tip to cut,.....with a bit of thought show should be able to design a Dremel (or similar) rotating (at the collar mount) through say 25 mm (being the spherical diameter of the bit plus say 5mm offset from the bit to the collet, the AXIS should be X,X1, Y, Y1, Z, and Z1, with (1) being the rotating around the X Y or Z axii.

now if you r able to set it up right the Z1 needs to have an auto zeroing offset which could be the 7th stepper........

I don't know who would be able to provide the programming but it should be available...try to google 6 axis and 7 axis........

No I am not a machinist, but I have a fair bit of conceptual design background, and I do want to build a similar machine...when I have the spare time and room.

[ger21]

Cut it in slices as was already mentioned. In addition to DeskProto, both MeshCAM and Cut3D will do slicing.

[NickAshton]

Just a thought, rather than a long bit, would it be practical to use a very wide bit? One wide enough for your Z axis to follow the bit into the hole??

Or at least wide enough for you to take it down in layers without the Z axis fouling the sides,

Nick

[MPCRice]

Chip removal will be another problem. If you are milling plastics, you will probably using a high speed spindle, which can generate a lot of friction and remelt the chips before you can blow them out. Blowing chips out of a deep narrow channel is difficult. A wide "spade" bit would also help in chip removal.

John

[rolfz]

Since your cutting foam, how about machining the plug in slices and then assembling/gluing the slices together to form the finished plug. The same way some padlocks are made.