Re: Why dovetails?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "mandras76" <mandras76@y...>
wrote:

>
>
> Hi,
>
> I'm new to this group and in fact new to CNC machining altogether. I
> have some experience in wood- and metalworking, and am in the

process

> of converting a Proxxon MF70 micro mill to CNC.
>
> In doing so, I'm dreaming of a larger mill already... I'm thinking

of

> building one from scratch, rather than converting a non-CNC mill,

let

> alone buying a CNC mill. Somehow I imagine that a design where the
> workpiece is at rest (or on a rotary table) and the spindle moves
> would be easiest and best. This would make the mill of my

imagination

> very similar to a CNC router, only somewhat stronger and stiffer.
>
> There must obviously be something wrong with this idea since this is
> not a design you normally come across among metalworking mills. Is

it

> because dovetails are so much better than any other kind of linear
> movement? I was thinking of triangular rails with ball bearings

loaded

> against each side, would that be less stiff compared to a dovetail?

I

> don't need the stability of a Bridgeport because I have all the time
> in the world and can take shallow cuts, but please tell me if my

idea

> is unsuitable even for my humble needs. It needs to be cheap in the
> first place, but also capable of cutting metals, at least non-

ferrous

> ones.
>
> Thanks a lot, and please excuse me if this has been discussed

before.

>
> András

Cutting Metal involves really high forces. Take a drill press and a
1mm drill into aluminum.

Put a dial indicator on the bench to the head. when you drill,
watch the indicator needle move.

Switch to a piece of steel and watch the movement.

Change to a 10mm drill and repeat.

Then look how massive the drill press parts are and how short they
are.

I seem to remember when I first started looking at units, there was a
machine with 3 inch solid linear rails that were about 4 feet long.

IIRC, it was painted yellow in some places, overhead rails most of
the pictures were from above, like it was sitting on the floor....
just in case it jogs someone elses memory.

The writer said it was not very good at cutting metal due to the flex.

The reason Bridgeports are so massive is the forces involved with
cutting a piece of metal.

One of the pressures involved in cutting is not cuttine stock, but re-
cutting chips. When the cutter swings around, it can grab a chip and
start to slide between the cutter and work. the chip acts like a
bushing to let the cutter slide, up to a point. At some point, the
cutter will bend and just not make a cut, or will cut thru the chip.

There is also vibration and harmonics that move the parts of the
machine at different speeds. When the vibratons align in a harmony,
the machine shakes much more than when they are not aligned.

Conventional tools need very sturdy structures.

If you want unconventual, look at a hexopod.

Dave