Re: [CAD_CAM_EDM_DRO] Why dovetails?

Andras:

Why dovetails? Three reasons 1) stability and accuracy thru length and mass of high tolerance dovetail connection 2) The design inherently "locks" the two mating surfaces together eleminating the requirement of any other method of maintaining engagement of the two surfaces. 3) surface area of the mating joint, distributing the load over a much larger area.

Traversing a carriage etc. over lengths of any type geometry supported at both ends will produce an undesirable surprising amount of deflection. Its lack of unsupported mass, deflection in the rails and surface area that are the culprit. I design and build line-side automation tooling and toil with traversing end effectors etc. Even using THK rails in parallel and two cars per side with a substantial moment in between them, Deflection is still a persistent problem. I then must make the end effector "float" as to overcome the deflection and engage the assembly operation. Ball bushings traversing on a round shaft are the worst about this.

The conventional vertical mill is a proven design, all other configurations have been tried over and over. Some failures and some successes. Successes that are excellent but far out of the realm of the home/shop. Designing and building a machine as you reference would require working with large weldments and puchasing expensive bearings etc. Price some THK components.

A design that allows excessive deflection would never be suitable to any appreciable amount of tolerance. Even a Bridgeport as massive as they are, have an amount of deflection that must be considered during a machining operation. The heavier and faster the cut, the greater the amount. The longer the tool, the greater the amount. Its starts at the cut deflects through the shank, holder and then through the spindle and frame of the machine. All materials, regardless of their mass and shape will deflect when an adequate force is applied.


Ron



mandras76 <mandras76@...> 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.

Andr�s









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