Re: [CAD_CAM_EDM_DRO] Why dovetails?

Hi Andras;

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.
>
>

You will probally wonder why you bothered, if that is the mill I think
it is it is awfully small! This will especially be the case after you
have a large CNC at your disposal.

>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.
>
>

This may not be as unusual as you appear to think. There have been and
still are a number of machines wher ethe table that holds the piece only
tranlates one axis. There are advantages and disadvantages of
course. What I'm thinking about here are planner mills, hand tracer
mills of the past and other beasts that escape me at the moment.

>There must obviously be something wrong with this idea since this is
>not a design you normally come across among metalworking mills.
>

I wouldn;t go that far. There are a number of examples of gantry type
machines and many CNC routers can do aluminum just fine. Of course
there are advantages and disadvantages either way you go. That is
what makes all the possibilities available in the market place.

> Is it
>because dovetails are so much better than any other kind of linear
>movement?
>

I have to wodner where this question comes from. You will not find
large machines with dovetails. At least not in my limited experience
with larger machines.

>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.
>
>

How big are we talking here? If you are talking sheet size goods the
you should consider some of the better CNC gantry machines on the
market. The trouble with cutting metal is that the forces increase
signifcantly over materials like wood. Thus you need to be able to
resist the twisting moment developed. This is not a modest
engineering effort and your materials bill will be big. For smaller
material sizes you should seriously consider a conversion of a chinese mill.

What you want to do is not imposible on a gantry type router. To get
good result though does required a well designed machine. You should
not delude yourself into believeing that this can be done cheap. Note
that I'm assuming you are working with sheet size goods. If what you
want to do can be handled on a smaller machine your costs go down
significantly. Depending on just how cheap you want to go you may want
to consider Gingery.

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

No problem but to be honest this is probally drifting off the focus of
the site. That is this is more of a machine rather than a controls issue.

> András
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