Re: [CAD_CAM_EDM_DRO] Why dovetails?

Thanks for all the replies so far, they have been very helpful.

> 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.

I actually started with a slightly bigger one, the
BFW40E-KT150-BFB2000 combo, which has an X-Y travel of 150mm
each. Having found it rather unsuitable for CNC conversion (it would
have required the leadscrews to be replaced altogether because of
excessive backlash, and the overall quality wasn't so great that I
wanted to waste too much time on this), I sold it and got the MF70
instead. Since I haven't done any CNC so far, I reckoned it would be a
good idea to practice a bit on something small and cheap before going
further and spending real money. The proper mill I'm dreaming of won't
be happening anytime soon anyway because I don't have the space for it
(even my first Proxxon was a bit too big). For the few things I want
to do in the near future, mainly routing in composite and metal sheet
material, the MF70 is big enough. Given that a ballscrew can easily
cost more than the MF70, I don't think I have lost a lot.

> > 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.

Ah, that makes sense. I somehow believed, maybe from seeing all the
big (for my anyway) machinery in the workshop at work, that these
tended to use dovetails rather than linear bearings etc.

> 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.

I think my Proxxon experiments will prove useful here. The MF70's
weekest point, as far as I can tell, is the aluminium dovetail, I
guess one made of cast iron would be a lot stiffer. The previous
Proxxon's weak point was play in the Z dovetail (poor design, gib too
short, etc.), the MF70 definitely looks better. It'll probably give me
a feel of what kind of stiffness is required.

As to size: I certainly want something of the order of 4' x 3' x 2'
for cutting timber, plastic, foam, etc., and it would be nice to be
able to use the same machine for metal as well. If that can't be
easily achieved, then in the end I might just settle for a large-ish
router and a smaller metal mill. I can't possibly see the need for
cutting metal stock that large...

> 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

OK, sounds good.

> 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.

Sorry...

Thanks again for your help,

Andras