Re: [CAD_CAM_EDM_DRO] Why dovetails?

Hi Andras:
What you're describing is in fact, a viable machine design, and there
are a few commercial machines that do work this way.
Okada is one make that springs to mind.
The biggest weakness of such a design, from the engineering point of view,
is that the Z axis travel is usually stacked on the other two axes, and this
makes the third axis intrinsically non-rigid, with varying cantilever loads
on the cutter head depending on how far the cutter head is hanging down.
A better design from this perspective is one that splits the axes up and
provides the same support for the head along its entire vertical travel.
A standard bedmill like the Sherline is an example of one configuration of
this design.
Okuma makes machines that combine two axes on a ram (Y and Z): these are
immensely overbuilt to accommodate the engineering compromises inherent in
this design.
To give you an example, the Y axis ram for a stroke of 24 inches is about 10
feet long and must weigh 5000 lb just on its own.

The take home message of all this, is that if you contemplate a stacked axis
design with long cantilevers, you must overbuild the axes to accommodate the
loads.
This brings us to the next design consideration; you're contemplating a car
on wheels as the sliding element.
This is great for low load applications and is used extensively and very
successfully for things like tool carousels (Haas does it this way), but the
line loads on the periphery of the bearings makes this design prone to
failure when it encounters shock loads or very heavy loads.
There's just not enough surface area in contact to support the loads, and
the bearings will either Brinell (the balls indent themselves into the
races) or the bearing shell will crack.
You need the support of many such rolling elements to overcome this
limitation, so you should be thinking of building an "18 wheeler", rather
than a "passenger car".
This will consume quite a lot of space.

On to the third point you raised: that of machine rigidity:
For this type of apllication, there is a lower limit of rigidity below which
the machine will not perform at all, so it's not just a matter of accepting
light cuts...you won't be able to take ANY cuts if you don't get above this
threshold.
Since it's so easy to design a rigid machine, it'd be foolish not to...after
all, you've got to do about the same amount of work to build a crappy
machine as a good one.
All you're really adding is mass and applying it strategically to counteract
the forces against which you'll be contending when the machine encounters a
load.
Try to reinforce in your design, against all those force vectors, and you'll
be in good shape regardless of how you choose to stack the axes.
This brings me to the last point...that of unsupported round rails as
machine ways like the Unimat lathe...don't bother, it's a stupid design.
Your ways must be supported along their full length to make even a
reasonable machine.
The big attraction to such a terrible design, is that it's easy and cheap to
get precision ground rod and round linear bearings; it's also easy to align
these elements but it makes a pig's breakfast of a machine for anything but
the shortest of machine travels.
I've seen a few reasonable successes with such a design; Kearney Trecker
made a universal boring mill this way, but the columns were massive and
short.
DoAll made a ginder this way too, and it was OK but not great.
So, despite the temptation, avoid unsupported ways of any geometry...they're
just not worth it.

Hope this helps

Cheers

Marcus

----- Original Message -----
From: "mandras76" <mandras76@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, October 15, 2004 10:59 PM
Subject: [CAD_CAM_EDM_DRO] Why dovetails?





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









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