Re: Why dovetails?

> 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.

Those 'engineering compromises' are what we call flex and allow for
vibration in the cutting.

> ..... makes this design prone to
> failure when it encounters shock loads or very heavy loads.

Normal cutting is a series of shocks as the cutter engages, then ends
a cut.

> 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.

This is partially correct. Metal will deflect and rebound without
any permanent change. Pushed furter it will bend and not
straighten. Also if you push on metal it the sufrace may give a
little, but if you hit it too hard with a point it will dent.

If the ball/surface ratio is too small the surface will dent or the
balls will. There will also be some flexing of the two but not any
signifigant amount. The arms and posts that are connected will bend
much more. longer and they bend more.

> 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.

Very well put. No matter how we might try, we have a very hard time
walking on water. buy, a 100 ton steel ship floats easily. One has
exceeded that threshold of floating.

If your machine has not passed it, the cutter will try to cut,
bounce, then the forces of the head and spindle and cutter banging
back into the part will take a cut and then bounce again. Although
we call it chatter, it is a reflection of being too close to that
threshold.

But, cutter size has something to do with that also. A small mill
can cut a small cutter. so, the threshold on your design has some
cutter that will work. The problem is it may be smaller than a
hair and you'll have a dickens of a time getting them that small.

If you want a mill, get a mill. If you want a router, get a
router. There is no shame in having two tools.

Dave