Re: What is the weak link in machine precision - Knee Mills

--- Cliff,
specmanship is just a bit of the fun on hobby CNC lists, as is
spending as little as possible and working as fine as possible.
Building for cents and working to tenths is a bragable achievement,
checkout cnczone
I find it difficult to understand the preoccupation with squareness
and parallelism on an enexpensive cnc retrofit because these
discepencies can be removed in programming.
An interesting read is Moore's book and the attention to detail it
takes to produce a jig borer or grinder. Invar metal is used in
conjunction with a heating element in the quill assembly.
I know from experience that a brand new BP with readout cannot be
used as a jig borer. But then again, if you are working out of the
basement without metrology equipment you can kid yourself!


CAD_CAM_EDM_DRO@yahoogroups.com, "metlmunchr" <metlmunchr@y...>
wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "zipdrive2k"
> <zipdrive2k@y...> wrote:
> Enco has a mill they state the specs are
> > 0.00078/ft. error for squarness and parallelism.
>
> Our friend Ballendo has often mentioned the overstatement of
> accuracies by sellers of low end machine tools. With this in mind

I

> dug out the original verification documents for one of my

machines.

> The limit for parallelism is .00078/ft, while the limit for
> purpendicularity (squareness) is .00118/ft. Notice this

squareness

> tolerance is 50% greater than what Enco states. The actual
> measurements for my machine all fell within the stated limits.

Some

> were substantially less than the allowance, while others barely

made

> it under the wire. It's possibly worth noting that these numbers
> are from factory tests on a machine that sold for slightly over a
> quarter million dollars. Do you think it's possible Ballendo has

a

> valid point regarding overstatement of accuracy? or does Enco have
> some secret no one else knows when it comes to machine
> construction? My offhand guess would be some of the better
> Taiwanese knee mills will equal Bridgeport's geometry numbers (I
> dont know what BP's specs are), but a further guess would be that
> most of the Chinese iron will fail to equal BP's numbers by a

factor

> of 2 or more. If the upper limit was indeed .00078/ft, then the
> average machine could be expected to have the geometric accuracy

of

> a Moore Jig Borer. I kinda doubt that's the case.
>
> Cliff