Re: What is the weak link in machine precision?

The same checks Jon Elson recommended for a used machine would also
apply to a new one of unknown quality. At the minimum, a good tenth
reading test indicator and a high quality precision square are
required for checking out a mill. The critical checks are to verify
that the 3 axes move at right angles to one another. These checks
generally require establishing a reference on one piece and checking
relative to another, as opposed to passing the elements under the
quill while it holds the indicator.

You asked if the imports are "exact copies" of a BP.....I'd say look-
alike is a more accurate term. If the machine was built in Japan,
there would be one level of accuracy. If it was Taiwanese, another
level, slightly inferior in general to Japanese. If the source is
China, then it can vary all over the place from good to junk. Jet,
for example, sells a BP copy that looks good and generally has a
good reputation for quality. Last time I looked, it's about $7K
with chrome ways (a good investment anytime they're available).
Personally, if I were buying a new full size knee mill to convert, I
would look first at ones made in Taiwan. Taiwan's long term
interaction with the rest of the world has taught them a lot about
what is required for accuracy to sell machines to industrial
customers. China, on the other hand, is still pretty much in
the "People's Milling Machine Company" mindset, where the government
is the producer and there's no competition. This will change with
time, but hasn't yet to any great degree from what I've seen. Good
quality Taiwanese mills are about exactly in the middle pricewise
between a Chinese mill and a BP.

You mentioned in an earlier post the need to maintain .001"
accuracy, but didn't state the size or type parts you need to
maintain that accuracy on. Does the mention of motorizing the knee
indicate the 5 inches or so of quill travel isn't sufficient for
your parts? You mentioned mold parts. Are these molds for rubber
parts made from mild steel, or are they something that requires the
use of some material like P-20? There are big differences in the
power and rigidity requirements, depending on what the material is
once you move away from mild steel. I realize you said earlier that
the requirement for speed isnt high on the list, but in many
instances, nibbling on a part makes it all the more difficult to
achieve high accuracy. It's one thing to pop some holes in a part
or mill the external dimensions to size and hold .001, but it's
something else entirely to machine a mold cavity and put that part
on a CMM and have .001 everywhere you check.

Please don't think I'm trying to pick on you. All these questions
have a legitimate bearing on what you really need in order to
accomplish your intended goals. I've made parts where the machine
was undersized and underpowered and not stiff enough or tight enough
for the job, and it's no fun. I did it because its all I had to
work with at the time, but no one would want to do it long term.

Cliff

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "zipdrive2k"
<zipdrive2k@y...> wrote:

> No worries, you are not bursting my bubble. I was looking at

buying

> an import because they are 'new' and I did not want to spend money
> buying an worn machine and have to spend time and cash to fix it.
> Are the imports not built well? Are they exact copies of the
> Bridgeport? Enco and Grizzly, for example, do they wear out

quickly,

> require maintanence? Has anyone noted problems with them? How do

I

> know a used Bridgeport is in good condition? What are the parts

of

> the machine do you recommend that I worry about when asking the
> overall condition?
>
> I am wanting a machine that is square, true, does not require

repair

> (or extremely minimal), and I can 'quickly' convert to a CNC

machine.

>
> Any suggestions are appreciated. Thanks.
>
> Tom.