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

--- Machine tools have a lifespan after which they are left out in
the rain to patinate and then end up on E-bay.
In many small "sweat shops" machinists are expected to produce
quality work on all sort of worn out equipment and a new Chinese
mill or lathe would be a dream. I cannot remember ever having
machines checked for accuracy as part of a mainenance plan at any of
the companies I have worked.
I have spent years in the telecom industry making precise plastic
moulds for miniature connectors and it takes a concentrated effort
to produce within a couple of tenths, even on CNC surface grinders.
It's much more fun and whole lot easier talking in tenths than
working in them <G>



CAD_CAM_EDM_DRO@yahoogroups.com, "David A. Frantz" <wizard@e...>
wrote:

> Hi Cliff;
>
> As far as your issue of squareness, lathes I use to work on

required

> checks for this and other things on a constant basis. In other

words

> this was a perventive maintenance issue. The machines had

enough

> adjustments in them that this could be handled rather easily.
>
> While I have little faith in the specs on some of the machines

that we

> are talking about here, I can say a few things that might stire up

the

> pot a bit. First it is far easier to machine the smaller parts

of

> these machines to high tolerances. Yes this means that there

has to

> be up keep on the production machinery building the parts. If

actual

> hand work is done on the machine it also makes for easier handling

and

> the potential for good results is much higher.
>
> A good portion of my career was spent on small high precision
> equipment. Expnsive stuff if you consider the size. Prior to

that

> I did have few years experience with the rebuilding of much larger
> machinery. There is a huge difference in effort required ot hit
> tolerances on the larger equipment. On the small stuff, after
> removing parts from the machine a minimal of hand work was

required to

> obtain results.
>
> So while QC may not be the greatest with said machinery, it

probally is

> not impossible to come close to the specs on the mall stuff.
>
> Thanks
> dave
>
>
> treadlemill wrote:
>
> >--- 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
> >>
> >>
> >
> >
> >
> >Addresses:
> >FAQ: http://www.ktmarketing.com/faq.html
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