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

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
>>
>>
>
>
>
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