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

metlmunchr 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.
>
>

When Moore made their Jig Bores, it was VERY hard to achieve that
sort of precision, and it was done by incredibly slow and difficult hand
scraping. Nowadays, a good CNC machine could mill the pieces to
accuracy pretty close to that needed for this level of precision. BUT, and
there's always a but, it would require constant surveillance to keep the
machine accurate, day after day, very frequent checking of the CNC mill
to keep it accurate, etc. Somebody making $250,000 machining centers
will have the on-site staff and expensive (Renishaw, Rank Taylor-Hobson,
etc.) instruments to maintain their accuracy. Somebody making $5000
manual mills in China is not going to go to this trouble.

Bridgeport still hand-scrapes their machines for final alignment. I suspect
very few clone builders are doing that, today! (Or, ever did, for that
matter.)

If you check out a used Bridgeport, anything over 10 years old, for
sure, the
table has drooped. The overhang of the table warps it, and it becomes
convex on top, concave on the bottom. This wears the top of the saddle
convex, and the table begins to move in an arc, rather than a straight line.
(The CNC Bridgeports had a wider saddle to better support the table and
prevent this from happening.)

I have seen some statements that were so patently ridiculous that I couldn't
believe the sellers were trying to pull this over on people. One common
misrepresentation is to list the dimensional accuracy of the leadscrew as
the total accuracy of the entire machine. Those of us who understand all
the other possible sources of error (straightness, orthogonality, thermal
expansion, hysteresis, deflection, .....) can laugh at such foolishness.

Jon