Datum planes, LCS, and base sketches...modeling challenge

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freedman
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Re: Datum planes, LCS, and base sketches...modeling challenge

Postby freedman » Mon Apr 20, 2020 10:53 pm

How do you machine the 2 corners between the 2 ribs ? You will necessarily have a round fillet somewhere.
EDM that corner out, money is no problem on this job. :)
drmacro
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Re: Datum planes, LCS, and base sketches...modeling challenge

Postby drmacro » Tue Apr 21, 2020 12:04 am

Zolko wrote:
Mon Apr 20, 2020 3:14 pm
... and impossible to manufacture. Apart by additive methods. I think it's a shame that educational exercises don't also think about real-world question, because such examples lead to designers that design parts without thinking how it's going to be made.
Hmm...well the exercise is taken from a college drafting text, the seventh printing in 1970, of a book originally printed in 1936. Maybe they had manufacturing techniques like the ancients used for Stonehenge, Machu Picchu, and the pyramids of Egypt. ;)

I don't think the object in this case was to actually manufacture the part. The chapter is about creating auxiliary views on a drafting board. 8-)
EDIT: and the model is actually wrong, doesn't correspond to the exercise.
RatonLaveur wrote:
Mon Apr 20, 2020 10:13 pm
I agree on the model not being an exact replica of the exercise.
Please enlighten us. :!:
RatonLaveur wrote:
Mon Apr 20, 2020 10:13 pm
My big issue with this drawing is the really weird distance call outs... they are all off by a factor of 25.4 exactly while the angular ones seem ok.
Is this a metric joke? 8-)
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RatonLaveur
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Re: Datum planes, LCS, and base sketches...modeling challenge

Postby RatonLaveur » Tue Apr 21, 2020 5:03 am

drmacro wrote:
Tue Apr 21, 2020 12:04 am

RatonLaveur wrote:
Mon Apr 20, 2020 10:13 pm
I agree on the model not being an exact replica of the exercise.
Please enlighten us. :!:
RatonLaveur wrote:
Mon Apr 20, 2020 10:13 pm
My big issue with this drawing is the really weird distance call outs... they are all off by a factor of 25.4 exactly while the angular ones seem ok.
Is this a metric joke? 8-)
My dear, look carefully at the left side of the part. One feature is missing (bottom left corner, milled flat corner under the part) and bottom right corner (face containing hole should not protrude further than general left face).

As for my joke...yes :D

Regarding the inner corner: you got me there. I tend to ignore this in CAD knowing that the machining process will necessarily leave me with tool-induced fillets or chamfers. Better fatigue resistance ;)
kisolre
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Re: Datum planes, LCS, and base sketches...modeling challenge

Postby kisolre » Tue Apr 21, 2020 6:09 am

RatonLaveur wrote:
Tue Apr 21, 2020 5:03 am
the machining process will necessarily leave me with tool-induced fillets or chamfers
Or not? I am sure there are not only hand chisels and if it is really needed it could be brought down to atom sizes - no real world rounding/chamfer...
Or this could just be intermediate sketch for the strength calculations and manufacturing methods could be selected later based on results from the calculation, selected materials, available manufacturing methods, necessary number of details,...
RatonLaveur
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Re: Datum planes, LCS, and base sketches...modeling challenge

Postby RatonLaveur » Tue Apr 21, 2020 8:59 am

Hello kisolre, yes you're right. I was only considering conventional methods.

The truth is most machines can operate dove-tail cutters of varied angles, that includes a 90° of course. So the right angled lips are not a terrible issue.
But the one place where it's going to be really uncommon to achieve the feature as Zolko aptly pointed out, is the inner corner itself (junction of 3 inner cutouts). Here it is almost guaranteed that without what is known in the industry as a "special process" or "non-conventional" the corner will actually be left filleted by default. If a fit has to be achieved there, then dog-bone operations or even a drilling of the inner corner to generate clearance becomes preferred.
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Zolko
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Re: Datum planes, LCS, and base sketches...modeling challenge

Postby Zolko » Tue Apr 21, 2020 10:38 am

drmacro wrote:
Tue Apr 21, 2020 12:04 am
EDIT: and the model is actually wrong, doesn't correspond to the exercise.
RatonLaveur wrote:
Mon Apr 20, 2020 10:13 pm
I agree on the model not being an exact replica of the exercise.
Please enlighten us
the dimension 2 3/8-th (what an unpractical way to note dimensions !), the lowest on the paper, is not respected.
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drmacro
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Re: Datum planes, LCS, and base sketches...modeling challenge

Postby drmacro » Tue Apr 21, 2020 2:21 pm

Zolko wrote:
Tue Apr 21, 2020 10:38 am
drmacro wrote:
Tue Apr 21, 2020 12:04 am
EDIT: and the model is actually wrong, doesn't correspond to the exercise.
RatonLaveur wrote:
Mon Apr 20, 2020 10:13 pm
I agree on the model not being an exact replica of the exercise.
Please enlighten us
the dimension 2 3/8-th (what an unpractical way to note dimensions !), the lowest on the paper, is not respected.
And, that is exactly why I ask others to look. :!:
Thanks.
Corrected:
Fig_9-56-2.png
Fig_9-56-2.png (50.33 KiB) Viewed 146 times
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Star Trek II: The Wrath of Khan: Spock: "...His pattern indicates two-dimensional thinking."

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RatonLaveur
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Re: Datum planes, LCS, and base sketches...modeling challenge

Postby RatonLaveur » Tue Apr 21, 2020 4:18 pm

Zolko wrote:
Tue Apr 21, 2020 10:38 am
(what an unpractical way to note dimensions !)
Haha. The proponents tend to say that fractions are awesome. To each their own.
drmacro wrote:
Tue Apr 21, 2020 2:21 pm

And, that is exactly why I ask others to look. :!:
Thanks.
Our pleasure. It's fun and these exercises can only improve the freecad community.