Re: little bit off topic (about inspection)

From: Priit Kull (
Date: Tue Mar 21 2000 - 20:40:17 EET


In the very original message Steve Pitt said that a few points are enough to
measure the features (plain, cylinder ...). That is true given that it is
known beforehand that there is this particular feature to measure.

The blessing and curse of the CMMs is that they deal with absolute
measurements. That makes them so expencive, all those ruby sensors and stone
tables. In a wide majority of inspection cases (if you try to think free) it
is not necessary to know if a hole is 10+0,01 mm in diameter. It is good
enough to know that the hole is 100%+1% from the theoretical. What is
actually needed is an analog comparison to a theoretically correct computer
model or to another actual part with known errors. This way we could
effectively eliminate the meter etalon in Paris.

In an ideal case the inspection machine should handle every surface as a
freeform surface. Now comes the difficult part. Historically there have been
developed certain criterias to tell if any particular feature measurement is
OK. These are mostly tool dependent, whoever has tried to inspect a
gearwheel understands me. Now with new possibilities and new tools we have
two ways, we can try to develop new criterias for new tools and get them
accepted, or we could try to emulate the old tools with our modern ones. If
somebody could develop an unified field theory for all the geometry we have
solved the problem (It smells after the fractal thing).

I was talking on this list on the same subject in June 98, when I tried to
invent a black box with billions of sensors that automagically tells me if
my parts are OK. The discussion continued off line for a while and several
inspection gadgets/software companies promised that the feature recognition
(that is figuring out cylinders, planes, cones, spheres etc) from point
cloud data will be there real soon now, in our next release, right behind
the corner. I got less promises on the line of finding my automagical sensor
tape with billions of sensors which does not cost more than 1 dollar per
yard, OK 10 dollars. Any progress on these lines?

Priit Kull

----- Original Message -----
From: "Chris Lawrie" <>
To: "'Tommy Tucker'" <>; "'Steve Pitt'"
<>; <>
Sent: 20. märts 2000. A. 23:05
Subject: RE: little bit off topic (about inspection)

> Tommy
> Have you had a chance to look at Delcam's PowerINSPECT
> (
> This product is designed to inspect the freeform shape in real time. The
> product has a short learning curve and automatically creates textual and
> graphical inspection reports in Excel using templates.
> If you or any other list members require more information about free-form
> inspection do not hesitate to contact us.
> Best Regards
> Chris Lawrie
> Tel: +44 (0)121 766 5544
> Email:
> -----Original Message-----
> From: [] On Behalf
> Tommy Tucker
> Sent: 20 March 2000 17:44
> To: Steve Pitt;
> Subject: RE: little bit off topic (about inspection)
> Steve,
> I thought your question was very interesting and was surprised not to see
> more discussion. Scanners and three-dimensional measurement equipment
> been discussed a lot on this list, but this is a subject that rarely comes
> up. Everyone thinks its great to inspect free-form shapes but doesn't say
> whole lot about what they mean by it.
> The main advantage in free-form surface inspection using
> technology has been the use of color mapping the errors from measured
> to CAD surfaces. You raise an interesting question as to whether this is
> enough. Most of the other features you mentioned require a tolerance.
> Free-form surfaces should to, but how are these spec'd out? Any input
> others on the list would be appreciated. My company is in a position to
> provide real innovation in this area based on input received.
> One area I have seen a tolerance used for free-form surfaces is turbine
> blades. Generally, these are spec'd out by cross-sections along the
> length. This has always bothered me because it takes a 3D geometry and
> simplifies it to 2D. With modern modeling systems, why can't a 3D
> tolerancing scheme be imposed? In any event, you may want to look into
> turbine blade inspection and how inspection planning is performed for
> products.
> Tommy Tucker
> (vc) 408-855-4372
> (fx) 408-855-4360
> > -----Original Message-----
> > From: []On Behalf
> > Of Steve Pitt
> > Sent: Friday, March 03, 2000 3:23 AM
> > To:
> > Subject: little bit off topic (about inspection)
> >
> >
> > Hello List,
> > I am Ph. D. student and my research topic is about inspection planning.
> > I have a question about inspection.
> > For inspection of freeform surface, what should be inspected?
> > There exist a lot of inspection features such as plane, cylinder, etc.
> > In that case, sampling several points is enough.
> > But I think that freeform surfaces are different from the features.
> > Just is it enough to see the difference between point data and
> > the original
> > surface?
> > Or the surface which is reconstructed from point data must be
> > compared with
> > the original one?
> > Which way is a CMM used for inspecting freeform surface?
> > I respect the answer from anyone who has expriences for freeform surface
> > inspection.
> > Tnank you in advance.
> >
> > Steve Pitt
> >
> > ______________________________________________________
> > Get Your Private, Free Email at
> >
> >
> > For more information about the rp-ml, see
> >
> For more information about the rp-ml, see
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