Re: about the measurement of mold built by SLS RapidSteel

From: Tom Mueller (tmueller1@mediaone.net)
Date: Mon Dec 21 1998 - 22:00:31 EET


I think the issue of accuracy in molds created by any RP process, whether direct
or pattern based, can be very confusing. It is very difficult to draw
conclusions about accuracy based on a limited number of measurements.

Paul Jacobs did some groundbreaking work to show that a significant portion of
dimensional error in rapid tooling is due to variations in shrink and that shrink
is a random variable normally distributed around a mean value. Furthermore, he
showed that the standard deviation of that deviation in shrink values is
proportional to the mean shrink rate. I expanded on that concept in a paper I
presented at the RPA Dearborn conference last spring where I included accuracy of
the RP system and of finishing in the analysis.

As a result, dimensional error in tooling is also a normally distributed random
variable. As we will remember from basic statistics, it is possible to make
predictions about error on a specific dimension only if we have information about
the population of dimensional errors.

I would suggest, therefore, that a better indication of accuracy in tooling is
the standard deviation of dimensional errors of all dimensions of the tool. Once
the standard deviation is known, we can then say with certainty, that there is a
68% probability that any dimension is within one standard deviation of the
nominal dimension, that there is a 95% probability that any dimension is within
two standard deviations of the nominal dimension and that there is a 99.7% chance
that it will be within 3 standard deviations.

We can estimate the standard deviation of the population by measuring a
statistically significant sample of dimensions on the tool and calculating the
standard deviation of the sample.

This measurement gives us much more information about the capability of a tooling
process than reporting the error in one or two dimensions.

I would not propose that this be done on every tool. It would be far too
costly. However, I would love to see this done on a few tools for each tooling
process that is being promoted and the results reported so that users can make a
much more informed decision about the tooling process they want to use.

Tom Mueller
TJ Mueller & Company
(847) 821-8809
(847) 821-1634 fax

Fusioneng@aol.com wrote:

> In a message dated 98-12-18 09:33:27 EST, mcs9413@lge.co.kr writes:
>
> << Dear all:
>
> There seems to be somebody who knows what the best method
> to measure 3D dimension of mold built by RapidSteel is....
> I think the dimensional accuracy of mold built by RapidSteel
> is relatively inaccurate to be used as an injection mold.
> So it is very difficult to define the origin and axis on it.
> That is why I've checked the accuracy of the whole mold surface
> using 3D laser digitizer(scanner) and then compare measured data
> with designed data.
>
> Is there anyone who has other opinion ?
> Chang-Shik,Min >>
>
> Chang-Shik,Min:
> My company uses 3D Keltool which is very similar to SLS RapidSteel. We
> normally put some kind of rectangular posts or set up references onto all of
> our mold cavities to verify part accuracy and ease machining after sintering.
> These are then removed for final finishing. This technique would be prudent
> for you to use with RapidSteel as well. We find the accuracy of 3D Keltool is
> well within the accuracy requirements of most moulded products. The majority
> of the moulds we produce in 3D Keltool are production moulds.
> Bob Morton
> Fusion Engineeering
> Fusioneng@aol.com
>
> For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/

For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/



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