Re: [CAD_CAM_EDM_DRO] Crashing CNCs: what gives ?

Hi Kim


I have one suggestion for you based on experience obtained years ago.
That is to glue your tool post in place with epoxy. Knocking the tool
post of the top slide on a lathe, in the event of a crash, is one way to
keep the grief level to a minimum.

Now I should mention that this approach was taken due to very unreliable
CNC controls that were attempting precision diamond turning. Since the
product, in the finshed state was only a few microns thick it didn't
take much of crash to damge things. Sometimes the diamond would
survive the crash and someitmes it wouldn't. The good thing though was
that the rest of the lathe did not suffer.

When a upgrade to better CNC equipment took place we moved away from
epoxy mounted tool posts. At that point very few crashes where due to
anything other than operator error.

As far as other aproaches do make use of your hard limit switches which
all machines should have. If you have critical tooling or product in
the machine and the machine supports it readjust those hard limits to
protect those items. Like wise careful use of soft limts on CNC
software that supports it will help. Additional switches wire in with
you hardstop swtiches may also be of help. All of these take setup
time so there has to be some pay off to implementing the protection.
Do realize that you have options to the normal hardlimit switch.

Though I've seen it attempted many times I've never seen an operator
actuated E-stop that really saves tooling. Generally things happen to
fast especially with machines running production code, so man in the
loop protection is not a good idea.

Generally fuses can help but they are rather slow. If the amplifiers
support it do use current limiting. This can impact your servo
performance and lead to debug issues so be careful and keep the
possibilities in the back of your mind. Certain types of crashed
involving your spindle can benefit from overload protection of the
spindle also, such an overload should also shut down your axis.

If you in mass production and the process requires operator tweaks do
consider moving the required intelligence to a PC. Have the PC
generate the CNC code and have it update the CNC controller. Anything
that removes manual computation and data entry should be looked into.

So for example if the operator has to monitior the parts for certian
dimensions that are key to the process and make program parameter
changes based on those samplings then it is a very good idea to have the
entire process computerized. To get further into this example if one
or more of the parameters that the operator is monitoring is directly
related to thermal growth of the spindle or growth of the leadscrews,
develope a computer program to generate the compensations. It is
amazing how many unexplained crashes go away once simple validation for
rational data is done. While I'm not currently in a situation where
I'm up to date on the latest CNC software available I do know that some
CNC packages can support such coftware developement within a modified
form of CNC code. With modern PC's running CNC interpeters, there is
a very real option of running your SPC software right beside the CNC
code. It should be noted though that you will have to figure out how
to get the CNC side of the machine to re read the CNC code
automatically. While I realize that these are techniques of mass
production where you want the cheapest operator possible running your
process, it can be very usefull for projects of lesser scale. It is
also surprising how many mistakes even a skilled and reasonably
intelligent operator can make when under pressure or simply
distracted. The trick is to use accumulated intelligence, experience,
a bit of engineering math to remove the human mind from the chain as
much as possible. This will save you time and materials due to mistakes.

Another thing that can be very useful if you go the software route is
the concept of traceability. It is amazing how usefull a change log
automatically generated can be. What I'm getting at here is to have
all paramters touched by an operator saved in a log file. At one
point we even had the lathes computing compensations on the fly and also
recording those parameters in the log files. So when thinking
automation software do not forget LOG FILES!!!!. A little bit of
information can go a long way to correcting or identifiying problems.
So things like time and date, before and after values of the parameter
in question and what ever other parameters that may be of importance is
recorded.

Log files are very usefull beyond the software maintenance issues
also. If you have a lathe making automatic tweaks to parameters, and
that lathe has issues, a log file can be very useful in determining what
went wrong. How the lathe was behaving when no body was around can be
very useful to the electromechanical maintenance people.

I hope this isn't long winded.

Thanks
Dave



Kim Lux wrote:

>I'd like to know what is supposed to "give" when one crashes a CNC
>tool/slide into a chuck, tailstock, etc ?
>
>Lets say we've got a servo/ballscrew set up that generates 1000 to 1500
>pounds of force when called on. Next, we accidentally command that axis
>to run into something immovable: a chuck, tailstock, etc. On a
>commercial machine, how is this handled without wrecking anything ?
>
>Right now we've got a quick blow fuse that pops in our equipment. It is
>the only way that I can see to limit the damage. What is everyone else
>doing ?
>
>BTW: now that we are experienced, we rarely crash our equipment. I'm
>just curious/wondering how others and commercial equipment handle this.
>
>
>