You are quite right; masking under a screw can create a serious
aesthetic problem. It's as mechanical designer, it's something that I
whined and complained about often in the past. The best we could do was
to minimize its necessity by making metal to metal contact BETWEEN the
metal parts being connected, rather than between the top part and the
underside of the screw head, and then from screw threads to the threads
in the lower part. This often was insufficient for safety purposes,
although it made a reliable contact for EMI reduction purposes.
I also have real problems with masking. It is very expensive and
requires care to be taken by what amount to low-paid personnel. The
result is often a lot of unavoidable sloppiness. People often fail to
understand the difference between the one or two prototypes they
carefully create in the lab, and the results that inevitably occur in
production when there are only seconds to do the job. It should be
avoided at all costs when possible.
Conductive paint as a safety connection is useless. Resistivity (even of
high conductivity silver or copper paint) is too high, and one cannot
guarantee it won't have flaked off.
Bob Wilson
TIR Systems Ltd.
Vancouver.
-----Original Message-----
From: Chris Wells [mailto:[email protected]]
Sent: March 22, 2002 4:53 PM
To: 'emc-pstc'
Subject: Masking - was: chassis bonding - star washers enough?
Thanks for all the feedback - some very good info!
Masking
As to all those who indicate masking off the metal what about oxidation?
I'm concerned about the appearance of masking off the painted areas.
Can the screw be coated after assembly?
Hard to take apart for sure..........
I have some problems with masking.
I have no issue with the electrical/mechanical principles....
But it seems that the appearance will be very poor and the effort to
accomplish high.
I suppose that is why plated surfaces are preferred......
What about conductive paint?
Too expensive?
Appearance?
Chris Wells
Senior Design Engineer
Cutler-Hammer
[email protected]
----- Original Message -----
From: Chris Wells <mailto:[email protected]>
To: 'emc-pstc' <mailto:[email protected]>
Sent: Thursday, March 21, 2002 4:40 PM
Subject: chassis bonding - star washers enough?
Looking for some feedback on bonding various parts of a painted
metal chassis together for safety and EMC performance issues.
I know we have a lot of experts out there on this topic and can
remember a number of threads on the topic.
Please help me pull it together:
* When screwing the cover on the base will star washers cut
through paint, even epoxy paint and provide a reliable connection?
* Are there special versions of electrical star washers that
work better than others?
* What about oxidation of the mating surface? I understand
star washers can make a gas tight seal.
* Should there be a special treatment to the painted area
where the screw/washer come together?
* Do PEM nuts act as a good mating mechanism for the screw?
There are many types are some better than others? What about the self
clenching types?
* One can get screws with captive washers. Are there any
issues with these? Preferred Vendors and types?
* There are various chemical materials to keep screws from
backing out and are applied to the threads of a screw such as those from
Loctite. Do these interfere or help? Are some formulas better than
others?
* Under the paint if the base metal is steel are there any
treatments or plating that help? For Steel, for Aluminum?
* If the chassis is plated steel instead like zinc chromate
yellow. What changes?
* Proper plating on screw and star washer? (assume painted
steel)
Any advice on the topic welcome!
Thanks in advance!
Chris Wells
Senior Design Engineer
Cutler-Hammer
[email protected]
