November 01, 2003
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Practical Issues, Creative Solutions
How to select your Plasma Cutter.
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Date: November 2003 - Practical Welding Letter - Issue No. 03
------------------------TABLE of CONTENTS---------------------------
==> 1 - Introduction
==> 2 - Article: How to select your Plasma Cutter
==> 3 - How to do it well: Grinding a Plate
==> 4 - Filler Metal: Silver Brazing Alloy selection
==> 5 - In the press: Recent Welding and related Articles
==> 6 - Terms and Definitions Reminder
==> 7 - Article: Manufacturing problems
==> 8 - Site Updating
==> 9 - Readers' Contributions
==>10 - Explorations: beyond the Welder
==>11 - Correspondence: a few Comments
==>12 - Bulletin Board
==>Unsubscribe link
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1 - INTRODUCTION
Welcome to this new issue of Practical Welding Letter.
We would like it to become an important and trusted reference
for our readers and a current source of information for
solving
their welding problems.
In this issue we present a new article detailing the important
factors to consider when selecting a Plasma Cutter: while
it is true that this kind of equipment may solve economically
certain cutting problems with high productivity, it would be
a mistake to assume that it is the best solution forall and every
situation.
For the Filler metals paragraph we introduce Silver Brazing
alloys, with a short primer on their selection.
In the section
on "How to do it well" we present what may happen when
grinding a plate and what to do to prevent that inconvenience.
Further down in this issue you will find an article explaining
why certain types of breakdown in manufacturing may occur out
of the blue and how to approach the search for causes and remedies.
As new readers subscribe almost daily we feel a mounting pressure
to fill this page with pertinent and helpful content.
We would like to see an increasing involvement of our readers
in establishing what really interests them. In fact one (!) of the
readers asked for some specific information that we hasten
to provide. (This question we take as a reader's Contribution!).
In the articles referred to in the section on the Press,
the case is made once more for the shortage of welding
professionals in all related jobs.
It again appears that welding can provide a promising career
for interested persons,provided they are willing to invest time
and energy in learning, practicing and staying up to date.
We present also a reference on modern developments in
radiography,
which is an important method of Non Destructive
Inspection of
Weldments.
We point out new pages added to the Site: we believe they may
interest our readers: let us know how you think this Letter
should be improved to help you even more:
Any questions or comments or feedback? Write them down and send them to us by e-mail.
Click on the Contact Us button in the NavBar at top left of every website page (www.welding-advisers.com).
We hope you will enjoy reading this Practical Welding Letter.
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2 - How to select Your Plasma Cutter
Plasma cutters have an interesting history of development.
It is reported that the first applications of plasma devices to
cutting of metal happened more or less by chance, when it was
observed that by driving a wire electrode (of whatever material,
even mild steel) from a Gas Metal Arc Welding or plasma torch
into hard to cut materials (like aluminum and stainless),
they were
easily cut through.
Plasma Arc Cutting (PAC) employs high-temperature, concentrated,
constricted arc between an electrode within the torch and the
material to be cut. The molten metal is continuously removed by
the jetlike gas stream blowing from the torch. Besides cutting,
the
method permits also piercing, manual or machine shape cutting,
beveling, stack cutting and gauging.
How to evaluate Plasma Cutting Equipment.
Material: establish first if your cutting needs include
stainless
steel and/or aluminum or not. If the answer is yes,
then PAC
is most probably the solution, outperforming
Oxyacetylene
cutting that cannot do the same without cumbersome additions.
Also other non conducting materials like rubber, plastic,
wood etc. can be Plasma Arc cut with a good quality surface to
thin tolerances. This is done with a minor electrical connection
change. Transferred arc occurs between electrode and electricity
conducting work, like metals, which becomes part of the circuit.
Non-transferred arc, occurring within the torch, excludes from
the circuit the non conducting material.
Thickness: PAC outperforms oxyacetylene only up to between one
and one and a half inch (25 to 38 mm) of mild steel (some put
the
limit at half an inch or 13 mm, but that may be questionable):
over
that dimension there is no claimed economical advantage using PAC.
For completeness one should also consider in the comparison the
newly available oxy-gasoline cutters which may have economic or
other practical advantages depending on the type of work at hand.
Consider the Maximum thickness you plan to cut.
Different models of Plasma Arc Cutters cater to different cutting
needs: inquire with manufacturers what they propose for your needs.
Also stack cutting may be easier with PAC because the plates
need not be tightly clamped together (as they need for OA).
Machine cutting, using the same machine for both processes
may result in productivity gains with PAC.
Furthermore there is no need to preheat the material, PAC cuts
a thinner kerf (the width of burnt material) and its effects
on the surrounding area are smaller.
Economy and Productivity...
...provide the complete answer as to which process is to be sought for.
Consumables: Of course OA (Oxyacetylene cutting) needs
supply
of the consumable gases but does not need any electricity.
Conversely PAC needs electricity and compressed air (or other
gas),
and sometimes water to cool down the underside and reduce
fumes.
Sometimes a dual gas supply is used.
Electrode and nozzle are paired consumables which need periodic
changing into the PAC torch depending on intensity of usage.
Compressed air, when used, must be clean and dry:
check it by blowing a puff on clean white absorbent paper:
if it leaves a stain it is not clean enough. Some cutters
are provided with an air compressor, other machines require
the use of compressed air from a cylinder or from a separate
compressor fitted with filter and dryer.
Input power of the proposed unit (which voltage and if single-phase
or three-phase) should conform to your available
power supply
from the grid.
Cutting speed is given in
inches per minute (ipm)
or meters per minute (mpm).
But the given figures may be based on different standards of
measurement, so that you should preferably check by yourself
if the available speed of cut satisfies your needs.
Duty cycle, meaning the percentage of available cutting time,
at any given power level, out of any ten minutes period, should
be considered because it affects productivity: it is most
important when long cuts are planned.
Portability, that is the ease of transporting and
setting up in place the unit, is important for field work.
For fixed cutting in the shop it should be of no concern.
The way the arc is started in the selected piece of
equipment may be an important issue to study, if it is feared
that high frequency spark might interfere with surrounding
equipment like computers. Manufacturers developed different
means to circumvent this problem.
Safety is always important, with any kind of equipment:
the necessary precautions must be learnt, implemented
and enforced.
It is recommended that you check the performance of the
proposed cutter for yourself, to be satisfied that it meets your
needs, before committing to any purchase.
Is this article helpful to you?
Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every website page (www.welding-advisers.com).
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3 - How to do it well: grinding and distortion
Q: A hardened steel plate was reasonably flat after heat treatment.
However, after grinding from one side as needed, it distorted
badly. What should be done to prevent deformation?
A: The hardened plate is subjected to internal stresses
in equilibrium,
(tension and compression) symmetrically
distributed from both sides of the mid plane.
By grinding from one side only, equilibrium is disrupted.
The remaining stresses rearrange in such a way that produces
distortion. The remedy would be, if possible, to grind
symmetrically from both sides.
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4 - Silver Alloy Brazing Filler Selection
The information assembled in this article is intended to provide
supplementary data to those presented in the page on Brazing
of the Welding Advisers Site.
Of the different filler brazing alloys available and suitable for many
purposes, we limit ourselves here only to those known as
Silver Brazing alloys, although, occasionally the silver content
may be less than that of copper or other element.
The selection of the most suitable alloy is subjected to evaluation
of several factors, like base materials, joint design, process to use
(Manual or machine torch, furnace, bath, induction, resistance
etc.), strength and temperature requirements, quantity and so on.
In general, the higher the silver content, the higher the cost
of the filler metal: however this is only one element summing up
to the total cost. It may happen that a more expensive material
permits a higher productivity process to be employed, providing
more economical an application.
When considering any brazing process one should be aware of the
stringent requirements that must be fulfilled in order to obtain
acceptable and repeatable results.
In particular one must provide the utmost cleanliness available
and one must pay due attention to the correct joint design and fit
of the assembly.
Successful furnace brazing is strongly dependent upon the quality
of the atmosphere, either vacuum, reducing gas (dry hydrogen)
or inert gas. In case of furnace brazing failure, the atmosphere
quality is the first item to be checked and cared for.
Brazing alloys melt generally within a range of temperatures.
The minimum temperature at which the alloy starts to melt is called
Solidus, and that where all of the alloy is liquid is called Liquidus.
These are physical temperatures that can be determined
scientifically with specially designed instrumentation at any
required degree of precision.
The above temperatures depend only on composition, i.e. on
the actual proportions of the different elements present and on
the level of impurities which are not generally reported.
As the materials are manufactured in batches starting from
different base metals, the actual temperatures found in a certain
batch of material may differ more or less from what reported in
the Table or in Specifications.
In general you will notice that certain alloys display a narrow
melting temperature range, while other ones quite a large
interval. A special case is that of BAg-8 which has range zero,
that is Solidus and Liquidus are at the same temperature.
This happens in the so called "Eutectics", meaning alloys that
behave like pure metals having a definite melting point
and not a melting range.
Alloys of narrow range are mostly used for capillary gaps,
where they can flow readily given the correct conditions.
Wide range alloys are more sluggish in their flow and they
are used sometimes to fill larger gaps that would not be
filled by other alloys.
One must take into account though that wide range alloys
have a tendency to separate in their basic components if
heated too slowly.
This characteristic is called "Liquation" and will interfere
with a correct brazing operation, hindering the normal flow.
Therefore it is always good practice to heat rapidly
through the melting range.
In practice the actual brazing temperature will be some
50 to 200 0F (30 to 110 0C) above the Liquidus, depending
on the process and the circumstances.
Different Silver alloys were formulated as filler metals
for Brazing. They are classified in AWS A5.8
SPECIFICATION FOR FILLER METALS FOR BRAZING
AND BRAZE WELDING.
Note: the following Table was prepared using
various sources and collecting as much information as was
available at the time of this writing. No claims of accuracy or
completeness are advanced here.
The user is always urged to verify independently
the data before applying them for practical use.
If necessary reference should be made to the original
AWS Specification.
| AWS A5.8 | Composition % | Temperature F/C | ||||||
| Class | Ag | Cu | Zn | Cd | Ni | Other | Solidus | Liquidus |
| BAg-1 | 45 | 15 | 16 | 24 | - | - | 1125/605 | 1145/620 |
| BAg-1a | 50 | 15.5 | 16.5 | 18 | - | - | 1160/625 | 1175/635 |
| BAg-2 | 35 | 26 | 21 | 18 | - | - | 1125/605 | 1295/700 |
| BAg-2a | 30 | 27 | 23 | 20 | - | - | 1125/605 | 1310/710 |
| BAg-3 | 50 | 15.5 | 15.5 | 16 | 3 | - | 1170/630 | 1270/690 |
| BAg-4 | 40 | 30 | 28 | - | 2 | - | 1240/670 | 1435/780 |
| BAg-5 | 45 | 30 | 25 | - | - | - | 1250/675 | 1370/745 |
| BAg-6 | 50 | 34 | 16 | - | - | - | 1270/690 | 1425/775 |
| BAg-7 | 56 | 22 | 17 | - | - | Sn=5 | 1145/620 | 1205/650 |
| BAg-8 | 72 | 28 | - | - | - | - | 1435/780 | 1435/780 |
| BAg-8a | 71.7 | 28 | - | - | - | Li=0.3 | 1435/780 | 1435/780 |
| BAg-9 | 65 | 20 | 15 | - | - | - | 1240/670 | 1325/720 |
| BAg-10 | 70 | 20 | 10 | - | - | - | 1275/690 | 1360/740 |
| BAg-11 | 75 | 22 | 3 | - | - | - | 1365/740 | 1450/790 |
| BAg-13 | 54 | 40 | 5 | - | 1 | - | 1340/725 | 1575/855 |
| BAg-13a | 56 | 42 | - | - | 2 | - | 1420/770 | 1640/890 |
| BAg-18 | 60 | 30 | - | - | - | Sn=10 | 1115/600 | 1325/720 |
| BAg-19 | 92.5 | Rem | - | - | - | Li=0.2 | 1400/760 | 1635/890 |
| BAg-20 | 30 | 38 | 32 | - | - | - | 1250/675 | 1410/765 |
| BAg-21 | 63 | 28.5 | - | - | 2.5 | Sn=6 | 1275/690 | 1475/800 |
| BAg-22 | 49 | 16 | 23 | - | 4.5 | Mn=7.5 | 1260/680 | 1290/700 |
| BAg-23 | 85 | - | - | - | - | Mn=15 | 1760/960 | 1780/970 |
| BAg-24 | 50 | 20 | 28 | - | 2 | - | 1220/660 | 1305/705 |
| BAg-26 | 25 | 38 | 33 | - | 2 | Mn=2 | 1305/705 | 1475/800 |
| BAg-27 | 25 | 35 | 26.5 | 13.5 | - | - | 1125/605 | 1375/745 |
| BAg-28 | 40 | 30 | 28 | - | - | Sn=2 | 1200/650 | 1310/710 |
| BAg-33 | 25 | 30 | 27.5 | 17.5 | - | - | 1180/640 | 1320/715 |
| BAg-34 | 38 | 32 | 28 | - | - | Sn=2 | 1200/650 | 1330/720 |
| BAg-35 | 35 | 32 | 33 | 17.5 | - | - | 1265/685 | 1390/755 |
| BAg-36 | 45 | 27 | 25 | - | - | Sn=3 | 1185/640 | 1260/680 |
| BAg-37 | 25 | 41 | 32 | - | - | Sn=2 | 1270/690 | 1435/780 |