CNC CONTROLLED PTA WELDING AND HARD FACING OF COMPONENTS

ARCRAFT PLASMA EQUIPMENTS (I) PVT.LTD.

◦ An extension to plasma welding when used for hard-facing

or cladding using powder as a filler material is known as

Powder Plasma welding. Also commercially known as PTA

i.e. Plasma Transferred Arc welding. Being a welding

process the advantages of the PTA are enormous compared

to other Flame or Arc spraying processes. Also as compared

to the commonly available welding methods the PTA scores

well above processes like SMAW, TIG, MIG.

◦ However all the advantages come at a cost in form of

“Automation”. PTA is a process which is not meant to be

used manually and call for an automation is to certain level

mandatory.

◦ The level & topology of the logic for the control of

automation may differ from manufacturer to manufacturer

however the automation can be broadly in 3 categories. 

1. Semi automated with timers

2. Numeric controlled (NC) with micro

controller, alpha numeric display, keypad

3. Fully computer based CNC system

• The basic differences can be judged from the working

capacity, the flexibility of the operation, repeatability of

the job, skill level of the operator etc.

• As the subject of automation is very vast and beyond the

scope the article, only the insight into high end

automation has been focused here in next few pages.

• The PTA process has multiple set of parameters to be handled which can be broadly grouped into gas, mass, motion

& power. The parameters covered by each group are as under.• GAS

Ø  Plasma gas flow & pressure

Ø  Shielding gas flow & pressure

Ø  Carrier or Powder gas flow & pressure

• MASS

Ø  Powder feed rate

Ø  Powder feed down slope

• MOTION

Ø  Surface speed of the job

Ø  Travel speed of the torch

Ø  Torch Oscillation

• POWER

Ø  Welding current

Ø  Arc gap

• Although the conventional method of handling certain

parameters like gas, powder flow, welding current can be

handled manually but it calls for the skill & the

experience of the welder / operator.

• Exploiting the advantage of the CNC topology,

practically all the parameters involved in the process can

be programmed and by allowing the computer to handle

the process the need for the skilled welding or operator

can almost be eliminated. As in any welding

qualification the initial setting of parameters can be done

under the expert’s guidance.  Once set the operator job is

only limited to the loading & unloading of the job on the

system. It is to be noted that any pre or post weld process

required to achieve the desired result have to completed

in any case.

• A typical CNC based system can be wired around high

grade of components which include.

1. Industrial mother board

2. High speed PLC configurable with external add on

modules.

3. High resolution graphic display with Touch Screen

facility.

• Other supporting devices like interlocks, sensors,

measuring devices are to be compatible with the CNC

configuration.

• To achieve the desired result, the compatible mechanical

system with high degree of accuracy is required typically

employing Ball screw mechanism, planetary gear boxes 

etc. is a must to squeeze maximum benefits out of CNC

system.

CNC controlled Plasma Transfered Arc (PTA) Welding system

oWith the necessary support of interfacing devices for signaling, measurement, control & correction a complete fail safe PTA

CNC base system can control literally all the parameters involved in the process which are listed as under1. Plasma gas flow & pressure2. Shield gas flow & pressure3. Powder or Carrier gas flow & pressure4. Gas pre flow time5. Gas post flow time6. Welding current7. Welding voltage / Arc gap8. Rotating speed of the job9. Linear travel speed of the torch10. Weld position of the torch11. Torch oscillation width]12. Torch oscillation speed13. Dwell time14. Weld current down slope15. Powder flow down slope

Display screen for Welding current

Display screen of motion control

o Other than the control of parameters the system employs typically single touch operation which controls the sequential

flow of operation automatically which is as under.

1. Sense the Park or No Weld position of the torch2. Bring the torch to Weld position above the job3. Initiate the gas4. Initiate the pilot arc5. Start the welding arc6. Start the job motion7. Start the torch motion8. Start the oscillation9. Start the powder flow10. Deposit desired bands of weld11. Deposit desired passes of weld12. Bring the weld current and powder flow to down slope mode at the end of the job.13. Stop welding & all the other motions14. Allow desired post flow of gas.15. Bring the torch back to park position

TYPICAL JOBS ACHIEVED BY SUCH CNC PTA SYSTEM:

Engine valve HardfacingID Cladding

◦ By allowing the CNC to virtually control all the activity of the process there is an endless list of advantages of such system

◦ Saving in Time

The time saved is what otherwise would be lost in activities like

1. Adjust the torch over the job every time the new job is placed

2. Manually raise the torch every time the job is finished

3. Manually adjust the powder flow rate for every new job.

4. Manually adjust the current for every new job

5. Manually adjust the speed of job, torch, oscillation of every new job.

◦ Saving in powder

1. Volumetric control ensures optimized flow rate.

2. Minimal  wastage of powder.

• Uniformity of weld deposit

1. Highly uniform weld deposit

2. No crater at the end of the weld

3. Optimum gas flow ensure no blow holes and no oxidation of the job.

4. Highly repeatable results

• The bottom line

As required by any industry is fulfilled in the form of

1. Reduced cost of deposit per job or per square inch

2. Reduced cost of labor

3. Reduced cycle time hence increase in the production rate

4. Very low rate of rejection

5. Reduced inventory of powder & gases.

• No dependency on skilled labor

1. Machine is used by Operator not Welder.

2. Increase in the rate of production.

◦ As general conception goes that CNC machines demand high capital investment does not really hold ground

here in the case of CNC based PTA machine.

◦ Carefully designed with optimized use of high quality of raw material, choosing the right option for the

electronics involved can make a highly efficiency system at much affordable price.

Arcraft Plasma Equipments (I) Pvt Ltd., 124, Diamond Industrial Estate, Ketkipada, Dahisar (East), Mumbai- 400068.

INDIA.

Visit us : arcraftplasma.com

Email us: salesarcraft@gmail.com

Contact us:

Ph : 00-91-22-28965890, 28965745, 28963247 Fax: 00-91-22-28966418