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The Lo-Pro Welding Turntable
 
  In 2010, I was commissioned to make aluminum capsules of approximately 8" height, and 5" diameter. To each, a circular lid was welded to the bottom and a similar lid was bolted to the top. The bottom, requiring a full welding bead about the circumference, was challenging to me.



   I needed to stop welding every couple inches to rotate the part back into my comfort zone. This subjected my weld (which needed to be as perfect as possible) to a visible discontinuity. The solution to my problem was to give rotation to the part while I held stationary. This ensures a consistent weld about the circumference of my workpiece.

  In fact, the shop already has a machine for this, and its drawbacks are immediately apparent. Especially as a novice welder, I prefer to be seated, as would anyone who works for extended periods. Our welding turntable, like all current models, is not at all compact in the vertical direction. As a result, a welder may not sit comfortably in a normal table and chair while working!

   Here is a picture of the completed design: The Lo-Pro Welding Turntable




Abstract


    A benchtop welding turntable has been designed and built which can be used by a welder to conveniently rotate a welding workpiece at a continuously variable speed up to 12rpm. The design is innovative in that its overall vertical height dimension is three inches, allowing for the welder to be comfortably seated at a table with the workpiece still below shoulder level. Furthermore, it uses a control system for the table’s speed that is both simple and easy to use and which utilizes the versatility of the Arduino microcontroller. Various calculations and design considerations were made with respect to material strength, power transmission, and availability of certain elements. Machine construction utilizes a maximum of standard and serviceable off-the-shelf components such as bearings, electronic power supplies, and chain/sprockets, while the frame and mounting hardware is custom machined.

  
Background


   In welding about the circumference of a circular object, it is very easy to weld the front side. However,  as you approach the sides, the weld becomes obscured by the workpiece or your hands. You could change foot stance to follow the weld, or stop to rotate the workpiece back into the comfort zone. This often result in a flawed or ugly weld. If relieved of the duty to constantly readjust stance or to reposition the work-piece, the welder can focus on producing consistent and interruption-free welds.



   There are products available for this purpose (above), however, they are both expensive and
tend to rotate the work piece at an uncomfortably high position above a human’s sitting height. The options shown above are the All-FabPS-1F table (left), ProfaxWP-250 table (middle), and Atlas XT-100 table (right). They are priced as 1,600$, 2,200$, and 1,100$. In the same order, the overall heights of each of the products are 15”, 14”, and 7.5”, which makes the Atlas model the shortest turntable in addition to the cheapest. A welder would almost certainly not be able to be seated at an ordinary table and chair and hope to comfortably look down upon the workpiece.

   To improve usability, this new machine should be of a convenient design for solely upright rotation and must absolutely be as low to the table as possible!


Design Constraints

   Certain aspects of our design are governed by strict requirements on size and weight. The primary goal is to design the turntable with as low of a profile as possible. However, the user of the product must also be able to lift and move the machine, and subject it to a large range of loads. These primary design requirements and justification are summarized:





Engineering Design Process


   The design of the Lo-Pro Welding Turntable progressed through many stages of development before culminating in a fully functional prototype. Many calculations concerning power transmission were iterative and required frequent reworking thanks to a large selection of mechanical elements.

Stage 1: Powertrain and Current Handling Concepts


   Here we came up with as many approaches as possible to delivering rotational power to the turntable platform while maintaining electrical isolation from primary welding currents. The design moving forward was based on a rubber coupling (insulating element) which would separate the input shaft of a 90 degree wormdrive gearbox mounted within the chassis from the neighboring 'external' motor. When we had verified that small lubricated and sealed gearboxes were available, as well as rubber couplings, we moved forward with this design.

   The current carrying element in the design, that is, the essential component which allows enormous current to pass unimpeded from the turntable platform (rotating) into the machine chassis/ground (stationary) is the grounding strap. This is a creative approach taken in lieu of carbon/copper brushes and commutators common to similar applications. The final design uses a strap which is fixed at both ends to the steel chassis and wrapped approx. 1/2 turn around the rotating central shaft of the machine. The strap is placed under tension and thus always maintains contact with the shaft during rotation.

   Some wear will occur inevitably but the softer copper strap is very thick and cheap (approx. 8$/ft @ 00AWG equivalent 180A). Because both ends of the strap are fixed to the chassis, the current capacity is doubled.


Stage 2: Survey of available materials

   In accordance with thriftiness, we first surveyed what materials were available to us. The
machine shop offered us a lengthy section of 10” diameter steel pipe, among some others. Other pipe sizes (shown below in Table 4) were also considered, but we opted for the piece which gave us the most internal space to work with.





Stage 3: Mathematical Model of Turntable and Load

   Next, the turntable platform, central shaft, and estimated 40lb load were considered to be
the only inertial elements (disregarding lighter motor rotors, rubber coupling, and
chain/sprockets). We found the moments of inertia for each, the total mass, and finally, the
torque and power required in accelerating the configuration. Although a very comprehensive
Excel spreadsheet was used in order to accommodate changing parameters, the general
approach is shown below:


Stage 4: Motor survey

   The selection of a motor is extremely influential on the remainder of the machine. It
dictates the power supply, motor controller size, overall height of the machine, and subsequent
gear reduction elements. We conducted a survey to learn what was available to us. Table 5
below illustrates our findings.






   Next, by exhausting testing of many sprocket configurations, gearbox sizes/ratios, and motors (with both ungeared and geared heads), three possible configurations which met our design constraints were selected. In all cases, overall height of the machine was considered the most critical criteria. The final design would use a 12VDC 1350rpm 1/20 HP Dayton motor (ungeared) coupled to a 30:1 OnDrive 90 degree sealed wormdrive gearbox, and placed in series with a 54:12 tooth sprocket and chain gear reduction arrangement. This allowed for an overall turntable height of 3”.




 

   Option 2, shown above, was the most appealing as it kept the motor moving at relatively high speeds no matter what speed the turntable was operated at. This ensured fluid motion with PWM input. Also, the 54T/12T sprocket ratio was compact enough to allow the gearbox (to which the smaller sprocket is mounted) a significant travel. This travel would be essential to assembly and chain tensioning. The 30:1 gearbox ratio was higher than we wanted, as the gearbox to be self-locking if back-driven by a load, but OnDrives claimed it would not. By this, the gear train ratio was established at a total of 130:1.

Stage 4: Sprocket and Chain Design.

   We simultaneously settled on a sprocket ratio of 54T:12T (4.5:1) and analysis was done to ensure the #25 roller chain was within its capacity. This design was approved only after meeting design requirements set forth in The Standard Handbook of Chains by the American Chain Association. From this text, we determined the following requirements:

     • A maximum of 6.7:1 gear ratio can be achieved using #25 chain
     • No less than 12 teeth on the small sprocket will ensure smooth rolling action
     • #25 chain carries a maximum load of 140lbf (480lbf for #35 chain)






Stage 5: Analysis of Gearbox Torque Handling

   Because power transmitted through a gear train is constant (when efficiencies of elements are neglected), the torque inputs/outputs of both the gearbox and each of the sprockets can be calculated using the gear ratios. Ondrive, the gearbox manufacturer, provided efficiency ratings for the gearbox and so losses were included in design.
 
   The final design, using the 30:1 OnDrive gearbox and 54:12 tooth sprocket and chain arrangement, powered by the 1/20th HP Dayton motor placed the following manageable torque loads on each of the elements:

     • The central shaft and large sprocket require 210.9 in-lb torque during acceleration
     • The connected smaller sprocket and gearbox output sees a 46.9 in-lb load
     • The gearbox input shaft and motor both see a load torque of 1.563 in-lb
     • During design accel. and load, maintaining this torque requires a min. 1/23rd HP

   Additional calculations ensure certain parts are able to sustain continuous service. This includes the chain itself and the keys on both the gearbox output shaft and the central shaft. As mentioned earlier, the chain load was limited to 95lbf (68% of its capacity). The key, for instance, on the central shaft was then analyzed. Its size was determined by the female keyway in the pre-manufactured 54T chain sprocket.

   Given the keyway’s length and width, a shear area was determined to be 0.125 in^2. The load
torque was then used in order to calculate the effective shearing force on the key. This stress
was compared to the shear stress rating for low-carbon steel and found to be less than 14% of
that stress required for failure. We were assured that our design was strong.

   Similarly, torque required of the gearbox’s output shaft was calculated to be 5.30Nm. This was coincidentally the manufacturer’s rating when operating at 500RPM (see below). Of worthy notice, we expected the motor to operate up to 1350RPM, for which the gearbox was rated at only 4.5Nm torque. The characteristics of DC motors (maximum torque at starting, minimum at top speed) led to the approval of this rating for our purposes since motor torque would continuously diminish to zero as the machine’s speed increased.




Stage 6: Solidworks Modeling.

   With all power transmission elements selected, the project could be designed around it. This was especially important for the electronics housing, which contains the power supply, etc. A detailed Solidworks model was created from which component drawings were produced.








Stage 7: Electronics wiring and programming.


   Knowing the Dayton Motor could draw up to 5.1Amps, a single-output Newark power supply (8.4A@12V, model:EMC100) which operates on 120VAC source. This adhered to a rule of thumb which says that a power supply should be 1.5x the motor’s maximum current rating. Additionally, with a usable output voltage of 12VDC, the Arduino and Cytron controller would be powered in parallel with the motor.

   The Arduino, like most microcontrollers, cannot provide the high current required by the motor. As a result, a Cytron motor controller/amplifier (model:MD10C) was used to provide current to the motor in a manner dictated by the Arduino’s signals. This is further regulated by a 7.5A fuse, which refers to another rule of thumb that claims a fuse should typically be 1.2x larger than the motor’s current rating.






   Above, the Arduino (blue), the Cytron motor controller or amplifier (black), and the Neward Power supply (yellow). The potentiometer and bus fuse (later replaced by spade fuse) are at top.




Expenses




Documentation

   During the modeling phase of this project. As each element was finalized, a detailed drawing was produced to take to the shop for manufacturing or to assist in assembly. Generally, these were indispensable and an excellent investment in time. Essentially, once the drawings are made, when you move to the shop you merely play the role of the machinist. You must only concentrate on meeting tolerances; the design is done!

... drawings to come


Microcontroller Program:

   Below is the program which was written for the Arduino microcontroller. When executed, everything below the line "void loop" is continued indefintely. In this way, the microcontroller is continuosly checking the position of the potentiometer and changing the turntable motor's speed to reflect its position. A 'dead zone' is programmed into the control so that it will but OFF during a predetermined postion range. This code basically replaces a polarity-reversing potentiometer (which doesn't exist apparently), and enables ON/OFF/SPEED/DIRECTION all with just one knob.


//This program controls a 12VDC 5A motor speed and direction using a potentiometer
//11-27-12
//Note: 'Enable' is the ON/OFF/PWM output pin on the Arduino
//The 'Cytron MD10C' is accepting the Arduino Output signals and regulating the motor current
//Power to the MD10C is supplied by the Newark ECM100 power supply @ 12VDC.
//Thus, this code requires the Arduino, MD10C, ECM100, and brushed DC motor.
//http://ruggedcircuits.com/html/rugged_motor_driver.html


#include <Stepper.h>

#define Enable 3 // Assign the 'Enable' (PWM) signal to Pin 3
#define Direction 12 // Assign the 'Direction' signal to Pin 12
intpotVal; //potentiometer position value is stored here as # from 0- 1024


void setup() //further define what each I/O pin is being used for in this section

{
Serial.begin(9600); //begin the serial monitor for debugging convenience
pinMode(A0, INPUT); //The potentiometer for speed/direction control assigned to Pin A0
pinMode(Enable, OUTPUT);
digitalWrite(Enable, LOW); //Enable (PWM) pin assigned as an output and set as LOW (forward)
pinMode(Direction, OUTPUT);
digitalWrite(Direction, LOW); //Direction pin assigned as an output and set as LOW
analogWrite(Enable, 0); // Set the motor as OFF just incase
}


void loop()

{
potVal=(analogRead(A0))/2-256; //Read Pot, map value 0-1024 onto range (-255)to(255), for PWM out
Serial.println(potVal); //display current pot value on the Serial Monitor for debugging convenience

if(potVal>=50){

digitalWrite(Direction, LOW); // Set Motor 1 forward direction
analogWrite(Enable, potVal-40); // Motor 1 ON at PotVal speed
delay(50); //Continue at that speed for 50/1000 sec. before restarting loop
}

else if(potVal<=-50){

digitalWrite(Direction, HIGH); // Set Motor 1 reverse direction
analogWrite(Enable, potVal*(-1)-41); // Motor 1 ON at PotVal speed. Math keeps PWM value positive
delay(50); //Continue at that speed for 50/1000 sec. before restarting loop
}

else{

digitalWrite(Direction, LOW); // Set Motor 1 forward direction
analogWrite(Enable, 0); // Motor 1 ON at PotVal speed
delay(50); //Continue at that speed for 50/1000 sec. before restarting loop
}
}



Machining

   Please see the "Turntable Machining" page which details the build process.




Results

   After a working on the physical construction of this machine for about 8 Saturdays (at a leisurely pace), the machine was finished and looking great! Following the completion of the final welding and assembly, an actual welding test was carried out using the machine for its intended application. This was a suspenseful moment!

   Several welding tests were conducted and the turntable worked with flying colors! During the tests, the machine control was simple and convenient and it did not feel at all awkward to use. Great! Some final specifications are shown below:




    The project was successful and the Lo-Profile Welding Turntable would be recommended for any welding jobs which suits its capabilities. Future recommendations include the integration of a foot pedal, a workpiece clamping system, and a power disconnect switch.

   Because the machine's ultra low profile is its real selling point, it should be noted that the machine could be made significantly smaller in height. The limiting factor in our design at first appeared to be the gearbox. Looking back, this ultra small 294$OnDrive gearbox was essential, but true gains in height minimization were severely restricted by a 18$no-name 54T #25 chain sprocket from McMasterCarr.com, as well as the 1-inch width of the grounding strap. These are both cheap elements! A thin sprocket, and a thin grounding strap (perhaps made from 'stacked layers') could have shaved a full inch off of the machines overall height!

  On the other hand, the Dayton 12V/24V motor used was very minimal in its size. Despite this fantastic motor, it still had a height which would ultimately determine the overall height of the machine. If I was to redesign this machine, I would find a smaller motor (perhaps by Dayton), and would merely limit the top speed/acceleration of the machine in order to minimize the height of the machine (more important).















References


Oberg, Eric, Franklin D. Jones. Machinery’s Handbook 28th Edition. New York, NY: Industrial
Press, 2008.

Alciatore, David G., Michael B. Histand. Mechatronics and Measurement Systems. New York,
NY: McGraw Hill Publishers, 2007. Third Edition

Budynas, Richard.,Nisbett, Keith. Shigley’s Mechanical Engineering Design. McGraw-Hill
Science, 2010.Ninth Edition.

Cengel, Yunus A, Micheal A. Boles. Thermodynamics: An Engineering Approach. New York,
NY: McGraw Hill Publishers, 2006. Sixth Edition

Mott, Robert L. Machine Elements in Mechanical Design.Upper Saddle River, New Jersey:
Pearson Prentice Hall Publishing, 2004. Fourth Edition

Callister, William. Materials Science and Engineering: An Introduction. New York, New York:
John Wiley and Sons Publishing, 2007. Seventh Edition

Wright, John L. Standard Handbook of Chains. American Chain Association. Boca Raton,
Florida: CRC Press, 2006. Second Edition.



Manufacturer Credits:



All-Fab Corporation: 1235 Lincoln Road, Allegan MI, 49010. Model PS-1F Bench top Welding
Positioner. Website: <http://www.allfabcorp.com/model_ps1f.htm>

Profax: 1603 North Main Street, Pearland, TX 77581. Model WP-250 Welding Positioner.
Website: <http://www.profax-lenco.com/logo.php?act=logo1>

Atlas Welding Accessories: 501 Stephenson Hwy. Troy, Michigan 48083. Model XT-100
Welding Positioner. Website: <http://www.atlaswelding.com/welding-positioner-xt-100.html>

OnDrive Precision Gears Manufacturing. Foxwood Industrial Park, Chesterfield, Derbyshire, S41 9RN, England website: <www.ondrives.com>

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