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Building The Lo-Pro Welding Turntable
 

Introduction


     This page will follow the build of the low-profile welding turntable during the the Fall 2012 semester. Construction is carried out per dimensioned drawings produced from the final design completed in Solidworks. Tools available to us include drill presses, lathes, mills, sanders/grinders, layout and measurement instruments, welders, and a waterjet. We chose to begin with the mechanical side of the project (the chassis and those elements within) while we await electronics to be shipped to us.

  
The Build

   The first part to begin with is the chassis. It is cut from a length of 10-3/4" O.D. steel pipe (3/8" wall thickness) on a vertical band saw. After being sliced off, it was held in a lathe's chuck from the inside so that the outside diameter could be reduced Then it was held from the outside so that the inside could be cleaned up as well.









    With the chassis mostly completed, the mild steel flat components were next cut on the waterjet. A piece of 1/8" thick steel plate for the 'bottom bolting ring (C-ring)' is laid onto the cutting table and the cutting file is loaded in the computer. The part is finished in less than two minutes and requires only some subsequent tapping for bolts to screw into. We repeat this process for the bottom cover, 'insulating plastic, supporting plate, and the top plate. We also used the waterjet to cut the 'motor hold-down straps' from 1/2" thick plastic.











   Some critical components were received in the mail during the early days of the build, they are shown below and are installed later on in this page. From left to right: Dayton 12V PMDC 1500RPM 5.1A 1/20HP motor, 9" turntable bearing (5/16" THK), OnDrive P30 30:1 worm drive 90 degree gearbox, Nylon flange sleeve bearings for 1" shaft, 12T and 54T sprockets, #25 roller chain, and 00 Gauge copper grounding strap (1-1/8" width).



    The two holes at the center of the 'top plate' were located, drilled, reamed to size, and counter-bored to accept the shoulder bolts which attach the 'central shaft.'


   The top plate was fixed to the central shaft by two shoulder bolts. This allowed the plate to be held in the chuck by gripping the shaft. At this point, the plate was surfaced completely and face-grooved with concentric circles.







   A large boring bar with a CNMG insert was used to create the concentric face grooves. Chatter became a serious problem. Different inserts/feeds/speeds/ were experimented with until finish was satisfactory.



     With the 'top plate' fastened to the 'central shaft', it was easily held it in the lathe chuck and was cleaned up. Vibration of the plate (kind of like a drum symbol) created an issue unless cuts were very 'light.' Concentric grooves were added after facing and a little sanding/polishing followed.





   The supporting plate has slots which required some milling so that they would have internal shoulders to capture the head of the gearbox's bolts. The insulating plastic layer is then screwed over the supporting plate to separate create electrical separation from the big bearing. Nylon bearings in the bottom and supporting plate capture the central shaft.








   At this point, the radial features are added to the chassis. This is the large 2" hole for the rubber shaft coupling and (4) tapped 10-32 thru holes for attaching the grounding strap. All work was done on the Bridgeport using simple layout methods. The large hole required 'stepping' up through several different sized endmills, plunging each. Note that this hole is not centered above the chassis center and could not be easily drilled.





    Next, the gearbox input shaft needed to be made. It is a bit frustrating that Ondrive doesn't supply this simple component with its P30 worm drive gearbox. This is magnified by that fact that the shaft's OD is 8mm and is thus not a common size in the states. I machined this component from 3/8" low carbon steel. It is fairly simple and merely required two c-clip grooves, and set-screw flats on one end. The lathe used is a tight Monarch 10EE toolroom lathe.





   A live center was used as the rod was turned down to 8mm.





   As the shaft approached final size, it was tested for the preferred slip fit into the gearbox. When it could be inserted without trouble, the OD was finished.



   The shaft was then grooved to accept C-clips. These clips would prevent the shaft from moving axially within the gearbox and are thus very important. A top-notch turning insert was machining to the required groove width and plunged into the diameter at the determined locations.



  After the shaft is turned to the finish diameter and measured, the grooves for retaining rings are added.



   The key is 3mm square, and is machined from 1/4" key stock. A keyset is made using an 1/8" F.E.M. cutter. Information regarding dimensions was found in a recent edition of Machinery's Handbook.








   Having made some 'test assemblies' to be sure everything was fitting together as designed, MIG welding begins. First the bottom bolting ring is laid on top of the bottom cover plate (to ensure the cover plate will sit flush after welding). The ring is shimmed away from the walls to ensure concentricity with the chassis. It is tacked, then welded more permanently.







  With the ring welded, the chassis is flipped over and the supporting plate (usually hidden inside the machine unless completely disassembled) is welded as well.




   With the bottom cover plate bolted on to the bottom bolting ring (above), the underside of the machine looks like below (picture taken after final assembly):



   Again, the chassis is flipped and components can begin to be attached to the top of the machine.







   With welding of the chassis finished, some assembly can be started. First the top platform is bolted to the central shaft and inserted with a nylon flange bearing into the hole in the supporting plate of the chassis. It is laid upside down on the table.

   The grounding strap is run around the shaft and clamped at both ends to the chassis



  
The gearbox is added. It may slide in the slots provided in the supporting plate. Sprockets are fitted to the shafts, and the #25 chain is slipped over them. The gearbox is pulled away to tighten the chain, and then itself is tightened.





   Flipped back right-side-up, most of the mechanical side of the assembly is finished. Next, the electrical enclosure and electronics must be built.
   This begins with fabrication of the electronics enclosure. This includes an 16g steel base welded to the chasses, with a bend sheet metal enclosure lid (designed in solidworks), as well as vents and a end-cap welded to the base.

   First, the base plate is cut on the waterjet: an Omax 55100. Mounting hole locations are piloted undersized for later drilling and tapping.


 









    The steel is conductive, and so a similarly shaped acrylic (plexiglass) plate is cut to lay on top of the above base plate. Additionally, plastic spacers (shown below) and nylon screws are used as standoffs to mount the circuit boards (power supply, Arduino, Cytron MD10C motor controller).







Time to Fix Some Mistakes!

   During some preliminary assembly, testing, and further work with the Solidworks model, it became apparent that some things simply would not work out. Mostly this was due to misalignment or interference issues. Here are the three big ones with shots from the model to illustrate:

1.   The output shaft of the gearbox (that was machined) was so long that it would not allow the bottom plate to close completely. It needed to be shortened by about 1/8". This was easily and quickly done.




2.   The ID of the flexible coupling (0.505") is larger than both the motor's shaft (0.313") and the gearbox input shaft (0.236"). Bushings must be made.




   Because its a quick fix, the bushings required to join the coupling to the two different sized metric shafts is shown below:



    Also at this time, it was realized that the OnDrive P30 30:1 gearbox did not have a flat on the input shaft to accept a set screw. This was a second disappointment in their design. Fortunately, the tools were available to grind a flat accurately. I used a Taft and Pierce surface grinder to do this. A magnetic chuck held a 2" x 4" x 6" precision block to which the gearbox was fixed (also with its shaft fixed) using vice-grip clamps.








3.   The grounding strap also still needs a tensioner spring to keep it pulled taught against the central shaft. This was also an easy fix. A spring was used and two portions of threaded 10-32 rod. A more elegant solution should be in order, but it works well in the meantime.





Electronics Phase
   The control devices are mounted on the enclosure plate and taken home for programming, wiring, and testing. Below is the program used to control the 12VDC permanent magnet PMDC motor using a Cytron MD10C motor controller and Arduino microcontroller. The complete program is shown on the sister page for this project (www.svenhoek.com/Welding_Turntable.html



   The components are wired together and the potentiometer and fuse are added. A grounded power cable is connected to the power supply so that it may be plugged into the wall. Our program is simple and reads the position of the potentiometer relative to its center of travel. Speed is then made to be proportional to this displacement. We programmed in a 'dead section' so that when approximately at center, the machine will keep the motor off.



   To ensure that the motor is reaching maximum duty cycle (and maximum speed), the oscilliscope is used to check the wave form. Its works just as expected. A slight whine is emitted from the motor. This is an apparent side effect of PWM (pulse width modulation) control of a DC motor.





   A cover is needed to protect these delicate elements. This was designed in Solidworks as a 3-sided cover which access for the control panel as well as the vent and power cable. It was 'unfolded' using the software and then cut on the waterjet as a flat pattern. Holes were included as guides to be transferred to steel tabs which were welded to the bottom enclosure plate.



   A brake press was used to bend this component. This is a hydraulic press operated by a foot lever. the lower die is stationary and is lowered or raised by a screw to control the magnitude of the angle of the bend.





   As a guide to locate the bends themselves, the waterjet was programmed to 'scribe' a bend-line using a blast of pure water (no garnet abrasive).



   The finished cover is shown below.





The base plate is welded to the Chassis. The motor is tested to see if it can control the speed and the direction of the table before proceeding. No problem!



   Electronics are removed and the side pieces are welded on.





   At this point, it was realized that the insulating plate didn't fit properly once the welding beads were included. The plate was machined down 1/8" on all sides in order to fit. It was clamped to another plate in a Bridgeport table vice, indicated, and cut.




   Finally, feet for clamping the machine to any table and also a grounding lug to which a welding machine's grounding clamp may be directly attached is added.








Final Assembly

  Final testing was successful! The end product looked and worked great during welding. Below are some shots of the final assembly, fully functional and operable. Of course, there are some things worth adding to the machine (more user-customizable control, different types of grounding lugs, etc.) but the final design is very satisfying to use.














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