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 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.'
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.
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.
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.
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
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.
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.