Post by geep on Jan 30, 2007 11:09:12 GMT -8
Last week I began work on a modeling table suitable for small to medium size work. Here are a few in-progress photos of my new modeling table and a description of the work to date.
Design: I wanted a modeling table roughly 42" x 70" to accomodate smaller patterns. My main requirement was that it be strong, stiff, but not incredibly heavy as it will eventually reside on a wood floor. In my specification I chose 1/100th of an inch as an acceptable deviation from flat as measured across the width of the table. Down the length and corner-to-corner a slightly larger deviation would be acceptable.
Due to the expansion and contraction of wood as the Equilibrium Moisture Content changes, I decided against all-wood construction. I wanted a wood surface to make securing plinths and models easy, but a steel frame to negate possible seasonal changes in flatness. Also, by building a steel frame it would be possible to incorporate an adjustment system to "tweak" the flatness of the table top.
The Frame:
I settled on a design made up of a 3" square tube backbone and a 3" channel frame. The square tube is very ridgid in torsion, which will prevent the table from racking. The channel makes joining the frame less complicated by using fewer welds. It also makes it possible for the CG of the welds to be arrayed around the CG of the section, resulting in lower distortion. An added bonus to the channel is it adds 1.5" of throat to the underside of the table for clamping purposes.
Materials for the frame:
20' 3" C-channel (frame)
5' 3" x 3" x 3/16" square tube (frame torsion spine)
10' 2" x 2" x 1/8" square tube (legs)
4' 1/4" x 6" flat (leg base)
4' 1/4" x 8" flat (frame stiffener/leg attachment base)
1' 3/8" x 2" flat (leg feet)
Additional materials required:
2 cans Rustoleum primer
2 cans Rustoleum "Dark Machine Gray" enamel
24 1/4-20 sheet metal "back markers"
16 3/8-16 x 1 bolts
16 3/8 lockwashers
Table top:
I settled on 3/4" 13 ply baltic birch cabinet plywood for the top. It will be finished with 4 coats of polyurethane to reduce the seasonal variations in moisture content. The top surface will incorporate a centerline to aid in centering the model, and the four edges will be square and parallel to each other within 1/64th so that they may be used as guides.
The top surface of the table will be drilled and counterbored to accept the 1/4" fasteners that to hold it to the frame. Underneath, 1/4" carriage bolts will apply preload up. Working in concert, the fasteners pulling down and the carriage bolts pushing up make it possible to adjust the height of the table at any location.
To ensure that the holes in the table perfectly matched the threaded adjuster locations in the frame I decided to make some back markers specifically for the purpose. Shown below, the shaft of the marker fits inside the minor diameter of the internal thread. A flange prevents the marker from falling through and the pointed tip acts like a center punch, marking the center of the hole.
The markers actually worked better than I expected. Once the plywood was cut to size I marked the locations of the four corners and placed the table top on the frame. Then I weighted the top and marked each location by whacking the table top with a deadblow hammer. I placed a phone book approximately 1" thick between the table and the hammer to soften the blow and prevent marring of the surface. Here are a few locations that have been marked:
I'm currently applying a finish to the table top. I'll have more photos later this week.
Design: I wanted a modeling table roughly 42" x 70" to accomodate smaller patterns. My main requirement was that it be strong, stiff, but not incredibly heavy as it will eventually reside on a wood floor. In my specification I chose 1/100th of an inch as an acceptable deviation from flat as measured across the width of the table. Down the length and corner-to-corner a slightly larger deviation would be acceptable.
Due to the expansion and contraction of wood as the Equilibrium Moisture Content changes, I decided against all-wood construction. I wanted a wood surface to make securing plinths and models easy, but a steel frame to negate possible seasonal changes in flatness. Also, by building a steel frame it would be possible to incorporate an adjustment system to "tweak" the flatness of the table top.
The Frame:
I settled on a design made up of a 3" square tube backbone and a 3" channel frame. The square tube is very ridgid in torsion, which will prevent the table from racking. The channel makes joining the frame less complicated by using fewer welds. It also makes it possible for the CG of the welds to be arrayed around the CG of the section, resulting in lower distortion. An added bonus to the channel is it adds 1.5" of throat to the underside of the table for clamping purposes.
Materials for the frame:
20' 3" C-channel (frame)
5' 3" x 3" x 3/16" square tube (frame torsion spine)
10' 2" x 2" x 1/8" square tube (legs)
4' 1/4" x 6" flat (leg base)
4' 1/4" x 8" flat (frame stiffener/leg attachment base)
1' 3/8" x 2" flat (leg feet)
Additional materials required:
2 cans Rustoleum primer
2 cans Rustoleum "Dark Machine Gray" enamel
24 1/4-20 sheet metal "back markers"
16 3/8-16 x 1 bolts
16 3/8 lockwashers
Table top:
I settled on 3/4" 13 ply baltic birch cabinet plywood for the top. It will be finished with 4 coats of polyurethane to reduce the seasonal variations in moisture content. The top surface will incorporate a centerline to aid in centering the model, and the four edges will be square and parallel to each other within 1/64th so that they may be used as guides.
The top surface of the table will be drilled and counterbored to accept the 1/4" fasteners that to hold it to the frame. Underneath, 1/4" carriage bolts will apply preload up. Working in concert, the fasteners pulling down and the carriage bolts pushing up make it possible to adjust the height of the table at any location.
To ensure that the holes in the table perfectly matched the threaded adjuster locations in the frame I decided to make some back markers specifically for the purpose. Shown below, the shaft of the marker fits inside the minor diameter of the internal thread. A flange prevents the marker from falling through and the pointed tip acts like a center punch, marking the center of the hole.
The markers actually worked better than I expected. Once the plywood was cut to size I marked the locations of the four corners and placed the table top on the frame. Then I weighted the top and marked each location by whacking the table top with a deadblow hammer. I placed a phone book approximately 1" thick between the table and the hammer to soften the blow and prevent marring of the surface. Here are a few locations that have been marked:
I'm currently applying a finish to the table top. I'll have more photos later this week.