1:72 Little Joe II QTV
Here are some of the various steps in producing the prototype 1:72 scale Little Joe II QTV. Some of the techniques are "high tech" and some aren't. Still, it should give you some idea of the kinds of things you might want to try on your next "super" scale model.
Most of my prototype parts are cut in machinable wax and then cast in a styrene-like white resin. The nice thing about wax is that you can collect all the shavings, "dust", and scraps and remelt them at relatively low temperatures (about 300F) into new machinable blocks...something that you can't do with any other machinable material. Plus, the wax can be machined at very high speed without lubricant or fear of melting the wax or dulling the cutting tools.
Another material that is good for making prototype or finished parts is Renshape which has recently become available in very affordable "hobby" quantities. Renshape is actually a very high density urethane foam (but you can't tell it is a foam) that also cuts quickly without dulling tools or requiring lubricants. It can also be carved and shaped with most any hobby tool that you would use to shape soft wood or plastic.
I'll cover some of these materials in the future elsewhere on this site.
Check back in on this page in the future. More from the prototyping of the 1:72 scale Little Joe II QTV will be added as the project progresses.
A block of wax about 2.3" square and 6" long is cut from a piece of machinable wax. The stuff is hard as plastic and cuts easily and quickly without dulling tools.
The square has its corners cut off to form a "roughly cylindrical" hexagon. Don't be misled by the term "wax." This stuff is very hard and durable...
The finished "rough" blank that will become the corrugated lower body of the Little Joe II. A bolt through the center will serve to mount it on the mill rotary table for milling.
The wax blank is mounted on a rotary table table in a Sherline mill. 120 lengthwise cuts (same as number of corrugations) make the "cylinder."
Overall mill setup. This is a CNC rig (less expensive than you may think). This rig has been considerably "updated" since the photo.
Grooves for the body rings have been cut and the longitudinal "corrugations" are being cut. The cutter is ground to the shape of the corrugations.
Finished corrugated body wax master. Finished (but not dusted off...yes, the wax makes "dust")
A preliminary trial run of the aft thrust bulkhead being cut in a scrap of MDF (med density fiberboard). A tiny .02" dia. milling bit is used.
A group of "proof" molded resin body sections used to estimate resin quantity and uncover any flaws in master.
Closeup of the crisp resin molding. Looks like an injection molded styrene part.
Once the CAM file is perfected the final wax master part is cut. In this case, two side-by-sides are cut.
Properly using casting resins and molding rubbers requires the use of an accurate scale to mix by weight.
The wax master mold and the two tins containing the very thin RTV rubber which mixes at about 2 to 1 by weight.
For small molds it doesn't take much rubber so VERY accurate measuring using a very accurate scale is required.
Though the mixing is done with a non-aerating electic mixer, the rubber is still poured in a thin stream to "pop" bubbles before reaching the mold.
As a final precaution, a bamboo skewer is used to make sure no air bubbles are trapped in corners. A vacuum pot can eliminate this issue.
Thin Plastruct "T" is used to form the basic mini-longerons.
Once glued into their slots, the "Ts" are milled to the proper thickness, length, and height. We're talking sub .01 and .02 overall dimensions.
A collection of some of the masters and molds used for making some of the various prototype parts.
After a few test molds and masters, the proper configuration for the bottom mold half is determined
A few of the finished resin moldings taken from the preliminary molds. One of these will be used to make the hard rubber injection mold.
A closeup of a "reject" (but still pretty good) prototype bulkhead.
A preliminary master mold and the resulting rubber mold for the longerons and antennas. Overall length of mold is only about 3".
Prototype molded longerons and antennas. From this point, a hard rubber injection mold is made.
L to R. Prototype body molding, centrifugal casting mold, and wax master.
"Family" shot of the components of the centrifugal mold and the prototype and wax masters. The yellow component is the rubber mold insert.
VIDEO Milling the Prototype QTV Thrust Bulkhead
For those of you who are interested in what an inexpensive mill can do for you and your level of scale modeling, check out these videos. The video is a series taken during the testing of the first prototype of the bulkhead using the first G-code file (for you CNC machinist types) to run the part. The code was later modified to be more efficient and eliminate unneeded or redundant moves.
Scale Projects-1/72 QTV Little Joe II
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