Preparing to Machine


Starting any major new machining project can be daunting. While this page is not descriptive of any portion of the Wren turboprop, it is designed to help newer machinists approach such a task.

The turboprop plans come as two bound booklets, a set of prints and an operations manual. The prints, of course, are standard machinist's drawings of the individual parts, and critically, the subassemblies, which detail just how everything goes together. The operations manual shows you how Mike Murphy, designer of the turboprop, goes about machining a specimen from start to finish. The quality of the Wren ops manual is superior, and while technically just the prints are needed for the project, the operations manual shows typical setups, and importantly the order in which the individual parts are to be machined.

If you have never attempted a project of this scale, I highly recommend you follow Mike's op sheets. Obviously, if you have a vertical mill, put it to work where appropriate, but in general it is tough to go wrong following Mike's directions. If you are an experienced machinist, the following may be redundant, but I post it here as one way to tackle a complex project.

First, before you turn one gram of swarf, study all of the prints, the ops manual, the subassemblies, etc., and have a firm grasp in your mind exactly how the turboprop parts relate to one another. The tolerances of critical parts on the print are certainly useful, but producing parts in an order such that part "A" can be used as a guage for part "B", makes the project simpler.

If you alter ANY component from print (NOT recommended), be absolutely sure you know how it will affect mating parts. For example, I already know that I am going to forego part 159, the cowl. Why? Because I like seeing neat bolt circles of stainless cap screws on the front of the gearbox, and it will allow me to sneak the prop shaft through a smaller hole in an airplane fuselage! This is purely a personal choice. But eliminating the cowl will force me to alter the Gearbox (155: constant OD forward of the rear bevel), Gearbox front (157: increase OD to match 155), the Prop shaft housing (158: add bearing retainer), and the prop driver (160: alter rear portion to better fit the prop shaft housing). So you can see that alterations have a cascading effect. Again, do no alterations unless you are sure that strength and functionality will not be affected.

Imperial vs Metric. Sorry, but I am still stuck in the Imperial world. All of my tooling, especially an extensive set of taps and fasteners, are imperial. Where appropriate, I am going to alter threads to imperial equivalents. Some examples are M2.5 becomes 3-48 (near perfect match), M3 becomes 4-40 (4-40 slightly smaller). In all cases, recognize that if the imperial equivalent is slightly larger, you must ensure that adequate material remains to accept the threaded hole. Where the imperial fastener is smaller, be sure that it is adequately strong for the task.

If you scrap a part, don't fret, start the part over again. Don't be satisfied with substandard parts. At the same time, if a dimension is non-critical, (example: 14mm OD for part 163, the prop shaft), don't spend 3 hours getting that OD to exactly 14.000 mm. Spend the time working the bearing journals to perfection instead. There isn't a machinist alive who doesn't have a scrap bin full of goofs. Learn from it and start over.

Finally, be persistent. Approach each part as a miniature "project" all by itself. If the prints are accurate, when you are done, you will have a "no-machining" kit of parts! Don't lose sight of the goal.

And I must repeat the mantra again. Study the prints. Study the prints. Study the prints.........

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