The ED Baby
This is part one of a four part construction series. Part Two will detail machining of the major crankcase components; Part Three describes the cylinder and piston assemblies, while Part Four presents the crankshaft and remaining pieces. Each part is accompanied with a single page of CAD drawings. We begin this month with the last page of the drawings that contains the General Arrangement. This is an important part of our CAD engineering process as it is put together using sectionalized parts cut and pasted from the other drawings. If it goes together well with all the important dimensions lining up, then parts made from the drawings should do likewise!
The drawings use European projection conventions, so may look a little odd at first to North American readers. The convention used for part numbering is worth a word, as well. It is derived from full size aircraft practice, specifically that used by the late John W Thorp in his T-18 Sportplane and other designs. Each sheet has a number. Parts depicted on the sheets carry "dash" numbers starting at one (1) on each drawing. Mirrored parts have a "L" and "R" suffix on the dash number, although there a generally none of those on a single cylinder model engine! Joining the two, we get a unique identifier for each part that also tells us which sheet number it appears on. For example, the general arrangement calls out the Crankcase as "1-1". This identifies it as part dash-one on sheet one. Some sheets contain notes. When these refer to parts on the same sheet, the sheet number is omitted so you know you don't have to go chasing to another sheet.
The drawings use imperial measurements. Non-critical sizes and "stock" sizes are dimensioned in fractions, down to 32nds of an inch. You should work as close as you can to the dimension shown, but if you miss by 5 or 10 thou, this can be compensated for elsewhere. Where three figure decimals are given, this either means you should work exactly to the dimension, or that there is no close enough measurable fraction we could apply. In such cases, the construction notes will give a way of arriving at the required tolerance. For example, the 3-7 Needle valve shows the point of the needle as being 0.425" from the face of the thimble. Rest assured, we'll present another way to achieve the correct line-up! Close tolerances and fits are not specified on the plans, but will be described in the construction "shop notes".
Below the identifier for each part is the material it should be made from and any required treatment. Screw and thread sizes are shown using British Association (BA) sizes as used for the original, but in all cases, equivalent American UNC/F threads are also shown.
The MaterialsThe crankcase for the Baby is rather complex for such a small engine and was die cast on the original. For modern builders, we are offering a casting set comprising the crankcase and backplate, sand cast in a heat treatable aluminium alloy. Alloy aluminium is easy to machine, especially if you squirt a little kerosene on it. Pure aluminium is not nice to machine. It comes off in hunks and tools tend to dig in. Surface finishes are bad.
The needle valve and spray bar are made from brass rod. This is available in small quantities from the K&S rack at your local model store. Free cutting brass cuts like butter and will finish out beautifully. A few well finished brass components will make your engine look good and you even better.
For the steel components, we recommend leaded steel stock such as 12L14. This is a joy to machine and well worth the investment. Small model engine components do not usually have to be extra strong. This material is great for cylinders without any special treatment. Fine grained cast iron pistons in leaded steel cylinder will wear well. Optionally, you can make the pistons from 4140 steel.
Building a crankshaft from nickel steel, or drill rod can be a real pain and a waste of time. Make it of leaded steel and case harden the crankpin. Buy a can of Kasnit Surface Hardening Compound from Blue Ridge. With it, and a propane torch, you can harden the surface of mild steel parts like cams and drive washers.
Good materials can be mail ordered from MetalExpress, N-15 W 22218 Watertown Road, Waukesha, WI 53186, telephone 800-657-0721. Also try Cardinal Engineering at RR 1 Box 163, Cameron IL 61423. For Mehanite (a trade name for fine-grained cast iron) try Commercial Ironworks, 14933 Whitram Ave. Fontana CA 92335.
The WorkshopThe engine can be easily constructed on any small lathe that has screw cutting capability. Naturally, additional workshop equipment, when available, will simplify some operations and reduce set-up time, but is by no means essential. The prototypes were made on a Myford Super 7B lathe. In US terms, this is a 7" center lathe, though the British, measuring from the axis, would call it a 3-1/2" lathe. The workshop is also equipped with an Asian, geared-head Mill/Drill, fitted with three axis digital readouts. One of the most useful accessories complementing this pair is a 6" rotary table fitted with a Myford style screwed center spindle that permits lathe chucks, complete with work, to be transferred back and forth between lathe and mill, in either vertical or horizontal orientation. A simple drill press with V-jaw vice, plus hand tools, soldering equipment and bench grinder completes the common workshop inventory.
The Workshop NotesWhen building anything, the starting point and order is generally arbitrary, but usually there is a definite sequence that will make life easier and more fun. For this series, we are trying a bit of an experiment in the machining instructions. Each part will be detailed under a common set of headings. These identify any part, or parts that should be made first, the tooling required and any specific things to watch out for while making the part (ie "gotchas"). Next follows a step-by-step procedure in workshop note style format. These notes employ a set of standard abbreviations:
|3JSC||Three Jaw Self-Centering (chuck)|
|HSS||High Speed Steel (tool steel)|
|4JI||Four Jaw Independent (chuck)|
|#||A number series drill size|
The part with no dependencies is the starting point. Next issue we begin on the Crankcase components, but the 1-1 crankcase has two dependencies; the 1-2 Back Plate and the 3-4 Venturi Insert! So, we conclude this part with an extract from Sheet 3 of the drawings and an example of the instruction format to prepare the way for the next instalment.
This part can be considered optional. In fact, not all original engines featured it! It does make the engine look nice, but complicates the crankcase machining procedure as it should be fitted after the cylinder thread is cut, but before the crankshaft bearing is reamed. However, boring for the liner and cutting the head thread should be done after the bearing is reamed and fixing the insert in place makes screw cutting the thread for the cylinder head close enough to impossible! We will present a sequence that side- steps this, but you should read the steps closely and if in doubt, just omit the insert altogether.
- 3-4 Venturi Insert.
- Things to Watch:
- Tooling Required:
- Sharp corner knife tool, part-off tool, #1 and #2CD, #36 and #33 twist drills.
- Chuck a length of 1/4" diameter aluminium bar stock in the 3JSC chuck with minimal protrusion. CD #1.
- Extend to 3/4" protruding stock and turn down to 0.149" diameter for approx. 7/16", leaving a square shoulder. Use kerosene, or Kool-Tool (the blue stuff) as a lubricant when turning or drilling aluminium.
- Drill #36 to a depth of 5/8", followed by #33 to the same depth (drilling initially 3 sizes under ensures the hole will end up close to the second drill size).
- Clean-up the OD to about 0.216" for approx 3/16" from the shoulder, then part off 1/8" from the shoulder and reverse in chuck, gripping lightly as the walls are thin.
- Lightly chamfer the edge at 45° and round off with a Swiss pattern needle file, followed by 600 wet and dry paper, wetted with kerosene. A finger or piece of balsa wood makes a nice form block for blending and polishing rounded edges like these.
- Countersink the inlet hole with #2 CD for approx 1/32" deep making the throat of the 60° opening about 0.15" in diameter (use the shank end of a #25 drill as a gauge).