Recreating the RA brakes

[Doug]

One of several missing components from my RA project were the distinctive brakes that were a signature feature of the model. While Douglas might have won a Senior and Sidecar TT with machines fitted with these brakes, the Works team only used them for two years and then moved on to a drum brake for the 1925 racing season. Numerous pictures of RA machines repurposed for cinder track racing show brake shoes and linkage removed or the entire lack of any brake components. Consequently, surviving brake hardware for the RA is rarer than the machines themselves. I was able to borrow a rear brake shoe and mechanism that looked like it had been removed unused from a machine. Yet another LDMCC club member had portions of the front brake mechanism and a rear brake ring. Work began on reverse engineering the hardware; as well as examining what was available in period photos or remnant with the few other surviving RAs. It became apparent that like the frames, there were early and late variants of brake hardware. As my RA is DF186 and fairly late in the scheme of things – '3rd variant' frame – it would not have the earliest version of the brake hardware like what would be seen on, for example, the Tom Sheard bike (1923 Senior TT winner). Most likely, being an Australian export circa 1925-26, it was sold directly for use on the cinders and never had brakes fitted. Much was missing and the entire bike will be pieced together from leftovers and numerous donors so we will never really know. For road use brakes, even mediocre ones, would need to be fitted.

Inner operating arm, rear brake. The initial design was a shaft and an arm permanently assembled. I think through silver brazing, though I was never able to determine the exact joint interface. Then superseded by an arm forged as a single piece.





Had my RA been fitted with brakes it would have had the later. For small quantities of reproductions, milling from billet is more practical than making forging dies, albeit wasteful of material and more time consuming. Even so, with some careful nesting the use of material can be maximized; though it meant a lot of milling. It is not as if I had to stand there at wait as the CNC machine was perfectly able to run itself unattended. The alternative to cutting out the blanks with a narrow milling kerf would have been sawing, but the method chosen had an advantage of work holding and uniformity. The billet was tack welded to a sub-plate, then milled through all but the last millimeter of thickness. Sawing would have yielded a narrower kerf, but it is not as if it would have gained five rather than the four blanks per slab size that I had to work with. Another alternative would have been waterjet cutting, but that would have entailed sending out, and I have had trouble in the past with residual abrasives embedded in the surface wreaking havoc with the cutters. Additional milling operations took it from a rough to finished shape.



Slab of raw material tack welded to a sacrificial sub-sheet to provide a means to hold the work.



Milled down to the level of the sacrificial sheet. Four operating arms and four islands of waste.



An original operating arm sitting on several blanks.



Twelve operating arm blanks; in case of attrition. The blanks are now surface ground to aid in uniformity of subsequent workholding.



Set vertical in the milling machine vice, with a fixture to support the operating arm (already drilled and reamed.) Preparatory to roughing the arm profile.



Arm profile rough milled.



Laid down on the milling table, preparatory to surface milling the contours of the arm.



One half of the surface contour of the arm finished.



Sequence of operations from rough profile to half surfaced, with original are the front.



Now flipped over and the second side finished, taking care to align both sides. The fixture/stud on the left remained clamped to the table and provided the registration from part to part, and both sides.



Again, sequence of operations.



Side by side comparison. Note faux forging flash under the flange of the eye to simulate the residual forging flash not entirely filed away on the original. As the original was hand fettled, the new reproduction are much more uniform in shape.



Preparing to turn the shaft.



Turning the shaft. Yet to do is the step where the square taper will be milled, and the threading the tip.



Done, except for milling the tapered square for the outer half of the operating arm.



I did not have an example of the outboard operating arm, so this had to be estimated from photographs. A plastic model was made using 3D printing technology, which was then posted over to the I.o.M. for a final check against the one on the late Bob Thomas RA outfit. Once the size and shape were validated, manufacture was similar to the inner arm; milling from bar stock in several setups to attack the workpiece from all sides. One deviation was that the small post for the return spring to hook onto was originally gas welded to the main forging. I opted to mill it all from one piece because I did not fancy welding thick and thin sections together and at the time I was in a 'lights-out' machining craze of getting the job started and then going to bed, so did not care if it added several extra hours to the job.



Initial work piece blank.



Faced, and tapered square hole added.



Turned on its side and the cup for the brake rod machined.



Turned again, preparatory for roughing the arm.



Roughing in process.



Roughing complete.



Finish pass complete.



A fixture to do the second side.



Work mounted to fixture preparatory to milling second side of arm. A square, tapered plug is fitted to the square tapered hole in the operating arm  under the bolt head to the right.



Roughing in-process.



Finish pass complete.


Probably the most technically interesting aspect was cutting the internal, taper square that connected it to the inner arm. I am not certain how they originally made this, but suspect it was hot punched, much like the later conical spline used on EW kick starter levers and subsequent models. Whatever the method, the challenge with machining would be getting a sharp internal corner. Electrical discharge machining would have been the ideal method, but I no longer had access to that type of equipment and did not want to pay the going rate to farm it out. So, I bought a Ø1mm endmill that had a 14mm reach.



Finish pass complete.


I bought two actually as insurance, in case the first one broke. As often typical with insurance, it was never needed. This was used to clean out 'the corners' yielding a 0.5mm radius which was sufficient to clear the corner break on the male portion of the taper. This delicate cutter did not do all of the work alone. The bulk of the material was removed with an 8mm pilot hole and larger diameter endmill cutters. The secret to success was a shallow depth of cut (0.15mm) and an aggressive (for the diameter) feed rate of 25mm a minute. This made sure the cutting flutes cut rather than rubbed, and combined with the monotonous, uniform feed rate of a CNC machine allowed one cutter to last the entire run of outer operating arms. I should add the new brake arms were made from heat treatable stainless steel, and stainless steel is prone to developing a hard skin through work hardening if the cutter rubs; either because it is dull or through too slow a feed rate. This hard skin causes the cutter to deflect, which causes it to rub and work harden more rather than cut, and pretty soon it deflects enough that the cutter breaks. While I had learned this approach years ago with micro endmills (Ø0.5mm and under) I had never tried it with one this long and slender.

I subsequently found that such a fine surface milling on faux forgings was not entirely necessary. A courser finish could halve the milling time at the expense of a little time spent at the bench with a mold and die polishing stone to smooth out the milling marks. If you have ever spent tedious hours rubbing parts with little bits of sand paper try polishing stones. They work so much faster that they are worth the extra cost.

To be continued...

-Doug