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 Then superseded by an arm forged as a single piece.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

[Doug]

RA Bit by Bit

The frame for my RA project was altered extensively to an approximation of the later DT model.  Missing from the rear lower chain stay was the anchorage lug for the distinctive RA brake system. The lug was subsequently used on the OB model when it came out in 1924. In fact, so cleanly has the lug been removed it appears as if was never fitted. Judging from period photos, into 1926 and '27 RAs were being supplied without some or all of the brake components as the former road racing machines were being sold stripped down for the rapidly growing sport of dirt track racing. Yet where it can be ascertained, they all seem to have that particular lug present. Indeed, it continued redundantly to be seen on early DT frames.

And I needed one. Originally these would have been a steel casting or perhaps even a forging, but for the sake of replicating one plus spares, milling from billet was more practical.


It starts with a block of steel.



Pivot holes and their faces (as counterbores) from the outer face; that side facing away from the tyre.



Stood on end and the hole for the chain stay tube bored.



Flipped over and the faces of the pivots counterbored from the inner face; that side facing the tyre. The smaller hole for the reaction pivot has no counterbore as it is flush with the billet surface



Rough pass, inner face. Note cut has broken through to the counterbore from the opposite side.



Second rough pass with ball tip cutter.



Finish pass.



Thread milling.



Checking with homemade thread go/no-go gauge.



Production!


The first side was relatively easy as there is a square block to grab in the milling machine vise. The second side required some fixturing to hold the part.



Sawing away some of the excess material.



The fixture.



With workpiece installed. A crossbar is passed through the bore for the chain stay tube.



Rough pass.



Second rough pass..



Third rough pass. As this setup would not be as rigid as the first side grabbed directly in the vise, the roughing was done in three stages rather than two.



Finish pass.



All done.


To install over the frame tube, I will either need to split the lug or de-tube the chain stay from the frame. That will be a big job for another day.

To be continued...
-Doug

[Doug]

There are a number of smaller components that make up the RA braking system. All of which were missing from my project. One of these is the link that gives the rear brake shoe a parallel motion as it engages the brake ring. One end is anchored on an eccentric to allow adjustment of the angle of the shoe. The first batch of eccentric bolts for the rear was based on the example seen on the Bob Thomas RA outfit. This turned out to be wrong as that was the 'early style' and my RA, if it ever had brakes, would have been the 'late style'. Oops!


Early style rear link eccentric pins. Eccentric split collet on right for turning and threading the stud offset from the pin journal.


Early style rear link eccentric pin and link on the left. Later style on the right.

The rear link is a made from folded bit of thick sheet steel. The one hole is quite close to the bend, so to avoid distortion – and also to get the holes aligned – drilling and reaming is done afterward. In the next picture, an original link can be seen on the right adjacent to sequential machining operations.


Sequential operations.


Fixture for profile milling the links; one side at a time.



To be continued...

-Doug

[karri]

Some lovely machining there Doug. Id kill to have a cnc at my disposal, but ill have to keep hacking away at it all manually. Thankfully havent had to take on something this intricate yet.

[Doug]

Id kill to have a cnc at my disposal, but ill have to keep hacking away at it all manually.

Well you can do a lot with a hacksaw and file. Most of the sculpted shapes here were forgings filed and sanded by hand and eye; likely by an apprentice. They did not need to be a precise surface milled to within 0.001 of an inch. The CNC mill is a convenient tool to do this sort of thing, but admittedly overkill and it could be done without. I still had to sand these by hand with mold & die polishing stones in lieu of additional time machining at a higher fidelity. At some point of diminishing returns it is quicker to stop milling and fettle it by hand.

[Doug]

The front brake for the RA is smaller and of less surface area than at the rear to befit the prevailing practice to avoid having the front tire skid on the loose road surfaces of the time. All that was available in original components to directly measure was a brake arm, the parallel link and its eccentric pin. Curiously, the link had been intentionally bent on an pronounced arc, though a study of period photos showed it was supposed to be straight. The brake arm formed the second link of the parallel motion and provided the correct hole center distance. As for machining new ones, it was fairly conventional work.



Long front brake arm blank, step milled to thickness. The concave ends to the steps is a result of using a face mill, and will be obliterated by subsequent milling.


The blank mounted on its corresponding step fixture. The hold down screws are standard button head hex screws with the head turned back to a cone for more clearance while milling.


First though, a profile cut. I do not seem to have taken a picture of the rough and surfacing steps of the elliptical sections between the bosses.


The fully machined arm, still in the straight condition. As the arm is symmetrical, it simply needed to be flipped over on the fixture to surface the second side.


The bending fixture. Heat is applied and the arm coaxed around the first bend with a hammer and soft aluminum drift. Then the block for the tip of the arm is bolted down to the fixture bed plate and the reverse bend executed. I found that I could not heat and hammer the arm hard up against fixturing to get the correct form. The fixture had to be cut back more so as to go past the point needed to allow for workpiece spring back. This fixture got it close, with some fine tuning at he bench vise to make the offset parallel.


A pair of finished arms with some of the hollow pins the RA used.


After making a batch of six brake arms and doing a trial assembly, a major blunder was discovered. The brake arm copied could not possibly be correct for an RA and a little forethought on my part would have revealed it was too long to line up with the brake cable running down the rear of the fork girder. The correct arm turned out to be (surprise) the same as used on the OB and CW models; though those used a dummy belt rim brake of different diameter. Eccentric pins and studs also appear to be shared between models. The sample I mistook for a RA front brake arm turned out to be a CW rear brake arm. I had machine the longer arm in the straight condition, then bent to the requisite joggle. Not only was that a bit of a pain in the arse, but I had doubt I would be able to pull it off on the shorter arm with its more aggressive bends. So, second time around I machined the arms with the joggle in-situ.



Roughing the steps on the first side from a block. The little triangles along the vise jaws are a form of inserted teeth for gripping the work securely by a very small shoulder. I bit of fancy tooling I was trying out at the time.


Roughing the second side and drilling and reaming the holes.


One blank at each stage for comparison.


Which in turn is bolted to its own custom fixture.


Profile cut.


Rough and finish surfacing cut.


First side done.


Comparison of the long (incorrect) and short brake levers.


Second side needs its own fixture.


Rough and finish surfacing cut.


Before and after, second side.


After heat treatment. Material is 17-4PH stainless steel.


Next were the pivot stud to make. Thread gauges had to be made and shipped globally to the interested parties to test in their girder fork lugs. Half those surveyed used a pitch diameter 0.006" over standard, and one had a pitch diameter 0.003" over standard, the remainder being standard pitch diameter. Why the variation I do not know. The 0.006" certainly seemed excessive for general manufacturing tolerances, and this is the only case I have encountered on a Douglas where there were various 'oversized' threads.


RA front brake anchorage lug with homemade thread gauge inserted.


Some heat treated posts and their nuts & washers. Note back end of post is drilled hollow to save weight.


Group photo.

The front brake arm forms one part of the parallel link mechanism, and a small link forms the second link. This too was milled from solid, in this case using a convenient size of round bar for material. One side was faced just to have a flat surface to work from, and then counterbored in depth to the face of the bosses. The opposing side could have the bosses faced with a face mill cut. Between the bosses needed to be relived as well, and pocketed in. Once profiled, it would no longer be a pocket.


The blank mounted on the fixture. The surfaces for the bosses top and bottom are already cut.


Profiled, and the step between the bosses cut. The step in the fixture to support the work level can now be seen.


Incremental steps. Top and bottom surfaces of the 'blank' and a finished milled link below. The second side of the link requires no further machining as the step down between the bosses was already cut with the pocket seen on the right.


The last step was to turn down the waist of the larger hub, to save a fraction of an ounce. The setup is not shown, but was simply mounting the lever on an arbor in the lathe and reaching in with a hook shaped tool, avoiding collision with the whirling arm of course!

To be continued...

-Doug

[Doug]

An overall look at what is being aimed at might be useful at this point. Component manufacture of the linkages, shoes, and brake rings have been covered in previous installments. Here is seen an original example on the Bob Thomas outfit.



Note the use of hollow pins and split cotters rather than heavier bolts and nuts. Below is the replica.





The threaded post (pivot for the lever) screws into the lug on the girder blade and is for adjusting the shoe axially to align with the friction disk. The small peg on the parallel (upper) link is for changing the angle of the shoe as it presents to the disk, so the leading edge does not dig in and self-energize. This was still very much in the era where an overly powerful front brake was considered a liability on loose and slimy road surfaces, and the front shoe is significantly smaller than the rear.

Next is an original rear brake on the same aforementioned bike.



This shows the 'early' style brake arm and eccentric pin which I did not copy as my RA is closer to the end of production than the beginning.



Or that was the plan. I inadvertently made a batch of the eccentric pins before I realized they were not correct for my project.

Then we have the replica shoe assemblies.





The brake arm lacks the large hub as it is now (originally forged) one piece. The eccentric stud now has the hex head inboard of the parallel link. Less convenient to adjust, but uses slightly less material and so save a gram or two of material. Axial adjustment is via the bushing for the operating arm being threaded on the outer diameter where it screws through the lug on the lower chain stay.

The variation in hue of the stainless-steel parts is due to some being bead blasted, some in the process of sanding with mold & die-stones, and others still in the as-machined state.

The final pic shows front and rear together for a size comparison.   



To be continued...

-Doug

[Jim]

 doug good post, very interesting, thank you Jim

[Doug]

The RA brake project required aluminum alloy castings in the form of foundation rings for the friction material and the brake shoes themselves. A fairly complete rear shoe assembly in Australia was on loan and copied. An original sample of the front shoe could not be borrowed, and it was reverse engineered with the combination of photos and field measurements of the one on the Bob Thomas outfit on the I.o.M. The shoe patterns required a core for where they fit down over the brake rings. The vee-shaped valley could have been machined out from solid, however there is an undercut at the root of the vee that would have made this difficult and tedious. Besides, it was hoped that the braking surface would cast close enough to the correct shape that a bit of sanding would suffice in lieu of machining; the rest taken care of by running in.

Like the rest of the brake hardware, the shoes were modeled in 3D CAD software. It was short step to go from that to creating the patterns and core boxes on a 3D printer in a UV cure acrylic material; primarily because I had access to a printer and wanted to see what it could do. The traditional method would have been to machine the patterns from wood. The pattern match board did one front and rear shoe per flask.


The front and rear shoe pattern mounted on a pattern board, but prior to applying gating. The reverse side of the board had the other, identical half of the patterns glued to it.


The core boxes for the front and rear shoe.

A local foundry did the aluminum castings, but they were more familiar with casting small brass hardware and not aluminum and the larger gating required for good flow. As a result, the first batch had to be scrapped and done over. At double the cost. At the same time the foundation rings were cast. The pattern for this was quite simple and just a loose ring laid on a pattern board and hand gated. These did not have to be castings as they are machined all over, but it was more efficient to cast rings than search for a large, heavy wall pipe to machine rings from, or alternately trepan or plasma arc cut them out of heavy aluminum plate.


Raw cast foundation rings.


Raw castings, front shoes and (right) rear shoes from the 2nd attempt.


What happens when the foundry forgets to insert the core into the mold.

The castings were then sent out for Hot Isostatic Pressing (HIP) and then solution heat treatment.  The HIP process involves heating the castings slightly below the melting temperature and then exposing the castings to a pressure of 200MPa or more, usually in an argon atmosphere. Internal porosity that might exist gets squeezed to nil, greatly enhancing the fatigue strength.

The brake shoes required very little machining, just a slot, face, and drill operation; though some substantial fixturing. This comprised of a steel plate held on edge and machined to the form of a segment of the brake ring. Since the rings are different widths, one fixture was needed for the front and one for the rear shoes. The same fixture with engineer's blue, was used to spot the castings and and a four-inch diameter right-angle grinder used to sand down the high spots until the casting fit down upon the fixture to the correct depth. The high spots were primarily at the ends where shrinkage caused the casting to curl in to a sligtly tighter radius.  A pair of clamps wedged the shoe onto the fixture, quite firmly, for machining. 


One of the brake shoe machining fixtures.


The fixture in use. The odd shape to the clamps is because they can be flipped over and used on either fixture to clamp a front or rear shoe.


Though not a lot of machining in the slotting and facing operation, after two dozen castings the chips start to pile up.


A partially machined front shoe next to an original rear shoe.

The same fixture on its side held the brake shoe for drilling and reaming. The ears had to be chocked with adjustable parallels and snubs with a clamp to prevent movement.


Drilling and reaming.

-Doug

[Doug]

Machining of the foundation rings for the RA brake project was more involved than the brake shoes. For something seemingly simple, it required a lot of operations and some head-scratching as when it came to figuring out the lacing pattern of the spokes. The lathe operations were straightforward enough, other than the first turning operation was actually done on the milling machine. It was more convenient to clamp it down to the mill table and interpolate the initial surfaces with endmills than trying to clamp and center it on the lathe faceplate, even though the milling took much longer than turning in a lathe would.


Raw foundation ring castings. Hot Isostatic Pressed and Heat treated.



Raw casting clamped to the table.  Shimmed to stop it from rocking.



Inner diameter milled.



Clamping transferred to the inside so there is access to the outside.


Once reference surfaces were cut, it could be transferred to the lathe. Now the challenge became avoiding squeezing the ring out of round. Large jaws had to be made for the 3-jaw chuck to fully encompass the outer diameter and the hydraulic chuck pressure set to the minimum and clamp the ring uniformly.


Chuck jaw design. One jaw inverted to show the standard serrated industry interface. 



The rough, flame cut plate steel blank for a chuck jaw segment.



Milling the pad for the chuck interface as well as some of the profile.



Serrations cut.



The three segments bolted to the lathe chuck.



After turning to mate to the brake ring blanks.



A semi-machined brake ring ready for the lathe operations on the face and inner diameter.



And after turning.



It was a bit messy, just remember to shut the door...





The foundation ring is laced to an outer, auxiliary flange and then shares one wheel rim spoke flange on the lefthand side of the machine. The shared flange is a very busy place for the heads of the spokes. Spoke clamps, like used with a dummy belt rim brake ring, also support the foundation rings.


A rear brake showing the plates clamping the back of the ring to the spokes for additional support.


The wheel rims are drilled for thirty-six spokes. Between the eighteen holes on the lefthand flange for the rim, are drilled nine holes for the foundation ring spokes. Nine more are drilled in the auxiliary flange. See the problem yet? Like the wheel spokes, the brake ring spokes are laced tangential, with crossed pairs. Yet how do you achieve crossed pairs with an odd number of spokes? Examining period photos was not easy as even in the best situations a portion of the pattern was blocked from view by the frame or fork girders. What could be seen and interpreted showed this was a puzzle for some even back in the day. 'Orphan' spokes and an odd-man-out spoke laced radial were used to span the gap between crossed pairs.

Period images:




 

 

 

Much experimentation on the computer and a physical mockup with lacing patterns was done to try and get something that looked like the photos, but it always came back to the immutable fact that nine is not an even number.

Finally, after putting the problem aside for a while (months) there was an epiphany.  Six plus three equals nine. Six gives three crossed pair, and leaves three orphan spokes to be distributed equally around the pattern. Those there are tangent, and oriented to purely supplement the braking reaction. Suddenly the apparently odd mix of double and triple crossed spokes in the pictures made sense; there was a pattern hidden in plain sight. 


Spokes of the inboard flange only. The fact that the rings are fully machined is a giveaway these photos are staged for the purpose of illustrating, well after everything had been figured out and done.





Inner and outer sets of spokes.





Judging from originals, the spoke laying on top of another was bent down at the crossing so the head was no longer than any other spoke.



The  progression around the brake foundation ring. This also had to be timed to the wheel spokes, and the spoke clips landed ion the correct place.



Busy, busy, busy! The spokes do not have any bends in this view to clear each other, so you do have spokes intersecting each other.

While the foundation ring spokes are tangent, the holes drilled for the spoke nipples were not. This was revealed by an original rear ring.


The large, spherical, countersunk holes are for the spoke nipples. The through hole is radial, not tangential. The holes between the nipple holes are for wood screws driven radially out into the friction material. On the top face are the tapped holes for the spoke clamp plates.


Looking at the brakes on the Bob Thomas outfit and the Tom Sheard bike one can see where the spokes are bent between the (more or less) radial nipples and the spokes assuming a tangent trajectory.


The ex-Bob Thomas outfit.



The ex-Tom Shead machine.


As an aside, the Tom Sheard bike is an example of an early hub where the auxiliary flange is scalloped to save weight. This weight saving step was discontinued and the flange became continuous. A final variation seen is where the hub flanges are not drilled for the brake ring spokes at all. Presumably used on late machines built directly for the dirt track, and not fitted with brakes. 

Somewhere around this point the holes for the spoke clamping plates were drill. The clamps appear to be standard 2-3/4hp and OB issue. Those have to be 'clocked' at the correct relative position to the foundation ring spokes, so that they fall over the positions that the wheel rim spokes cross.


 

Next the foundation rings needed holes for the spokes and the wood screws that secured the friction material to the ring. Typically, this would be done in a mill/drill with a dividing head to position/index the ring at the desired angle. The CNC lathe that I have has a built in capability to mill and drill, so that was used instead. First a faceplate to secure the rings to was required. A scrap VW diesel Rabbit (Golf in the EU) flywheel provided the necessary base.


Preparing the VW flywheel in an engine lathe (1942 Lodge & Shipley).



Mounted in the CNC lathe. Note the spoke plate holes have already been drilled in the face of the brake foundation ring. The ink marker line on the periphery of the ring and faceplate ensure that they will be 'clocked' to the spoke holes properly.



The milling head. One for the drill (shown) for the wood screw holes, and another for a ball tip end mill to machine the countersink for the spoke nipple.



 

Video of the preceding machine operation. (19Mb)



The finished stack of rings. Note the nipple holes have yet to be drilled.


I could not continence bent spokes where they exited the nipples, even if it was authentic; and surely it made lacing and truing that much more difficult. With only a slight amount of experimentation, the correct angle and amount of offset to get tangency was arrived at. It did not need to precise to three decimal places, aimed in the general direction was better than Kingswood achieved.

Update. A shorter version of this article originally appeared in the Sep/Oct 2023 issue of the New ConRod. Since then, information from another RA restoration has come to light that steel cup washers were fitted under the heads of the spokes. This allowed some angularity of the spoke nipples through the oversized holes of the brake foundation ring. Not quite enough it seems to allow them to lie perfectly tangent.

The computer provided the proper angle to tip the head of the mill/drill slightly to aim the spoke at either the inner or outer spoke flange as appropriate. Offsetting the spindle from a radial position provided the angle for the tangency.

It ended up being rather simple with a fixture and a ball stylus in the drill chuck to align the spherical socket for the nipple.





With the ball tip stylus seated nicely in the spherical countersink, the clamps are nipped up to hold the ring in position.



A drill is substituted for the stylus, resulting in a hole aligned with the spherical countersink and at the proper angle for the spoke.


The tricky part was numbering the positions and keeping track to drill the correct pattern of in & out and right & left. A case of check, double check, and check again!

All that remains to describe is the countersink for the heads of the wood screws between each spoke that secure the friction material. As these are on the inner diameter of the ring, a reverse countersink is used. These are quite common in the aircraft industry, but in the USA, are standardized on a 100 degree head angle. So a commercial purchased cutter was re-ground to 90 degrees, which seems to be what Kingswood used.


The mandrel for the reverse countersink poked through the hole. The odd cuts at the bottom are a bayonet fitting to capture the cutter head and drive it. This actually preceded the drilling of the tangent holes for the spokes (note their absence.)



The cutter head installed. Ready to drill UP.






All done!



Image credit: Martin Fridel, Austria. On his RA with a set of new replica brakes.



Rex Judd astride a Works RA.


This concludes this thread.

-Doug