Thursday 29 November 2012

More Progress with Forks


I was expecting to have to get the blowtorch out to help remove the twist from the blades, however, the torsional stiffness turned out to be quite low so no heat was needed.
 
Twist Removal
To minimise damage during the untwisting process I knocked up some tooling to hold and twist the blade.

Twist removal tools
The wooden blocks where used to clamp the blade in the vice, while the alloy plate with arm attached applied torsion.

The processed was surprisingly easy and was finished off with straightening out a lateral bend and a couple of passes through the profile forming tool

Having achieved a reasonable result for the basic blades I now took the plunge and attacked the original forks with there welded on blades.  These turned out to be surprisingly easy to remove.  To begin with I simple sawed off the old blades below the weld line. Then being able to see what was inside (bits of squashed tube join re-enforcing material), I made my way up towards the crown, cleaning away as much of the old blades as I could without damaging the crown. With hacksaw and chisel I split each blade vertically into two parts and then using my blow lamp applied heat.  With the blade starting to glow red I carefully hammered and chiseled the joint to separate the blade remains from the crown.  This turned out to be much easier than expected.
First fit of new blades

 With a bit of a clean up, trimming the blade ends and addition of 1/8" pins it is starting to look good.

Fork Crown Revealed















 How to form the blade ends?

This has been puzzling me for a while, should I make inserts or should I attempt to flatten the ends of the blades.
The worry with the later is how to ensure the finish is neat and professional looking.  With the old fork blades in my hand inspiration struck - why not section them and see if there are any clues.

Lo and behold question answered:


 The old fork blade has a neat rounded transition to the flat section.

Close examination showed a third layer of metal sandwiched between the two flattened sides of the blade.

In this photo the insert has been removed leaving a gap





Cutting the blade lengthwise revealed that the interleaved layer extended into the blade and was profiled to support the blade while the end was flattened.

The sectioning made it difficult to see exactly how the insert was profiled.







Fortunately I had two blade so could open up the other one more carefully to extract the insert, shown here

The material of the insert is much harder than the blade itself so with an appropriate outer forming tool the blade end can be flattened while the insert prevents unwanted squashing occurring




















Monday 26 November 2012

Fork Blades Bend at last   

Having done loads of sums and a lot more thinking I decided that a more traditional pipe bender was needed. Rummaging around the garage I found some nice 1" thick oak left over from the window ledges in our lounge extension. This combined with an angle iron fence post bits of old bicycle frame tubing some nuts and bolts and other scrap material and one of the rollers from tube bender Mk1 yielded the following:
Tube bender Mk2 initial version

The oak former was made in two parts using a 1/2" radius cove router in my milling machine.  I should have taken some photos but didn't.  The technique was to rough out the shape using a jig saw before mounting one piece on a rotary table.  The 11.5" radius arc was then cleaned up using a plane parallel router before changing to the cove router to add the quarter circle profile using the cove router - several passes where required as the miller speed is considerably less than the router speed in order to avoid excessive vibration and noise..
The second piece was then processed in the same way - but the other side up and the two parts glued and screwed together.
Knowing that a high force would be required the lever arm was extended using the bike tube and further extension was also possible giving a good 6ft of leverage.
Alas even 6ft failed to be enough to cold form the tube so heating had to be brought into play.

Bending under way

A little heat gets the bending going
Although heating the tube got the bending on the move, the oak former did not take too kindly to the very high temperatures needed and it was necessary to keep putting out the fire!!!!



Burnt remains

Bend nearly there
Fortunately the former survived long enough to make two bends, but only just as can be seen from the burnt remains

 The loss of the correct profile on the second bend resulted in a small kink but I hope to dress this out.




A word on the choice of bend geometry

Deciding on the fork geometry has been quite a problem.  Without an identity for the machine I have decided to use the study guide machine (see possible match photo) as a baseline.  This machine has a 30" front wheel and a 28" rear.  The geometry of the replacement forks is clearly all wrong - they have negative trail which makes the machine behave really bad. So some experimentation was called for. I made up some bracketry to attach to the forks so that I could move the wheel around an try different sizes.


After trying out the original 28" wheel in various locations and getting some improvement I took a trip to see Ray and borrowed a 30" wheel from one of his machines and tried that out.  It had to be tested up hill as the the brake could not be fitted and the sprocket is a free wheel.  Nevertheless, it was found that the 30" wheel worked well and even made the bike look better.

The final choice was to have a 30" wheel with around 3" rake on the forks.  This was about the minimum required to give clearance on the pedals when steering.

Applying some artistic thought then led to aiming for a bend radius of around 17" along the fork centre line.

So how to achieve this. From Bobs Blogg I learnt that when ovalising the fork blades some straightening would occur.  I made and estimate of how much by assuming that the inner and outer curved lengths would remain unchanged when the fork section was changed from round to oval.  Additionally I also expected some spring back after initial bending.  This was difficult to estimate but from various sources something like 5 - 20 percent seemed likely
So to finish with a 17" radius and bend angle of 31degrees I estimated that I needed to start with an 11.5" radius and 45degrees.  Just in case I then made sure that I left sufficient straight length on either side of the bend to be able to make any adjustments to get the desired rake etc on the finished article



























Back to the Blades

Having at last managed to bend the 1" dia tubing the next step is to change the profile using the ovaliser.  I did consider changing the crank to make it easier to turn but having replaced the broken a gear wheel with the last 45 tooth wheel from my Myford decided that keepin gthe smaller crank would reduce the risk of another failure. - the theory worked but instaed of breaking a gearwheel I managed to inflict a nasty spain on my thumb - Oh well some you win some you loose

 As noted by Bob the curved blades feed through the profiler with little problem.


The final result was two blades which were virtually idendical but unfortunately have a slight twist as they go from straight to curved and back again.  I have yet to work on that but think I should be able to straighten them with a bit of heat a torque.

Checking the final radius I am pleased to report that the target 17" radius was closely acheived.


Once again the wisdom of doing some sums rather than just suck and see is proven






Saturday 17 November 2012

Two Steps Forward One Back

 It's taken a little while but eventually after scouring the junk box and filling a few holes I managed to transform some scap material into three rollers to use in the bender to form the rake on the fork blade tubing.
Ready to roll
Manual crank
 Thinking that I might save my good material I searched aroung for some scap 1"dia tube and found an ax Halford's bike I had been given for spares that had 1" frame tubing. So another digression while I stripped it down and extracted the tubing only to find that it was best quality gas pipe with amazingly thick walls - so scrub that idea.
 
 Anyway I found somewhere to mount the bender and using my good 18swg tubing set about trying to set a bend. First problem the rollers could have done with being a bit bigger as there was not much travel on the
Oops no teeth

adjusters and it was apparent that to set a bend a good allowance for spring back would be needed.  Undaunted I forged ahead and cranked up the adjusment towards its limit and then disaster struck - the load on one of the gearwheels was exceeded  and teeth were shed.  The trouble seems to be that the gear wheels are cast iron which is not good under high tensile or shear loads.  The question I now have to ponder is can I salvage this basic design or do I need to try something completely different.


The trouble with posting bloggs as you go is that when you realise that you have made a big gaff you have already told the world. Well so be it.

 I have been doing what I should have done all along and got out the paper and calculator and rapidly realised that the forces required to bend a 1" dia tube in my design of rollers are absolutely enormous - no wonder it broke.  So I am now doing more research (and calculations) to work out the best way forward.  With my background I should have known better - thats why we had designers and stress men etc to make sure our concepts stood a chance of working before they were built.  My trouble is being a thermodynamasist / fluids mechanic engineer (fancy names for rocket scientist) I tend to gloss over the mechanical stuff.









Sunday 4 November 2012

Development Work

Fork Blades 

Profile Forming Tool
Bob's Geared Facile blog (http://geared-facile.blogspot.co.uk/) describes an adaptation of a leather cutting machine he used to form the fork blade profile. Being unable to find such a machine I rummaged in the scrap box and borrowed some change gears from my Myford to come up with this alternative.

The machine has a couple of rollers machined to match the oval profile required for the fork blades.  The rollers are driven by the hand crank via the change gears and the spacing between them can be adjusted
Trial Fork Blade - formed from 1" x 18swg Mild Steel

Fortunately, when checking the outer perimeter of the remains of the original fork blades this was found to be 3.142 inches - i.e. equivalent to a 1" diameter tube.  First trila using some scap alloy tubing showed promise so some 1" x 18swg mild steel tubing was ordered and a short length used to check it would still work with the stiffer material.  Result was good, but suggested a better lever would be desirable (this is in hand see later)





Next stage is to sort out best way to form rake a nd taper on the blades...........

Wheel Rims

Mean while success with this and more info from Bob's blog turned my mind to making wheel rims.  At first I was tempted by the method Bob described of using tubing sections cut in half, nested together and welded. However, my welding skills have a long way to go to be good enough for that.  Sturmey's Indispensable Handbook describes The Club Hollow Felloe and says it is constructed of a steel tube rolled into a crescent section without join.  Now this appealed to me more and having already made a rolling machine it seems worth giving it a go.

First stage involved  lot of calculations and to my amazement I found that to form a crescent like the Roadster model illustrated in Sturmey's book and suitable for a 0.75" dia solid tyre a tube of 1" diameter was required. - how lucky can you get...... well no, I guess it makes sense as those early bicycle makers were a wily lot and were bound to pick materials available off the shelf.

Trial forming tool with first and final attempts
Final attempt after dressing
Thinking that the forming would need to be done in stages and not wanting to spend money and time making sets of rollers that would finish up as scrap, I decided to start with a simple formers to form a short section of tubing by squashing in a vice.  After a few trials I found that I could get a fairly good result in one step which would only require a little subsequent dressing to finish off  The first attempt was made without the guide rods and as can be seen went lopsided.

Rolling machine

Rolling machine drive side
Rolling Machine - awaiting Rollers
Meanwhile while thinking about all of the above I had started putting together a machine for rolling fork rake and now rolling rims.  It is awaiting rollers and strength testing but now I need to order the material for this and also find some scrap 1" x 20swg tubing to test it for suitability for the rims.  Again the basis for this has been the scrap box and Myford change wheels (shared with the forming machine).  Note also the better (longer) crank handle which will also be shared with the other machine














Friday 2 November 2012

What is it?


Having searched through many catalogues & books on the National cycle Museum web site, and other sources I have been unable to identify the manufacturer.  The nearest that I have got is in the Veteran-Cycle Club cross frame study guide, which has a picture of a nearly identical frame, shown here.

Possible match
Unfortunately the owner of this machine does not know what it is and I am unable to get access for a closer examination.

It would be appreciated if anyone out there can offer any suggestions as towho may have made it.








Some close up details are below:-

Rear Fork End

Fork Crown with sawn off plunger brake guide

BB & Chainring - with donar cranks

Steering head with frame number 3557

Seat pin clamp bolt










Wednesday 31 October 2012

Barn Find

or should I say floorboard discovery...

As Found
The Crossframe bicycle that I have acquired and hope to restore was discovered in a cellar under the floorboards of a cottage in France.  The machine dates from around 1888 to 1892 and has had a somewhat chequered career.

My best guess is that sometime between the wars the owner decided to upgrade from a solid tyres to a pneumatic tyres and enlisted the help of the local blacksmith and the remains of another somewhat more recent machine.

Most of  the conversion was simply a case of  replacing parts - wheels, handlebars, saddle pedals etc. but the front wheel and brake created a problem.  I now believe the front wheel originally was 30" dia  i.e. larger than the 28" donar wheel (this will become clear later) and as a result the donar stirrup brake would not fit. To the blacksmith the solution would be simple - cut of the old fork blades and weld on ones frm the donar machine - job done....

Well thats a start more later