Hi!
I am working on a project that was supposed to take only a "couple of weekends", but it is taking longer. I used this project to practice some frame building skills, particularly that of building a jigless frame. Albeit the project is a motorbike, the building technique is just the same that can be used on a fuselage frame. During the building process I have spotted some flaws, that have arised the somehow inevitable question; how to achieve perfect symmetry with basic tools, or how good, is good enough?
A friend donated me a scooter that he used back when he was a teenager, the scooter has been sitting for a number of years on my garage, and I have decided to recycle the parts and build a very basic 49cc race motorbike, somehow similar to a Derbi Ran. A particularity of the original Derbi Variant model (picture 1), is that the whole engine swings around the silent blocks next to the cylinder, pretty much like a scooter; in this case the engine is attached only by a single bolt and the shock absorbers, a small plastic beam slides on the rear part of the engine, to keep the rear wheel aligned. Except for the head tube, the original frame has completely been replaced by a truss type 16*16*1,5mm squared tube frame (picture 2). I planned to do a very simple raw build; no lathe parts, and only using stick welding.
Most of the frame is a 2D tube, this part has been done simply by welding the tubes on a flat table, in where the frame unions were drawn. The again, using a flat surface both sides have been weld together, matching both parts has been somehow easy, by simply using the symmetry between those. The tricky is connecting the frame with the head tube, for this I used a vey basic jig (picture 3), that is typically used for motorbike builds, but that can too serve for a fuselage, if the beam is long enough.
I tried to cut all the tubes perfectly equal, using the one on one side as a reference to the next one being cut, to ensure the necessary perfect symmetry. But no matter what, there is one side that is at least 4mm shorter than the other one (picture 4, notice the gap on the bottom of the left tube, that connects the frame to the head tube). So my question is; is this somehow inevitable in this type of jigless builds?
I asked a professional welder friend, who told me that it was all right. Of course if the head tube is properly centered, the front and the rear wheel will be aligned, and the motorbike will work fine. But it pisses me off, to not be able to make all the parts perfectly symmetrical, like if the motorbike was factory build; except for the stick weld of course.
Any thoughts? Is this normal when building jigless frames?
As a side note,
After stick welding the frame and removing it from the jig, to check (picture 2) if the proportions were right? The wheel did not touched the spark etc. I noticed that the head tube was 2mm more on one side than the other!
I have realized that the wood flat surface, in where the frame sits, was not properly aligned with the jig. So I have cut the welds, connecting the head tube and the frame, and now I am on the process of fixing that part. After having fixed the alignement of the frame with the jig. I will double check if the proper alignement of the frame and jig has truly been achieved. Perhaps the gap is telling me that something is wrong.
This somehow proves how difficult it is, to achieve perfect symmetry on the shop.
Sorry for the long post, thanks for your comments!
Oriol
I am working on a project that was supposed to take only a "couple of weekends", but it is taking longer. I used this project to practice some frame building skills, particularly that of building a jigless frame. Albeit the project is a motorbike, the building technique is just the same that can be used on a fuselage frame. During the building process I have spotted some flaws, that have arised the somehow inevitable question; how to achieve perfect symmetry with basic tools, or how good, is good enough?
A friend donated me a scooter that he used back when he was a teenager, the scooter has been sitting for a number of years on my garage, and I have decided to recycle the parts and build a very basic 49cc race motorbike, somehow similar to a Derbi Ran. A particularity of the original Derbi Variant model (picture 1), is that the whole engine swings around the silent blocks next to the cylinder, pretty much like a scooter; in this case the engine is attached only by a single bolt and the shock absorbers, a small plastic beam slides on the rear part of the engine, to keep the rear wheel aligned. Except for the head tube, the original frame has completely been replaced by a truss type 16*16*1,5mm squared tube frame (picture 2). I planned to do a very simple raw build; no lathe parts, and only using stick welding.
Most of the frame is a 2D tube, this part has been done simply by welding the tubes on a flat table, in where the frame unions were drawn. The again, using a flat surface both sides have been weld together, matching both parts has been somehow easy, by simply using the symmetry between those. The tricky is connecting the frame with the head tube, for this I used a vey basic jig (picture 3), that is typically used for motorbike builds, but that can too serve for a fuselage, if the beam is long enough.
I tried to cut all the tubes perfectly equal, using the one on one side as a reference to the next one being cut, to ensure the necessary perfect symmetry. But no matter what, there is one side that is at least 4mm shorter than the other one (picture 4, notice the gap on the bottom of the left tube, that connects the frame to the head tube). So my question is; is this somehow inevitable in this type of jigless builds?
I asked a professional welder friend, who told me that it was all right. Of course if the head tube is properly centered, the front and the rear wheel will be aligned, and the motorbike will work fine. But it pisses me off, to not be able to make all the parts perfectly symmetrical, like if the motorbike was factory build; except for the stick weld of course.
Any thoughts? Is this normal when building jigless frames?
As a side note,
After stick welding the frame and removing it from the jig, to check (picture 2) if the proportions were right? The wheel did not touched the spark etc. I noticed that the head tube was 2mm more on one side than the other!
I have realized that the wood flat surface, in where the frame sits, was not properly aligned with the jig. So I have cut the welds, connecting the head tube and the frame, and now I am on the process of fixing that part. After having fixed the alignement of the frame with the jig. I will double check if the proper alignement of the frame and jig has truly been achieved. Perhaps the gap is telling me that something is wrong.
This somehow proves how difficult it is, to achieve perfect symmetry on the shop.
Sorry for the long post, thanks for your comments!
Oriol
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