Friday, May 9, 2014

Home-made Tools - Hand-powered Sander - Precision Picture Frames

There is a preamble to this blog post which discusses the use of bicycles as upcycling material - a link to it can be found at the bottom of this article. It will also take you to a copy of the film: Home-made Tools - Hand-powered Sander.

Architectural drawings from The Builder 1884

 A whole pile of architectural drawings, patiently awaiting frames


Introduction

From time to time I need to cut mitred corners for picture frames. This is something I find impossible to do free hand - if I'm following my 45° line I invariably do not keep the saw cut square to the upper face of the wood. I have a mitre block to aid me in this operation but I never seem to obtain beautifully butting joints. I've seen mitre saws for sale but I dissuade myself from purchasing one as I keep thinking of how little I'd use it. So, when I found myself with the task of making frames, I thought about rough cutting the corner and then achieving the final satisfactory joint by sanding the face to the prescribed finished angle. I knew of powered disc sanders which could tidy up my cutting but I wanted something that would: 1) be difficult for me to 'over do' the sanding operation, 2) be accurate and 3) be cheap.

This led me to think:-
The Final Design
1) a hand-powered machine with 

2) a support table at 90° to the vertical face of the sanding disc and guides attached to this table at the mitre angle of 45° and 

3) to make it myself out of recuperated materials.




Design

Hand-sander prototype
My first idea was to mount a disc, with the sheet of abrasive glued to it, to a wheel spindle from a bicycle front wheel. The hub in which the spindle ran was to be fixed to a support. Attaching a crank-handle to the free end of the spindle would enable it to be turned by hand.

The first idea worked up to a point but highlighted a crucial error, it was impossible to mount the sanding disc to the shaft such that there was no wobble (I call it nutation in the film but that is not correct). This was a consequence of attaching a 230mm diameter disc to the hub face of about 40mm diameter.

Developing the design further I decided on using a more robust arrangement for supporting the disc, that is, using the crank assembly of a bicycle and attaching the sanding disc to the face of the pedal crank.

Most of my projects involve the use of untreated pallet wood. Sometimes on my pallet collecting 'runs' I find a pallet with a chipboard top. Now, I do not like this material because of the toxic bonding agents used in its fabrication, nevertheless, being manmade it normally has the property of being flat and the surfaces are relatively smooth. So, provided that the pallet has not been exposed to rain, or excessive loading, a chipboard pallet-top can be used for making the sanding discs.


Fabrication 

Bicycle bottom bracket, chain wheel and cranks

Bicycle bottom bracket
Ready for deburring
The first thing I did was cut the mechanism (with the pedals still attached) and the lower part of the frame away from the rest of the bicycle. The two horizontal arms that go to support the rear wheel I kept with the mechanism so as to furnish a means of attachment to the sander support. These two arms are at 90
° to the vertical face of the chain wheel. The larger diameter downtubes of the frame were cut close to the bottom bracket. After cutting, all the exposed edges were deburred.

Bicycle crank wheel
Crank wheel with crank removed
The crank levers connecting the pedals to the spindle were then cut, I used a grinding disc. The lever on the crank wheel side was cut close to the cotter pin, the other was cut longer, as this was to be where the turning handle was going to be attached. As I had decided to mount the sander mechanism onto a small wooden pallet, the handle length had to be shorter than the height of the pallet top from its base.

Bicycle pedal crank
From pedal to handle



At the free end of the shortened lever I attached a wooden handle to enable the crank to be turned by hand.

 





The mechanism was then screwed to a small wooden pallet, ensuring that the turning handle was not obstructed. Onto this pallet I also attached a worktop of chipboard with a plastic surface (again recuperated from a pallet). 

I had to have two chipboard discs on this sander. The chipboard sheet from which these discs were cut was checked for flatness by using the flat edge of a try-square, although the edge of a steel rule would work as well. 

Once cut from the sheet, both discs' edges were 'cleaned-up' using a router. 


I have received a few comments on the film regarding my use of power tools to produce a hand-powered machine. I do this because filming a project as it progresses, takes at least 4 times longer than if I was just making it and the power tools allow me to finish the projects in my own lifetime. Nevertheless, most of my projects can be undertaken solely with the use of hand tools. As a case in point instead of machine cutting a circular disc, it is possible to cut an octagon using a hand saw and having the same diameter, this would work just as well. As our plans for the future are to be completely off the grid, I intend to reduce the use of power tools but whilst I still have mains power I shall continue to use them.
 
The disc which was to be attached to the face of the crank wheel had a clearance hole cut in its centre to fit over the portion of the crank lever that remained. There already existed 3 holes in the crank wheel that I believed would be suitable for the mounting screws but, finally I drilled three more holes on the perimeter of the crank wheel to ensure the disc ran 'true'.  



The second disc was fixed to the 'crank wheel disc' with 3 screws. Prior to the second disc being fitted I used a PVA glue to adhere a sheet of sandpaper to its outer face. The size of the sandpaper sheet set the maximum diameter for the disc  i.e. 230mm. 



Depending on the thickness of the chipboard it may be necessary to provide a clearance for any portion of the crank lever that protrudes above the outside face of the crank wheel disc. In my case it merely meant cutting a 5mm-deep recess in the centre of the discs' rear-facing surface.



 


I attached two guides on the worktop set at an angle of 45° to the sanding disc face. 



  


Hand-powered sander

Two guides were necessary so that the mitre face at each end of the frame piece could be sanded by the disc moving downwards. 





Hand sander in action
Thus the sanding disc would rotate clockwise to sand one end and anti-clockwise for the other. If the workpiece were sanded with the disc moving upwards it would 'chatter' and be less precise



Left: Sanding disc rotating anti-clockwise.



Does it work?


 



The results were completely satisfactory. The finished mitre made a perfect corner for the frame





The film and a run down on bicycle upcycling can be found here


Thanks for dropping by and hope to see you next time. Cheers, Andy



© Andy Colley 2014