Many parts will be cut from pieces of ready cut strip or drawn wire, but some things, like mainframes, boilers, or cab fronts and sides will have to come out of sheet metal. Most, but not everything following, will apply to other materials, the exception being plasticard. To enable one to cut shapes from metal, the shape must first be draw or marked out on the metal. This is not difficult to do provided one follows some simple rules.
1 From the original drawings do a sketch, complete with dimensions of the section(s) that are to be marked out, then check that you've transferred the dimensions correctly. This drawing does not have to be an exact machine drawing, but all the important lines and dimensions must be included.
2 Where possible all the dimensions should be measured and drawn from two lines drawn at right angles to each other called datum lines. These lines are usually drawn at the bottom and on the left hand side.
If for example you are marking out a main frame, the most important lengths will be the overall length and the overall height. These should be drawn first, to be followed by other internal lengths, such as distance between hornblocks. See the drawings for the right and wrong way.
3 Measure twice – cut once. In this case measure twice – mark once.
4 Never scale from a drawing or photo. There is no substitute for properly done research.
If you're not familiar with the term 'scale' let me explain. There are many loco drawings published, especially the general assembly (GA) ones. Some are beautifully drawn but they are definitely not accurate enough, even if printed as 'drawn to scale', to use as a basis for marking out.
Similarly, counting the bricks along a wall in a photo to calculate the length of the wall simply does not work. Even if you know that a certain loco had six foot driving wheels, you must not 'scale' from the photo, to find the height of the cab say, the drivers may have been turned down once or twice!
Once the drawing is done double check all the dimensions. Add the various sub-lengths and check against the total length. Make sure that you have used radii and not diameters.
Above all, sit back, have a cup of tea or a fag, and then look to see if it makes sense!!
O.K. if you are happy that the drawing is correct the next thing is to transfer it to the metal.
Ruler For marking out you will need a steel ruler as described in Part 1. This is 0.4mm thick which brings the divisions closer to the metal and makes the scriber more visible. It also has a satin finish to aid readability.
Scriber There are a lot of good scribers to be had. Proper engineer's scribers are about six inches long with one straight point and one bent at right angles. The scribers have a twist in the handle and are properly hardened, which is important.
If, however you wish to save money at this point, a thick sewing needle or fine darning needle placed in a broach holder is an excellent substitute.
Set square Engineering set squares have one thick side, which is held against the metal and one thin edge against which the scriber is drawn. A four inch set square should be large enough for most jobs.
Dividers Dividers are used to mark arcs and circles on the work. They should be of good quality with hardened points, of equal length. The adjusting screw must be easy to use, accurately allowing fine adjustments. There should be no wobble to the legs and should be large enough that they do not have to be opened more than about fifty degrees to do the work.
A firm surface, not necessarily a bench, and a good light are essential.
Make sure that the sheet of metal is clean so that any mark made will be visible and unambiguous. Clean the surface with a glass brush or fine sandpaper.
Glass brush is great for cleaning work but can be an irritant if pieces get into your skin. Try to work away from the bench on some old newspaper that can be discarded Whenever possible I use a glass brush under running water. Just try to ensure that you don't get a build up of glass brush on the bench and DON'T blow the bits around.
If you have a problem seeing the scribe marks then cover the surface with 'engineers blue' if you want to be posh or thick felt tip pen if you're common like me.
The first and most important thing is to establish a datum point and datum lines on the metal. The long edge of the metal should be checked for straightness with the ruler by holding one against the other and holding them up to the light. Correct any faults with a file. File a second edge at right angles to the first. Check with the set square.
This right angled corner is your zero point or datum and the two edges are your datum lines.
All measurements will be taken from these datum. (Alternatively, you can scribe a line, say 5mm, parallel to each edge, and use these as your datum. The point that the two lines cross will be the zero datum point.)
Take a minute to plan the sequence in which you will work. For example start with the main outline, including any radii that the profile might have, then any cut outs, such as horn blocks, and then the centres of any drill holes.
Remember that the centre points of some radii may lie outside of the profile so, if you have enough metal, leave room on the metal for these centres. If the finished piece has one long straight edge, like a main frame, plan to leave one of the datum edges as that edge.
Make the initial marks very lightly, so that if you have to correct them there will be less confusion.
To mark a straight line, place the work on a firm surface and carefully mark the position of the line relative to the datum line. This can either be done directly from the ruler or transferred from the ruler to the work using the dividers. Make a small mark approximately at each end of the line to be drawn.
Place the tip of the scriber on one of the marks, preferably the one furthest from you and, with the ruler held down with the fingers of your left hand, slide the ruler up to touch the scriber.
The best working position is to have the ruler pointing away from your chest. That way you will be able to see and reach the edge which is touching the metal. Turn the work around to find a comfortable position.
Looking from the top, note the relative positions of the ruler and the mark, (the thickness of the scriber will hold the ruler slightly away from the mark) and holding the scriber and the top of the ruler in place, slide the lower part of the ruler up to the second mark so that it is the same distance away from the mark as the top part of the ruler.
Making sure that the ruler is now held firmly down to the metal, draw the scriber down towards you with enough pressure to leave a fine line. The line can be a little longer than necessary.
This is not as easy as using a pencil and paper and you can slip, so exercise caution and practice until you feel confident.
Using a set square will also need some practice. The thick arm is held against the datum edge and you scribe up against the thin arm. You cannot do this flat on the bench because of the thick edge, so rest the work on a book or something to raise the thick edge clear, but make sure that the thick edge is up against the work and not the book. It is usual to hold the set square with the thumb and the work with the fingers of the left hand.
When using dividers the need for caution is just as great. You must calculate and mark the centres of any arcs in the profile or holes to be drilled. Apply some pressure to the point of one leg of the dividers, at the centre point of the circle, so that the point digs into the metal and anchors itself, remembering that with thin metal you will mark right through if you go at it like a mad bull!
Most dividers have a small handle on the top. Having set the dividers to the radius of the arc, grip the handle between thumb and index finger and place one point back in the centre mark. Holding the work down firmly, gently scribe the arc or circle, using the thumb and index finger. Use as little wrist movement as is comfortable. As with the straight lines mark a little longer than the actual length needed.
The marks that you make should be good enough to be legible but not so hard that a fine sandpaper will not remove them.
Once the work is complete, check your measurements again to be quite sure that everything is correct before you start cutting.
I can not within the scope of this article give geometry lessons but just one or two things that may help.
If you want to mark a line at right angles to another and can not for any reason use the set square, first mark the point at which the two lines will cross. With the dividers on this mark, make marks on the line about one inch to either side. Open the dividers about another inch and make two marks above the first mark. A line drawn from where the two divider marks cross and the first point will be at right angles to the first line.
If you are marking out a piece of metal that is going to be bent at right angles then allow half the thickness of the metal extra for the bend. If, however, you intend to mitre the corner, for a cleaner corner, then this rule does not apply and less allowance for the thickness should be made. In this case make a test piece. Make a mark, say one inch exactly from the end of a piece of metal, file the mitre exactly on that mark, put in the bend and measure how much longer than inch it turned out.
As you can see from the above, marking out is not very hard if you follow some simple guidelines, no pun intended. Most of the work is done with pencil and paper and some little thought. If the marking out is done accurately you will have confidence that the piece you cut out will be the correct size and shape.
© Peter Wilson
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