Introduction.

CNC machining is now such a part of modern manufacturing that everyone takes it for granted. There are CNC lathes, milling machines, lasers etc. I first became involved with laser cutting when I worked for an American organisation producing small conveyors. All the flat plate was laser cut, even if it was a rectangle! The benefit was that everything fitted. I had just hacked out my Holmside frames and had cuts and bruises to show for it. Three days of hot sweaty work drilling, sawing, milling and grinding produced a set of frames which, like the curate’s egg were good in parts. Watching the laser cut 6mm plate like butter to an accuracy of a few thou convinced me there had to be a better way. So I resolved to have a go myself and the following are some of my experiences and where I am now.

What is a Laser

Basically a laser works like a fluorescent tube. Gas is held in a glass tube and at each end is a metal plate which is at high voltage. The gas in the case of a modern 3kw laser is a combination of carbon dioxide (the lasing medium), helium (assists in removing heat) and nitrogen (assists in coupling the electrical energy from the discharge into the carbon dioxide). The electrons in the atoms are excited by the high voltage and go from the lower level to a higher level of orbit in the atom. They then fall back to the lowest level and in doing so emit a photon of light. The unique thing about laser light is that it is of one frequency and is in phase. This means it is extremely strong light, of one colour and can transmit energy in the form of heat. The light goes back and forth in the laser tube, reflected by metal mirrors at either end. All the mirrors have to be water cooled, otherwise they quickly burn out. One mirror is called a “Leaky mirror” and the light escapes through here. It is first condensed before being reflected through tubes by mirrors to the head of the machine. The light is focused on the material by a small convex lens. This is very similar to what we used to do on the school field at lunchtime on a bright summers day with a lens and a piece of paper. The movement of the head is controlled by a computer which is fed with G code. This then is how a basic laser works. The cut is only a few thousandths of an inch wide so the power does not have to be great. A 3 kw laser can cut through 15mm plate.

What can it do for Model Engineers?

I first considered the most obvious application which is to cut frames and hornplates. These are usually fairly thick pieces of steel and are of a complex shape. They also need to be accurate otherwise we are into offset bushes for the coupling rods etc. The thickness is ideal for laser cutting with the majority being in 3mm plate, but I have cut 10mm for a Thomas 2 - very impressive! The thinnest is usually for Gauge 1 or O gauge in 1.5mm. All material is now metric which does not usually create a problem for us, the few thou difference being easily accommodated.

 Not only can the laser cut the shape it can also spot the holes. To cut a hole the laser starts at the centre and cuts to the outside. It then goes around the circumference finishing at the place it started from. To spot a hole the laser is switched off a few milli seconds after it starts to cut, thus leaving an indentation in the material at the centre of the hole. When I first started laser cutting I cut through the material. However this leaves a heat affected zone which on drilling can make the latter wander or even worse break the drill. The spot I now make is easy to find with a very sharp centre punch and leaves no heat affected zone. Even 1/16” drills can go through with no problems. The frames for my 5” Black 5 tender took 30 minutes each to drill and every hole was in the correct place.

The inner frames were even quicker and when offered up to the outside frames all the holes lined up!The next stage was to look at what else was possible. There are the obvious parts like the buffer and drag beams. Although these are fairly simple parts the cost of having them laser cut is fairly small, they come out right and I don’t have any bruises or broken band saw blades. The drag box is a complicated fabrication involving nine separate parts. These can be cut out as on the plans but with a little thinking “outside the box” it can be made a much easier task to manufacture. The secret lies in breaking the box down into the flat plate components and then adding what we would call in the woodwork classes mortice and tenon joints. The laser will cut any shape. It has very few limitations and one needs to think about how this can be put to best use, not only in cutting the part but also in silver soldering it together. In fact you do not have to silver solder them. If you are a good welder, TIG is a definite possibility with a lot of these parts.
The first step then is to draw the parts in CAD. It needs to be a good package as we need 3D to check for fits. Having drawn the views one then needs to pull it apart, insert the tabs and slots, put it into 3D and finally create the file for the laser. As an example I propose to look at the inter stays on Don Young’s 5” Black Five tender. My first idea had been to make up one piece which would be folded and have some more parts for the inside. This did not work out well as it is very difficult to accurately fold the material and then the inside plates would need to be held in place for silver soldering. Therefore a fabricated solution was sought. The obvious is to have two sides, a top piece and two inside spacers. The spacers would hold the frames together whilst the silver soldering was being done.

Three views were drawn in CAD, side, plan and end. The top was drawn first with the edges castellated. The castellation helps in positioning the sides and top. The inside spacers were developed with two tabs, one on either side. I could have had another through the top but experience has shown that this is superfluous. The sides had two slots in to accommodate the tabs on the spacers. In order to check that the parts would fit a 3D drawing was done with the parts in different colours. This aids recognition whilst creating the 3D view and also in seeing any conflicts.

The final stage is to nest the parts for the laser. This puts all the parts in a kit together on one sprue which has two advantages, cost and ease of storage. When costing up a component there is an initial set up charge, the material cost and finally the cutting cost. The least number of parts is therefore a cheaper option. With regards to storage some of the parts are extremely small and would easily get lost whereas in one piece the plate is quiet large and all the parts are kept together. With the file checked for errors it is ready for the laser.

Building the part

Building is similar to a plastic model kit. The first operation is to clean up the steel. I find this is best done using a mop head in the angle grinder with the parts still fixed together. Once cleaned up the parts are cut off the sprue as required, the edges cleaned up with a file and slotted together. It is preferable to clean up the slots with a needle file. Once the components have been fitted dry they can be taken apart again for fluxing up and soldering. The tabs have been designed over long so that they can be slightly twisted on final assembly thus holding the stay together whist silver soldering takes place

I soldered mine upside down with solder laid in the base against the upright sections. Due to the excellent fit very little solder is actually required. When cool the tabs can be filed down and the stay cleaned up. A small mini drill with some carbide burrs is extremely useful for this operation.

The total build time for the four inter stays was two hours. Cleaning and painting the stays probably took as long as the fabrication. I was very pleased with the final stays which fit perfectly in my tender.

Conclusion

The inter stays are only one part of the tender. The frames, drag box, draw box and the other stays have all been produced in this manner.

The total build time for the whole chassis was only 15 hours.

Whilst the purists will say this is cheating I believe it has helped me to actually get the frames built to a reasonable standard. I have all the admiration in the world for someone who can take a piece of black steel from the scrap bin and make a perfect tender chassis. Unfortunately I have neither the time nor the skill to do this. What I am using is a different type of skill to achieve the same end product but in a shorter time scale. If I consider the number of band saw blades I would probably have broken and the fact I would have had to source the material the overall cost is very similar.

I am constantly developing this technique, my latest addition being the weighshaft bracket and girder for the Black 5. There are no castings for this part and it is an obvious candidate for fabrication. I have included a picture of the finished item. I suspect there are many more parts which could be candidates for this process.

If any member would like to contact me to discuss further possibilities you can reach me on (01302) 721611, via the web site on www.modelengineerslaser.co.uk or by e-mail on malcolm.high@btinternet.com.

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