Latest! Cab sheets arrive.

While the other posts on this blog have been recapping what has been done in the past, this post relates to progress right now (July 2016).

It would be feasible to make new sheeting for the cab in-house by plasma-cutting, hand-finishing with an angle grinder, marking out and drilling holes.  However this would require a lot of time and effort, and it would be difficult to get really straight vertical edges where the sides are joined on with angle iron.  It takes much less physical effort to measure up the old cab sheets and produce drawings in AutoCad.  CAM (Computer Aided Manufacturing) can then produce new cab sheets with a high degree of accuracy an no hard work!

A CAD drawing of the cab sheets was produced after careful measurement of the old metalwork.

Upper & Lower Cab Front CAD Drawing

There are three main ways of producing metal sheets using CAM; plasma, laser and water jet.  Plasma cutting is the fastest, but is less accurate and requires some finishing.  It is not very good for cutting small holes (eg. 10mm), but you can use it to produce a shallow cross to mark hole centres.  Laser cutting is very accurate and requires little finishing.  It is also good for fine detail and holes.  Water jet cutting is often used for cutting thick materials, but is also good for sheet metal.  It is a bit slower than the other methods but very accurate.  Because there is no heat-affected area around the cut, there is no distortion and no finishing is required.  Water jet cutting uses a very fine jet of water at 60,000 pounds per square inch pressure, either with or without an abrasive (depending on the material).

Initial thoughts were to use plasma or laser cutting, and there are companies that can do this in Aberdeen and Glasgow.  However a recent volunteer at the locoshed is a machinist at Forsyths in Rothes.  While Forsyths traditional market was making and maintaining whisky stills, they also do specialist work for the oil and gas industry, and have a water jet cutter.  Jim took away a memory stick with the AutoCad drawings, and most unexpectedly a couple of weeks later the new cab sheets arrived at Aviemore sheds!

Upper rear cab

Lower front cab

Lower cab sides

The finish is truly amazing, and the cut edges are perfectly straight with no burrs.  They arrived as bare metal, so were immediately sanded and professionally painted with red oxide primer.  It is important to apply primer carefully (avoiding sags and brush-marks) to make life easier when applying the final coats.  Otherwise it would require a lot of sanding down to obtain a good finish.

Detail showing clean edges and neat holes

This slot (for the spragging lever) shows the precision of the cutting process

Lesson Number One

Assume nothing.  There are 4 "identical" handrails on the sides of the cab, and on the CAD drawing the fixing holes are all the same distance apart, based on the measurements of one old sheet.  However, it appears the handrails were not accurately made and differ in length by a few mm, so the original cab sheets must have been drilled to fit the handrails.  Some dressing of the holes will be required, but it won’t be noticeable when the handrails are fitted.  This is the first job using CAD/CAM, but there can be no excuses – what you get is only as good as the drawing!