Wednesday, 29 September 2010

Guest Blog - Bending and jigs

Today's blog entry is a guest article from Alan Ross who I interviewed earlier this year.

Alan Ross: I had to braze up a large CD rack frame , so I took the opportunity of photographing the process. This is the third incarnation of this jig, simple and effective typically accurate to about 1mm.

Alan Ross: I make a lot of these and they have to be accurate, setting this up takes a few minutes but avoids lots of measuring and the chance of messing things up.

Project inspiration

Alan Ross: Back to curves and bending, I recently had to make a large dome structure as a memorial to an old Hippy's wife, a fabulous job, we met discussed it and how it would go together, then i got on with it.

Alan Ross: To bend up the metal I had to buy a Metal Craft Power bender, the Big Daddy version of the other tool. See photo's, a truly awsome piece of kit.

30x5mm on edge
30x5mm on edge

25mm box section

13mm bar

Workshopshed: Are those your 'rivet welds' in the next photos?

Alan Ross: Yes those are the 'rivet welds' a process I devised for myself. Basically I bolt the structure together, then lop the back off the bolts leaving the head on one side and plug weld the back side of the bolt through the hole to leave a mock rivet. The bolts are dome headed so the front looks like a large rivet. Harder to describe than to to do.

Workshopshed: Thanks again to Alan for spending time taking photos and answering questions. You can reach Alan and see more of his wonderful work via his websites.

Thursday, 16 September 2010

Victorian Metropolitan Plumbing

Thames Water has been in the news recently about their plans for a Thames Tunnel running down the river. Not to be mistaken for the other Thames Tunnel which goes across the river at Rotherhide.

This giant 7m diameter tunnel follows the line of the Thames and is needed to take the overspill from the existing Victorian Sewers designed by Joseph Bazalgette.

I've been reading about the original sewer project in
"The Great Stink of London". There are interesting similarities in the enormous costs, political conflicts and motivation. Both projects were initated because of sewerage getting into the Thames. The Victorian design was put in place to take both household waste and surface water and deposit them further down the river by Becton. During the course of the project they realised that even this was not going to be acceptable and ended up taking the solid waste and dumping it at sea. More recently they have started burning it and using the heat to drive turbines and generate electricity from human waste.

The problem with the existing Victorian sewers is that they just don't have the capacity to handle all the households and all the rain water. Hence when we have a bit of rain or Sophie Dahl shows us a new recipe for prune curry then the system overflows. So the new tunnel will take that extra and get it down to the power station rather than discharging into the river. You may not agree with the costs or the disruption caused by the works I am sure this will be as facinating an engineering project as per the origional project.

The Great Stink of London

Friday, 10 September 2010

Model Engineer Exhibition 2010

As autumn approaches the model engineer's thoughts turn to what new bits and pieces they can buy at the model engineer exhibition.

Model Engineer Exhibition 2010 10-12 December 2010, Sandown Park Racecourse

Model Engineer Exhibition 2010 10-12 December 2010, Sandown Park Racecourse
This year's 3 day exhibition is at the Sandown Park race course again and as per usual there will be a courtesy bus to take you from Esher railway station to the exhibition halls.

There will be a large selection of suppliers and exhibits from clubs and competition entrants. There will be live steam engines and traction engines for you to see in action and a selection of aircraft and marine models too. If you have a model or tool that you want to show off then there is still time to enter as the closing date for entries is 30th November 2010. For those with a more zany taste the Stirling society will have a large range of engines on show.

The Society of Model & Experimental Engineers will be providing a series of lectures and demonstrations, please be aware that the show web site is currently displaying last years programme.

If you book early it's possible to stay on site at the race course, I stayed there last year and the rooms although basic were comfortable and a good cooked breakfast was provided too. The staff were helpful and very friendly.

Model Engineer Exhibition 2010 Show Website

Buy tickets for Model Engineer Exhibition 2010

Wednesday, 8 September 2010

Perfectionism gets in the way of completion

Last year, I was looking at some articles by Steve Ogden and Bre Pettis who were discussing the relative merits of getting things done vs getting them perfect.

Surely the answer is to get them done right and to use Engineering parlance, to make them within tolerances.

"TOLERANCE is the allowable variation for any given size in order to achieve a proper function."

However, how do you know what tolerance to use? Well back in the days before mass manufacture every shaft and hole had to be made as a pair otherwise you could not guarantee that they would fit. So some bright spark came up with the idea of standardising so that you could have two manufacturers make items and they would fit together.

The way this works is that you have a defined minimum and maximum size for each size of hole and the same for the shaft. Even if you had the hole at it's smallest size and the shaft at maximum you would guarantee that they would fit ok and even if you had the other extreme the fit would not be too loose. Dependant on the type of fit you need e.g. does it need to rotate or slide vs a press or force fit then the dimensions are slightly different. This also makes the job easier for draftsmen as they don't need to specify the detail of the dimensions on the drawings simply put the nominal size and have the fit type specified. I expect clever CAD software will put the detail of the dimension in a table automatically for you. The machinist / CNC programmer would then look up the dimensions from a chart and determine how to machine the parts. Again it's possible that some CAM software will understand how to apply the tollerances directly.

As well as standards for holes there are equivalent standards for geometrical tolerances. For example when working with castings you could machine all of the surfaces to a high standard of dimensional accuracy. However, that rather wastes the point of casting them in the first place. Instead you need to know which surfaces need to be parallel, perpendicular, concentric, in line or at a defined angle and hence machine just those to a suitable standard.

When designing parts you need to understand how those will be made and hence don't over engineer them with a greater tolerance than necessary. Machining parts to a defined tolerance means that you won't need to spend hours trying to make them to fit.


Tolerance, limit and fits
Tolerances and fits, terminology and definitions
Tolerancing System
ISO Tolerances For Fasteners
Fits for holes and shafts - tables ISO 286-2
Geometrical Tolerances
Idiots Guide

Workshop Practice Series