How to mark out and machine a radius

Hi I have bought a kit to make a stother and Pitt beam engine their is quit a bit of machine work to do for me a novice. On one crank there are several radius internal and external and I was wondering if somebody could point me in the right direction in how to machine a radius.

It depends on the size and position. Sometimes it is possible for an internal radius to carefully drill a hole so that it forms the radius of perhaps mount on a lathe faceplate for smaller items and bore the hole before other work is done.

For outside radii of small size research "filing buttons" and for a large radius look for the old ways fo making stephenson's valve links. I say old way becasue a lot of the web searches will now come up with stuff about CNC and rotary tables which you don't need.

Maybe a drawing would help people see what you are intending to make.

First mark a centre line and the two hole positions 18mm apart and dot punch them.

Take a pair of dividers and mark the 10mm and 15mm external radii

Mark the 10mm and 8mm widths and join these up to get the tapered lines

An easy way to mark the internal 6mm radii is to take a short faced bit of 12mm rod hold the end onto the work and adjust the position so it is tangental to the arc and line then just scribe the 6mm radius.

To machine will depend on what you have available but

First drill the two holes, saw off the waste, make arbours to fit the holes and mount the work in a chuck on the rotary table and using a 12mm (to give 6mm internal radii) cutter round the ends stopping at the taper lines. Reverse and do the other end. Then either mill or file away whats left.

Finally use the arbour again to mount in the lathe and reduce the thickness

Of course filling buttons and a couple of files will also do the job. At the other end of the scale if you have a CAD package you can use that to get co-ordinates of the 6mm radii and just drill them to 12mm and join up between with the mill.

What does Anthony Mount say about the part?

Edit just looked at Anthonys build article and he does it as I described but say what machines and tooling you have available as there are always more than one way to do the job.

J

Edited By JasonB on 28/04/2013 07:56:25

Richard you may also want to take a look at Jo's threads on MEM, she is building both the 1/12kit and a larger 1/6 scratch built version and has a few threads showing various parts being made

here

here

Many parts like this (external radii) are more quickly dealt with using a combination belt sander/disk sander. It perhaps won't be as accurate as machining the part (depending on what tooling you have available) but if time is valuable to you then grinding is a good method. The finish is also very good. A rotary table with a chuck should also be high on your list of things to buy, you'll find it has many other uses apart from this type of work.

For the belt sander, make a proper table for it adjusted to exactly 90deg, and if you run the belt right up to one edge (and slightly beyond) it curves around the edge a little allowing tight radii to be formed. The disk sander (typically 6" diameter) is good for putting the final finish on the part.

This is a rather poor picture of mine, on the left, but you get the idea..

I would always use buttons for a part like that, but one day I'll get a linisher/disk dsander and try Chris' approach.

I've used the mill/rotary table approach for connecting rods, and it works, but I'm not sure it is significantly faster or easier than buttons for a single part.

Don't forget that, within broad parameters what matters here is getting the part to look right, so expect hand finishing anyway. The crank below was entirely done by filing buttons and hand, aside from making three holes. ( the top counterweight has a plug in it).

Neil

For us newbs please exactly what is a filing button and how does one get some and then how does one use them ?

This thread might assist.

Basically you make them yourself to suit the job.

Regards,

Phil

Rick,

It's just a bulky washer with a screw through the middle. Some people leave them soft to be kind to the file, but I make mine from from silver steel and harden them and don't temper them.

Neil

I make my filing buttons from EN1 or EN3, usually from the offcuts bin, leave them soft, and throw them away after use.

Andrew

I make mine from odds and end of scrap steel, and case- harden it with Kasenit, I,v got a whole stack of them on the window sill, some times I even find that I have old ones that fit the job. Ian S C