Harold Hall

Workshop Processes

First Page

The method relies on a close fit and it is essential that the hole is parallel. Also, if several parts are to be machined, the hole in each one must be made with minimum variation, say within + 0.01 -0.0mm. Even a small difference may, prevent the component fitting onto the mandrel if small, and be loose if large. For multiple parts therefore, a reamed, or accurately bored, hole will be necessary. Ideally, if bored, the parts should first be made with the hole slightly small and each one then returned for final boring. In this case, bore the first one returned to the diameter require and then lock both the cross and top slide. Subsequent one can then be bored at that setting ensuring that they are all the same diameter.

I fully appreciate that the initial shape machined may not permit the part being returned a second time. In this case, particular care will be necessary when machining the bore to size, each one being done individually.

It is though preferable, even for a one off that the hole is parallel even though the bore diameter, in some cases, is not critical as the mandrel can be made to suit.

The viewer will most likely appreciate that even a new drill will produced a hole marginally on the large size. What though may not be realised is that the hole size reduces to drill diameter as the drill breaks through. The larger the drill is the larger the distance over which this occurs. This can cause confusion when attempting to make a mandrel to fit the hole especially if it is made to the size at the smaller end. Because of this, a drilled hole will only be suitable if the reduced diameter is removed by some means, a triangular scrapper being a good method.

Another important factor is the hole diameter to length ratio. It should be obvious that, with a part having a large hole but thin, typically of washer proportions, that fitting the part to a solid tapered mandrel would be

difficult. Unless some form of guide were employed, say the tailstock barrel, getting the part to run true would be difficult. Even then, machining would have to be carried out with extreme care. Hole diameter no greater than twice its length would be ideal but greater will be OK providing the limitations are taken note of, Photograph 2 being a typical example.

Unfortunately, there can though be no strict rules, as it depends on so many factors. These include, outer diameter large, intermittent cut and difficult material such as stainless.