Having used a fly cutter in the previous instalment to machine the rough block it seems right to look at them in a bit more detail this time. Where a large surface is to be machined the fly cutter is a good choice as it can cover the area in a single pass thus giving a uniform finish unlike using a smaller diameter cutter that will leave a series of visible lines on the surface which although they don't leave a physical step can look unsightly. They also work out to be quite economical in use as the one usually HSS tool bit will do a lot of cutting and is easily sharpened on a bench grinder unlike milling cutters that need expensive tool grinders or insert face mills where the cost of inserts can soon add up. Most fly cutters take the form of a cylindrical head on a straight or tapered shank, the end of the head is cut at an angle and slotted to take a toolbit which presents the tool in such a way that it clears the work below the head. Photo 84. There are two or three grub screws to retain the toolbit in the slot. For very large areas or to machine a concave radius the head can take the form of a flat disc mounted on a shank and a toolbit can be held facing vertically downwards for surfacing cuts or protruding from the edge when used for cutting concave surfaces. Indexable fly cutters are also available and often take the form of a shanked body and adjustable arm that takes the insert, these are a bit on the large side for the average bench top hobby machine though a smaller version could be made. However, if you do want to go down the indexable route then it is often simpler to mount an indexable Left Hand lathe tool in place of the HSS bit.
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