The Lathe - Headstock & Backgear

The Lathe - Headstock & Backgear As its name implies, "backgear" is a gear mounted at the back of the headstock (although in practice it is often located in other positions) that allows the chuck to rotate slowly with greatly-increased turning power. For a novice the ability to run a workpiece slowly might seem unnecessary, but a large-diameter casting, fastened to the faceplate and run at 200 r.p.m. (around the bottom speed commonly found on a lathe without backgear) would have a linear speed at its outer edge beyond the turning capacity of a small lathe. By engaging backgear, and so reducing r.p.m. but increasing torque, even the largest faceplate-mounted jobs can be turned successfully. Screwcutting also requires slow speeds, typically between 25 and 50 r.p.m. - especially if the operator is a beginner, or the job tricky. A bottom speed in excess of those figures (as found on most Far Eastern and some European "Continental" machines) means that screwcutting - especially internally, into blind holes - is, in effect, impossible. These lathes are advertised as "screwcutting" but what that really means is just power sliding - a power feed along the bed. With these machines even if you go to the trouble of making up a complex pulley system to reduce the spindle speed (like the early Atlas 9-inch)  you will find the torque required when turning large diameters at slow speeds causes the belts to slip. The only solution is a gear-driven low speed - and so a properly-engineered small lathe, with a backgear fitted, not only becomes capable of cutting threads but can also tackle heavy-duty drilling, big-hole boring and large-diameter turning and facing; in other words, it is possible to use it to the very limits of its capacity and strength. To show how important backgear has always been considered examine the small English-made metal turning lathes made from the mid 19th century onwards: nearly every one was so equipped. For a further explanation of the desirable features required in a small lathe, click HERE.

 

 

بقیه در ادامه مطلب........

منبع : http://www.lathes.co.uk