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Hollow Spindle Engine Lathes. The Solution for Some Hard to Hold Parts
by
Mel Joelle
Hollow spindle engine lathes come in sizes from roughly 5 inch thru hole on up to over 20 inch hole. Lathes with spindle holes of 3 or 4 inches may be referred to as big-hole engine lathes, but are not hollow spindle lathes and generally do not have the ability to mount a second chuck in the rear.The reason people buy a hollow spindle lathe, is, most often, to do end work on cylindrically shaped parts that require end work or ID work and long bars or pipes. But, there are many parts that are ideally suited for a lathe than can swallow the part deep into the chuck and headstock. These parts are often found in the aircraft and hydraulic industry long parts of not too great diameter that need to be machined on the ID and held very securely for minimum deflection. In a pinch you could machine these parts on a regular engine lathe by holding one end in the chuck and the other with a roller steady rest, but the rigidity and therefore accuracy of such a setup is questionable.
The most common arrangement for hollow spindle engine lathes is with a front and rear chuck. The rear chuck is bolted on the back of the headstock to a special flange built into the machine specifically for this purpose usually an A type mount. Using both chucks achieves a two point, extremely rigid and accurate way of holding a long part since both chucks are fixed to the spindle and are as close to dead nuts as can be achieve with that lathe the part run out is basically the same as the spindle run out you can not do better than that. Having two 4 jaw chucks also allows the part to be held off center in the spindle so boring or machining an off-center hole can be achieved which would be very difficult with an ordinary engine lathe. 4 jaw chucks are very popular with hollow spindle lathes for another reason they allow a flexibility in set up when a part is out of round or irregularly shaped that 3 jaw chucks can not match.The most often thought of use for a large hollow spindle engine lathe is in the oil patch. Drill pipe is held in the spindle with two chucks so the special API (American Petroleum Institute) both internal and external tapered threads used to connect rotary drill stem elements can be machined. These very unusual threads allow miles of drill pipe to be connected quickly with a single turn and connected so well that they rarely break in the ground. Because of the rough treatment these threads receive (see Discovery Channel shows on oil drilling platforms) they can be damaged and require repairs, which is why you always see hollow spindle lathes nearby anyplace where a lot of drilling is on. The drill pipe is upset forged on both ends where the machining needs to take place. One end of the pipe is a male tapered API thread and the other end is a female API thread. Cutting this forged steel in a regular lathe using a steady rest would be a slow and difficult task, but with a hollow spindle lathe the work area is so close to the chuck and headstock and the part is held so well, maximum cutting forces can be achieved resulting it better quality and higher productivity than could ever be achieved in a regular engine lathe.Another place where hollow spindle lathes are required is the aircraft industry. There are a number of parts in an airplane that need to be swallowed and held with two chucks in the spindle of a big bore lathe. The two parts that come to mind are the landing gear piston assembly that acts as the shock absorber when planes touch down and the center core parts of jet engines these parts are long and have a lot of ID machining that needs to be done near both ends ideal for a hollow spindle lathe. In repair work, a hollow spindle lathe can save a lot of disassembly and reassembly when a part has a narrow section then a bigger diameter section think of a pipe with a flange on the end. These parts can be machined on both ends using a large spindle hole lathe without the need to remove the flange and reattach it this could save hours and hours of labor.In production work, CNC hollow spindle lathes have largely taken over from manual lathes for the bulk of this work. A lot of these lathes are designed for a particular family of parts for example, drill pipe, or aircraft strut manufacturing. These lathes are built around a hollow spindle headstock with front and rear chucks then are customized for the family of parts they will be making. Often, tailstocks and long beds are dispensed with in favor of a shorter, more compact design using less floor space for those shorter aircraft and hydraulic parts. A lot of the oil patch parts are perfect for hollow spindle holding these are parts designed to attach to the drill string and go down drill holes. There is a heavy emphasis on ID and OD threading on the ends of these parts that is an ideal application for hollow spindle CNC lathes. Some of the newer flat bed big hole engine lathes are being built with CNC controls as a higher productivity option to replace the older hollow spindle engine lathes, some of which go back to the 1930ss and are still in production mainly for repair work or in less developed countries where speed and labor costs, up until recently, has been less important. The accuracy and part quality achieved by a modern, CNCized hollow spindle engine lathe more than pays for the cost difference, assuming one can come up with the money or the financing. There are only two negatives with hollow spindle engine lathes. The first is the cost they tend to cost much more than an ordinary engine lathe the large diameter bearings and the chuck mounting flange on the back of the headstock and that second chuck add to the cost, plus these lathes are designed to handle heavier weight parts and more spindle loads so they tend to have larger gears and bigger parts throughout their headstocks. The second negative is they are always going to have lower spindle speeds than typical engine lathes of equal swing. The large diameter bearings can only go so fast and the bigger the spindle hole, the larger the diameter of these bearings and the slower the top speed of the lathe. The bottom line is, whenever you have some quantity of cylindrical shaped parts where there is a lot of end work and/or ID work that needs to be done, a hollow spindle lathe should be considered. The time saved in setup and the speed and accuracy gained due to the rigidity advantage often can make acquiring a hollow spindle lathe a logical and cost effective choice.
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