Workshop Hints
Dead centre turning
Replacing a damaged end of a centre wheel arbor
By S Callihan CMC FBHI
Cylindrical parts were produced between centres long before chucks or collets were invented. The simplicity , versatility, and accuracy of the technique are the main reasons it is still used today.
In its simplest and most accurate form two fixed centres support each end of the workpiece with a bow being used to drive the work directly. In the watchmaker's lathe a centre is mounted in the headstock and a lathe dog is used to connect the centre to the workpiece.
This convenient arrangement is less accurate than fixed centres by the amount of the headstock radial play plus the degree of eccentricity at the centre. These sources for error are usually minute in a lathe in good condition. However, if the results of your efforts are not truly cylindrical look well to these areas.
The versatility of centre turning lies in the fact that there are a number of different ways to support the work. A little creativity will help a great deal when deciding on the best way to set up a particular turning job.
The normal mounting method is {if the part is large enough to permit drilling a female centre on each end] to mount it on the lathe between male centres. Use female centres on each end of smaller parts.
These are fine when the part is robust enough not to self-destruct while removing the necessary material to form the part. What is to be done when one end of a workpiece is too fragile to support the cutting loads?
These special cases are where creativity plays a role. One example is the centre wheel of a common Pendule de Paris round French clock. Often the taper pin hole for the minute hand splits out. These arbors are often tapered and therefore are hard to grip in three-jaw chucks or collets.
Even if a collet will support the pinion end, there is still about an inch and a half hanging out from the face of the headstock.
Of course, the wheel and hub could be removed, the work completed and then the wheel and hub could be remounted. That's a lot of work and there are many ways to get it wrong. But then I wouldn't get to describe the nifty way of using dead centre turning to solve this problem.
Prepare a special female centre for the pinion end by drilling a hole in the end of a brass rod just .01mm larger than the pivot three times deeper than the pivot is long. Make certain the pivot is a slip fit in the drilled hole and that the hole itself is on centre.
Make or buy a hardwood disk about one inch in diameter and about 3/8 inch thick. Hobby shops sell wheels for wooden toys that work well. Drill a hole in its centre just big enough to be a press fit on the OD of the female centre described above.
Select a piece of annealed steel or brass wire of a convenient size. This wire will be used as a driving dog to turn the workpiece. The length will be determined by the distance from the face of the disk to the wheel crossing. Drill another hole about a 1/4 inch from the edge of the disk and press the wire in place.
Use a very sharp lozenge shaped graver to cut a female cone using the hole as its centre. When the female cone is properly formed only the shoulder of the pivot will contact the brass.
The centre shaft taper is a disadvantage as far as using collets is concerned. However, for this repair it can actually be made to work to our advantage.
Now that we've provided a way to support and rotate the pinion end, we can turn our attention to the broken end of the centre shaft. Most watchmakers lathes are equipped with a tip-over tool rest. This can be easily used as a base for a dead centre jig. A suitable size angle iron can be clamped to the lathe bed for a tool rest while the tip over is used as a jig. {Unless of course, you have 2 tee rests.}
Begin by slipping the tee portion of the rest from the base. {Don't panic the rest itself will not be modified in any way.} Measure the diameter of the rod part of the tee. If you're lucky it will be a standard steel size. If not, turn a piece of steel 1.75 inches long to the diameter you measured. Make sure its a nice slide fit into the rest. Press it all the way to the bottom.
Install a very sharp male centre in the head stock of the lathe. Attach the tool rest with the rod in place to the lathe and slide it carefully at right angles to the ways so that the centre scribes a line on the rod. Remove the rod and cross drill a hole through it exactly on the scribed line. The hole needs to be 25% larger than the diameter of the centre shaft at its end. Place a 60 degree counterbore in one end of the hole. Polish the counterbore.
Locate a tapered reamer or broach that closely matches the diameter and taper of the centre shaft. It doesn't need to be a perfect match, just close.
Select a piece of brass rod 3/8 of an inch long and about 3 times the diameter of the end of the centre shaft. Chuck the rod up in the lathe and through drill a hole slightly *SMALLER* than the diameter of the centre shaft end. Turn a 60 degree angle on the end of the rod. Polish the cone to provide a smooth bearing surface. Turn the work piece around and {using the broach you selected}ream it until the centre shaft will go far enough to allow turning on the free end. Press the male cone firmly in place on the shaft. DO NOT REMOVE IT UNTIL THE REPAIR IS COMPLETED.
Install the female centre in the headstock. Don't forget to install the lathe dog. Place the pinion end of the centre arbor in the female centre. Then slide the tool post jig toward the headstock until the cone on the centre shaft is captured by the counterbore on the rod. The work should be adjusted to turn freely with no end or side shake. Tighten the tool post and use a small clamp to prevent the rest from lifting while turning. Place a drop of oil on the cones to reduce friction.
Make a note of the precise distance from the cannon pinion shoulder to the centre of the taper pin hole. Also make an estimate of how long the finished shaft would have been. Install another tool rest in a convenient position to face off the broken end of the arbor. If no extra tool post is available, clamp a suitable size angle iron on the ways to use as a temporary tool rest.
After facing the end flat, select a 1 inch length of high carbon steel rod about .003 inch larger than the end of the shaft. *Do not remove the male cone from the centre shaft*.
Silver solder the steel rod to the end of the centre shaft. The slightly oversize rod makes the alignment a little less critical. Carefully turn away the rod material to match the original taper of the centre shaft. Mark and drill the hole for the taper pin. Part off the excess rod length and finish shape and polish the repaired shaft.
Remove the male cone from the shaft.
Steve Callihan, CMC, FBHI Deltona, FL
