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            Making a Jacot Tool


My 20 year old home made Jacot tool is worn and past its sell by date, it’s not surprising the amount of work it has done over the years. It was made to be mounted on the lathe cross slide and overcome the limited space on a Unimat lathe.

1 The old Jacot tool

One of the drawbacks to this design is that each runner has to be centred before use. I looked at several pictures of the traditional Jacot tool and noticed that there was no self centring device fitted to the examples I saw. I wanted to make a tool that would be easy to set up and finished with the tool in this article. I made the body of the tool from Ύ” square steel and drilled 2, 10mm holes for the tailstock spindle and the Jacot disk spindles. I had to reduce the tailstock spindle slightly to fit the chuck capacity of the Unimat. I made a mistake on the centre distances of the 2 spindles and this resulted in my disks being oversized. I made mine at Ύ” distance and a better measurement would be ½”.   As stated above, my spindles were made from 10mm stock but a better option would be ½”.

 With ½” spindles you can get 8 or 10 divisions for each disk but you will have to make the body of the tool from 1 inch stock to give the locking screw more threads.

You can use the lathe dividing gear and a drilling spindle to make the divisions or do as I did and use a piece of scrap hexagonal brass or steel as an indexing device. My dividing gear was tied up on another job at the time and I resorted to using a jig for the dividing. Having scribed a line around the diameter of the spindle I used eyeball methods to make the divisions. Use a sharp No1 centre drill and drill a shallow cone shape into the outside diameter of the spindle.

Don’t worry if the holes are not 100% on the centre line as a later stage in the making of the tool will overcome any discrepancies. My Jacot has 6 disks and the runners are from -6mm up to 7-6mm in 2/10mm increments. The first 3 disks are 1/8” thick and the others are 3/16”. The 6 spindles were faced off in the lathe and drilled and tapped 4ba for a depth of ½”. Don’t forget to turn a small depression on the inner most portion of the spindle to avoid a wobble on the disk. The disks were cut on the band saw and you can use the scrap plate from earlier clock projects as I did. The size of the rough disks can be calculated as follows. If you are using the ½” spacing for the spindles, this will give a disk diameter of 1 inch or 25-4mm plus the largest runner diameter and a little extra to be safe. As an example take, a disk that has its largest runner as 6mm, 3mm will be below the centre line of the lathe and 3mm above, add 2mm for safety and we end up with a diameter of 30-4mm. The rough disks were drilled for a 4ba clearance and counter sunk to a depth that completely hid the screw head. Each disk was secured to its spindle with a good dose of Loctite and really tightened with a screwdriver, as an added precaution the 3 larger disks have 2 taper pins inserted either side of the securing screw. The body of the tool is tapped 4ba and an indexing screw is made with a tapered point that engages the holes in the spindles of the tool. Install the tool in the lathe tailstock and secure. Place a spindle into the tool and lock in place with the set screw, the disk is now centred and can be drilled to the chosen runner size. Using a No 1 centre drill, spot all the disk positions at one session. The smaller holes do not present a problem but a word of warning when drilling the larger ones! An ordinary twist drill will try to act like a wood screw when drilling brass, this can ruin the part you are working on or worse, cause injury. I have a limited amount of modified drills up to ½” diameter (for brass only) and, my ordinary drills are a mixture of imperial-metric and numbers/letters. When you get up to about 4mm it pays to be careful and open the holes in small bites. The way I got around this problem was to drill the disk 4mm and mark the hole with a felt tip pen, index round for the next one and drill this one 4mm and all others on that disk. Replace the drill for a 4-2mm and drill the next hole after the pen mark (mark this hole) and drill all the others 4-2mm and so on. When the disk is marked at every runner hole you can go onto the next disk. Some of the runner sizes proved to be beyond my range of drills and so I cheated, I used a cutting broach! I have a 7mm drill but not a 7-2mm or 7-4mm. I gently placed the broach into the drill gauge at the 7-2mm hole and marked with a pencil, I broached out the hole from either side down to the mark. I did the same for the 7-4 runner and anyone who is doubtful of this idea should take heart. I measured between the pencil marks and this was 6-5mm, so to open the hole from one size to the next the broach will have to move a full 6-5mm so the error should be minimal. Turn the disks down to 1 inch diameter and they are finished. All the runner positions are clearly marked with number punches and in the future when I am doing pivot polishing, all I will have to do is fit the worn pivot into the drill gauge and select the correct runner. As the tool is self centring I will not have to worry about setting up. The wheel carrier was made from a slice of brass bar 1 inch diameter, drilled 3/16” to accept the hollow cone punches from my staking tool, and has a set screw to hold the carrier. I have used the same punches in my Colin Walton depthing tool for smaller clock work.

2 The Jacot tool set with 2 extra
holes for future disk additions

3 Drilling the divisions by eye methods


4 and 5-- A French intermediate
 wheel set up in the 1mm runner

6 The Jacot tool next to the staking set


David Creed     

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