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Modifying a piston selector gun to rotate. By Stuart Calvert After having modified the rudder on the Artemis to make her more manoeuvrable, she still didn't turn that well. It can never be expected that she will out turn a DD, but a good captain attacking the Artemis will know to keep away from dead astern and therefore the gun will be almost useless. I decided to come up with a method of rotating the gun bearing in mid the limited space in the stern. For this modifiaction, avoid using any kicker to make the CA glue cure quicker. When kicker is used, the CA glue will lose a lot of it's viscosity and through capillary attraction, may run into the bearing causing it to seize. It is much better to let the assembly sit there for a while and have a bearing that works! |
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The original setup showing the gun in a simple bracket made form 3mm plywood and mounted to the bottom of the hull. This also shows how close the rudder servo is to the gun. |
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The "T" of the gun is almost perfectly vertical in the bracket with a lean of a few degrees to allow the bbs to feed onto the top of the piston correctly. The slight depression of the barrel was done by bending the copper riser as close to the elbow as possible. The reason for this will be apparent later |
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The 2 parts necessary for the rotation modification. A ball raced 1/4" ID bearing (as airtight as possible) and a bearing holder turned form a piece of aluminium. The bearing holder is stepped and is resting upside-down on the smaller OD section. Ruler for scale. |
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The bearing sits in the bearing holder. The excess solder has been filed from the copper riser so the bearing holder can sit as low as possible so the deck will still fit on. |
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The bearing was 3/8" in height. Therefore the copper riser was cut 3/8" above the top of the "T". The centre of the plastic gear was drilled out to just under the size of the top part of the holder. The aluminium holder was then heated with a blowtorch and the plastic gear melted on. After cooling some CA was added for good measure. The outside of the bearing was then glued into the bearing holder. BE VERY CAREFUL not to use too much glue as it may ooze between the top face that seats in the holder. Any glue around the bearing cover will find its way in and make it seize. |
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The cannon (with 3/8" of the riser only) was placed
back in the bracket. After degreasing the cannon and aluminium bearing
holder, a small amount of CA was smeared around the riser and the bearing
holder positioned. It is very important to make sure that the bearing
holder is as horizontal as possible in both planes. If it is sloping down
toward the stern for example, when the cannon rotates to the side there
will be no depression. This is why the depression must be achieved by
bending the riser near the elbow.
Getting the top surface of the holder horizontal was done by placing a sheet of glass with a small flat metal shim in the centre of the glass on the holder as the glue cured. With the ship on a flat surface the edges of the glass were chocked up to the same level on all sides of the ship while the centre still contacted the bearing housing. |
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The detached elbow (with barrel fitted) was placed in the top of the holder. Remember to remove any burrs from the inside of the riser first as these may interfere with the bb going up the riser when the gun is fired. With what seemed like a reasonable depression, it was glued in place with a little CA glue. Again, use only a LITTLE CA glue. Any glue that runs down between the riser and bearing may glue the rotating surfaces together. The drepression can be carefully adjusted later by bending the riser just below the elbow |
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The final setup with chain drive and rotation servo. There is very little friction when using a bearing so a servo is all that is required. The Cannon is held in place with a cable tie (not present in photo) which keeps it in the correct position and the chain tight. |
| Note: If the bearing is completely airtight then there should be no loss of air pressure and the gun will fire as hard as previously. Any air leaks can be fixed by squeezing a small amount of CA onto a piece of plastic and applying very small amounts with a needle, small length of fine wire or other similar object. | |
| Back to the USS Artemis page to see the finished result. | |
| Back to the Contsruction page and "How To" articles | |
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