Introduction

[rpbidgood]

Hi,
    I'm a teacher (Physics) in a fairly large comprehensive school . I am interested in a number of different branches of model making, aircraft, cars etc. but my first love is probably boats. I intend taking early retirement next year  , and hope that this will give me a lot more time to devote to my hobbies. I am currently trying to finish a slightly enlarged version of the Santa Ana, a Graham Goodchild design given as a free plan in Model Boats, August 1989. I finished the boat several years ago, but I only had access to a few hand tools and a kitchen table for a bench, and, although I was satisfied with the boat itself, I had difficulty with the pitman arms etc. and had to build in a fair bit of slop in order to get the paddle wheel to turn effectively, as a result, when the motor turned, the drive mechanism clanked and rattled alarmingly. Since then I have acquired a nice shed complete with a small lathe, pillar drill etc. Having access to all this equipment I reasoned that it should be relatively easy to improve on my previous effort. Unfortunately, despite a few weeks effort, I am still having difficulty with the cams/ pitman arms locking as they rotate. I have turned 4 disks that act as the cams, and have made them as identical as I think I can. Similarly, I have made the arms as accurately as I am able. It seems to me that the tolerances are tighter than my abilities. Has anybody else had similar problems? I could increase the diameter of the cams by nearly 50% - would this make the tolerances easier?
Thanks,
           Keith.

[Dinosaursoupman]

Hello Keith,

Welcome to the group.

The problems inherent with trying to duplicate the action of pitman arms has been a subject for discussion before. Check this thread;
http://www.paddleducks.co.uk/smf/index.php?topic=2838.0.

With your background in physics you should be able to pick up on the difficulty we have trying to duplicate the motion accurately. Me, it took awhile to figure it out.

Anyhow, welcome and enjoy what the group has to offer,
Randy

[derekwarner_decoy]

Hi PD's...& welcome Keith...setting accurately, the lengths of components be they wheel cranks or what ever comes back to basic mechanics & you may need to improvise with the aide of some simple but robust jigs

If you needed say 3 1/8" pin to pin for your wheel cranks.....a 5" long steel plate with two vertical pins inserted @ the required center distance
would enable you to use this as a soft soldering jig....& hey presto...both arms would technically be identical in pin to pin dimension

 .....if I am on the right direction with your question here, it is not the ultimate accuracy of 3.125"...as if both arms were 3.127" pin to pin all would be fine & any cumulative dimensional errors are taken up in clearance bolt holes etc - Same applies to all other "pairs or sets" of components in a similar construction

http://au.youtube.com/watch?v=56HjudkACdQ&feature=related

Interestingly.........the Delta Queen appears to have her wheel cranks set at approx 30 degrees off axis........Derek

[rpbidgood]

Derek and Randy,
                        Thank you for both replies. (i) The sketches of the different motions are fascinating - I might even attempt the crossed coupling. (ii) I did use a very simple jig to set the distance between the cams?, but it clearly needs a more sophisticated jig (plan C). It is a very interesting problem, and one that is clearly a lot more difficult than I naively imagined. I will keep you posted.
Thanks again,
                   Keith.