Stuart Plans, I dislike under constrained dimentions

[Richard Carlstedt]

During My career as a Manufacturing Engineer , it was my job to look over drawings released by the professional machine designers.....believe me it was not a simple task, even though they used CAD for the drawings (2D) The error rate was 50 % and these were mechanical designers. In our hobby I find many errors in the various concepts and here are some reasons why.
First, If it is a casting kit, the person making the drawings did the work before the castings were made. SO expect variations in dimensions
Second, there are concepts that home shop guys may not aware of - Like what are "Air Fits" ? --that is when a Dimension has no relevance to the operation of the machine (steam engine)- you see it on some drawings that might show a dimension from the edge of a casting to a bearing mount for instance.. Bogus dimension as the casting can (1) change (2) has no effect on the bearing, the crank centerline is the deciding factor.
When ever a dimension is given that only sees air...it is not important. A good example is if the drawing says the cylinder should be 1-1/2" OD and you measure the casting and it is 1-7/16 or 1-5/8" ---it means nothing , what is important is the distance from the cylinder centerline to the valve chest. And last is the absence of Tolerances- They can give you pleasure if you stay inside them or can be a nightmare when not on the print. The Designer expects you to "Know" what the range should be. For guys not familiar with machining , that is a destiny of failure and it's too bad, because we lost a participant in our hobby.
Many more things, but not here
Rich

 

[Jasonb]

Regarding tolerance, little point in giving a beginner hobby guy say an H7 tolerance on a hole if he does not have the ability to measure the hole to a tenth or two. Elmer was on the right track just giving "Close, smooth or press, etc" though still needs experience to know what each should fit and feel like.

 

[Charles Lamont]

Print this, laminate it and keep at the drawing/workshop desk

 

[olympic]

During My career as a Manufacturing Engineer , it was my job to look over drawings released by the professional machine designers.....believe me it was not a simple task, even though they used CAD for the drawings (2D) The error rate was 50 % and these were mechanical designers

Not surprising. Any school system of which I am aware defines 50% as a "pass", and there are very few "100-per-centers" out there.

I am an amateur musician, but all musicians are expected to perform flawlessly. On one occasion, our conductor decided to emphasize this by using the 50% standard, asking us to play every second note wrong in a piece. You should be able to imagine the results---but we passed....