It generally means that the 4 holes will have their ctrs placed evenly around a circle of 7/32" diameter but that sounds very small. What engine is it?

Following your edit something just over 3/4" PCD would be more likely.

Edited By JasonB on 21/06/2021 19:03:12

Hi Roy,

When calling for holes on a pcd it is necessary to describe the hole and specify the pcd.

e.g. 4- holes 6mm dia on 24mm pcd.

I suspect something is missing from your drawing.

David.

It means it is a misprint I think. Pitch circle is the diameter on which the 4 holes are positioned. In this instance it must be more than 1/2 and less than 1 1/16. Scale it off and let us know what it comes out as. I wouldn't be surprised at 27/32, making the holes 7/64 from the outside edge and 11/64 from the bore

How to post pictures here

Yes 3/32" hole size and I'm with Howard 27/32" sounds likely.

Roy,

The side view drawing shows the holes between 1 1/16" and 3/4" so 7/32 must be an error or they end up in the bore.

ME drawings seem to be full of errors and omissions in general. It's wise not to make stuff at random without comparing the fit.

As Duncan says 27/32 (0.844" ) plots Ok and looks like the drawing.

The PCD or Pitch Circle is the diameter of the green dotted circle that should have been 27/32" not 7/32" and that is what the ctr of each of the 3/32" holes sits on.

Edited By JasonB on 21/06/2021 20:01:54

As most drawings are produced by amateurs working in home workshops, and detailing what they have actually made the drawings are only as good as the skill of the designer in fully detailing ( or remembering ) any changes which had to be made as the build progressed. All drawings thus should be checked as far as possible for correct dimensions as it can lead to complications. I am making a loco donkey pump at the moment, using magazine drawings. The height if the pump block was given in its main drawing at 1/16” less than shown in the later drawing of the water passageways . This I missed this, so it gave me headaches to sort out, solved by making the top plate 1/16 thicker to make the block big enough for the stroke of the pump , but I still had another couple of mods to make to sort things out. The drawings for this loco build are tremendously accurate with tiny clearances and I trust them! It is essential as even with careful checking, a component assembly may have been made a long time before adjacent components are fitted, and mistakes in the drawings may not be spotted even with diligent care But as seen problems still arise. I suspect the problem this time was with the draftsmanship of the magazine employee rather than the designer.

if you make and measure where do you start? Something is needed to measure! It’s all part of the process as experience and knowledge of interpretation of drawings and the reality of construction grow. But it also needs common sense, you can’t drill holes in fresh air! Your case was a simple printing error . One of the advantage of producing 3D cad drawings is that the dimensioning is automatic to what has been created and such printing errors in both your and my case would not take place! But again, that depends on the designer actually doing that my designer does not use cad, and I am constantly amazed at his skill.

It depends upon the quality of the drawings. The drawings for my traction engines are riddled with errors and omissions. So I am re-designing in 3D CAD from whence I can create 2D engineering drawings. I also create assemblies in CAD so I can check fit and function before generating drawings, like this assembly of the valve gear:

Apart from the errors and lack of information on the drawings other reasons for CAD modelling are to account for metric, rather than imperial, material and to add features that are on the full size engines but not on the drawings.

I'm idle and I expect to make parts once, and also expect them to fit each other even when made months apart. I aim to do my faffing and fitting on the CAD system, not in the workshop.

Andrew

Since you have already identified two errors in the drawings, it would be time well spent in comparing the dimensions of parts meant to fit together to ensure that they are compatible, and correct.

You don't want to find that the piston is drawn as 13/16 diameter for the 3/4 bore!

"Measure twice, cut once" is a good maxim to remember, especially when you are gaining experience.

Howard

Hi Roy!

We all learn something, often every day.

Mistakes are not unusual (We are all human and fallible ) the gag is to spot and correct them before too much is lost.

Don't be like me and rush! things

You will always find help and advice on the Forum.

VAST amounts of experience of Engineers and Model Makers on here, no matter what the specialisation.

You will find it helpful. also to find and join a local Model Engineering Club, for face to face help / mentoring / demonstration.

Keep up the good work

Howard

Roy,

A lot of us do not have a machining background, me included.

Finishing the Rob Roy might make more sense, it's much simpler than Virginia, but still a proper engine + far more people have built one.