Re-scaling a drawing

[Mark P.Participant @markp]

[Mark P.Participant @markp]

Hello to all,can anyone help me with an answer to this question.I am getting ready to start building a horizontal hit and miss engine from a set of drawings which I have aquired,I wish to build it twice the size of the drawings,so do I double the size of everything including the valve sizes?

 

Regards Pailo.

[Steve GarnettParticipant @stevegarnett62550]

It’s hard to be 100% sure without seeing the drawings, but generally the answer is a straightforward ‘yes’. Certainly this would be true for dimensions running at right-angles to each other – measurements from a single reference point generally fall into this category. One place that you might have to watch this a little more carefully would be on a ‘helpful’ dimension given that looks like the hypotenuse of a triangle, where the other two side lengths (the ones at right-angles) were given. That certainly wouldn’t double – you’d need to apply Pythagorus’ theorem to arrive at the correct answer for that. But generally I don’t think that this applies to engineering drawings, so it shouldn’t be an issue. But you should note that angular dimensions wouldn’t change – these would remain the same regardless of the scaling. So 45 degrees remains 45 degrees, however large you make it!

 

As for valves – well if you have radius or diameter dimensions, then yes, double them – they will remain to scale. But as I mentioned, not any angles – they stay the same.

 

If in doubt, download one of the free CAD packages and have a go at redrawing some parts of the engine with your new scaling, and see how it looks.

[Mark P.Participant @markp]

Hi Steve,thanks for the reply.It is as I thought double all dimensions except angles.I wasn’t sure about re-scaling the valve dimensions.

Regards Pailo.

[Gordon WParticipant @gordonw]

Don’t forget the cyl. capacity , varies by the square of the bore. So you might have to resize valves, induction , exaust etc.

[Steve GarnettParticipant @stevegarnett62550]

 

Posted by Gordon W on 09/10/2011 15:23:10:

Don’t forget the cyl. capacity , varies by the square of the bore. So you might have to resize valves, induction , exaust etc.

I don’t think that this is too much of an issue, mainly for two reasons. If you increase all of the dimensions, then you are also increasing the amount that the valve will open. And since doubling the diameter of a valve increases its area by a square law ( πr² ), you’ve pretty much covered this, I’d say; you’ve doubled the circumference, increased the opening proportionally, and also increased its heat dissipation. Chances are that if you are being really fussy, you could probably improve the gas flow by careful attention to the port design anyway. Is this really critical on a hit and miss engine?

Edited By Steve Garnett on 09/10/2011 22:30:48

[Martin WParticipant @martinw]

Steve

 

Just a quick observation re scaling factors and triangles, if you double the length of two sides the third side will double in length as well. The classic right angled triangle of 3 x 4 x 5 scales up to 6 x 8 x 10 as will all straight sided triangles provided the the two known sides are scaled by the same factor.

 

Cheers

 

Martin

[Steve GarnettParticipant @stevegarnett62550]

Posted by Martin W on 10/10/2011 00:06:22:

Steve

 

Just a quick observation re scaling factors and triangles, if you double the length of two sides the third side will double in length as well. The classic right angled triangle of 3 x 4 x 5 scales up to 6 x 8 x 10 as will all straight sided triangles provided the the two known sides are scaled by the same factor.

 

I don’t think I’d had enough coffee at the time of the original answer. I thought about it later, and realised that it is indeed true! It really is only angles that you have to leave alone, isn’t it?

Edited By Steve Garnett on 10/10/2011 00:56:05

[Eric CoxParticipant @ericcox50497]

Yep, leave the angles alone

To take it to an extreme, if you had two sides at right angles to each other and you double the angle ie. 90 to 180 then the two sides would be in line with each other. !!!!!

[Donald WilliamsonParticipant @donaldwilliamson36047]

Hi Pailo,I have just finished building the Little Angel hit and miss engine at twice the scale and I didnt have any problems,it runs well.

Donald Williamson

[Author]

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