Imp to Metric conversion round up or down

This topic has 8 replies, 6 voices, and was last updated 16 December 2019 at 18:00 by not done it yet.

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16 December 2019 at 15:33 #441854
Garry Smith 7
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@garrysmith7
Because of the tooling I have I want to convert my mill engine plans to metric from imperial. Is it better to round up the conversion or round down. Ie 7/16 converts to 11.11 mm do I go to 12 mm or down to 11 mm. Thanks for reading
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16 December 2019 at 15:33 #9985
Garry Smith 7
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@garrysmith7
Converting plans and drawings
16 December 2019 at 15:36 #441855
not done it yet
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@notdoneityet
You won’t find many 11mm screws, bolts, taps or dies on offer?

Edited By not done it yet on 16/12/2019 15:38:07

Edited By JasonB on 16/12/2019 16:14:39
16 December 2019 at 15:42 #441856
Lainchy
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@lainchy
Are we talking threads? Or just machining in general

Edited By JasonB on 16/12/2019 16:14:54
16 December 2019 at 15:58 #441864
HOWARDT
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@howardt
You can round to what ever you like. If a part is made from stock, then it is the nearest either way. Other than the look of things it doesn't matter what you make something so long as it fits together. Remember that most standard size reamers are whole numbers, while upto 10mm is easily available in 1mm steps, over 10 may be not. Also screw sizes, whilst small screws are available in the odd 0.1 steps bigger ones are in 1mm. I use metric conversion on the fly and just check what the mating part is and make to suit.
16 December 2019 at 16:12 #441868
JasonB
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@jasonb
A lot will depend of what you are rounding. If it is stock diameters then you will need to go down for some eg 1/8" becomes 3mm, 1/4" becomes 6mm and up for others like 3/8" which becomes 10mm. 7/16" and 9/32" are ones that can go either way.

When it comes to the engines layout you can keep things like stroke (piston and valve), conrod length and engine centers the same as a direct conversion is possible and you don't have to start thinking about valve events being altered or parts clashing if your rounded up stroke means the piston moves further in your rounded down cylinder length.

There is also another option if you don't mind having a slightly larger or smaller engine and that is to use the conversion factors of 1/16" = 1mm for a smaller engine or 1/32" = 1mm for a larger one. I have used this quite a few times and generally it works out well with the odd unusual size being tweaked for stock sizes.

What was the mill engine again?
16 December 2019 at 16:17 #441871
Garry Smith 7
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@garrysmith7
Thank you for the replies. The engine is Tina from GLR Kennions. I was intending to use metric stock and my milling cutters drills etc are in metric. I have just ordered metric hand reamers so hence the question.

Edited By Garry Smith 7 on 16/12/2019 16:18:01
16 December 2019 at 16:29 #441875
Bazyle
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@bazyle
Posted by HOWARDT on 16/12/2019 15:58:53:

Remember that most standard size reamers are whole numbers, while up to 10mm is easily available in 1mm steps, over 10 may be not.

An when you order an 11mm reamer from an ebay supplier "location London" 14 miles away it will take 2 weeks and be 7/16. 'Why you complain? loose fit easier to put together'.
16 December 2019 at 18:00 #441901
not done it yet
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@notdoneityet
I’m not sure how I double-posted on this particular thread! Maybe JB moved my (identical) reply from a duplicate thread?

Bazyle,

They had to order it from china is why it took so long! Likely ordered the cheapest they could find, and nearest to your order size, to maximise their profit on the ‘deal’. I was caught by this trick once, when I wanted the identical part offered from china, but I needed it much sooner. They complained about my negative feedback but did not pursue it.
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