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By mistake I ordered a set of Forstner bits that are all metric (mm) I am seeing g that for wrenches there is this mapping

enter image description here Does it work for Frostner bits or anything else as well?

MiniMe
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    it works for drills – Traveler Dec 23 '22 at 01:13
  • More or less according with this https://tengtoolsusa.com/blogs/news/sae-to-metric-conversion-chart – MiniMe Dec 23 '22 at 01:47
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    Ironically, the worst application for that chart is probably hex sockets and drivers. For any high torque or small size hex, if the sizes don't match well something will strip. Drill sizing being a bit off is much less likely to cause issues. – blarg Dec 23 '22 at 03:21
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    this chart works for wrenches if you dont mind rounded fasteners – Tiger Guy Dec 23 '22 at 07:21
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    That chart is ridiculous, and an invitation to damage both tools and the equiment being worked on. – kreemoweet Dec 23 '22 at 17:03

3 Answers3

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No, they are not the same sizes.

That chart you found, while it says what you want to hear, is not the truth. The only one that is quite close is 3/4" and 19mm. The others are quite far off, as much as 0.5mm off.

An inch is 25.4 mm by definition. So you can simply take the fraction you want, multiply by 25.4, and that will tell you the exact metric size that it is. Then you can round up or down to the bit you have, as suits your application.

Harper - Reinstate Monica
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It is close.

If you need exact size can use any search to find. 1/2 inch is 12.7mm, 11/16 inch is 17.4625mm.

crip659
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  • Yeah I am seeing a complete comparison here https://tengtoolsusa.com/blogs/news/sae-to-metric-conversion-chart. I am going to use european hinges for the cabinets I will buil and I will build some jigs as well for which I could use metric machine screws so I am keeping them – MiniMe Dec 23 '22 at 01:45
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    Converting can be hard. It is best to keep to one system than converting, can see this and see better people than us make mistakes. https://www.mentalfloss.com/article/25845/quick-6-six-unit-conversion-disasters – crip659 Dec 23 '22 at 02:30
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Just buy both sets.
Though 99% of everything I do is metric, there's still the occasional ancient structure using imperial… so right down the bottom of the spare toolbox I have a lot of old spanners & sockets in 'old money', even ASF, BSF & Whitworth. I don't really know [or care] exactly how they differ, but that's what's stamped on the body & sometimes it's just the right one for the job.

It's no good buying expensive spanners if the first time you try on a rough approximation of the right size you round off either spanner, bolt head or both.
There's always that 'fun' moment too, where it half rounds but then jams solid & you have to bang it off with a hammer, or it suddenly skids & you skin all your knuckles.

I also don't see the point in trying to buy a spanner of 17.4625mm when there's one already made in that size - it's just that they call it 11/16"

At least in the US, as far as I'm aware, you only have 'one set' of inches.
There's one more 'fun' fact.
This was resolved in recent times, but historically, a US inch is 25.4000508 mm, a UK inch is 25.399977 mm so if you bought spanners/wrenches/sockets from Germany - & why wouldn't you, they make some great tooling - then you wouldn't be absolutely sure which inches you were getting. Though the legislation on inches finally resolved itself to being 2.54mm, this was amongst the reasons the rest of the world moved from medieval "one open, one shut, the span of my hand & half a brick" measurements based on the length of some king's nose or arm [US, Imperial, ASF, BSF, Whitworth, etc etc are all different "inches"]… to metric.
Just look at the madness "inches" were in the UK…

enter image description here

Image - http://www.baconsdozen.co.uk/tools/conversion%20charts.htm

Tetsujin
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  • For a time there was a tiny difference between British and American inches as you point out, but that changed for practical purposes in the 1930s, and for legal purposes in the 1950s. Both countries now use the definition that an inch is exactly 25.4 mm. – Mark Dec 23 '22 at 18:47
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    @Mark - you tell my old chunks of iron in the garage that. They won't believe you;)) They're not going to change their sizes because some legislation decided they should - this stuff all still exists, hence the toolbox of 'old money' spanners.. Same as telling my old wiring it should be brown & blue is not going to change the fact that it's actually red & black;) – Tetsujin Dec 23 '22 at 18:51
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    @Mark - tbh, the last bit is all just fluff. I'm going to bin it - it's not really relevant to the answer itself. – Tetsujin Dec 23 '22 at 18:59
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    While I agree that existing devices don't magically change themselves to comply with new legislation, I think your fun fact is not completely accurate. The inches were defined differently in the past, but have been exactly the same for over fifty years. – Mark Dec 23 '22 at 19:00
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    @Mark - see next comment above. Last bit gone, not really relevant. I'll delete my comments too in a minute – Tetsujin Dec 23 '22 at 19:00
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    I like the information - I wasn't aware of that fact. I would favor keeping it there but just noting that it was in the recent past and has been resolved. – Mark Dec 23 '22 at 19:01
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    @Mark - OK, I rejigged it to be a bit of 'fun history'. Better? – Tetsujin Dec 23 '22 at 19:04
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    BSF and Whitworth tools are sized by the side of the bolt shank, not the size of the bead or nut, SAE and metric tools are measured across the flats of the fastener head "A/F". – Jasen Dec 24 '22 at 01:40
  • You want something crazier? I have a car built in 1973, that has 2BA bolts in one part of the steering, standardised in 1903.... yes the car is English; I didn't really need to state that. – Criggie Dec 24 '22 at 09:25
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    @Criggie - In the 90s I had a BMW 735SE [2nd hand - very posh, but very cheap, ex-demonstrator model so it had every toy known to man in it]… which had metric wheels. I actually never even knew that all vehicles around the world have wheels sized in inches, to this day. They did it so you could only buy one specific tyre to fit it, rated to 175mph. If they'd put regular wheels on, it would have been possible to put a lower speed rating on. [That made them damned expensive too:\ Eventually, they stopped making those tyres so I had to change the wheels. – Tetsujin Dec 24 '22 at 09:35