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Metric/Imperial Threads

Pushrod Twin

Well Known and Active Forum Website User
VOC Member
Another weird way of defining head hexagons is wanting a spanner for a BSW bolt 5/8 or so, you cannot tell from that type what size the hex head is across flats. Got a different screw with unknown thread, so what spanner will you hunt in your garage for this case ?? So get out a vernier and take the size. Then look up a file to compare that size to any norm that happens to have same spanner ?

There is actually no need to define head hex sizes. Our British ancestors, to wit; Mr Whitworth, had a perfectly logical system. The spanner was determined by the important part, the bolt shank size, eg, a 5/16" fastener used a 5/16" marked spanner. He had determined what he considered the optimum hex size by calculation, that became a standard, and the man on the tools selected his tool to fit the relevant shank, easy.
Unfortunately, it came undone when the head size standard changed by 1/16" to BS, British Standard, that does create some confusion, and requires a little more simple education.
The real confusion came after WWII when Captain America bullied the British into "Unifying" thread systems across the pond on the premise that if they were to fight wars together again, then they should share a common thread system across their munitions to avoid some of the mechanical disasters which reputedly happened as a result of not being able to screw each others bits together. (for want of a better expression!) :mad:
The US conceded to a few minor changes to what had been SAE and called it "Unified"
They must have been a little disappointed when the UK "metricated" in the '70's along with us Antipodeans & the Canuks!
To bmetcalf & Mike 40M's point regarding vehicles which suffered the evolution of starting with BS Standard fasteners then progressed to Unified, British aircraft which had a longer life, could have every thread system known to industry, Whitworth, British Standard, Unified, and Metric, not to mention weirdo fractional inch hex sizes which fit neither BS nor AF standard spanner sizes. Long live Messers Rolls & Royce!

Mike 40M

Well Known and Active Forum Website User
VOC Member
Of course the whole thing is blured now most of the time on a metric lathe I need a imperial dimension. So I open it to the imperial measure I want then zero the gauge switch to metric. then measure what I need to take off and measure and cut till I get to zero. I keep promising to fit a DRO system to my Harrison but I am quite happy....
Lathe DRO is the best thing to have since beaded edge tyres came out of fashion. Or pre sliced bread.
Push a button to change between metric and inches. Another button to change between radius and diameter. No more need to remember how many revolutions you've turned a handle.
Even more useful on a mill.


Well Known and Active Forum Website User
VOC Member
Adding a DRO to your lathe is extremely effective as well, when you got a number of change tool holders - a necessity in itself - so you can number them for quick change of tool reference in the DRO. You DON`T dial in the amount of material you want to machine off until you get zero shown. This would be ineffective when you want a number of steps on the workpiece. Logic is to zero each tool on the front face and do a skim anywhere for measuring diameter, check after skimming the diameter and dial THAT size into the DRO for actual tool number. Do same for other tools with their respective numbers. You then just change toolholders and know what size you machine, no need for checking every minute what you actually have. It is a very quick operation like that for several workpieces you want to produce, really.
After 30 years of having my fat lathe I added a DRO finally a few weeks ago for machining two sets of big dual 200 mm drums for two Bramptons, would have been extra lengthy to do without DRO. And my smaller lathe in second floor with DRO would have been overwhelmed by the 250 mm size of plates. So these days get your DRO for fun in the workshop, no more expensive from China - and you´ll never regret this. Instead you´d like to kick your ass for not having done this a lot earlier !



Vincent Brake

Well Known and Active Forum Website User
VOC Member
With these no more surch for correct spanner.

The best invention after the hex. Nut.

Knipex. With the 180mm i can undo even a 1/2" whatefer... Nut.
NO slipping at all.
NO damage to any Bolt.
Even better grip than an open spanner.
Only disadvantge is they are bigger to the outside...

BTW. German invention...
After they were confused reading this thread...