# Metric / Imperial conversions



## David Morrow (Sep 17, 2008)

In "Steam and Stirling" there is an interesting article on page 39 about converting plans from Imperial to Metric or vv. He gives an example of a Dutch builder who simplifies the conversion by using 1/32 = 1mm ( instead of 1/25.4 = 1mm). This gives the builder an engine that is about 20% bigger. I would like to build one of Jan Ridder's creations but convert on the basis of 1mm = 1/16 which would result in a somewhat larger engine. Maybe 1mm = 1/32th inch for a smaller engine ? 

I realize that not everything will convert perfectly but many people don't always build exactly to plan anyway.

Any thoughts ?


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## rake60 (Sep 17, 2008)

When I'm converting Imperial to Metric I multiply by 25.4
Metric to Imperial I multiply by .03937

It's not perfect to an infinate number but it is close enough 
for anything I can see on a mic!

Rick


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## David Morrow (Sep 17, 2008)

rake60  said:
			
		

> When I'm converting Imperial to Metric I multiply by 25.4
> Metric to Imperial I multiply by .03937
> 
> It's not perfect to an infinate number but it is close enough
> ...



Yes, but the use of 1mm -> 1/32 in. makes the need for a calculator or conversion chart all but unnecessary. And, much more common fractional & decimal results.


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## greenie (Sep 18, 2008)

OK, here's another way of doing the conversion and it's real easy too, it is not 100 % dead nuts spot on, but it's that close it actually works.


0.040" = 1.00 mm

0.020" = 0.50 mm

0.010" = 0.25 mm

0.004" = 0.10 mm

0.002" = 0.05 mm


It's so quick and easy to do any converts from either Mongrel to Metric, or the other way round, it gets you that close, then it's out with the appropriate micrometer and you can then get it dead nuts spot on.

The machinery I work with, lathe and mill, are all graduated in Metric and to get it converted to Mongrel, well it's just a "chunk of urine" doin' it this way

OK, the purist are gunna shoot this lot down, just 'cause it aint 100% dead nuts spot on, all I'm sayin' is, give it a go and see how easy it is to do the converts.

regards greenie


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## John S (Sep 18, 2008)

I work on a wide variety of parts, some going back well over a 100 years and in that time frame i come across imperial metric and where threads are concerned there is no standard at all.
The metric of 100 years ago it's the metric we know today.
There was a lot of kludging during wartime years to try to achieve a standard that met the axis powers requirements.

We also had a big transition period in manufacturing when we swapped from imperial to metric in the 1970's something the US hasn't had to do yet and when they do [ or if ? ] that will also be different as we didn't have the range of CNC machines that are available now and had to rely on manual machines with limitations.

A classic example of this was Wadkin, the large woodworking machine makers. When they changed over from imperial bearing to metric it was a simple design change to alter the print from say 1.250" to 30.00mm where it got sticky was that 1.250" x 16 thread nut that held the bearing on wouldn't fit, the lathes of the day wouldn't do metric without a lot of shagging about so the designer in his infinite wisdom specified 30.00mm x 16.
Not a problem to the turner as he had to screw cut it anyway.

Result is now we have a shed load of machines, still in everyday production with these bastard threads. Loose a nut for a spindle moulder and you are stuffed. Wadkin's only stock the newer 30.00 x 1.5

I work with what Greenie has said and round up until I get close. Greenies way is only like using verniers to rough out and a mic to finish off. It's well worth learning or even making a cheat sheet up to go above the lathe.

Not wanting to get into the imperial / metric debate as everyone has fixed views but there is one big difference.
Imperial has two methods of measurement, metric only has one.

In metric you get something like 23.81 there is no other way to show this.

In imperial you have decimal and fraction.

The other day I had to do a 3/16" keyway 1 - 1/8" long. Set up on the will plunged down set the DRO and I now have to go 1-1/8" minus 3/16" in distance so working in my head that 15 /16" ??? the DRO don't read in 16ths of an inch.

If that had been a metric keyway it would have been 28mm minus 5 mm so cut along 23mm

.


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## Circlip (Sep 18, 2008)

Trouble is John, we never did TOTALLY go Metric did we ? Not like they had to in OZ. Said before component leg pitching was dictated by the LOTF in the electronics industry for IC's and Processors etc., don't know if they still are but it made laying out PCB's interesting using bisexual measurements n the panels.
 Dave, do the conversions LOGICALLY, if a shaft is called up as 1/16" (1.62mm) and you're using Metric material YOU choose, - 1.5 or 2mm dia, just make sure that the corresponding part is the same. If a block is called up as 1" square material you ain't going to get a piece of 30mm square and machine it down to 25.4mm. Every fortnight I used to spend a couple of hours converting photocopy's of the next installment from ME of an imperially dimensioned twin steam engine cos of my metric machines. :wall:
 Regards Ian


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## David Morrow (Sep 18, 2008)

Circlip  said:
			
		

> <snip> Every fortnight I used to spend a couple of hours converting photocopy's of the next installment from ME of an imperially dimensioned twin steam engine cos of my metric machines. :wall:
> Regards Ian



Is "fortnight" Imperial or Metric ?


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## mklotz (Sep 18, 2008)

There's a simple trick for mentally converting fractions to metric. It relies on the fact that 256 is very close to 10*25.4 = 254.

Keep doubling numerator and denominator until the denominator is 256. Then the numerator divided by ten is the equivalent in mm with an error of only 0.78%.

13/64 = 26/128 = 52/256 -> 52/10 -> 5.2 mm (correct value = 5.159375 mm)


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## John S (Sep 18, 2008)

David Morrow  said:
			
		

> Is "fortnight" Imperial or Metric ?



True story, When we went metric in the 70's we had to hand in all our money and get it changed to new currency.
As you would expect there was load of bitching about this and at the time I worked for a haulage company, one of my jobs was to go down the row of trucks waiting to diesel up at night and check for defects, check lights and fit new bulbs etc

Invariable you would have a crack with the drivers and at this time it was all about the change over to decimalisation.
I started telling them that if they thought the currency was a lot to take in just wait until we went on decimal hours, 10 hors am, 10 hours pm.

Straight away they clocked onto the fact they would be out of pocket, next thing they had a meeting and went straight into the office to get it sorted ;D

Of course the office hadn't a clue what they were talking about .............. ???


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## baldrocker (Sep 18, 2008)

John Stevenson youre a nasty nasty man :big:


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## Kludge (Sep 18, 2008)

baldrocker  said:
			
		

> John Stevenson youre a nasty nasty man :big:



YEs. Yes, he is. Admirable trait, isn't it. ;D

Best regards,

Kludge


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## David Morrow (Sep 19, 2008)

greenie  said:
			
		

> OK, here's another way of doing the conversion and it's real easy too, it is not 100 % dead nuts spot on, but it's that close it actually works.
> 0.040" = 1.00 mm
> 0.020" = 0.50 mm
> 0.010" = 0.25 mm
> ...



At first I wasn't sure where you came up with your metric/imperial conversions but I made up a table on Excel using 1mm = 1/25 inch ( instead of 1/25.4) and your numbers all showed up. All of the imperial equivalents come out to nice clean numbers to 2 decimal places. This looks like the way to go.

BTW, normally I create conversion tables on Excel and then print them for hanging up on the wall by the mill.


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## rake60 (Sep 19, 2008)

If you look at the top left of this web page you will see a *Unit Converter* under the *Menu*.

There are many ways of doing the approximate conversions.
You have a perfect easy to use converter in front of you right now.
We placed it there to be used.

Rick


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## mklotz (Sep 19, 2008)

What is it with you guys? Don't you have a calculator lying on your workbench?

Want the decimal equivalent of 39/64? Just divide 39 by 64

Want to convert 6 mm to inches? Just divide 6 by 25.4

Want to convert 3.5 inches to mm? Just multiply 3.5 by 25.4


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## David Morrow (Sep 19, 2008)

mklotz  said:
			
		

> What is it with you guys? Don't you have a calculator lying on your workbench?
> 
> Want the decimal equivalent of 39/64? Just divide 39 by 64
> 
> ...



As opposed to just looking up at a set of tables grabbing a nicely rounded number? Not poking around at a calculator and getting long strings of odd digits? What's wrong with simplicity? The side benefit is it generally leads to fewer errors. Why make a task more difficult ?


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## David Morrow (Sep 19, 2008)

I made up a table in Excel for both the simpler 1:25 conversion as well as the 1:25.4 conversion. It's a 1 page pdf document which you can print and hang up in your shop for quick & easy reference.

http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item45


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## MadKad (Sep 21, 2008)

This is me and what i do, as I work online if I am not in my shed I do all my working out next to my computer also and I just search google for any working out (waights, sizes etc)

[ame]http://www.google.co.uk/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-GB%3Aofficial&q=3.5+inches+to+mm&btnG=Search&meta=cr%3DcountryUK%7CcountryGB[/ame]


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## chucketn (Jul 3, 2011)

I found a great comparison of imperial and metric bolt/screw sizes here:

http://www.physics.utoronto.ca/~astummer/pub/mirror/Docs/Engineering/Bolt%20sizes%20and%20dimensions.pdf

I copied the chart to Excel and made a printable version for the shop. The web page is helpful but a lot of flaffing about to get the info into Excel and print it. The format of the webpage doesn't favor printing directly.

Chuck in E.TN


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## mike os (Jul 3, 2011)

David Morrow  said:
			
		

> Why make a task more difficult ?



I take it you are not an engineer then? ;D ;D ;D ;D


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## Tin Falcon (Jul 3, 2011)

Dave :I read the same page in steam and sterling and I agree it makes a lot of sense while not a true conversion it allows for a translation for imperial to metric materials and visa versa . And if someone wants to do a true conversion or an approximate one that is ok too . 
There are lots of ways to skin the proverbial cat or as i sometimes say remove the fur of a feline. 
Machining is of of those things ask 8 guys the best way to perform a task and get 10 different answers. 
have fun make parts . 
Tin


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## mzetati (Jul 4, 2011)

mklotz  said:
			
		

> What is it with you guys? Don't you have a calculator lying on your workbench?
> 
> Want the decimal equivalent of 39/64? Just divide 39 by 64
> Want to convert 6 mm to inches? Just divide 6 by 25.4
> Want to convert 3.5 inches to mm? Just multiply 3.5 by 25.4



I have a couple of 'Euro' converters I bought purposely: I set the 'currency' conversion factor to 25.4 to see the metric result as soon as I key in the Imperial value digits.
Works with "39/64=" too!! ;D

Marcello


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## jonesie (Jul 4, 2011)

i have to agree with marv.go to the dollar store and buy some dollar calculators and just do the conversions so you have exact number and then round off if you need to.it is as fast and simple as finding the right chart. iwill take a print and convert all the numbers before i start a project. just my thoughts jonesie


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## mklotz (Jul 4, 2011)

Yes, go to the dollar store. But don't buy a cheap four function (add, subtract, multiply, divide) calculator for a dollar. Instead, buy a cheap scientific calculator for $5. That way you can throw away your trig tables since all those functions are built in, as are the conversions between decimal and deg.min.sec angles. Most of them can do fractional calculations directly (as, 3/4 + 1&7/16 = 2&3/16) and convert the result to decimal with the push of a button.

And for David, the OP, who can't cope with "long strings of odd digits", scientific calcs have built-in formatting functions that can be set to automatically round the display to any number of decimals.


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## Lew Hartswick (Jul 15, 2011)

Yea Marv. If only those cheep ones used RPN I'd have one every where I sit down. 
  ...lew...


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## mklotz (Jul 15, 2011)

Lew Hartswick  said:
			
		

> Yea Marv. If only those cheep ones used RPN I'd have one every where I sit down.
> ...lew...



Oh come on, Lew. If you're clever enough to use RPN, you're clever enough to retrain yourself to use algebraic.


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## Maryak (Jul 15, 2011)

Whose a Hewlett Packard fan then ;D ???

Best Regards
Bob


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## mklotz (Jul 16, 2011)

Maryak  said:
			
		

> Whose a Hewlett Packard fan then ;D ???



I, enter, am.

The calculator program I wrote (it's on my page) is RPN and includes a few features that HP left out.


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## boxcarmj (Nov 8, 2011)

Fast & Easy -- http://www.kbismarck.com/weights.html


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## Lew Hartswick (Nov 9, 2011)

Maryak  said:
			
		

> Whose a Hewlett Packard fan then ;D ???
> 
> Best Regards
> Bob


Me!! It's the ONLY kind to have.  
  Who needs an = sign? And especially to re-enter a solution to part of a problem to 
do the next step.  BAH! 
  ...Lew...


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## Swede (Nov 9, 2011)

Am I the only Imperial guy who almost never uses fractions? In my head, they're all decimal. 0.250, 0.500" etc . 1 inch is divisible by 10, 100, and 1,000 just like a mm or cm.

The usual grumbling from Metric guys is "those fractions are a pain to work with." Yes they are, but why bother with them? As model guys, we rarely work with feet, only inches. And there are 1,000 of our favorite "units" per inch. I don't know if this is making any sense. I don't think 9/16ths, I think 0.562", or 0.500" or whatever.

That said, I've never had problems working on Metric plans. Especially if you have a DRO, it is child's play. About the only thing I have trouble with is a mental "rule of thumb". I can visualize what a 0.100" cut looks like in my head, but not so well when using mm.  ???


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## Tin Falcon (Nov 9, 2011)

I am with you swede i can easily convert fractions to decimals most of the time in my head. I also convert ounces to pounds regularly (I am a scale mechanic) we have two weight kits one the small weights are decimal pounds the other fractional ounces.Tell me that is not confusig at times. You would be surprised or not the number of folks that have no idea how to convert ounces to decimal pounds. Easy for a machinist who is used to converting inches in 1/16ths to decimal inches or the other direction.
I also worked in a steel fabrication shop for about a year. the lead guy in the shop had been a dentist. Yeah not kidding. smart guy 4.0 average all the way through medical school . but he needed a construction calculator to add and subtract fractions. I did it in my head then checked with soap stone on steel. 

Tin


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## lazylathe (Nov 9, 2011)

I grew up on metric and have a hard time figuring out fractions....

Just a story now:

I went to Home Despot to get some plywood cut for my bench.
I whip out my diagram with measurements on it and they are in cm.
I tell the guy i need four bits cut to 98,5 X 85,5.
He looks at me and tells me the sheets of plywood are not big enough for my measurements!! :big:
When i tell him they are cm's he pulls out his tape measure and asks if he can convert to the nearest 1/16th.
Confused the heck out of me!!!
Even though his tape measure had inches and cm's on it he could not do the measurement in cm's!

It was quite a fun evening!!

Andrew


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## Tin Falcon (Nov 10, 2011)

Andrew when I was in high school in 19XX there was a big push on metric . the us was converting to metric and metric had to be taught and learned in school . Edmund scientific had kits with metric measuring tapes and other aids for learning metric. So I learned metric. I used metric for engineering classes and chemistry in college. 
I admit metric makes more sense in most cases. but I still live and breathe imperial measurements for the most part.
Tin


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## Lockstocknbarrel (Nov 11, 2011)

Hey Tin,
You must be as old as me. ;D..I remember when 342434 was just an amassing measurement then they added...ABCD :bow: 
But now Metrics have come along it does not seen the same.
But I must admit using metrics to measure a flat surface that man is deserving of knighthood.


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## Swede (Nov 21, 2011)

I do believe that metric is, overall, a superior system. When you look at prints for a steam engine, you see the dimensions like

<-- 5 -->

Or

<------------ 3.2 --------->

And you rarely need to mess with hundredths of a mm, since 0.1mm = 0.004"

It makes for a clean and logical layout of components. But in the end, I guess yo use whatever you grew up with. It's interesting that the USA switch to metric decades ago failed so badly. I suspect it was because we had billion$ if not trillion$ of $$ worth of Imperial machinery, tools, etc. Entire industries, giant factories, were all inch, like the factories that pump out imperial fasteners by the thousands of tons annually.


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## Russel (Nov 21, 2011)

As I understand it, a thousandth of an inch (0.001) is a tolerance that can be machined and a ten thousandth (0.0001) is a tolerance that can be precision ground. So, imperial measurements kind of falls into place for machining. I know this isn't carved in stone, but it is a fair rule of thumb.

Unless I'm machining something that doesn't need accuracy, I usually machine to a thousandth of an inch. A thousandth of an inch in millimeters is kind of an odd number, at least for me, 0.0254mm. Machining to a tenth of a millimeter is a little course, 0.1mm = 0.0039, and machining to a one hundredth of a millimeter is a little to fine 0.01mm = 0.00039.


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## greenie (Nov 28, 2011)

A thread that is 3 years old has surfaced once again, amazing, eh.

Russel, have you read the third post of this thread, if not, here it is again.


-- "OK, here's another way of doing the conversion and it's real easy too, it is not 100 % dead nuts spot on, but it's that close it actually works.


0.040" = 1.00 mm

0.020" = 0.50 mm

0.010" = 0.25 mm

0.004" = 0.10 mm

0.002" = 0.05 mm


It's so quick and easy to do any converts from either Mongrel to Metric, or the other way round, it gets you that close, then it's out with the appropriate micrometer and you can then get it dead nuts spot on.

The machinery I work with, lathe and mill, are all graduated in Metric and to get it converted to Mongrel, well it's just a "chunk of urine" doin' it this way

OK, the purist are gunna shoot this lot down, just 'cause it aint 100% dead nuts spot on, all I'm sayin' is, give it a go and see how easy it is to do the converts.

regards greenie - "



Instead of trying to remember all those little numbers, just make them into these 'full numbers' and it actually works.

Easy as falling of a wet log, eh.

regards greenie


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## AussieJimG (Jan 7, 2012)

Life for the imperial guys would have been much easier if the micrometers etc. used octal (at least up to 1 inch). Then 0.1 = 1/8", 0.01 = 1/64 and so on. 

The fractions are all basically binary: 1/2, 1/4, 1/8 and so on. The crazy part is converting to decimal on the dials.

Of course once you get over 1 foot, you are on your own. Or as Bernd says: You can't fix Stupid. 

Jim


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## Peter. (Jan 7, 2012)

I can't convert the fractions to decimals in my head - but I know the 1/8th sizes by learning them rote, anything more than that and I grab a calculator. I just bought a couple of hundred imperial taper-shank drills, but I do all my work in metric. Do I care? Nope - I just measure them with a caliper - same as the metric ones I have. Some of them make great tapping-size drills.


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## terryd (Jan 7, 2012)

Tin Falcon  said:
			
		

> I am with you swede i can easily convert fractions to decimals most of the time in my head. ...........
> Tin



It's really easy to convert fractions to decimal and to metric if you require, what is more difficult is multiplying or dividing fractions without converting. Try dividing 3 7/8 by 1 9/16 ???

T


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## mklotz (Jan 7, 2012)

terryd  said:
			
		

> It's really easy to convert fractions to decimal and to metric if you require, what is more difficult is multiplying or dividing fractions without converting. Try dividing 3 7/8 by 1 9/16 ???



3-7/8 = (3*8+7)/8 = (24+7)/8 = 31/8 = 62/16

1-9/16 = (16+9)/16 = 25/16

(3-7/8)/(1-9/16) = 62/25 = 2-12/25 = 2.48

While it's not that hard to do the above mentally, any cheap scientific calculator (~$5) will have a fraction key that will allow you to enter problems such as the above directly.

Please don't take this as an endorsement of the Inferial system. However, even if one uses metric, one should still know how to manipulate fractions.


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## MachineTom (Jan 14, 2012)

A question for you metric only guys. Having to tap some metric threads the chart I had listed some letter drills as the tap drill need, some fractional sizes and some metric sizes as the required size tap drill. Another chart I found listed metric only tap drills, but is 7.25mm common drill size? My metric drill set is by .5mm and there are .1mm sets as well. Thats alot of drill bits, or do you by the .5mm sizes and add what needed for tap drills.


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## Admiral_dk (Jan 14, 2012)

> but is 7.25mm common drill size?



No - Absolutely not ! What do you need that size for ? M8x0.75 ? Not a standard size ....

Some of the values you get from the table or easy calculation, will give you a value that isn't a .1, .2, .3 etc.

If I for instance should do a M7x0.75 a very common size on ALL Japanese motorcycle carburetors (Air Screw), I would select a 6.3mm. drill bit.

All metal workshops I know have drill sets in .1 increments and just about all private persons I know buy drill sets in .5 increments ....


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## arnoldb (Jan 15, 2012)

Tom, like you mentioned, the "normal" metric sets runs in 0.5mm steps. I just buy the 0.1 in-betweeners as needed. When it comes to drills for taps and reamers, I buy the appropriate drills when buying the taps or reamer.

Some of the metric fine sizes will have "funny" metric drill sizes; for these, just use the nearest larger 0.1mm size, like Admiral_dk said.

There are 0.05mm increment drills available, but they are a bit more expensive. Here's a link to some

There might be 0.01mm stepped metric drills available too; I didn't search.

As for "Thats alot of drill bits", I don't think so. To get a set of Imperial drills from .001" to 1" in one thou steps are 1000 bits. To get a set of metric drills from 0.05mm to 25.40mm in 0.05mm steps are 508 bits. If metric did 0.025mm steps, it would be the near-equivalent of one-thou steps, but in metric thinking, the next step down would be 0.01mm requiring about 2500 drill bits. IMHO, I think drill differences at this small level becomes a moot point in a hobby shop, as drills are rather crude at cutting accurately-sized holes ;D

I've had very limited exposure to imperial drill sizes, but here's a rough equivalent:
When I grew up, my dad had a set of drills in an index from "small" (I can't remember the size) to 1/2". My equivalent metric drill index ranges from 1mm to 13mm in 0.5mm steps for just about the same set of drills, with just about the same amount of drills in each. So it becomes more an issue of using the closest size drill needed for a job, and matching up the bits & pieces that must go into the holes.

Regards, Arnold


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## AussieJimG (Jan 15, 2012)

Like Arnold, I purchase the appropriate tapping drill when I buy the tap. In fact, I usually buy a couple of tapping drills and mark one with tape as the new (unused) one. And I buy a clearance drill a little bit larger (say 0.2mm) than the nominal size.

I work only in metric but over time, when I see specials, I have collected a set of number drills, letter drills and imperial drills. These are not usually of top quality but they are only used on special occasions when that "little bit larger" or "little bit smaller" hole is needed so they don't get much work and last for a long time.

Jim


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## Peter. (Jan 15, 2012)

I go up to the next available size drill I have, be it a metric or imperial size. Often you don't need, or want the exact tapping size anyway.


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## mu38&Bg# (Jan 15, 2012)

Yes, the M8x0.75 needs a 7.25mm drill. You can order a single out of a catalog. So if I'm ordering an odd tap like that I just order the odd drill. I have a 7.25 drill in my box for this thread. I didn't have a letter/number set when I bought it or I'd do just like Peter suggests if it were a one time thing, but I plan on using it more than once. A few % of thread depth won't make a difference, and if it does you know what to do.

Greg


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