Metric / Imperial conversions

[mklotz]

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.

 

[MachineTom]

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.

 

[Admiral_dk]

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 ....

 

[arnoldb]

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

 

[AussieJimG]

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

 

[Peter.]

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.

 

[mu38&Bg#]

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