Metric system tolerances

GaryK

GaryK

In Memory
Rest In Peace
I am thinking about making something using plans that are in metric. Rather than convert everything to inch I was thinking about just using metric.

Having never done this before I am just wondering what tolerances are typical? In inch generally .005" is acceptable in most cases.
(Though being as anal as I am I try to keep everything within .002")

This question is mainly for people that use metric as standard. What tolerance do you use as acceptable? .1mm, .2mm...?

I want to get comfortable using both systems.

Thanks
Gary
 
I look forward to hearing from folks on this and wish you, Gary, a lot of luck.

We all know the metric system is rooted in scientific/mathematical logic and carries with it great simplicities. In my own mind, I can mentally comprehend and visualize the basic distances (kilometer, meter, centi and milli). For some reason though, when I see the metric equivalent of 2.75" +0.000, -0.002 represented as 69.85, +0.00, -0.05, my brain locks up and I can't get a sense for the tolerance part. The 69.85, I can visualize OK the fractional tolerance doesn't register (yet).

Ray
 
Mentally I convert the number 1 in mm to 4 in inches. It just depends on the decimal point, so 1 mm = .040", .1 mm = .004", .01 mm = .0004". This not quite accurate but it will allow you quickly to understand the dimensions involved as you grab a calculator to really dail in on the part.
Pierre
 
G'Day Gary,
Down here in Oz we switched to the Metric system back in the 1970s, the transition was a bit painful at the time, but you get used to it, while I don't know what you are making and not wishing to sound like a condescending smartarse, most mating turned or milled parts are made to a tolerance of .01mm, the thought of a tolerance of .1mm is a worry as you would be able to see daylight between them.
Regards,
Martin
 
I've made chips using both systems and I actually prefer metric vs inch. If the drawing, tooling and measuring is all metric then there is little thought involved. It is a little strange to bounce back and forth. The thing to remember is that .001" equals .0254mm or 25.4 "microns" which is the standard nomenclature for 1/1000 of a millimeter. If the drawing gives a tolerance of +/- .100mm (or 100 microns), that is actually .003937" which rounds off nicely to +/- .004". Here is a real world application. I was once a machine operator at a Japanese automaker here in the states. Everything was of course metric. A cast iron cylinder bore had a spec of 87mm +0.000/+.030mm (87.000-87.030mm). That 30 micron range equals .0012". Thats right, every cylinder bore that came off that line was within a thousandth of an inch! We made well over a thousand engine blocks a day. Here is the part that really gets your attention, that 30 micron range was divided into 3 different sizes of bores, #1, #2, #3, each one being within a .010mm or 10 micron window. We would target the #2 range and if something wandered smaller or larger we could bring it back to the center of the spec with out making a bad part. About 80% of the cylinder bores produced were #2 and within a 10 micron tolerance window or .0004". The scrap rate for oversize cylinder bores was less than .5%.
 
It really is up to you and the fit the parts require. I usually work to a tolerance of around 0.02 mm for dimensions below 100 mm, mainly because I can achieve that on my machines without too much fuzz. But like toolman49 said, if dimensions are critical, e.g. because you need a good fit, working to 0.01 mm or better is not unusual. (But on our hobby-machines it's not always easy. Mostly it's better to resort to lapping).
If dimensions are not critical, do as you please. A tolerance of 0.1 mm is generally perfectly acceptable. But it depends on the size of the part. If the main dimension is as small as 1 mm you might want to do better, if it is 1000 mm (i.e.1 m) you might be complacent with a tolerance of 0.5 to 1 mm.
Regards,
Honore
 
Usually the tolerances are marked on the drawings. Either to a specific dimension, or a general table in one of the corners.
 
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