Practical Screw Thread Information & Tolerances

[Gordon Clarke]

Someone I know has made this free website and I find it has really good information.

https://calc.homepage.dk/indexeng.htm

Basically the only problem I have with it is in the part dealing with Metric screw thread tolerances.

https://calc.homepage.dk/metricthread.htm

My concern with this is that it gives min. tolerances on d1 and max. tolerances for D. If these two tolerances are used as in the table they will allow a “sharp” thread profile and thus give the possibility for shearing the thread, especially with a load bearing construction. I’ve always believed that min. d1 and max. d values were for stress calculations only. Am I right or wrong in this assumption?

The guy who made this has followed https://mdmetric.com/tech/M-thead 600.htm and believes this to be 100% correct.

My question is relatively simple – am I right or wrong to believe that a min. d1 or max. D tolerance should never be used for manufacturing purposes?

All answers and inputs will be appreciated.

 

[stefanhg]

Hi Gordon,

These limit sizes (max.D and min.d3) are according to ANSI B1.13M and are used in the design of gauges.
ISO 965 prescribes min.d3 and max.d3 for normal external thread (fasteners) and only min.D.

External thread
d1 (=D1) is Flat-form minor diameter
d3 is Rounded-form minor diameter

 

[Gordon Clarke]

Hi Gordon,

These limit sizes (max.D and min.d3) are according to ANSI B1.13M and are used in the design of gauges.
ISO 965 prescribes min.d3 and max.d3 for normal external thread (fasteners) and only min.D.

External thread
d1 (=D1) is Flat-form minor diameter
d3 is Rounded-form minor diameter

Thanks Stefan for your quick reply. The guy who made the free website is great at doing this, but has no screw thread knowledge and won't read a standard so - do I have reason to worry that he gives a min. for d1 and a max. for D in his website? I've advised him not to put these two values in his website but he insists he is right as it "says so" in the Maryland table and website. I emailed "Maryland" several days ago but have not received an answer.

 

[Gordon Clarke]

Again, thanks Stefan but could you give me a reply (a reply from anyone else too would also be greatly appreciated) I can send to the guy who made the useful website re whether or not his values for min. d1 an max. D should be in the programme he's made?
From what you write I can see that the minor diameter on a metric thread has both a d1 and d3 tolerance and what he gives seems to be d3, whereas he refers to d1 in his table.
I repeat, my only problem is that if anyone uses his metric table to manufacture from, then the result could be borderline criminal - to put it very bluntly :mg:

I don't mind being told I'm wrong ad he is right but I'd like some verification for my own peace of mind. As things are just now I'd love to put his website on my website as a link, but don't dare until I get "a second opinion"

Bobdoering: thank for your thanks, but I'm just the "delivery guy"

 

[Gordon Clarke]

I've come to the conclusion that if d1 in the programme https://calc.homepage.dk/metricthread.htm is changed from d1 to d3, then all values are correct.

Ladies and gentlemen, find your corner The "yays" in the left corner and the "nays" in the right corner

 

[stefanhg]

The standard ANSI B1.13M prescribes min.d3 (rounded form) and max.d1 (flat form).
The same values are used in the both web-sites:
https://calc.homepage.dk/indexeng.htm
https://calc.homepage.dk/metricthread.htm.

The upper limiting profile for rounded root form (max.d3) allows no tolerance for flank wear of a toll producing it and is therefore not to be used as a design profile. It is an indication of the limiting acceptable condition for the rounded root form which will pass a GO thread ring.

 

[Gordon Clarke]

Stefan,
So the values (everything?) in the tables are correct and I have no worries?

 

[Gordon Clarke]

I mean of course the metric thread tables

 

[stefanhg]

Hi Gordon,

The values are correct, but the designations are max.d1 and min.d3.