Measuring Screws for Replacement

[friquant]

For machines where parts are not directly available, how does one measure the diameter of a screw to determine what size it is?

To determine thread pitch I've been using this SAE pitch gauge: https://www.amazon.com/dp/B0BGRB96DP and viewing through a jeweler's loupe the pitch gauge snuggled up against the screw for comparison.

To determine the major diameter (aka outer diameter, see https://en.wikipedia.org/wiki/Screw_thread#Major_diameter ) I've been measuring the threads with the fat part of the digital caliper jaws.

Here's the hard part for me:
For the nominal fractional sizes (1/16, 5/64, 3/32, 7/64, ..., 1/2) commonly sold as SAE screws and used on sewing machines, what is the actual major diameter for these screws that calipers or a micrometer would measure?

If we were to assume that the nominal size matched exactly its actual major diameter (outer diameter), we could simply generate a table with a little math and multiple by 25.4 if we want millimeters. Here is such a  table:

NOMINAL  | Theoretical | Theoretical
 1/16 in | 0.0625   in | 1.587  mm 
 5/64 in | 0.078125 in | 1.984  mm 
 3/32 in | 0.09375  in | 2.381  mm 
 7/64 in | 0.109375 in | 2.778  mm 
  1/8 in | 0.125    in | 3.175  mm 
 9/64 in | 0.140625 in | 3.572  mm 
 5/32 in | 0.15625  in | 3.969  mm 
11/64 in | 0.171875 in | 4.366  mm 
 3/16 in | 0.1875   in | 4.762  mm 
13/64 in | 0.203125 in | 5.159  mm 
 7/32 in | 0.21875  in | 5.556  mm 
15/64 in | 0.234375 in | 5.953  mm 
  1/4 in | 0.25     in | 6.35   mm 
17/64 in | 0.265625 in | 6.747  mm 
 9/32 in | 0.28125  in | 7.144  mm 
19/64 in | 0.296875 in | 7.541  mm 
 5/16 in | 0.3125   in | 7.938  mm 
21/64 in | 0.328125 in | 8.334  mm 
11/32 in | 0.34375  in | 8.731  mm 
23/64 in | 0.359375 in | 9.128  mm 
  3/8 in | 0.375    in | 9.525  mm 
25/64 in | 0.390625 in | 9.922  mm 
13/32 in | 0.40625  in | 10.319 mm 
27/64 in | 0.421875 in | 10.716 mm 
 7/16 in | 0.4375   in | 11.112 mm 
29/64 in | 0.453125 in | 11.509 mm 
15/32 in | 0.46875  in | 11.906 mm 
31/64 in | 0.484375 in | 12.303 mm 
  1/2 in | 0.5      in | 12.7   mm 

But I don't have enough evidence yet to say whether this is the best course of action, or if there is a better table somewhere based on actual values instead of theoretical.

 

Photos

LED loupe, thread pitch guide, and machine screw

 

Matching up threads under magnification

 

Measuring the major diameter with digital caliper

[AlZilla]

I would just call Toledo Industrial and tell them what I need a screw for ... 😬

[friquant]

32 minutes ago, AlZilla said:

I would just call Toledo Industrial and tell them what I need a screw for ... 😬

Of course. For Singer machines that is a viable option, provided you can find the part number. I have a few screws I need to find the part numbers for so I can order them that way. But when I get off the beaten path:

• Manufacturer went out of business
• Manufacturer considers my machine obsolete
• Manufacturer has no dealers from which I can easily and affordable purchase parts
• Brand name on the machine is not the actual manufacturer
• Parts list for machine has no photos so you don't know which set screw it's referring to
• The ease of a midnight transaction on Ebay calls my name

then I want to measure the screw and order it by its dimensions. Given that some of these sizes are uncommon outside of Sewing Machine Land, someday I aspire to look up which singer parts are the same thread pitch, major diameter, length, and head style  and then order by part number through a singer parts dealer even though the screws are going into some other brand of machine. But that requires a lookup table and the only one I've seen is a good start but lacks screw length and head type: https://www.singersewinginfo.co.uk/screw_threads/

But for today I want to be able to accurately measure screws and at least know what size to ask for at the common internet retailers.

[kgg]

1 hour ago, friquant said:

For machines where parts are not directly available

Interesting. Finding bolts that fit properly for sewing machines can be a chore at times since a lot has to due with whether the manufacturer is using standard bolts with standard thread pitches or they are using a particular manufacturers specific bolt thread setup like Singer and to lesser extent Juki and Juki clones. Sometimes you can sorta kinda get a Imperial bolt to fit in a metric hole but most times you have to re-thread the hole to get a proper fit. Most machines these days are made using the metric system (International System of Units - SI).

The problem of trying to match bolts sizes tends to be a more of a problem in the US as most of the world uses the metric system (International System of Units - SI). Even in Canada and to some extent the US it is not that uncommon to see three systems ( the Imperial, the Metric and the America ) of measurement being used.

kgg