Strange thread on the hold-down screws of a domestic machine

[DrmCa]

Hi all,

The domestic machine in question is held on its swing-up mount in the table by the two thumb-screws with the 28 TPI thread, and they look like 1/4" but they are not.

The major diameter is between 0.227" and .0228" or 5.8mm. I am leaning towards the inch size because it is precisely 28 TPI but no match on the metric side.

What thread could this be? I need to chase one of the threads because the screw sticks, but I do not have the matching tap and will have to order it.

Thank you!

[Northmount]

1 hour ago, DrmCa said:

machine in question

You should identify the make/manufacture of the machine.  Singer has their own thread sizes.

Tom

[CowboyBob]

 There's 7/32 & .2188 or 15/64 @ .2344.

[DrmCa]

This is Kenmore.

I thought of 15/64 when I saw the diameter on the caliper, but why on Earth would one insist on such an oddball thread when there is 1/4???

[LatigoAmigo]

3 hours ago, DrmCa said:

why on Earth would one insist on such an oddball thread when there is 1/4?

That's an engineer for ya.

[chrisash]

Probably to make each item only suitable for each job, so you cannot put the wrong length item in and damage something, or maybe a pissed off designer sneaking it in and laughing when nobody noticed

[Gunnarsson]

Also, manufacturers can use their own special thread to make sure you buy parts from them and not some one else, or use standard screws from the hardware store. It is still possible for others to make copies, but non standard threads makes that a more expensive project, many won't bother. And then there's the old companies that started making things before there were thread standards to follow (or the existing ones were unsuitable), if there was no standard they had to figure out their own.

[Cascabel]

Singer has always used their own proprietary thread sizes on their screws.   No standard screws will fit.  I have made a few as needed with my lathe.  Some of them are close to standard sizes, and most are not.  It becomes a problem with an older machine that is no longer shown in the catalogs.  You are then forced to find somebody that can make what you need, or drill oversize and tap to a standard thread.   Depending on where you live, you can sometimes find a dealer that keeps older machines on hand, and can cannibalize what you need from his junk pile.