Elliott Omnimil 00 screw threads

[avivParticipant @aviv]

Sorry for the basic question, but does someone know if the bolts in the Omnimil are or Unified?

they are not metric for sure.

Thanks.

[Wade BeattyParticipant @wadebeatty78296]

I’ll go check

[Wade BeattyParticipant @wadebeatty78296]

The belt tension holding bolt on the vertical head is 1/2 × 13 and one of the hex head bolts holding the flange for the horizontal drive is 1/4 x 20, both fit well on my 60 degree thread gauge set

[aviv]

On 23 March 2026 at 10:35 aviv Said:

…the bolts in the Omnimil are or Unified?

There appears to be a word missing before ‘or’. Could it be Whitworth?

Find something with a hexagon head on it. If a fractional spanner fits, it is likely unified. If it has some strange A/F dimension, it is likely Whitworth.

Find something on it 1/2″ nominal diameter and measure the thread pitch.

Find something on it with a fine thread and measure the thread pitch.

[Wade Beatty]

On Wade Beatty Said:

…one of the hex head bolts holding the flange for the horizontal drive is 1/4 x 20, both fit well on my 60 degree thread gauge set

1/4 – 20 tpi is used both for Whitworth and unified. Your thread gauge test is incomplete as you did not assess the fit of the 55 degree Whitworth gauge.

If you are trying to do science, please do it properly.

[Wade BeattyParticipant @wadebeatty78296]

Oh! Einstein shows up!

[BazyleParticipant @bazyle]

An unified nut does not fit happily on a Whitworth bolt but works the other way round.

Worth knowing if you have a Boxford or SouthBend lathe as some bits don’t fit eg chucks.

[Howard LewisParticipant @howardlewis46836]

In addition to the thread form, the pitch may give a clue, as BSF or UNF

Unfortunately, BSW and UNC tend to have same pitches for the same size, but 1/4 BSF is 26 tpi where 1/4 UNF is 28.

5/16                                                 3/8

BSW   BSF   UNC   UNF                      BSW   BSF   UNC   UNF

18      22      18     24                         16     20       16    24

Although a good guide, don’t be too fixated on the hexagon size. Sometimes you can come across A/F headed Whit form fasteners; fortunately only very occasionally! I met a 5/8 A/F bolt with a 3/8 BSF thread!

Howard

[Wade BeattyParticipant @wadebeatty78296]

A  1/4 x 20 NC nut fit on the smaller bolt and a 1/2 x 13 nut turned easy on the larger, also a 3/4 wrench fit fine on the hex on my mill.

[DC31k]

On DC31k Said:

On Wade Beatty Said:

…one of the hex head bolts holding the flange for the horizontal drive is 1/4 x 20, both fit well on my 60 degree thread gauge set

1/4 – 20 tpi is used both for Whitworth and unified. Your thread gauge test is incomplete as you did not assess the fit of the 55 degree Whitworth gauge.

If you are

On DC31k Said:

On 23 March 2026 at 10:35 aviv Said:

…the bolts in the Omnimil are or Unified?

There appears to be a word missing before ‘or’. Could it be Whitworth?

Find something with a hexagon head on it. If a fractional spanner fits, it is likely unified. If it has some strange A/F dimension, it is likely Whitworth.

Find something on it 1/2″ nominal diameter and measure the thread pitch.

Find something on it with a fine thread and measure the thread pitch.

Indeed I meant to ask UNC or BSW. Fractional spanners fit well on the nuts and bolts. I will try to find 1/2 fastener on the mill and measure its TPI count with a thread gauge.

Aviv.

[BazyleParticipant @bazyle]

BTW are you a member of Groups.io/elliott

 

[Bazyle]

On Bazyle Said:

An unified nut does not fit happily on a Whitworth bolt but works the other way round.

Worth knowing if you have a Boxford or SouthBend lathe as some bits don’t fit eg chucks.

Don’t need Einstein!  1/2” is probably the only size that has different pitch between Whitworth and UNC.

Whitworth is 12 tpi, while UNC is 13 tpi.

 

OOPS.  Meant to quote Wade, not Bazyle.

[Bazyle]

On Bazyle Said:

BTW are you a member of Groups.io/elliott

 

I’m afraid not. Could you share a link?

[BazyleParticipant @bazyle]

https://groups.io/search

One of the files of the parts diagram quotes table locking screw of 1/4 x20 UNC.

 

[RobCoxParticipant @robcox]

Having stripped down my Elliott Model 00 in the past to refurbish it I can say with all confidence that they’re UNC threads.

[avivParticipant @aviv]

Thank you all!

[vintagengineerParticipant @vintagengineer]

Head size doesn’t always relate to thread size. Some early Morris cars had Whitworth headed bolts with metric threads.

[BazyleParticipant @bazyle]

Aviv, assuming you are new to the Omnimill it may be worth checking these previous threads for interest.

https://www.model-engineer.co.uk/forums/topic/elliott-omnimill-00-axial-spindle-lock-ideas/.

https://www.model-engineer.co.uk/forums/topic/elliott-omnimill-modification/

 

[avivParticipant @aviv]

Thank you.

[avivParticipant @aviv]

And BTW, I’m also looking for an original slotting attachment for the Omnimil, so if anyone knows about one for sale I would appreciate a link.

Thanks.

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