Threads for gun optic

[BillGEGHV]

Hi All, Im looking for some input on making a threaded part that fits to a gun optic. Im not well versed in metric threads especially larger fine-diameter ones.  I do not know what the thread is on the actual optic Im trying to fit to so Im just trying to figure it out.  The part Im making will be 3D printed and is just a simple sunshade for this optic. Im pretty sure the threads are .75 pitch as they are very fine. BTW this is for an airsoft application for my sons gun so not real steel.  

Here is what I have done. 

The ID of the part (internal thread) is 27.4574mm  (1.081 inch)  

I made a 27, 28, 30mm cylinders then added  X .75 thread on the cylinder just to test and see if it would fit. I do not have any way to gauge or verify the actual thread on the optic here at work. They do not fit 27 & 28 is to small, 30 is to big so I wanted to try a 28x.75. BUT there is no .75 for a 28mm diameter so i want to ask why this is?? Ive looked at all the destinations and do not see .75 for 28mm???

Since Im just trying to wing it here I was hoping to get a better understanding of how to go about measuring the ID of this existing thread bore and knowing what actual diameter to be using in Fusion on the cylinder and how to know what the actual thread pitch is.  I don’t have thread garages here at work but do have some at home guess I need to get them. So when figuring out out threads do I need to know just the OD of the cylinder OR the min diameter?

[MRWakefield]

The minor diameter of the internal thread is pretty close to 1-1/8 British Brass Thread. Do you have any more information on manufacturer/brand name etc?

[Drewpan]

Hi,

I think your problem may not be the thread but shrinkage from your 3d print. The 0.75 is the standard for a fine thread

so this will be correct. I am sure that an M28 exists but there are gaps for reasons I have never bothered to find out

about in the standards chart. I don't think an M29 exists for example. Also above M25 the gaps between are bigger.

The probable reason of why an M28x75 does not exist is that once you get that big the precision of a fine thread sort

of becomes a bit redundant. The standard tells you how to cut custom threads so if you need one you often custom it.

Fusion will give you the capability to model with the accuracy you need but depending on the filament type there can

be shrinkage of up to 5% on some and it may not be evenly distributed. It might be a case of keep trying until you get

one that does fit or literally print a 28mm piece of plastic and use the metal thread of the sight to cut the actual thread.

Information about metric threads with the formulae and stuff is here:

https://en.wikipedia.org/wiki/ISO_metric_screw_thread

Cheers

Andrew

[BillGEGHV]

Here is the scope. 

https://hammersbinoculars.stores.turbify.net/reddotsi1x.html

[BillGEGHV]

The Prusa printers we have consistently print parts within.003 fitment, Yes there is always shrinkage but in this case Im way off so far.  and yes I may just have to keep tweaking the OD till it works.  OR per your suggestion of threading the print to the metal part we already tested that yesterday and that is probably the option we go with.   It works fairly well.  

[MRWakefield]

Thanks. I can't find any info on it other than the very limited info on the product page.