Drilling & tapping for a screw whose major diam. is undersize

Apologies if this rather elementary question has been answered before.

I've recently bought some knurled M5 stainless steel thumb screws for a project. The holes they will go through will be in brass.

An unsatisfactory aspect to them is that their major diameter comes up rather small for M5, being typically around 4.65mm [and in a few cases even smaller]. This means that when I drill 4.2mm and tap with an M5 0.8 tap, there is a lot of slop between the male and female threads.

Logic tells me that, to reduce slop, apart from drilling slightly undersize I need an M5 0.8 tap with a smaller major diameter than is usual, but is this realistically obtainable? If not, is there a practicable workaround?

Bill

The screws are undersize, I would think 4.8mm is the lowest tolerance.

If you have to use them put a couple of pieces of nylon fishing line in the hole before inserting the thumb screw.

Emgee

I would be very tempted to try tapping 2BA

... I will leave you to check the dimensions for yourself, Bill

MichaelG.

Thanks for your suggestion, Emgee.

I don't think I can safely do that; the finished item will be subject to repeated heating of 150 degrees C or more.

Edited By Bill Phinn on 06/03/2021 19:36:37

Thanks for the suggestion, Michael. I'll have to price up a tap.

Some solution at least will be needed, I think: I have twenty of the thumb screws at £1.25 each.

Well, according to thread standards an M5 has a 'NOMINAL' maximum diameter of 5.00mm but by the time you have crest radiusing the major diameter falls to between a high of 4.976mm and a low of 4.826mm. These male threads therefore do not meet the ISO standards. The more crucial Effective diameter is given as between a high of 4.456mm and a low of 4.361mm.

Thumb screws are usually 'panel access' fasteners so probably explains the 'loose' quality.

Following Michael G's suggestion you could also try finding a French 50 degree series M5 tap!

Martin

If they are new they could be #10-32 UNC for the American market which is about M5 pitch but smaller OD

Have you tried drilling a smaller hole eg 3mm then opening out to 4.2, it may be that the drill you are using s cutting oversize thus creating the sloppy fit.

Many thanks for everyone's replies, especially for the heads-up on the three different kinds of threads that are a close approximation to M5 coarse. The screws were bought new, yes, from a Chinese eBayer, and advertised as M5, but I suppose that's no guarantee of conformity.

Howard, it is definitely not the drill cutting oversize, but thanks for mentioning it.

I have just bought a single BA2 tap off eBay for £2, and will see how it goes. Given the fact that I have another unexpected solution, explained below, it will be interesting if nothing else to have a go with the BA tap.

The unexpected solution is that the screw part of the thumb screw and the knurled nut are in fact "loctited" together.

This wasn't ideal anyway given that the screws are going to be repeatedly heated in use, and it now allows me to disassemble them and use either a known high-temp loctite or [what I'll probably use] silver solder to join a piece of M5 stainless studding in to the original thumb nut.

I've just heated two of the screws to a dull bronze colour and this allowed me to disassemble them easily with the help of mole grips on the screw and a leather-gloved hand on the knurling. Interestingly on both of them the part of the screw that is inserted into the blind nut has a 2.5mm hex recessed into the end. Possibly the hexes were originally intended to allow the screw to be tightened down into the nut, but not wanting to have to trim off the ends afterwards they just inserted them the other way round.

If I'd known the screws were going to be this undersize I'd have just bought the blind nuts rather than pay almost twice as much for the full screws.

As Jason suggestion of 10-32 would be a better choice than 2BA because of the different thread angle, the thumb screws may still be loose though as OD is stated as 4.826mm.

Emgee

Reading your last post indicates the screws are just grub screws for use with a hex key.

Edited By Emgee on 06/03/2021 21:16:15

Try tapping the 2BA thread into a scrap piece of brass. Then try one of the thumbscrews. If you do not get a decent fit try returning the screws to the vendor.

.

Do we have any evidence [except for the fictitious M5 description] that these screws have 60° thread angle ?

... Just asking

MichaelG.

No Evidence Michael, only the fact they were listed as M5 so 60 deg threadform, not a very good match with a 47.5 deg threadform.

Finding 2BA stainless grubscrews with provision for hex key may also prove difficult.

Emgee

Edited By Emgee on 06/03/2021 22:18:13

I would question whether the screws are made to any standard.

My opening line was “I would be very tempted to try tapping 2BA” ... and I stand by that

Even if the screws are tight in 2BA tappings; it is much easier to open-up a 47.5° angle than to close-down a 60° angle. ... Bill could even modify one of the actual screws, to make a ‘finishing-tap’

MichaelG.