Missmatched threads

[old martParticipant @oldmart]

I was making a backplate in Meehanite for an er16 collet plate to fit the Emco SL1000 we are getting ready for the model club to use. The collet plate uses four 5mm screws to hold it onto the backplate. I drilled and tapped for 5mm and tried using 5mm SHCS which took up too much room as I was going to have a register with about 0.004″, 0.1mm loosness to allow fine adjustment and bolts with hexagon heads would be better. I found some 10-32 bolts for Rolls Royce Gem engines which fitted the 5mm holes perfectly. In theory, 10-32 is smaller diameter than 5mm, but aircraft quality threads are right up at top limit sizewise, and the thread pitch error is 0.8 verses 0.79375 and even with 2 1/2 diameters engaged I couldn’t fault the feel, and the hex heads are only about 4mm thick which gives plenty of spanner room.

The 2″ diameter by 1″ long Meehanite came from a supplier new to me, delivered very quickly. Their name is EKP Supplies, and considering the recent demise of some of our old favourites, they are worth checking out. They also sell Kennions locomotives.

http://www.ekpsupplies.com

[Nigel Graham 2Participant @nigelgraham2]

Yes -EKP is well-established and I have bought quite a number of things from them, most recently only last week.

[Robert Atkinson 2Participant @robertatkinson2]

Got to watch out for those 10-32 UNF fasteners. A British Airways BAC 1-11 captain found himself half way out of the cockpit at 17,000 feet when someone used 8-32 UNC bolts to hold the windscreen in. Same thread pitch but about 26 thou underside. An interesting tale of many mistakes..

https://asn.flightsafety.org/reports/1990/19900610_BA11_G-BJRT.pdf

Robert.

[Mark RandParticipant @markrand96270]

There are an awful lot of threads of about that size which nearly fit. 3/16bsf is another. Most of them will screw into an M5 hole for a bit as well. It can take a bit of care when sorting out boxes of assorted finds/gifts/inheritances 🙂

I might get nagged about my obsession, but ekpsupplies don’t claim to sell Meehanite.

 

[old martParticipant @oldmart]

Mark, you are mistaken about EKP, I just had to log onto ebay to check where I bought the 2″D X 1″ long Meehanite and it was from them, £8.50 post paid. That’s the stuff I parted off with a 2mm insert in a 26mm blade very easily at about 500rpm.

[Mark Rand]

On Mark Rand Said:

[…]

I might get nagged about my obsession, but ekpsupplies don’t claim to sell Meehanite.

 

On ebay they do … albeit in parentheses 🙂

That said … my Henry vacuum cleaner is not a Hoover !

MichaelG.

[old martParticipant @oldmart]

Too quick for me, Michael, I just checked both their website and the ebay listing where I got the (Meehanite).

[Mark RandParticipant @markrand96270]

Had fun today trying to work out what a thread of that general size was. There wasn’t much length of male thread to check but had to get a screw through a longer distance in the existing hole. Turned out it was 2BA.

 

Meehanite:- https://meehanitemetal.com/licensees/

[old martParticipant @oldmart]

Whatever the cast iron actually was, it certainly did machine and part of better than the common stuff. The swarf was less powdery and about half way to that produced from SG grades. The skin was very thin and not any problem and the OD cleaned up with no more than 0.020″ reduction in diameter. I cut the four 5mm threads plus the centre 10 x 1mm thread nicely. I used inserts intended for aluminium throughout including the parting off. I would buy it again if I needed some round stock. The cut ends were as near perfect as a bandsaw could produce, within 0.005″ of square even when chucking off the skin.

[Clive FosterParticipant @clivefoster55965]

Interestingly there appears to be no direct pitch error tolerance specified in the ISO metric thread, or any other bolt thread, specifications. Just a simple number.

Apparently the pitch error falls out as a subsidiary of the major and minor dimension tolerances. I did find an attempt on Stack Exchange to generate results mathematically. I’m unsure as to how effective the maths are but the guy certainly desires and A for effort.

If you are using Go-NoGo thread gauges an ISO metric bolt thread must freely enter a gauge of the appropriate class by at least 2 complete turns of a if it is to be accepted as within tolerance. Interestingly UN threads need to freely enter by 3 complete turns to be within tolerance.

In principle the pitch tolerance for a pair of perfect threads with the specified clearance between male and female threads could be derived from the 2 turns into the gauge rule.

Along the way I tripped over a thread reference handbook from ISCAR whose opening 20 or so pages have some very clear graphics covering the various thread forms and other relevant data without delving too far into complexity. Good educational resource for folk who don’t really know about the variety of such things.

https://www.iscar.com/Catalogs/Publication/english_1/Threading_Handbook/Threading_Handbook.pdf

Clive

[Bill PhinnParticipant @billphinn90025]

That’s a great source of information, Clive.

The closeness of 10-UNF to M5 was already known to me.

What wasn’t, until recently, was the existence (assuming it’s real) of a 14-24 UNF thread. Tracy Tools sell taps and dies in this size, but I can’t find it mentioned in my 1965 copy of Machinery’s Handbook.

The reason 14-24 UNF is problematic for me is to do with a forum thread I started on here in 2024 called “Whit Form Threads”. I had assumed that the screw I have an example of that prompted that forum thread was a Whit Form 1/4”x24. Given that its major diameter is in fact 6.15mm, I’m thinking it may actually be an example of 14-24 UNF. It’s very difficult to tell whether the threads are 55 or 60 degrees because the crests are quite rounded off, though this is clearly not a result of wear as the screw is a very good fit in its original female thread with almost no detectable wobble.

If anyone can shed light on the status of 14-24 UNF that would be marvellous. Is it perhaps a relic from the United States Standard or American National series that preceded Unified threads?

[Michael GilliganParticipant @michaelgilligan61133]

The mighty GoogleAI sayeth:

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… but of course it could be fibbing

MichaelG.

[old martParticipant @oldmart]

Over the years there have been lots of threads that have been discarded, now threadcharts go from 12 series to 1/4″ unified. Number 12 is 0.216 not much smaller than 1/4″.

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