Boring Head – Thread Quality

[Steve CrowParticipant @stevecrow46066]

I'm making a small boring head with an M3 "leadscrew".

I can single-point cut the thread but I've considered using an M3 screw as I can retain the hex socket on the head for adjustment.

My question is, in practical terms, would give this measurably inferior results than a single cut thread?

I know this depends on the accuracy of leadscrew on the lathe doing the cutting.

On a side note, are commercial screw threads cut or rolled/formed?

Cheers

Steve

[Steve CrowParticipant @stevecrow46066]

[Anonymous]

Posted by Steve Crow on 17/11/2021 16:46:43:

On a side note, are commercial screw threads cut or rolled/formed?

Almost always rolled, as it's cheaper and much faster than cutting. A caveat is that specialist fasteners, such as BA bolts, will probably be cut.

Andrew

[Brian WoodParticipant @brianwood45127]

Hello Steve,

They are roll formed, a necessary method to get the strength into the finished screw and to cope with the sheer number produced in the various length for each thread form that they are supplied in.

All thread is also made in the same way and some time ago I did study the pitch accuracy of the standard supply, but to a size of not less than M6. The accuracy is really remarkably good considering that miles of this useful material must be produced daily. I found that M6 was good to an error value of +0.005 mm in 100 mm of thread and M8 to a value of -0.005 mm in 90 mm

I think your M3 screw will fall into similar error values which might be a trifle better than you might achieve screw cutting on a ;lathe, but I do question the size you have selected from the point of view of wear.

Regards

Brian.

[Steve CrowParticipant @stevecrow46066]

Thank you Brian, I thought that they'd be rolled. Should be a better surface finish than cut as well.

I'm going to go for the commercial screw, even though it feels a bit like cheating.

M3 sounds really small but it is a tiny boring head, copied from this one but with an MT1 Sherline shank.

Cheers

Steve

[Brian WoodParticipant @brianwood45127]

That really is rather small; Dinky I think the Americans might call it!

I hope it goes well for you

Brian

[John BaronParticipant @johnbaron31275]

Posted by Brian Wood on 17/11/2021 17:13:42:

Hello Steve,

They are roll formed, a necessary method to get the strength into the finished screw and to cope with the sheer number produced in the various length for each thread form that they are supplied in.

All thread is also made in the same way and some time ago I did study the pitch accuracy of the standard supply, but to a size of not less than M6. The accuracy is really remarkably good considering that miles of this useful material must be produced daily. I found that M6 was good to an error value of +0.005 mm in 100 mm of thread and M8 to a value of -0.005 mm in 90 mm

I think your M3 screw will fall into similar error values which might be a trifle better than you might achieve screw cutting on a ;lathe, but I do question the size you have selected from the point of view of wear.

Regards

Brian.

Hi Brian, Guys,

I've used a lot of M6 left and right hand all thread for lead screws on several items ! The 1 mm pitch and a graduated dial with 40 index marks on a 50 mm diameter dial varies less than one index mark in 40 turns, so I would say that the error is negligible.

[Nigel Graham 2Participant @nigelgraham2]

My work used a lot of standard M-thread studding at work, from M3 to M24, mainly in stainless-steel; and what was clear was that its finish was better than on the mild-steel versions.

We were not concerned with accuracy because this was for holding things together, but I would think it as good as John Baron has found.

Something worth considering if you want to use studding for lead-screws.

[Michael GilliganParticipant @michaelgilligan61133]

Probably just for drooling over: **LINK**

Introducing The World’s Smallest Lead Screws

MichaelG.

[egaParticipant @ega]

I understand that Rolls-Royce used to use screw-cut fasteners.

I think they also claimed not to need lock washers, etc because each screw was precisely tightened to a degree which prevented loosening.

[Bill PudneyParticipant @billpudney37759]

Posted by ega on 18/11/2021 23:46:17:

I understand that Rolls-Royce used to use screw-cut fasteners.

I think they also claimed not to need lock washers, etc because each screw was precisely tightened to a degree which prevented loosening.

Both of these statements are intriguing, do you have any evidence for either or both??

cheers

Bill

[egaParticipant @ega]

Bill Pudney:

I'm glad to have aroused your interest.

Years ago, K R Whiston of New Mills, Stockport advertised R-R cut thread screws in his famous "cats". I cannot now lay hands on any of these and may well have binned them but someone may be able to check and confirm.

I don't recall the source of my other comment – maybe someone was just "torquing" R-R mythology – but, again, can someone say whether it is likely to be true?

On the same subject, Clutton and Stanford in The Vintage Motor Car mention the "Rolls-Royce torque tube and rear-axle unit sewn together [in Royce's own phrase] by innumerable rings of tiny bolts".

[Bill PudneyParticipant @billpudney37759]

In the 20s and 30s RR did a lot of basic research into many engineering issues. One that has always stayed with me was, they investigated the optimum number and size of fasteners in any joint, whether structural or fluid retention. The conclusion was that "more small fasteners is better than less big fasteners". It sort of makes sense, but it's nice to know that RR worried about these things!!

cheers

Bill

[Howard LewisParticipant @howardlewis46836]

In the days of the R-R C range engines, in the early 60s, Big End bolts were waisted for equal strength and tightened to a known increase in length.

Cylinder Head bolts were tightened to a stated torque, everything else relied on a calibrated , or not, hand..

Apparently, in the early days, De Haviland relied spanners having a sharp edge, so that when the discomfort became too great, the fastening was tight enough!

At Perkins, for a long time all critical fasteners were torque tightened. When we started investigating yield tightening, is interesting to see how Ford and BMC went about things.

Where Perkins used a large number of air guns to tighten the non critical fasteners, Ford used speed braces!

When Cylinder Head fasteners were tightened to yield, the incidence of head gasket and head bolt failures dropped to zero. I did spend six months proving off the 32 spindle machine, before it was signed off for use on the production.line. Yield tightening also allowed us to standardise on one type of head gasket, instead of a number related to applications and outputs, as well as dispensing with the need to retighten during test or at first service.

At the time, it was the worlds largest yield tightening machine, and to see 32 fasteners tightened to yield, with a 9 ton tensile load in seven seconds was quite something!.

Ford and BMC used the same machines, but only a single spindle, so that despite having fewer fasteners, the entire process took longer.

Each spindle sensed yield by monitoring the torque, and the rate of change of torque vs angle. As soon as the rate became non linear, (Hooke's Law ) the fastener had just gone into yield, and taken a permanent extension of 0.001 – 0.002". By keeping the extension so small, it was possible to reuse the fasteners , should the need arise. In service tightening was either by means of a sophisticated electronically controlled wrench which again compared torque vs rate of change of angle / toque or using a snug torque plus angle method.

Howard

[egaParticipant @ega]

Howard Lewis:

Thanks for these words from the horse's mouth!

You have reminded me that part of the torquing process is lubrication; I think that Hillman Imp cylinder head bolts had to be dipped in a specified oil before being installed (I don't know if that applied to all versions of the engine).

[Bill PudneyParticipant @billpudney37759]

A long time ago, I had to make a test rig to demonstrate the importance of lubricating threads when tightening to a specific torque. It had to convince a very dubious Electronics Engineer,,,,,,,,,,,

cheers

Bill

[Howard LewisParticipant @howardlewis46836]

ega,

One of the stages in commissioning the machine was find a supplier who would, or could, ensure a consistent level of lubrication of the fasteners.

In the event, the difference between 5% and 15% oil on the bolts made little difference. The essential thing was to have lubricant present.

That ensured a consistent coefficient of friction between male and female threads, allowing the sensors and electronics to do their magic!

With plain torque tightening, consistency was even more important.

Howard

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