Brass plug

[martyn nutlandParticipant @martynnutland79495]

I'm afraid this is a very open ended kind of question but I really would appreciate an experienced practitioner's view to help me think sensibly.

I restore vintage Austin Seven motor cars as a hobby. One of the many, many, many machining jobs I am undertaking (or contemplating) is making the rather sexy little brass cap that closes off the eye for the king pin boss in the stub axle. (You unscrew it to fit an extractor for the king pin and/or pump in grease).

It's brass. About half an inch in diameter. About a quarter inch thick and threaded, I think, but am not yet sure, to a Cycle Engineer's Institute 'spec' but possibly its BSP.

Would you try and buy one – simplest, but I learn nothing and gain no satisfaction?

Buy a die – extremely expensive solution at this largish size for something I will use once or twice in a life time?

Or could I chase it by hand on brass bar and would the skill to do that be very hard for a rank amateur to learn?

I really would appreciate a sensible view to help me 'wise up'.

(Might be a case of life's too short to shell a prawn, eh?)

Martyn

[martyn nutlandParticipant @martynnutland79495]

Austin Seven king pin cap

[Phil WhitleyParticipant @philwhitley94135]

I would make it on the lathe, and single point thread it . Contrary to what ppl believe, threading on a lathe isnt hard at all, its just that we all believe its hard till we do out first. Once you have done one, and this could be the one, its easy.! It was reading explanations and watching videos on this and other sites that got me up enough bottle to make a part I could not buy anywhere, Remember you can practice on bits of scrap aluminium till you get it right. First job is to get some thread guages, find out what thread it is, then see if you have the gears to cut it with.

Phil

[martyn nutlandParticipant @martynnutland79495]

Thanks Phil. Sounds sensible, fairly straightforward and satisfying if I pull it off.

Martyn

[Speedy Builder5Participant @speedybuilder5]

Hi Martyn. As others have said, cut with single point 55deg tooling. Chase not necessasary. Cut oversize. It's difficult to check if it fits without having the hub to check the size. Oversize – chuck it in the bin. Undersize and sloppy no real problem for your application. (Fellow A 7 owner) . bobH

[AjohnwParticipant @ajohnw51620]

If you want something that looks exactly like the original part there are several ways of going about it. Using a single point screw cutting tool wont achieve this as the general idea is that the bar is turned to a size under the normal thread dia that removes the need for the rad on the crest of the top of just about every imperial thread form. As it's a pointed tool it needs to cut deeper than the correct form as well. You should be able to find calculations for this about on the web. The information needed is available when the full details of the thread are given, crest and root rads etc..

I've ignored that you mentioned the word cap so if it is the above needs reversing.

If you have a feeling it's a cycle thread it's probably a brass thread, whit form rather than 60 degrees. Brass is usually reckoned to be 26 tpi but I have a feeling 32 is used as well. At all diameters. The idea is not to weaken tube wall too much as say using 10 tpi which might be more sensible at some diameter. Thread gauges should come in whit and unified as then it's possible to see what the thread angle that is being copied is. Often now they only come in metric and unified so if the flank angle looks to be wrong using the unified guages it's assumed to be a 55 degree whit form.

Screw cutting with a chaser isn't really any different to cutting with a single pointed tool other than when the end of the thread has been cut the chaser has to be wound out of contact otherwise it will mess up the thread. A single point tool can be left in contact as it will just turn a groove at the end of the thread. In that case screw cutting just has to be disengaged. With a chaser the tool has to be wound out of contact and screw cutting disengaged at the "same time"- very slightly later. Not that difficult to do with the lathe running slowly providing that there is a bit of space available at the end of the thread for the tool to run on a bit. A typical chaser may need modifying on an off hand grinder. Some may have a bit of a lead in. That needs grinding off slowly without over heating the chaser and wrecking it's temper. If it is a cap it wouldn't be a bad idea to cut a recess at the end of the thread to provide some space for either type of tool to run into. Sometimes when a single point tool is being used the noises change when the tool enters the recess. Some fit a long travel DTI to the lathe bed for tricky jobs and use that to gauge when the end of the thread is reached.

Not often mentioned but for internal threads and a single point tool there is another technique. Run the lathe backwards with the tool upside down or even cutting on the opposite face to the one usually cut. This way the tool runs out of the work rather than in. Problems – screw on chucks might unscrew and normally screw cutting is engaged some distance from the work so that when the actual cut starts all play has been removed. Might work out with a decent sized recess. With a chaser too if it can fit into the recess. It would be best to only have a few teeth on it, only 2 are needed really for machine chasing.

Once upon a time many threads were cut by hand with chasers. They can also be used to correct single point cut threads to the correct form or correct the pitch slightly when some one has an imperial lathe and wants metric. Chasers self correct as they cut which is why hand chasers have so many teeth on them. The modern machine chaser is in the form of a disposable carbide tip. Advice from a now rather old clock restorer is to make up a handle around 18in long for fully cutting threads with them from scratch. What I found is that it's best to lightly score the work with the chaser stationary 1st and the use this as a guide to feeding the chaser along by hand. Higher speeds for finer threads seems to work best as it's easier to match the required feed speed.

My boxford came with it's hand turning rest – make me think it must have been a model engineer that initially bought it.

John

–

[martyn nutlandParticipant @martynnutland79495]

Wow. John, how long did you spend explaining this to me? I rely do appreciate it and hope I am a better educated soul as a consequence. Thank you for your trouble, many times over.

And Bob, from the kingdom of Austin Sevens. Helps. I really do feel ashamed and belittled and inadequate when I have to bin a job. Do you think Herbert Austin or Henry Royce ever made a bollocks of a part and had to bin it? It would make me feel better if they did. I doubt it though.

Martyn

[Phil WhitleyParticipant @philwhitley94135]

Hi Martyn, just to clarify the above point from John, is the thread male or female? Johns post above is absolutely correct, but, without igniting a flamewar, I would suggest that as Austin sevens were built to a price, the original component would be made on a capstan lathe, possibly using an auto die head, or just a tap for internal, neither of which methods produce a perfect thread form, . A thread in use mates on the flanks, it is virtually impossible, and also unneccasary to machine perfect crests and troughs. Can you post up a pic of the plug?

Phil

[AjohnwParticipant @ajohnw51620]

LOL not too good a post really as I stopped from time to time. It's easy to loose the plot then.

Some of it relates to cutting internal threads – mention of a DTI to show when the end of the cut is reached. Also running backwards but that may be of use in other situations. If a chuck unscrews = mess, a big one.

One other thing as well. Say it was a cap. It might be better to screw cut a length of tube and then make a top for it and loctite it in place or even use solder paste if it's brass. When something is difficult to machine fabricating it cab often help.

John

–

[julian atkinsParticipant @julianatkins58923]

hi martyn,

can i suggest you think outside the box plus check the thread tpi and form!

on miniature loco building i regularly do 1/2" ME threads in 26 tpi as its a standard size for us on boiler work, and i also have the dies for 1/2" x 32 tpi etc. the dies are very cheap and easily obtainable but need using in a lathe with a tailstock die holder for accurate work. on brass they are dead easy. if the thread equates to the ME type then most miniature loco builders could make you a few in 30 minutes.

cheers,

julian

[HopperParticipant @hopper]

If that is the plug with the screwdriver slot in it, easy way to make a few would be to screwcut a length of brass bar in the lathe, then part off pieces the required 1/4" thick. Then put the screwdriver slot in with a hacksaw. You can make a little jig to guide the hacksaw quite easily.

[martyn nutlandParticipant @martynnutland79495]

I think I'll be able to do this now. So many thanks everybody

Yes, it is the plug with the screwdriver slot across the top. The cap is the male part, the stub axle boss carries the female. And yes Phil, absolutely right, these things were built down to a price. Earlier cars have a blind bush at the bottom which presents it's own problems for extraction! Some Austins also close the holes with a welch plug which is not as elegant as the brass cap but simpler and, of course, cheaper.

As the company needed thousands of these caps I suspect they were produced on a dedicated machine. Probably stamped out like coins then thread-cut with an automatic die and had the slot milled at more or less the same time.

Thank you again everyone.

Martyn

[AjohnwParticipant @ajohnw51620]

i tried to find a picture of the part out of curiosity. The only brass blanking plug I could find fits on the steering axle assembly. Image here that even shows what appears to be poor workmanship – blunt tools probably

**LINK**

I don't want a flame war either but I can assure you that die heads can produce very accurate threads especially compared with modern usual standard nuts and bolts. Or even compared with modern precision stuff such as high tensile cap head screw etc. It would be interesting to know how they did make them. It looks to me like a part designed before design was more closely tied up with ease of manufacture and some parts were even machined by hand. It might even be a tapered thread on the original but it looks like it tightens down on a bush so no need.

I worked for Lucas who owned Girling and had no idea that they were once into suspension parts – king pins by the look of it. I also spent a short time in an auto shop setting during training. They generally use methods where the accuracy of the basic lathe like machines hardly matters so that high accuracy parts can be produced easily

Oh – if you hacksaw the slot obvious but some might not know the width can be set by fitting more than one blade. Also cleaned up with a needle file more easily then if some one is thinking of entering certain types of show competitions..

John

–

Edited By John W1 on 06/07/2015 10:30:14

[Phil WhitleyParticipant @philwhitley94135]

You are right John, die heads do produce accurate flanks, but rarely cut peaks and troughs to 100% depth, If you consider a thread cut to 100% of its depth and with correct peak and trough profiles, it would be almost impossible to fit a nut to it without having clearance "top and bottom" It is the flanks that do all the work, so you can afford to leave clearance on the peaks. I have used the two blades in the hacksaw trick, and this helps to produe a decent square sided slot, of a decent width. One of the constant frustrations in engineering repair is the removal of slotted screws, there doesnt ever seem to have been a standard for the thickness of the screw slot/driver blade, and things like these plugs, you have to search for (or make) a screwdriver with the full width and the perfect thickness to fit tight in the slot, then push HARD and turn gently, because it is a one chance job! If you worked for Lucas, you will know a lot more tricks than me John.

Phil

[Michael GilliganParticipant @michaelgilligan61133]

Martyn,

Idle curiosity led me here

£2.30 seems pretty reasonable

MichaelG.

.

P.S. … The curved slot would be tricky to do with a hacksaw. 

Edited By Michael Gilligan on 06/07/2015 23:07:38

[AjohnwParticipant @ajohnw51620]

Not looked for years Phil but my recollection is that nuts and bolt in BS form are sized to give little but some clearance on the crests and valleys. The die heads I have used do provide the correct form but yes the flanks do the work so an accurately pointed tool cut thread will be as good except for weakening the core of the male part. That often doesn't matter on the female part.

Yes my soul was sold to Lucas for my indentures and I was paid so little even bus fairs were a major expense. I never left really but was sold on to other companies 3 times.

John

–

Edited By John W1 on 06/07/2015 23:46:11

[martyn nutlandParticipant @martynnutland79495]

John

Your illustration shows exactly the part I was talking about. It blanks off the bushed top boss for the king pin on the stub axle. The trouble in the Austin Seven context is that ham-fisted owners/mechanics put them in as if they were securing a girder on the Forth Bridge. The screwdriver they use to try and get them out again then buggers the slot and helped by dirt and age the blanking plug won't come out so you have to drill, drift, or use some other butchery to remove it.That's why I need some!

Martyn

[Michael GilliganParticipant @michaelgilligan61133]

Posted by martyn nutland on 07/07/2015 06:20:21:

The trouble in the Austin Seven context is that ham-fisted owners/mechanics put them in as if they were securing a girder on the Forth Bridge. The screwdriver they use to try and get them out again then buggers the slot and helped by dirt and age the blanking plug won't come out so you have to drill, drift, or use some other butchery to remove it.That's why I need some!

.

Martyn,

I think your time would be well-spent making a proper tool to fit these "coin-slots"

It would be a very simple job to fit a suitably sized part-circle into a modified socket spanner.

MichaelG.

[alan smith 6Participant @alansmith6]

Martyn,

I have to agree with MichaelG on this thread. Better to make a simple extraction tool and buy in the caps as they are as cheap as chips! There must be more challenging items to make that will give that feel good factor.

I owned and rebuilt many A7`s in my younger days and at one time owned a stable of ten. unfortunately the terrible traffic problem in the UK and the owners of modern cars that don`t understand about the minimal braking effeciency of the A7 can make a journey a misery.

One adventure that I had was when I converted to hydraulic brakes on a 1932 saloon the first test crash stop made the car swerve into the kerb, hitting it quite hard. A couple of years later. I lent it to my brother and he phoned me in a panic saying that the nearside stub axle and wheel had come off and could I come and help.

I went into the garage and picked up a spare stub axle and kingpin and drove to the stricken car, the wheel had rotated under the wing and was acing like a jack. it was a simple job to put on the spare stub axle and wheel and as it was only a few hundred yards to my garage, just drove the car there without front brakes.

luckily the police, for once, did not make an appearance and what was the reason for the problem? The king pin had sheared two years earlier and the car had been driven around without problem and as luck would have it the incident ocurred at slow speed with no apparent damage to the car.

Alan

[Phil WhitleyParticipant @philwhitley94135]

I know the feeling John, when I was an apprentice, I worked Saturday mornings, and the bus fare was more than I got paid for the mornings work!

Phil

[martyn nutlandParticipant @martynnutland79495]

Alan

Enjoyed your posting and although this is not the place to talk 'Austin Sevens', identified with what you say.

The swerving (loss of steering) I think emanates from a change in castor angle from positive to negative when a badly designed steering/suspension system is subjected to very heavy braking forces (can you get very heavy braking forces with an Austin Seven!?). I have had this on an Austin Ten; and it's scarey.

I have a 'chamber of horrors' in my workshop that includes a bent and broken king pin I removed from a spare Seven axle that had been damaged in some unknown violent mishap. But damage like this is not confined to Austin Seven's of course. I had a friend who drove his Speed Six Bentley into the back of a fellow Bentley club member's Three Litre model. The Speed Six appeared to suffer little more than a buckled registration plate, it was only much later that a king pin was found to be broken in half. Salutary lesson: potentially lethal damage can go undetected. Moral, perhaps: if you bend the number plate check the king pins!

Thanks again for the sound advice.

Martyn

[alan smith 6Participant @alansmith6]

Martyn,

The swerving was caused by an imbalance between the front brakes which had to be sorted out. Seem to remember that it was caused by air in the right hand brake pipe or wheel cylinder.

As for A7 braking, the best that I ever did with standard brakes in an MOT using a Tapley Meter was 60% in my 1928 chummy which had uncoupled front and rear brakes so that one could stand on the footbrake and at the same time pull on the handbrake and the car would stop. The coupled brakes suffered from the problem that front axle wind up under braking tended to release the pull on the front brake levers and reduced the braking effect..

Exactly the same as what happened on Vintage Bentleys!

Martyn, I seem to remember that you are a motoring journalist

Alan

[Russell EberhardtParticipant @russelleberhardt48058]

Didn't Herbert Austin say something to the effect that, "Good brakes encourage furious driving"?

Russell.

[martyn nutlandParticipant @martynnutland79495]

Alan,

Right on all counts!

And Russell, he also said four wheel brakes weren't dangerous (not his uncoupled variety at any rate) because people would be so busy trying to steer as not to have a hand free to put the front ones on at the same time as the back!

Martyn

[alan smith 6Participant @alansmith6]

Russell,

I`ve owned vintage cars since I was seventeen and the poor braking on most of them certainly taught me anticipation and to look far ahead to identify incidents in the making that would require braking. This has stayed with me whatever car I`m driving. I now swan around in a vintage growler with an engine six times the cubic capacity of the A7 and it can be scary. If you miss that change down to third gear in the crash box when approaching a traffic roundabout at speed you`re in trouble!

Well maintained modern brakes and good tyres can allow drivers to drive like hooligans and mostly get away with it but what about you Russell? Do you find that having all that modern technology in your car affects your driving?

Alan

[Author]

Posts