Best way to turn long, thin brass job

Can someone please advise me the best way to turn a carburettor float needle for a pre-war motorbike carb with remote float chamber? The needle is approx. 3 inches long but only about 1/16th inch diameter with a 3/8" long conical end diameter about 1/4inch. I had in mind to make it from 1/4" rod, hold it in a 3 jaw chuck steadied with a centre at the other end. Turn the conical end nearest the chuck then reduce the dia. along the rest of the length. When all is shaped, cut the new needle at each end 'out' of the rod.

The cone end seals against the float chamber to cut off petrol supply.

Does this plan sound feasible?

Thanks in advance

Hi C I would personally think about making it from two pieces and soldering it together. Use a piece of 16th diameter for the stem and drill the 1/4 inch to take the stem which was finished to size.

David

Perhaps a fabricated construction here with a plain 1/16 inch pin from silver steel fitted into the conical section; that avoids all the flexing difficulties you are bound to have making it from solid.

I don't understand the finishing detail you describe

Regards

Brian

I think a box tool would be better as it supports the rod right at the cut all the way along, with minimal flex. After cutting OD, part off the needle and chuck it in a 4 jaw to centre it perfectly. Then put the cone on the end with the cross slide set at the needed angle. Leave just enough of the rod projecting from the chuck to do the length of the cone.

https://www.youtube.com/watch?v=X-TkuQDWdbA

If you can get to the above it might just be helpful.

M

Edited By Watford on 21/12/2018 18:17:05

Jeff,

Can you please enlighten me what a box tool is? Getting parallel cuts on thin bar is something that crops up from time to time in what I do, and I don't yet have a fully satisfactory method for doing this

Regards, Chas

Roller Boxes and Vee Boxes are - or used to be - common capstan lathe tools. A biggish casting projects toward the chuck/collet from the tailstock or capstan, with an adjustable toolbit to turn the diameter and either 2 rollers or a vee to support the workpiece against the cut. The roller boxes were used for steel components and could take very substantial cuts in an industrial power machine - one of the first exercises in the Government Training Centre course in the 70s was to take some 25mm A/F hex bar and roller box-turn a bolt with a 10mm diameter mirror-bright finish 100mm long. Then index to a second roller or vee box, turn 8 dia x 12 mm long, index to a die box and thread the small diameter M8. The rollers in the box tool followed closely behind the toolbit and polished the surface. The vee box wasn't as good and was more often used from brass and other soft metals.

You'd need quite a cute one to turn a long 1/16" diameter - the sort of tool you'd be more likely to see on a Swiss Auto in the clock or instrument industries.

https://www.youtube.com/watch?v=zw7xRZsORug

Edited By Mick B1 on 21/12/2018 20:54:34

Interesting video.

Thank you, bear..

Is this for a type 76 or earlier Amal Carb ? If so I will pull my spare down and have a look for ideas.

Box tool I had in mind was a homemade type used for various production jobs in toolroom and in screw machine room in the old days. Consisted of a rectangular mild steel or tool steel bar held on the compound rest or carriage, with a central hole a close fit on the rod stock to be cut. On the back (right) side a HSS tool would be mounted and adjusted to cut the exact size needed. The adjustment was often done with a screw behind the short HSS tool. A cap plate was usually fitted over the HSS tool to firmly hold it clamped in place. The bar was relieved at the right side to allow oil flow in and chips and oil out. If a lot of chips were being removed the tool would be planned to operate upside down to let the chips and oil fall out by gravity.

The youtube video posted above has roller or vee versions of this tool. Much nicer build than the rough ones we used to make, and roller one in particular would operate with much lower friction than the basic ones with a hole that we used to make. The ones in the video operate on exactly the same principle though - of holding the work close to the tool , closely controlled, regardless of length of cut.

Why use brass? Piece of 1/16" silver steel rod with a piece soldered on the end would be easier.

Sharp HSS tool side cutting tool plenty of rake, light feed. Dead on centre height. Cut in one pass from maybe 10 - 12mm bar. this should self support itself depending on the length you require.

Effectively you are leaving a 1/16 thick long pip!

Oh and allow for a few tests to get it right.

It is not adviseable to turn down brass rod of this length from 1/4 to 1/16 ,the core of a drawn rod is very weak. if you take a piece of 1/16 brass rod say brazing rod it can be bent and twisted without breaking,now turn a piece of 1/4 rod and turn say a 3/4 inch length down to approx 1/16 dia does not matter if its tapered or a bit rough and try bending it ,it has nothing like the strength of the brazing rod and easily breaks. In this case of a long needle a two piece fabrication is essential, you might say that the original was one piece,but they were probably made in high volumes and used some heading process to form the larger end. This weakness in the centre of round or hex bars occurrs in other metals so be aware of the problem ,It does not ocurr when making one off bolts as the reduction in diameter is about half, but in the case of the needle the reduction of four times is too great and down to a ery small diameter.

i have just got back home from work and I have to say a sincere thank you to all who took the trouble to offer me the benefit of their experience, I really appreciate it.

My first thought had been to make the needle in two parts but the conical end must be 'square' to the needle to seal the valve and I don't think I am good enough to get it accurate enough.

Chris, this is for a Dell Orto, similar to Amal type 76 but the needle is about 1/2" longer and seems to be unobtanium.

The box tool method looks good but I don't have one so the cost is prohibitive.

If I have a go at a two-piece needle using silver steel can I soft solder it? although I don't have much confidence in getting that square either!

i might have posted an image of similar needle below. Thanks.

Looking at the image I would try soft soldering a brass end to a piece of silver steel and then holding the silver steel in a collet. Turn the brass part in situ. You will have to lap it in to the float chamber even if you could buy a new one.

To be honest, much as I enjoy making small pieces for this bike, if i could find a new replacement I would buy it. I hadn't thought of doing it like that. Why silver steel? is it simply the strength of it?

Hi Jeff,

Thanks very much for your reply. A proper roller box would probably be a bit more than I need but the simpler version sounds like a great idea which I intend to make. It sounds like a great idea for an article in MEW if someone had the necessary drawing skills. It should be possible to use different-sized hardened bushes for the hole making it quite versatile.

Chas

Why silver steel ? it is available in lots of sizes and ground to size, will or should be straight and is very strong. Not expensive to buy.

Thanks for that, Chris. Sorry for my ignorance. I'm mostly trying to remember O level metalwork!

Charles - could do an article I guess, I do have more than adequate drawing capability, but at the end of a day doing machine design on CAD (my day job) the last thing I want to do is draw some more! The other thing to consider is that the box tool is usually made to fit a particular lathe in the height and clearances so they can not easily be designed as a universal fit. (At risk of boring you with legal matters I don't really want my effort / drawings to become intellectual property of My Time Media or whoever for minimal if any cash.)

You could use bushings in the tool for various sizes of shaft IF a) they are well secured against falling out b) there is enough adjustment length designed in for the HSS tool to cope with the diameter range of the bushings.

Again these tools were typically made for a particular production job making hundreds of thousands of parts, so each was purpose built. They were only an afternoon's work for a toolmaker so designing in variability / wide adjustment range was not a focus. We'd make them, adjust them to run the job well, and after that they made parts until sharpening time for the HSS tool, or the setup was torn down to do a setup for a different part. On some of the tools used often, we would make a gauge plug with a tail the rod size and an end at diameter to correctly set tool position. Using this plug, setup after sharpening took seconds - just move the tool up to the setup diameter and clamp it, start the job again. These plug gauges were stored on the tool in a drilled hole with a homemade brass set screw in a drilled and tapped hole at 90 degrees to the storage hole to secure them. Brass screw prevented marring the OD.