cutting a scroll inside a bush

While working on various old british motorcycle engine and gearboxes i occassionally need to replace a bronze bush. It is a simple matter to turn the internal and external diameters and the length. However these bushes usually have a scroll internally through them for oil to enter the bush. When i say scroll i mean a thread with a large pitch , 1/2" or 3/4" at a guess depending on the size.

How are these scrolls cut ? Surely not by standard lathe threadcutting techniques ? Any ideas ?

just like you say ..

if deep then probably Screwcut . if very shallow( and left hand)..done by rapid withdrawal without lifting tool off work

There were specialised machined used to cut the butterfly style oil grooves in bearings, so may be something along those lines was used?

Andrew

Ronan,

Some years ago when helping a friend rebuild a vintage bike we came across the same problem. How to put the figure of "8" oil grooves inside a bronze bearing.

I think they would have done this on an hydraulic copy lathe of sorts.

We managed to replicate it quite well but it meant heavily modifiying a little Unimat lathe. We removed the leads screw so that the carriage could be push up and down by hand. We cut a piece of tin plate to wrap round the chuck, which was cut like a sine wave, so it acted like a cam, a piece of steel bar with a little roller bearing on the end was clamped to the sadddle to act as a follower. The top slide held an internal grooving tool. Slowly turning the lathe by hand with a little crank handle in the spindle, the saddle could be made to follow the cam and the cut gradually put on with the top slide. After the first half of the figure of 8 is cut the "cam" was moved around 180 deg and the second half cut.

Hope that makes some sense. Barry

Edited By Saxalby on 16/10/2013 00:43:07

Can use the same system to press something like a cog onto a mandrel instead of using a key

bore the cog out undersize then cut a large pitch thread into it, then press(or batter) it on with a hammer and drift

Did the small cog on my backgear like that a couple of years ago and it's been fine so far, done some serious work too

Used to cut figure 8 grooves and similar inside bushes with a small chisel with rounded end. the bushes were bigger than usual model eng. ,about 1" to 2" dia. but could be done.

As always thanks for the replies. Some good ideas on how to achieve this seemingly simple task. On BSA bantam fork bushes, i notice that they are a flat piece of material folded around a circular former, it would be easy to stamp the scroll on the flat plate before forming.

.

Ronan,

Quote: On BSA bantam fork bushes, i notice that they are a flat piece of material folded around a circular former

It was almost exactly 50 years ago that my Dad helped me refurbish the forks on my D1.

We very carefully measured the wear, then removed the bushes and re-fitted with a suitable shim on the outside of the bush. This closed-up the bore, and all was good !!

I think this was a trick they developed when he was working on Spitfires, out in India during WWII.

MichaelG.

On a Beezer Bantam chances are it wasn't even a circular former

The shape of the oil groove isn't critical so couldn't you cut a straight axial groove instead? You could do that in the same way as cutting a keyway with a suitably shaped tool.

Russell.

I just came across this YouTube Video - pure thinking out of the Box. Actual Oil Groove Cutting starts at 6:40.

https://youtu.be/9EPfRd79FPM

That’s astonishing, Bruce … in so many ways

Thanks for the link.

MichaelG.

Truly an astonishing video, absolutely fascinating, never seen a set up like that before so ingenious, Thanks for the link Bruce.

I have never seen the machine but I went to the same college( Bromley) as the apprentices from Molins in south london ( who made cigarette making machinery), and remember them saying they all spent time on a machine dedicated to putting the elliptical oil grooves in bronze bushes of varying sizes. I believe it was a commercial made machine and was German. It was regarded almost as a punishment job to spend time on it, a pig to set up then days of repetitive boredom. I remember thinking at the time that it couldn't be much worse than having to chisel the things by hand, only ever had a few to do but I was working with Mr Superfussy at the time!

I've always wondered what the machine was like and how it operated, any links or pictures to anything similar would be much appreciated and of great interest. The only thing remembered from their description is that gearing was changed to alter the stroke of the machine, but it was a long time ago!

Dave

Edited By Dave Wootton on 27/12/2022 08:03:21

Fascinating video Bruce - just shows how a bit if thought and some quick welding can solve a multitude of problems!

Also the heath and safety footwear and going by the background noise child minding as well no wonder we cannot compete with them.Excellent find and fascinating to watch

We did compete, but then we got civilised. Go back in 100 years and they'll be asking the same question.

Regards Ian.

Edited By Circlip on 27/12/2022 11:39:15

That's old mate from Indonesia, Mekanik Mesin. I posted some of his amazing videos a while back here LINK Well worth a look. The guy is a genius at doing almost anything with almost nothing. A very clever machinist with many great ideas relative to the home workshop where we don't have the resources of industry at our command.

This latest scroll machining attachment is pure genius and his best effort yet! But sheesh, he is not shy about arc welding right there on his lathe is he. No anti-splatter spray I am sure.

Edited By Hopper on 27/12/2022 12:01:09

FWIW Steel backed shell, bearings, and bushes all start life as a huge roll of steel onto which the, for want of a better word, "white metal" bearing material has been bonded by immense pressure..

Having been sheared to the required size, the embryo bearing shell is wrapped, (pressed ) and broached.followed by the oil grooves being machined in, before the anti rotation tang is formed.

(Any swarf is collected, and recycled )

The suggestion to produce helical oil groove ( s ) by slowly withdrawing the tool as the chuck slowly rotates is the method that i would use. Preferably before final boring to finished size.

Ideally the groove stops short of the edge of the bearing, to retain oil.

The depth of the groove is probably not that vital., since in a non pressure fed bearing, the oil provides boundary lubrication, because the oil molecules are smaller than the depth of the groove, and distribute themselves along the groove.

On a pressure fed "white metal" steel backed bearing, the clearance between shaft and bearing will be about 1/1000 of the journal diameter. The clearance provides for an oil flow to act as a coolant to keep bearing temperatures where the bearing overlay material does not start separating into it's constituents.

In a less highly loaded, and non pressure fed bearing, the clearances may be a little less (Again, boundary lubrication ) to minimise the impact loads from irregular loading.

Where the bearing is a thick wall, bearing (Such as metalled in the rod or crankcase ) the clearances are likely to be closer, because of the lack of support provided by the steel backing..

Howard.

Saxalby's interesting method using his Unimat lathe is very much in line with a lot of repetition-turning in years past. It didn't seem to need modifying his lathe particularly, simply removing the leadscrew.

Prior to hydraulic copying, and now course NC, lathes, a task like this could have been done on cam-controlled, capstan lathes. These typically had a big cam-drum on the back of the headstock, below the bar-feeder. The cams themselves were steel strips profiled to task, and screwed to the drum's outer wall; to reciprocate the linear motions via appropriate followers etc.

Some bearings are made with straight oil-grooves fed centrally by the oiling point. On a flat external surface like a cross-head, it was often a circular groove presumably cut by a boring-head on a milling-machine. (The slide-bars on P.S. Waverley's engines use such.)

What seems to matter less is not the profile but the groove not meeting the bush end. The more elaborate patterns can hold more oil, over a larger surface.

Edited By Nigel Graham 2 on 27/12/2022 13:02:11

My firm used to make large quantities of bushes for aircraft with beautiful internal grooves down to 1/2" bore on CNC machines. The last time I wanted a groove, I used a dremel, the oil will flow even in a shallow groove.