Turning a tapered tube queries

I have a couple of tapered tubular bearing spacers to make, approx length 65mm, major dia 38mm and minor diameter 25 ( both outside) and a wall thickness of 3mm.

My plan is to use some 2" steel stock that I have to hand.

Chuck in the four jaw and drill through 19 mm. Then set the compound over at the appropriate angle and turn the taper. As far as I can see I will have to turn with the major diameter away from the chuck.

I do not have the benefit of a steady.

Beyond personal pride dimentional tolerance and surface finish is not critical.

Should I turn the outside or the inside first.

My lathe is a sturdy industrial lathe. ( a Leinen LZ4S without taper turning )

What snags does anyone see.

Machine the inner diameter first. Boring bars always tend toward chatter, and that can be made worse if machining a thin walled tube etc. So finish machine the inner bore before going on to machine the outer. With 3mm wall thickness you should not have any big problems machining the OD smoothly.

As an aside, it's counter-intuitive but if you were wanting to turn the wall thickness down to a very thin shell, say 0.5mm or something, the way to do that would be to finish machine the inner first, then do the outer diameter in as deep a cut as you could. The strength of the unmachined material supports the point being machined and the thin shell left behind needs no support, having been machined already. But if you do what seems intuitive, ie machine down careflly in small cuts, hoping to take a light finishing cut, the remaining shell gets too thin too soon and starts moving and chattering before you get down to size.

You can't turn the taper with the compound? That's a pretty steep taper with a set-over and tapered inside and out makes it worse to hold. I'd drill some 1.5" stock 19mm and then turn the inside taper until the outboard ID is 32mm then repeat it on the outside taper with the same angle setup. Part off and de-burr to finish.

Hello Andy,

I would tackle that slightly differently. Drill out the centre as you have said and then fit a plug at the tailstock end to support the work. I don't see any need for internal boring if the drilled hole up the centre is adequate.

Align the compound slide to the appropriate angle to leave what will become the minor diameter at the tailstock end and machine the outer surface up towards the chuck, you can check your compound angle geometry as well as you go until eventually you have the taper you want over the whole length. I am not a fan of tailstock offsetting for taper turning on a length this short, it is usually a pig to get the proper alignment correct again afterwards

If the motion available on your compound is shorter than 65 mm, you can do this in two bites with a shallow radius shaped step between them in the middle as a 'design' feature, linking the steps, you say the quality of finish is not critical

Cut off the short stub left in the 4J chuck and deburr.

Regards

Brian

WhaAfter further consideration this is what I really need. I think it will be simpler to make in three pieces as it is, after all, only a spacer between two bearings of different sizes. I’m currently thinking if I turn my stock to 36mm then bore 19mm and turn the 50mm length inside and out with the narrow end towards the chuck I can then part off and have the bit in the chuck to use for my 14.5mm parallel tube. I can then make the smaller diameter parallel bit from a suitable size of feed stock. I could also make the wall thickness 4mm.

What could possibly go wrong

Edited By ANDY CAWLEY on 25/11/2018 14:15:31

Hello Andy,

Now that you have thoughtfully provided a drawing, forget my contribution, I misinterpreted your description completely! For a start I took the wall thickness to apply to the minor diameter end

Retiring to think again, sorry

Brian

Brian Wood:

They say a picture is worth a thousand words!

I'd just turn as one much like hopper says, done far longer and thinner wall traction engine chimneys like that. Drill to 18mm, then drill part way with 25mm drill followed by boring bar. Turn outside then saw/part off and finally reverse to skim the other end.

If you do it in three how do you propose to join them and keep it all concentric?

As ega says, a drawing helps a great deal.

I might now make that shape from tube, it will save a great deal of swarf making. Joining the three sections I would do with short step joints and silver solder the assembly well cramped up to preserve the alignment

The central tapered tube could be made from 36 mm O/D tube split down the length with a narrow triangular cut to then ease it together to join on the cut edges making the taper. Bind wire round that before sweating together

Just my thoughts

Brian

Hi Andy

As these don't have to be too accurate, I believe they where made with a form drill. Have you got enough room in the hub to fit a straight stepped spacer?

Check measurements

Set the cross slide to the correct taper.

Set a solid piece of about 50mm thickness round in the lathe.

face both ends, set length to 69.5mm length.

Drill and bore it to 19mm.

Counter bore the open end to 30mm dia by 14.5mm deep.

Bore taper watching the internal corner where it touches the 30mm x 14.5mm and the 19mm x 5mm bore length is set.

Set a piece of scrap in the lathe to make a plug taper using the same cross slide settings.
I would only worry about the taper section no need to do the ends.

Place the previously turned piece over the taper and set a centered washer about 19mm dia over the end.
Use the tail stock to apply pressure.

Turn the outside features.

As the plug was turned in situ all should be concentric.

I use the method John describes above to make tuned pipes for my tether car engines, wall thickness is usually 10 to 12 thou, I machine sockets on the one end of each tapered part the silver solder together.

Measure twice, cut once!!

Thank you John Mc

Fortunately I haven't   cut yet!!

Edited By ANDY CAWLEY on 26/11/2018 17:11:07

I use the method used by John for parallel tubes that become displacers and hot caps for hot air engines, that way you can take the thickness to less that ,010", but for the job in hand, use Hopper's method.

Ian S C

This ^^^^. Would make life a lot easier for yourself. Turn it out of one piece of bar, stepped along the way to clear the axle inside and the hub outside.

We don't always need to go the easy way. It wouldn't be too hard to make in one piece.

Ian S C

I hope to give it a go this weekend.

Harrumphj, managed to break one of the screws on on my 4 jaw chuck late Friday afternoon. Won't be making swarf today.

At last I made the spacer!

I held the stock in a 4 jaw chuck and series drilled through the centre ending up with a 3/4" blacksmith drill. I then opened out to 20mm with a boring bar,

I was amazed with quality of finish with such a long extension of the boring bar. The above photo shows this. It also shows the spindle for which the spacer is being made. I measured the angle of the taper as 5 degrees and set the compound over at this angle. Having reduced the overhang of the boring bar I then bored the 30mm short parallel.

To bore the taper I extended the boring bar and set it in line with the spindle axis. With the compound slide wound well back I then advanced the cross slide to the bed stop, wound the boring bar in on the compound until it nearly touched the shoulder of the 30 mm bore. Having measured the length of the taper I then marked the compound slide with the length as below.

I then bored the taper winding the compound slide in by hand. Running at 500 rpm it took 0.5 mm cuts quite happily producing blued steel chips and giving a good finish.

To turn the external taper I made a plug for the 20mm bore that the was supported with a revolving tailstock centre.Again turning at 500 rpm with a 0.5 mm cut.

This what the finished assembly looks like, well, it will be improved with a bit of Scotchbrite.

In the end it was a lot easier than I anticipated thanks to everyone's advice..

Edited By ANDY CAWLEY on 16/12/2018 13:47:02