It is hard to achieve a really good finish by hand-feed. Could you not have used fine feed on the lathe?

Also, that taper is usually from the boring-tool springing as you suggest, and a couple of spring cuts can often cure that. It's best to use the largest boring tool the cylinder can accommodate, if necessary using a smaller one for the initial cuts.

It can be set up for re-machining but being so close to finished size, will need a lot of care to ensure that 0.1mm each side does not become 0.2mm from one. A boring-head won't necessarily give you a better result as the tool can still spring; and you'd need consider which would be the more rigid machine, as well.

The best result if your lathe has a T-slotted cross-slide, is probably from a between-centres boring-bar with fine feed, though it may involve a lot of setting-up.

Exactly, before rushing off to use a boring head the question that should be asked, and resolved, is why the bore is tapered when using the lathe? A difference of 0.2mm is huge. I might expect a thou or two difference due to tool spring, but no more, and after a couple of spring passes you should be able to achieve well under a thou. Tool spring should stay pretty constant throughout the cut, so I suspect the taper is caused by something else. Like feeding with the top slide when it is not set parallel to the lathe axis?

Andrew

Andrew, I agree, why would tool spring cause tapering? The spring forces at the cutting point are the same when starting the cut as when finishing. A lot of people seem to think this is a thing but I have never understood why. Bed wear, poor lathe setup, the workpiece flexing (least likely with anything substantial and well secured) or material not being homogenous are all I can think of to cause tapering.

Martin C

Edited By Martin Connelly on 22/05/2021 08:56:19

I wonder if most people just measure each end and assume a taper as they don't have the tools or ability to take multiple fine readings along the length to determine the exact shape of the cut

It's just one of the great model engineering myths.

Nick: Compound slide is the same as the top slide; the slide on top of the cross slide than can be swivelled for taper turning or screwcutting.

I've just had a quick look at the rear wheel hubs for my traction engines, which were bored in the lathe:

Tool is a Glanze 16mm boring bar and stick out is around 130mm. I measured both ends and as near as I can tell both readings are 1.629". I don't see why only measuring at both ends should lead to assumptions of tapering. Apart from a very short lead at the start the bore ought to be parallel, if the tool is being moved parallel, as the cutting forces should stay constant.

Andrew

Not convinced that taper bores due to boring bar spring is a complete myth.

Given the basic quality, age and less than stellar condition of lathes commonly affordable by relative beginners in the past its pretty certain that bed wear would be greater near the chuck. Flat edge beds, whether dovetail or Myford style, generate greater inaccuracy for any given amount of wear than the prism guide style used on professional machines. They are also harder to set just so for best results on a worn bed. Setting involves a few tricks too which are not as commonly promulgated as they should be.

Hence I find it reasonable to suppose that cutting forces could deflect things a bit as the saddle moves over the worn area.

Especially if there are various infelicities in the tooling such as imperfect shape, inadequate sharpness or errors in the height of the cutting edge creating excessive push out forces.

Most especially where relatively small bores are concerned boring tools need deceptively large amounts of heel clearance. I've more than once been too casual about shapes in the past and found that the cut was being made by the heel rather than the tip.

Even if the heel isn't actually cutting it can drag on the newly bored surface slowly forcing the tip out of cut as the bore gets deeper. Instant taper.

Something I've actually deliberately worked through in the past either to keep a stronger tool or because I didn't want to modify the one I had to hand which was fine for the finish cuts. Just not enough clearance for the first few cuts from the starter hole. So long as the last cut is right it matters little what goes on before.

I have a printed out, pictorial, crib sheet to remind me of heel clearance angles for smaller bores.

Obviously the cutting tip needs to be on centre and the lead in angles such that cut forces don't tend to draw the tool into cut or push it out of cut. Again with slender boring tools in small bores getting the shape and sharpness right so it behaves itself with a proper cut is important.

Mucking about rubbing a couple of tenths off each pass is not the way to go. Even razor sharp tools rarely behave themselves on hair fine cuts. I reckon that anything under 10 thou cut is asking for trouble due to the inevitable limitations of hand ground and honed tooling made by relatively inexperienced folk with relatively limited facilities. My experience is that a misbehaving tool tends to make a measurable taper.

The cure is to sharpen the thing properly rather than assume it was ok last time. I usually get bitten by one dragged out of the back of the tooling drawer for a special job that none of the general use collection can handle. Generally on a quick "Can you sort this?" job I don't want to do but can't politely turn down. Hurry-up and don't wanna are a potent combination.

It all gets much easier once up in the 16 mm – 3/4" (ish) bar size range. Even then excess stick out is a gremlin magnet. I've worked at 10 times diameter (and more) stick out when needs must but keeping under 5 times is far more restful.

As Geo. H Thomas was wont to say the common "mutant golf club" boring tool is something of an abomination. Expecting a neophyte to get good results from one is asking a lot. Quite apart from mechanical limitations getting a well shaped, sharp tip is hard. If only because of the difficulty of holding it up to the grinding wheel at the correct angles on the normal toolrest. My preference in smaller sizes is to use round HSS held at a slight angle to the lathe axis so you don't need to grind a neck behind the tip. take off just enough to give heel and to clearance with a short cutting edge.

Clive

Hopefully, the taper is not the result of slightly different measurement techniques. Check, and double check!

Riding a hobby horse.

Does the lathe turn parallel on ODs?

If those cuts are tapered, maybe the bed is twisted. But to produce 0.02 mm taper over the short length of a cylinder, there would be a lot of twist! Whatever twist is present, it should be possible to reduce, if not eliminate it.

(Read Ian Bradley's "The Amateur's Workshop", or his "Myford 7 Series Manual" for the method. )

But if the problem is caused by wear, the same problem would be present when turning outside diameters.

Howard