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Is LBSC correct

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Eric Cox03/02/2013 11:47:00
544 forum posts
37 photos

Reading LBSC's article on the Bantom Cock he describes boring the cylinders ME 4449 p 221.

At the end of the article he states "when the bore is the correct size, take a couple of traverses through the bore without shifting the cross slide. This will make up for any springing in the tool"

Surely, if the bore is the correct size then you don't need to make any more passes with the tool.

CHRIS WOODS 103/02/2013 12:06:24
38 forum posts
3 photos

Further passes with the tool at its last setting will reduce the amount of taper in the bore inevitably left by most model engineers working on small machines, also it is likely an improvement in the finish will result.

As the piston will be turned to an appropriate fit in the bore an extra thou. or two on top of the nominal size will not matter. Or you can approach the final size and before reaching it you can do as LBSC suggested to get a parallel bore with a good finish at the drawing size.

NJH03/02/2013 12:10:39
2314 forum posts
139 photos

Hi Eric

I suspect he is implying that the bore will not be correct throughout its length. It is after all more important that the bore is parallel that that it is dead to size.



colin hawes03/02/2013 12:21:39
558 forum posts
18 photos

Boring tools will always spring away from the cut and leave a small taper; how much depends on the rigidity of the tool. I think LBSC was referring to the entrance to the bore where it is easiest to measure it. To be sure the bore is parallel it is wise to make a plug gauge that will travel along the whole length of the bore before you set up the boring operation. LBSC is right. Colin

John Stevenson03/02/2013 12:24:19
5068 forum posts
3 photos

"When the bore is the correct size" probably done as well as to add " As measured at the throat"

But hindsight is a wonderful thing and bear in mind it was probably a weekly mag at this point in time and he did a lot more than a lot of modern writers in that he supplied far better descriptions than " turn the piston rod to size "

Stub Mandrel03/02/2013 12:39:30
4315 forum posts
291 photos
1 articles


He's right the tool only has to spring by a tenth of a thou for there to be a noticeable difference on a second cut and the bore will become more consitent along its length..You may be surprised how much metal comes out on a second or third pass.

For any critical job I always take multiple cuts at the same setting when approaching size.

Don't forget to wind the tool out of contact with the bore before winding it back.


Bazyle03/02/2013 13:39:03
6324 forum posts
222 photos

I don't think LBSC was doing any more than repeat a standard term of phrase both he and countless other writers had used before because it was common knowledge that that was how it should be done. Just like every hole drilling is along the lines of 'start small and open out in stages'. There will always be one newcomer for whom this was their first reading of ME so it would help though most regular readers would already know.
Some of this is also instruction for people using amateur equipment at home because there would be some 'experts' and 'toolmakers' whose work equipment allowed boring dead true and drilling a 2 in hole in one pass and were too dumb to realise they couldn' do that on a £5 lathe.


chris stephens03/02/2013 14:41:00
1049 forum posts
1 photos

Hi Neil,

"Don't forget to wind the tool out of contact with the bore before winding it back." Why?

I find a cut taken "backwards" often produces a far better finish. This applies to internal and external turning. Can you explain why it should be necessary to do as you suggest, is there something else I don't know?frown


David Haynes03/02/2013 15:13:02
168 forum posts
26 photos

Okay, so the boring tool has been passed through several times without altering the cross slide, but LBSC does not go on to recommend lapping in order to remove the tiny undulations that may be present along the length of the bore. Is this because they will be taken up by the piston packing?


Keith Long03/02/2013 15:24:46
877 forum posts
11 photos

Hi Chris

Depending on the design of the lathe and the wear and slackness in the gibs it is possible for the saddle to twist ever so slightly in going from rightwards to leftwards travel or vice versa. This movement can get magnified by the boring tool sticking out of the tool holder and can easily put an an extra cut of several thou just when you don't need it.


Edited By Keith Long on 03/02/2013 15:25:28

Russell Eberhardt03/02/2013 16:03:00
2736 forum posts
86 photos

"Don't forget to wind the tool out of contact with the bore before winding it back." Why?

Because, on a lathe with flat ways rather than inverted "V", if the saddle gib is not correctly adjusted or the bed is worn, the saddle can twist slightly when winding back. This causes the tip of the boring tool to cut a little deeper and produce a nice helical groove. Don't ask how I knowsad


Stub Mandrel03/02/2013 18:13:04
4315 forum posts
291 photos
1 articles


It has been answered for me. Even on a bed with an inverted V like my lathe there is still the possibility of slight twist. What matters is that all finishing cuts are in the same direction so each cut matches the others.

Also, using power feed can help improve the finish.


Sandy Morton03/02/2013 18:25:24
104 forum posts

It will be a very brave engineer who will argue with the writings of LBSC surprise

NJH03/02/2013 21:03:58
2314 forum posts
139 photos

Hi David

You say :-

LBSC does not go on to recommend lapping in order to remove the tiny undulations that may be present along the length of the bore. Is this because they will be taken up by the "packing?"

Do Steam engine cylinders NEED lapping? I suspect that in LBSC's day this would have been an unnecessary refinement - do folk even do it today?


Edited By NJH on 03/02/2013 21:07:12

6196203/02/2013 22:58:42
65 forum posts
1 photos

I think you mustn't forget that in the days when LBSC was writing most model engineers didn't have access to the equipment that most of us have now. Micrometers - telescopic bore gauges, digital measurement have all become common since LBSC was about. Then many people were working with inside and outside calipers and made go-no go gauges if they needed great accuracy.

I always take the back cut and measure that when I'm going for high accuracy. I can measure the forward cut and it gives me a good idea where I'm at and I can decide whether it's finished or whether I need the extra thou.

Some very good model locos that were built in the fifties had lapped bores, and plain pistons with no packing or rings. It's a labourious process, (I remember doing a CI engine) and very much likely to go wrong if you are impatient. I've never done that with a loco myself, always using piston rings with a relatively easy fit for the pistons. Works for me. I've been considering buying some hones for the latest creation - any experience out there?





Edited By 61962 on 03/02/2013 22:59:01

NJH03/02/2013 23:38:16
2314 forum posts
139 photos


Hi Eddie

I have used a hone from Polly Model on a hot air engine and, with care ( and patience!), it produced a very nice smooth bore. I mounted the cylinder, inside a plastic jar ( to stop spraying honing oil around the workshop!), and set up on my pillar drill. With a slow speed and constant  steady up and down movement a good result was achived. It is important to keep the hone moving and this is easily achived with the drill handle.





Edited By NJH on 03/02/2013 23:46:42

john kennedy 104/02/2013 07:18:59
214 forum posts
24 photos

Why would a boring tool spring out further at one end of the bore to the other,thereby producing a taper ? The overhang of the tool doesn't change. Is the bore bigger or smaller at the beginning of the cylinder. ??

Michael Gilligan04/02/2013 08:58:55
20182 forum posts
1053 photos
Posted by john kennedy 1 on 04/02/2013 07:18:59:

Why would a boring tool spring out further at one end of the bore to the other,thereby producing a taper ? The overhang of the tool doesn't change. Is the bore bigger or smaller at the beginning of the cylinder. ??

That, John, is a very good question !

The hole will be bellmouthed rather than truly tapered

... There is a reaonable explanation here


IanT04/02/2013 09:01:19
1993 forum posts
212 photos

I have one of those hones Norman - but not used it just yet. I will certainly use your idea when I do.

Thank you.




PS How are you holding the part? The jaws seem to be clamping the lid.

Edited By IanT on 04/02/2013 09:02:47

Ady104/02/2013 09:39:45
5089 forum posts
736 photos

Assuming all things being equal I would think that only a between centres boring bar would create a proper straight hole

Especially on a hobby lathe

You can even shove a short workpiece over with your finger pressure when there's a dti on the back

This gets harder, the closer you get to the chuck

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