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.

Regards

Norman

"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 "

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.

.

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?

Dave

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 know

Russell.

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

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?

Eddie

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

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. ??

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

Thank you.

Regards,

IanT

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

Hi Ian

The cylinder had a flange with mounting holes on the bottom. I fixed a couple of wooden blocks with screws and silicon sealant to the inside of the lid and screwed the cylinder to these blocks. I clamped my 4-jaw independant chuck to the drill table then adjusted the jaws to position of the cylinder in it until it was centred beneath the chuck. Popped on the cover with the previously cut hole and inserted the hone such that, with the quill fully retracted, less than 1/2 the length of the hone stones were out of the cylinder. I then lowered the quill such that, again, less than 1/2 the length of the stones were out of the bottom of the cylinder and then set the depth stop. Added honeing oil from squeezy bottle with fine spout and set drill running. Fairly slowly and steadily wound the hone up and down the bore adding more oil from time to time. I found it helped to count steadily to keep a steady rate of movement – an appropriate piece of music would be better! It does take qiute a time and is pretty tedious but you can always stop for a cuppa or indeed leave and continue another day! As said before a nice smooth bore results.

Here is a pic of the inside of the lid.

Good Luck !

Norman

The thing with a back cut is to remember to take it under self-act. All to easy to run back by hand far to fast and leave a 'spiral scratch'

Joey