Line Boring Cylinders

[arch1947]

Hi Folks, In a recent issue of EIM there was an article about line boring cylinders in the lathe using a boring gar between centres. Not that I am ready to do such things now, in the future I plan to build a 5" Evening Star and this technique looks good but I have questions that someone my be able to help with.
1. Is set up difficult? It is necessary to get the cylinder block lined up at the correct height above centre and from centre to centre.
2. Are there commercially available boring bars to do the job? My sense is that the bar was probably home made. Do they have some kind of fine adjustment on the tool so that you can gradually open the bore to size.
3 Are they equipped with carbide tipped tools? If not you be forever removing the cutting tool to sharpen, a real pain at the best of times.
Very interesting and much closer to what would done in the full size world.

Cheers,

Arch.

PS The fire situation in Victoria is expected to get worse again this weekend with strong winds and high temperatures.

[welshy]

Hello Arch, Boring with a bar between centres is pretty straightforward. I have a home made bar which is a 1" dia steel bar about 15" long which is centre drilled at each end and has a 3/16" round HSS tool bit cross drilled into the boring bar approx midway along the bar. There is no micro adjustment on the cutter I just measure across the bar and the cutter tip.
You need to set the cylinder casting cast bore on the centre of the boring barby packing up the underneath on the crossslide and adjusting the cross slide. I bore the both cylinders to within about 10 thou and then resharpen the tool to finish both bores off exactly the same dia. Even with large cast iron cylinders the tool should remain fairly sharp so you should not need to sharpen it too many times.
Mike

[Shawki Shlemon]

I always bore cylinders between centres , I made the tools and very simple . I have thread here with photos called boring between centres. on the question of the bar and the tool i do exactly like welshy except I use two bars ,one to get close and one for finish and keep the finish tool at the same setting and use to finish the second cylinder .

[albert]

Hello all,
I am with all the above,but I have more than one boring bar made over the years.I use 1 to get the bore very near to size and then use another for the final cut and nothing else.It may be more trouble this way but all bores are exactly the same,you also finish one bore before starting the second--no resetting up.
The bigger the bar in O/D and short as possible the better.I only use high speed cutters.
Albert.

[russell]

I made a boring bar to Geo. Thomas' design. It has the hole for the cutter at 40 deg to the axis of the bar and a 40 tpi screw behind it. Thus 1/4 turn of the screw advances the cutter 4 thou. I'm sure you metric guys could come up with a suitable alternative design.

Russell.

[jgb7573]

I too have bored cylinders between centres. I find it a very satisfying technique. You're not taking a lot off, nor are you going fast, so resharpening the tool is not an issue. As an added advantage, by machining (flycutting in my case) one end of the cylinder while it's still mounted on the cross slide, you can be sure that the rear cylinder cover is truly at right angles to the bore.
Between centres boring bars are available commercially. Arrand do them. They include micrometer feed of the tool bit.

[drjohn]

Hi Arch

Without referring to acerbism, what I want to tell you is that the back of the cylinder (the end that the piston rod goes in) MUST be absolutely at right angles to the bore.

Boring between centres to make prissy little holes is ok, but unless you flycut the back of the cylinder in the same machining operation, you're wasting your time.

Put it in a 4 jaw and do it like us non-engineering types do -

dr-john.org/images-web/simplex/smaller/cylinders 005.jpg[/img]

Face it off and bore it all in the one setting. Alright, that was a pic I took to show blowholes in a cylinder I was machining.

The front end doesn't really matter - it's just a cover plate and can be machined later.

ALthough, having said that, I did machine the front to give a reference surface when I slapped it up against the face of the 4 jaw.

But then, I'm not a trained engineer ;D

DJ

[jgb7573]

Hi Dr John,
A question from another non-engineer. How do you ensure the bore is parallel to the bolting face of your cylinder? From your picture it looks like you are relying on the jaw of your 4 jaw for this. Or am I missing something?
John

[drjohn]

Hi John

I faced off the end of the cylinder and then bored the hole without taking it out of the chuck.

To me, that's about a s square as a Chizaou family lathe allows. ;D

I then wrote on it with a magic marker "B" to remind me that was the back.

DJ

[durhambuilder]

I've always been a little puzzled line boring cylinders in this way. The lathe saddle and cross slide has to have a certain amount of slack / working tolerance to enable it to move and also general wear and tear, backlash etc. So, as the cutter goes round it will try to push the saddle down, push it towards the operator, lift the saddle up and then push it away from the operator. Now I accept this may only be a thou or so and does this really matter?

[jgb7573]

Hi Dr John,

Sorry I wasn't clear in my question to you. I quite agree that with the chuck method the rear face of your cylinder is going to be as square to the bore as near as is necessary. What I was trying to ask about was the surface that is used for bolting the cylinder to the frame. With the between centres method, if you mount the cylinder on the crosslide with the "to the frames" bolting face at the bottom on suitable parallel packing, you can ensure that as well as the rear face and bore being perpendicular, that the bore is also parallel to the frames.
Maybe I'm being over-fussy here (as I said above I am a non-engineer and you have a working loco), but it seems to me that this would be a good thing(TM)!
Best wishes,
John

[albert]

Hello All,
I wish at times I could have mounted my last 3 cylinders in the 4 jaw, and I have a Colchester lathe.
From the casting shop they weighed 84lbs.
Albert

[welshy]

Hello John, To answer your question on the squareness of the bolting to the cylinder faces.
To machine a cylinder in the four jaw the first stage is to centre the unmachined casting in the chuck so that the bore centre runs true.
Second stage is to machine the bore to to size and face end face to the correct depth.
Third stage is to take the casting out of the chuck and turn a mandrel to a hand push fit in the bore of the cylinder.
Fourth stage is to push the cylinder onto the mandrel with the machined end towards the chuck and then face the other end, this will ensure that the second face is square to the bore and parallel to the other end face.
Fifth stage is to mount the cylinder either in the milling machine vice with the two end faces in between the vice jaws or mount in the four jaw chuck again with the two end faces between two of the jaws.
The bolting face is the last face to be machined and it is the machine vice or the 4 jaw that ensures the accuracy of this face to the end faces, altough as DrJohn pointed out the top end face of the cylinder is not as critical for squareness.
Hope this helps
Mike

[drjohn]

Mike explained almost what I did, but I missed out the mandrel for squaring off the cover face, just used the 4 jaw.

DJ

[chameleonrob]

if you at all unsure of your lathes ability to turn long lengths parallel then don't use the four jaw to hold the cylinder, you really don't want a tapered cylinder, even by a few thou, my old lathe was like that so I bolted it to the cross slide, tightened the cross slide gib strips, drilled them, used a homemade between centres boring bar to rough them out then used a small boring head that fit inside the bore and to finish them used a fly cutter on the rear face. if you tighten the gib strips and take light cuts then backlash and free play isn't a problem, when mine were finished they sealed near perfectly with the home made rings in them. would I do the same thing again, no, but then I trust my new lathes ability to turn parallel more and the amount of free play in the saddle less so it would be a 4 jaw job, but thats because of tools rather than tecnique.

rob

[jgb7573]

Hi Mike (Welshy).
Thanks for that explanation.
John

[welshy]

Hello again John hope it was of help.
Mike

[baggo]

Feb 11, 2009 12:31:48 GMT durhambuilder said:

I've always been a little puzzled line boring cylinders in this way. The lathe saddle and cross slide has to have a certain amount of slack / working tolerance to enable it to move and also general wear and tear, backlash etc. So, as the cutter goes round it will try to push the saddle down, push it towards the operator, lift the saddle up and then push it away from the operator. Now I accept this may only be a thou or so and does this really matter?

Interestingly, Keith's column in this issue of ME mentions exactly the same problem.

John

[alanstepney]

Feb 11, 2009 12:31:48 GMT durhambuilder said:

I've always been a little puzzled line boring cylinders in this way.
...
Now I accept this may only be a thou or so and does this really matter?

I've always bored cylinders this way, and never had any problems with them.
So, in practice, "the odd thou or so", doesnt matter.

In fact, it is VERY hard to get a perfectly circulary bore.
A reamer wont do it, and a single point tool will give a result that is closer to round.

Finishing the bore with a lap can be an improvement, but only as long as it is to a reasonable degree of circularity beforehand.

Fortunately for "us", (or those of us who are not professional engineers) one can get away with a less-than-pefect bore and still have a working engine.

[ettingtonliam]

Hi
If you adjust the saddle gibs to be a little stiff, then unless your lathe bed is worn unevenly, in practice there won't be a problem, epecially on a light finishing cut where the forces are small compared to the weight of the saddle. Depending on the design of your lathe saddle, you might find you have to add an additional clamp to stop the saddle lifting off the back shear if you are taking a heavy roughing cut. The bigger and heavier the lathe, the less chance there is of this happening. Clamping things down on the saddle to bore them used to be a common practice in industry, but few modern lathes apart from 'home use' ones seem to have tee slots on the carriage anymore.
Hemingway do a kit for 3 between centres boring bars, of a design where the cutting head moves along the bar, so you can clamp the saddle down, and avoid any movement there. The downside of this is that if the bar itsef is not perfectly straight, then the moving toolhead will tend to produce a barrel shaped bore. Personally, I prefer to stick to the simple between centres bar with a fixed cutter, and I've never had any problems this way with tapered/out of round bores. I set them up so that the critical end (The crosshead end) is nearest the headstock, and then face that by bolting the topslide to the faceplate with a cranked cutter in the toolpost, set the speed to dead slow, and give the handle a tweak each time it comes round. I know it breaks all the safety rules I was ever taught, but I've been doing it for a lot of years (behind closed doors), and still have all my fingers. You just have to be careful. Posh people have proper boring/facing heads, but I don't.
Richard