Between centres boring bar

[ettingtonliam]

I need to make a between centres boring bar of the type which has a head carrying the cutter, which moves along the bar itself, actuated by a long feedscrew. Now I'm sure I've seen a published design for one of these, but can't for the life of me remember where. if it was in MEW, thats great, because I have most of them. Anybody any ideas?

Regards
Richard

[SteveW]

Richard,

I understood the "Between Centres Boring Bar" was just that. A bit of bar that runs between centres and driven via a drive-dog from the chuck end. Around the middle is a simple lathe tool that sticks out at 90 degrees.

The parent metal to be bored is mounted on the saddle with the axis of the required hole at/on centre height and this a hole big enough to take the bar and the cutter.

The trick is the saddle moves the work past the rotating cutter rather than your more complex "cutter moves on the bar". The bar need to be long enough either side of the cutter to be able to clear the work before/after the pass. This is a very simple and accurate method of boring bigger/longer holes on a lathe.

[jasonb]

Sounds more like a line boring tool than a between cts boring bar, sorry not seen any articles.

Jason

[ettingtonliam]

Steve
What you describe is the normal between centres bar. I have a situation where the work is too big to fit on the saddle, indeed, will only just fit on the bed, so I need the moving head type of between centres bar. Before anyone asks, no it won't fit on the mill either.

Richard

[Deleted]

Richard: any chance of a photo to help with the creative process.....
JB

[ettingtonliam]

Umm- Not really, partially because I'm away from home and the workshop until Easter, and partially because, despite being given guidance, I've never mastered posting photos on this site. I can email you photos after Easter though.

Richard

[kneedeepinswarf]

In Guy Lautard's "The Machinist's Second Bedside Reader" ISBN0-9690980-3-0, pp96-97, is a description of a device which might be what you are looking for. Although this is not a constructional article, there is enough information given to enable one to be made.
It consists of a steel bar, mounted in bearings at each end, upon which a tool block slides. The tool block is driven along the bar by a lead-screw, which is fitted with a star-wheel. The whole assembly is rotated and, as it rotates, the star-wheel contacts a pin which causes the tool to be advanced along the bore.
The unit described here is actually a free-standing device, driven by its own motor, but I see no reason why it could not be used on a lathe, as you propose.

[ettingtonliam]

Yes that sounds like the sort of thing. Unfortunatly, I don't have a copy, and as far as I can tell from the Camden Steam website, they seem to be out of print. any possibility someone could scan it for me. I have to say I hadn't thought of a complete free standing machine, but I do have a very robust machine bed casting under the bench which might do very nicely. Oh dear, another project starts!

Richard

[kneedeepinswarf]

On a lathe you would'nt need the spider assembly or the bearings, just mount the bar between centres and fit the star-wheel indexing pin into a block clamped to the lathe bed

[SteveW]

Richard,

If the moving tool version will fit your lathe and you still have to make it have you considered making what is in effect a relatively simple sled on to which you mount your work piece. The sled would be made to ride accurately on the bed.

You would then use the saddle to move the work using the lead screw as before.

The big problem with moving the tool along the rod is that it'll deflect the bar differently depending where it is on the bar. Maybe the simplest method would be to find someone with a bigger lathe. If the work is that big you might just need a more robust machine anyway.

[steam4ian]

G'day Richard.

The one thing in favour of you proposal is that it is protypical of cylinder reboring machines used at 12" to 1'. A true scale model would only have its cylinders bored with one of these!

Regards
Ian

[ettingtonliam]

Steve
Yes I had thought of the sled method, and probably its current favourite. One of the castings will only have 1/8" clearance above the bed though, so it will have to be very thin. I was going to provide motive power for the sled by pushing it with the carriage.
As you point out, if the boring bar is not absolutely straight, the resulting bore diameter will vary. This isn't a great problem with these particular bores, because they are fairly short, but would certainly be a problem if I was, for example, doing a cylinder with it. The other problem is that the smallest bore is 1 1/4", and so far, I haven't come up with a design of moveing head which will fit. I'd like a bar dia of 1" for rigidity, 3/4" at the very least, and there isn't room for a 1/4" dia leadscrew outside even the 3/4" bar, and still get it all to fit down the pilot hole of 1 1/8" dia. I don't want to groove the boring bar to take the leadscrew, because the boring bar would undoubtedly distort to banana shape in the process.
By default, I'll probably end up with the sled method.

Yes I know the solution to many machining problems is a bigger machine, but given the problems I had getting the present machine (Raglan Littlejohn Mk 2) into the space available, and that to do the job comfortably I'd probably need an 8" (16" in US terminology), that just isn't going to happen.

Thanks for all the help gents.

Richard

[SteveW]

Richard,

I haven't yet spotted how deep you need to go and I guess the work shape precludes mounting that on a face plate and boring from the saddle.

What about the sledge method or just mount on an overhang off the saddle and use a long boring bar mounted off the face plate or use a boring head with a long tool? Agreed, if the head axis/bed aren't in line this is the classic way of getting a tapered bore, the between centres method solves this one but I'm not sure a traveling tool type will be as fool proof particularly if home brewed (no offense intended).

[ettingtonliam]

Hi
I think its going to be the sled/sledge method. These are big castings, engine bedplates, around 14" long, 6" high and 6" wide, around 10kg+ in weight, far too big for faceplate mounting, and definitely not suitable for any type of overhung fixing. I need to bore from one side to the other, around 6" total travel, so overhung tooling probably isnt the best plan either.

Thanks for all the suggestions, its helped me make up my mind.

Regards
Richard