Boring Bar

[David S Newman]

Anyone got any ideas for adjusting a between centres boring bar to get the last thou spot on ? My bar is completely manually adjusted ie tapping the cutter in or out, which is a pain and ends up with a few parts in the scrap bin. Love to know a simple method. David

[camdigger]

Fine thread set screw feed.

[John Stevenson]

David,

many moons ago we used to line bore generator bearings in situ after having new babbits cast in situ as well.

Because we had a couple of oversizes happen we developed a couple of rough and ready jigs to stop this.
Real crude, lumps of pipe, bigger lumps of weld but worked.
A nicely made set would work well, in fact as well as ower bodge it and scarper models.

I'll try to do a couple of sketches later as we have company at the moment.

[jdleach]

You can mill a flat on your boring bar perpendicular to your cutter hole, and just wide enough to permit placement of a depth gage. Then after you make your initial roughing cuts, you can slightly loosen the cutter set screw, and use the depth gage to push the tool out to the required distance..

Dave

[JCHannum]

I have shown this before. It was made by a machinist years ago for just this purpose. I have used it several times myself, and it works quite well for measuring the position of the cutter on a boring bar. A workable copy could be made by clamping a depth micrometer to a V-block.

[Weston Bye]

One picture, Jim. Worth a thousand words. Thanks for posting again.

[TGTool]

MEW a while back had a nice design with a screw adjustable cartridge that held a square toolbit. I made one for a job and it worked nicely.

Working side and adjusting side below.

[Paul Alciatore]

You can mill a flat on your boring bar perpendicular to your cutter hole, and just wide enough to permit placement of a depth gage. Then after you make your initial roughing cuts, you can slightly loosen the cutter set screw, and use the depth gage to push the tool out to the required distance..

Dave

I had exactly the same problem a few months ago and this is exactly what I did. It is a bit hard to see here, but the top of the boring bar is flat by the tool bit.



Here's a shot of the flat being milled on the lathe.



I found it quite easy to adjust the bit with a depth mike and a donut shaped spacer that sat on the flat. The spacer was a piece of 1" diameter stock with the ends turned parallel and a 1/2" hole drilled down the middle. It kept the mike perpendicular to the flat and parallel to the tool bit. It would take different lengths of tool bits and spacers depending on the diameter. I was boring aluminum so I choose a borken milling cutter for the bit and ground a proper tip on it for boring.

Of course, it took three hands to position everything and tighten the set screw, but I learned that trick many years ago. I could easily set it to a half thousanth.

[ptjw7uk]

Paul
nice use of a unimat business end!
Peter

[EVguru]

Mount a centre in a boring head and mount that in the three jaw.

Wind the boring bar off centre to increase the cut.

[spkrman15]

Hey TGtool,

Do you have drawings for that? That is a great idea and i would like to copy it

Rob

[camdigger]

The Southbend "how to run a lathe" book shows two configurations for adjusting "between centres" boring bars. My copy of revised edition 56 shows the illustrations on page 91.

One for the common bars shows round cutters being used in a stepped cross hole. The smaller step is threaded for a fine thread set screw, the larger step is for a round cutter bit.

The other configuration is for a mini boring head arrangement where the set screw is arranged along the axis and has a tapered end to force the cutter out.

Measurement could be done with a clamp arrangement right out of Gingery's lathe book and a set of feeler guages.

[TGTool]

Hey TGtool,

Do you have drawings for that? That is a great idea and i would like to copy it

Rob

Rob

The original article is in issue #136, March 2008 of Model Engineers' Workshop. The design is in metric dimensions on a nominal 1-1/4" boring bar. I made some revisions for a 1" bar, then for a 3/4" using an Everede triangular toolbit to save space since it becomes a lot of features to shoehorn into the diameter without compromising strength too badly.

The first was just notebook sketches, though I used CAD to look at webs and clearances for the second. I don't have anything like real drawings but I'll see if I can collect enough sketchup and dimensions to be useful. It's probably worth looking up the original article since the holes and machining sequence make for some head scratching at first.

[David S Newman]

Thanks for all the help and advice, gives me a few ideas which I will try over the next few days. David