Packed Boring Bars

Boring accurate holes to a good finish can be difficult on our smaller lathes as the job is generally quite small and the boring bar correspondingly slender. Relatively long slender boring bars are inevitably weak so only small cuts can be taken. Even with small cuts they are prone to chatter.

One way of pretty much eliminating chatter is to add packing to the boring bar close to the cutting tip so it is supported against the side of the hole it has just made. Obvious problem is how to make the packing exactly the right size. This YouTube video **LINK** shows an elegant way of getting the packing exactly the right size by using a sacrificial wood block. The sharp edge of the bore cutes the block exactly to size.

The job is bit larger than what we usually deal with but the principle remains the same. Obviously the system works best with a fixed, on centre, bar with cutter adjustment to set diameter. But it can be done with an ordinary "mutant golf club" style tool. Its generally only the finishing cut after all. Flow coolant is pretty much essential of course.

If using tooling with a parallel shank an extra fixed steady bearing on the tool shank can be helpful to keep things stable for the first part of the hole. A proper steady is overkill for this job and almost certainly too large to fit easily. A simple L shaped plate with nicely finished sides will do the deed and need only be 1/4" or 3/8" - 6 or 8 mm thick and be much easier to install. Smart and Brown used the L shape plate system for their travelling steadies, albeit in expensive phosphor bronze, so the idea has good pedigree.

Clive

Thank, Clive. And, of course, there are cheap bendy/springy boring bars and there are good quality - less springy/bendy - ones which cost a fair bit more than the former. Choosing the best tip is also beneficial.

But anything like Clives’s post above is relevant, even with better quality boring bars.

Question arises as to whether it is better to ream rather than bore to size for these more difficult diameters and lengths.

What is being shown in the video looks to be a modern version of the "Spill boring" process that used to to be used for the final finishing to size on gun barrels after drilling and reaming. A wooden packing "Spill" rides against both the tool and the gun barrel on the first pass. Then more thin paper sheets are inserted between the spill and the tool on each pass.

What GHT likened to Harry Lauder's walking stick!

Thanks for the link. Those prepared timber blanks look like oak or ash; presumably turned and then split in two. I wonder how this method would work in a blind hole; in the video the chips seemed to be exiting at the chuck.

Most of mine are solid carbide or densimet. I have only ever bought one, a Garant 6mm, but it cost a lot for a box of inserts as they are tiny proprietary ones. Don't think that a top quality steel boring bar is any less bendy than a cheap one, size for size,Youngs modulus for steel doesn't vary much.

ega,

I saw only coolant pouring out of the chuck, not chips. I did also wonder if the timber " cushion" was being shaped by the cut edge of the hole being bored as the bar was powered down into the hole. They were described as sacrificial so I assume new ones, roughly shaped, were fitted for each tube being bored.

Clearly the lathe in use had a long bed; it was rather frustrating not to get a view of the holding arrangement on the saddle end of the boring bar. The process did leave a nice looking finish ready for honing to size.

Thank you Clive for the link, it was interesting and a process I have not seen before

Regards Brian

I think you demonstrated my point, short of quoting the difference between steel and carbide. (Numerically 200 versus 500+?)

Edited By not done it yet on 24/10/2019 11:42:35

True, but a top quality boring bar may not be steel at all, or at least only partly made of steel. Plenty of choice if you're rich! There seems to be a pecking order, where the cost of a boring bar rises dramatically with the sophistication of it's anti-vibration technology:

1. Ordinary Tool Steel
2. Alloy Tool Steel containing heavy elements like Tungsten intended to decrease vibration frequency by adding mass.
3. Solid Tungsten or Tungsten Carbide (heavier than 2.)
4. Passive damping achieved by pre-tensioning a outer sleeve with an internal steel rod. Some of these may be made entirely of steel. As they are easy to make, but not easy to get working properly, some of them may be snake-oil.
5. Tuned Mass Damping, probably mixing positioned weights, pre-tensioning and a fluid (possibly Mercury) to absorb chatter under particular conditions,. Possibly adjustable.
6. Active damping, where tool vibration is detected by the holder which applies a counter vibration in the opposite sense, and/or dynamically moves weights inside the bar, or alters tension and damping to reduce it.

I guess most of us are using ordinary bars, Class 1 or 2, with a sprinkling of 3's and maybe a few 4s. The really advanced bars seem to be designed for rapidly boring deep accurate holes under CNC, not for occasional use on manual machines. Unless something special is going on, I think owning one would be a waste of time and money.

I like the idea of adding a wooden cushion. Must give it a try.

Dave

For precision work in many machine shops, SOP is drill to rough out and rapidly remove metal to near size, then bore the hole to ensure location and alignment, then ream to achieve final size and finish.

Interesting video!, but in our sizes I struggle to see how this would work

At the start of the cut, the boring bar was held in an exact position by the bushing steady, meaning that the bar could not spring during this initial cut before the wood block has been shaped by the sharp edge of the tube thus forming the moving internal steady.

I think that without this bushing steady the wood will not be cut to the exact size required and the boring tool would be forced upwards. It’s one of those situations where the industrial process where multiple parts are being produced makes the investment in tooling viable as opposed to our normal one off component machining tasks. With a bushing steady fitted, it would probably work, provided the wood support to the bar did not crumble. I would also think that there would be few situations where such a long component (comparatively for our much smaller jobs and machines) would need boring

edit.......  the bushing steady in our size would probably need line boating to the exact diameter of the Boeing bar, which must also be absolutely perfectly aligned with the lathe axis. The great Martin Cleeve (?) got a lot of use out of his, but he was earning his living from his vastly modifier ML7

Edited By Zan on 24/10/2019 13:50:51

Brian

Thanks for your comments on my post. There certainly was coolant but at times I struggled to see where the large chips being produced were going. The video did indeed make clear that the timber cushion was cut to size by the sharp edge of the bore.

Plus one for your comment about the holding arrangement. The secondary steady reminded me of the bushing steadies favoured by Martin Cleeve.

I recon all the chips were being expelled forward, the coolant leaking back was free of them, and there were turnings at the chuck end. I noticed the hole in the tube was not round, or was eccentric with the OD, the cut varied from 1/8" to 3/8" which was another reason for the wooden damper. Also, I would guess that the boring was carried out in one pass, although it was odd to see an area at each end turned for using the steady, maybe the boring was done from both ends due to the length of the tube. Checking out the video again, I realised that two lengths of tube featured, the short ones did not need turning around.

Edited By old mart on 25/10/2019 19:49:24

The video maker said that the tubes were bored with a 0.005" taper, can anyone tell me how he did it, was it magic?

I think he meant that he ended up with 0.005" taper which is not a bad error on such along bore

Zan

As ever when scaling down industrial size concept to smaller machinery a certain amount of low cunning and, probably, verbal encouragement will be needed to get it working well. My machines are industrial size (Smart & Brown 1025, 10" swing and Pratt & Whitney Model B 12 x 30 true swing 13 3/4" ) so I'm unlikely to need the technique personally.

However, remember that its a technique for finishing cuts, not roughing cuts, so forces will be relatively low. With appropriate care it should be possible to get a nice finish running far enough down the bore to produce a self sized support immediately behind the cutting tool. I guess 3/8" to 1/2", 8 to 12 mm, or so should suffice. If you are using a round boring bar with inserted cutter an L shaped, plate, steady, Smart & Brown style as mentioned in my first post could be used for extra support if the material proves stubborn. Once you have a start all should proceed to plan. Hopefully your plan, not the shop Gremlins plan.

As I see it this is more a get out of jail technique rather than the normal way of doing things. I recall doing a boring job many years ago on one of my SouthBend 9" lathes whose surface stubbornly came out a series of very shallow grooves perhaps a couple of thou deep at most rather than smooth. Many spring cuts later it was acceptable. Reason for the problem was that the variation in cutting force between hollow and peak of the grooves sprung the boring bar just enough that it pretty much followed the grooves rather than cut the peaks away. A bigger bar would have solved the problem but my next size up boring bar was fraction too big so I had to make do with one that as really too small. A D bit would have worked if I'd had the right size material to make one. As would a reamer but buying one for a one time job made no sense.

Just another concept for your toolbox to be hauled out when you've not got the paper tool to do it "just like that".

Clive

Edited By Clive Foster on 25/10/2019 21:23:04

One dodge I've read about to help stop chatter while using a boring bar is to wrap the bar with o rings, sometimes big ones overlapping. Seems to help a little.

John

If you're doing the same jobs over and over again then there's bound to be all sorts of dodges and techniques to make life easier

It is well worth perusing other videos by the same man. He does some very interesting work.

His lathes are repurposed very large turret lathes, so the cutting tool is held in the turret, hence the need for cut adjustment at the sharp end.