Boring bar height.

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I have 'heard' that the cutting edge of a boring bar should be slightly higher than center.

Urban myth.? Or if so how much is 'slightly' as that is quite a loose term.

I have 12 & 16mm boring bars if that should be of any significance.

Thanks, Nick

Centre height should be fine. That is how I set mine. Yes, there may be a bit of deflection but that should be negligible on fine finishing cuts.

As David suggests, the cutting edge should be working at centre height ... If you need to set it higher [to accommodate deflection] then the stiffness of the bar is questionable. Anything more than a couple of thou' probably means that the bar is too flexible to do good work.

MichaelG.

[ just waiting to be shot down by the Gung-Ho brigade ]

Just set it to as near to centre height as possible, you won't go far wrong then

Tony

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OK thanks guy's

Nick

Urban (or should that be Workshop) myth. More that it shouldn't be below centre height. Especially on the small(er) jobs and small machines folk like us use.

The important thing is that the tooling be really sharp, have sufficient front clearance not to argue with the side of the bore and be of the correct shape. For small jobs I did some drawings of the front clearance needed which are great help. Compared to ordinary tooling the rakes needed are huge and my instinct at least is always to leave too much metal on when grinding. Shape is important as its easy to end up with something that will push off or drag in if you aren't careful. Don't assume the basic shape on the as bought tool is good. Assuming you have the clearances needed to get the tool into the starter hole you are much more likely to have trouble from drag in or push off than anything else.

Always remember that a tool must be made to cut if results are to be reliable. Shaving off half a thou at a time is absolutely not how its done and if you regularly have to resort to this its time to take a good look at your sharpening skills and material stocks. Unless you are working with "incrediblyhorribletanium" from unknown source donations, scrapyard finds and odd leftovers twenty thou is the sort of finishing cut you should be aiming for with good sharp tooling. Under ten is usually asking for trouble. One day I shall steel myself to take my stock of "incrediblyhorribletanium" to Mr Scrappy despite it being such useful sizes and spend some money on decent stock stuff.

The above centre height thing come from big machine practice with big tooling, big jobs and big cuts. Shipyard, railway and similar shops where you can reliably put a few thou bend on a bar or tool and expect it to hold steady under cutting loads.

Clive.

It's pretty standard to set it a some thou over centre when boring and the opposite when turning. The reason is simple in both cases and sometimes boring needs more. When boring if the tool deflects down from on centre it will take a deeper cut and deflect some more etc etc. Set it above and if it deflects the cut reduces.

Turning should be set on or below because above reduces the front clearance and what happens if things do deflect when it's above centre - deeper cut.

It's no urban myth. It's often impossible to bore a hole without some deflection and often the result is that the same cut may have to be run several times to actually remove as much metal as the cut should take. Sometimes if the boring bar is going to bend rather a lot when it takes a cut there are 2 options - set it even higher or accept that it will take a bigger cut. The other factor though is that once it goes below centre the clearance angle is reduced - rapidly on smaller holes. The tool can easily finish up rubbing and scraping rather than cutting,

How much above centre - depends on how much the boring tool will bend.

John

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If you set the tool 10 thou above centre on a 1/2 inch bore and it deflects down by 10thou it will make approx 1/10thou difference in cut or 1/5 thou in diameter assuming the tool only deflects downwards. I would suggest that setting slightly above centre hight will not matter in terms of cut and it's easier to set slightly high to avoid being under centre. Referring back to the copious jottings on parting off I would suspect that the biggest issue for the dimension of the bore would be the force required to push the tool into the work which also gives a component in the radial direction so push off and tool spring is going to be the dominant issue.

Martin

I suppose some one could take cuts so fine that a boring tool like this doesn't bend. It projects about 60mm and is made from a 1/4 hhs tool bit. That tends to be stiffer than typical dedicated boring tools.

Even if it didn't flex the work itself depending on size might bend due to the cutting forces or there might be slight looseness in head stock bearings so the chuck and work is likely to lift.

The bar may also flex away from the cut. I tend to angle the end of them to provide some force that will counteract that. It also allows bored recesses to be finished at the bottom with the same tool. Many boring tools assume that the cut will pass all the way through the work - even RGD indexed tip ones to my annoyance.

The normal technique to take care of accuracy variation is to take at least 3 cuts of very similar sizes measuring and refining the results. I find with what I have at home though that if I want say a 0.001" fit the best option is to get close and then reduce the cut to 0.001" or even less. I also find that when boring cuts often need to be taken repeatedly at the same setting to run them completely out. In my case this is probably because the front headstock bearing on my Boxford has work very slightly oval as all lathe bearings do.

John

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My first two boring bars were made by Eclipse, a 1/4", and 3/16" hockey stick shaped piece of black HSS.

Ian S C

I've always thought those were a bit daft - the hockey stick shape increases the min bore sizes. Lots of boring bars are made with that sort of problem. Done sensibly the bar could have a bigger section. They are good for making internal screw cutting tools. Lots can usually be ground off the tip.

The smallest and stiffest boring bar are probably best made of of round HSS. Grind half way through the thickness to form the cutting end then flatten one side of the the bar to provide some clearance.

John

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John, after a while they end up as a straight bar, you just grind that roughly in the form of a D bit, and carry on. Back then I knew nothing of what was required for a lathe tool, saw these in a shop that used to be the local agent for Myford lathes, so assumed that they must know about these things.

Ian S C

Not a briliant pic, but I find one of these, angled slightly is the stiffest style for any given hole size.

Neil

By setting above centre height you can sometimes use a thicker boring bar without scraping the bottom of the hole. This is obviously more rigid. Dial increments will be different though.

Neil, not clear from your pic but is the round tool steel just ground on the top to about half way and on the end only, leaving the rest of the bar round, with the slight angle providing the necessary clearance? What size are we talking here?

Chris

Have to say that in order to accommodate my boring bars I have often run them way over mid point, and I mean way way over! Result...works fine for me.

I have always set boring tools a few thou above centre,to ensure the cutting tool edge has clearance to cut cleanly,if the tool is dead on centre or above the front clearance on the tool will rub and will not cut correctly, if anyone sets a tool above centre because of deflection then they should get better and stiffer boring tools that will not deflect on normal or fine finishing cuts,deflection on heavy roughing cuts where metal removal and time saving is essential when earning a living is acceptable.The HSS tool bit ground to the required shape is/was popular with time served craftsmen as the tool is very rigid, preformed tools like the Eclipse hockey stick mentioned above gave poor results they deflect and never seem to hold their edge as well as a toolbit The practice of finishing a bore by passing the tool down the bore two or three times at the same index setting would have been frowned upon when I was an apprentice,you would have been told to sharpen the tool correctly and do it in one pass.

Likewise.....

I have several SCLCR boring bars 8-12 mm in size and have to set them above CL to avoid catching on the bottom.....unless the hole prior bored is greater than the clearance required....

I set all by indexable bars on ctr height, that way they can also face the botton of blind holes without having to faff about resetting the tool holders.

The bigger the hole and bar the less critical it is to have the tip of the tool at ctr height, I find once I'm upto 1" dia bars a 1/16" either way does not make a lot of difference.

J

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Thanks to all who have responded to this.

Regards, Nick