You seem to be assuming that the cutting tip is level with the top surface of the boring bar. If you make your boring bar with the cutting tip close to the centre line of the boring bar then the diameter of the main part of the boring bar can be closer to the starting hole size.

Les.

You need space for the chips that are produced to prevent packing of the hole so for short pieces you can use less clearance but longer bores you either add clearance, take smaller cuts, use some kind of flush or retract the tool more often.

If you are not happy with your boring efforts, I would suggest drilling, boring nearly to size and finally reaming to size.

Reamers are more expensive than boring bars, but provide accurate and reproducible results.

William

I've had similar boring tool clearance issues. Its very easy when grinding HSS tooling for small bores to underestimate the angle needed under the tip to clear the bore you have just cut.

I ended up doing a quick bit of CAD work to produce a diagram clearly showing suitable grind angles to ensure clearance on various sizes of holes.

For my drawing I standardised on 1/16, 1.7 mm clearance between the bottom of the heel and the hole and reckoned to use mostly 1/4" , 6 mm blanks for the cutters except for 3/8" to 9 /16" , 10 to 15 mm, holes where I drew it for a 3/8" , 4 mm cutter blank.

The small one needs 45° rake to get clearance in a 3/8" , 10 mm Ø hole.

With 1/4", 6 mm cutters you need:-

50° angle in 9/16", 15 mm Ø holes;

60° in 3/4", 20 mm Ø holes;

65° does 1" to 1 1/2", 25 to 35 mm Ø holes;

Beyond 1 1/2", 35 mm Ø holes 70° does fine. I don't see much point in using a shallower angle for larger holes as the gain in strength / heat capability is tiny and unnecessary multiplication of tooling wasteful.

Above 2" , 50 mm Ø holes I tend to switch to 1/2", 12 mm tooling, again at 70° angle.

Above 4", 100 mm Ø holes 80° works fine and cuts down on grinding.

Obviously this doesn't take into account the space needed for the bar or shank carrying the actual cutter.

In the smaller sizes the tip has to be ground integral with the shank so you will be doing well to accommodate a basic shank diameter of 2/3 rds hole size. Which will need a fair bit of careful grinding to maximise support and retain some space for chips. As the actual cutting tip has to be ground at the centre line of the blank things rapidly get crowded.

PM me if you want a PDF of the picture.

Clive

There is a lot going for the double ended type boring bars, on a lathe that only has 5/16" between bottom of tool holder and ctr height you can still use a 1/2" dia bar which will be stiffer than a 5/16" one as the cutting edge of the toolbit is only a couple of mm above the bars ctr. The small HSS bits are easy to grind and cheap to buy or just use old ctr drill shanks.

Your 3/4" bore could easily take a 1/2" dia one of these, maybe even a 5/8" one depending on your toolpost. and a 3/8" one would have started the trunk guide before swapping to a 1/2" one for the last few passes

Not hard to make if you don't want to buy them, I did a lot of my early boring with this type and still use large ones (upto 28mm dia) for things like engine cylinders upto 6" long.

I, for one, would like to see Clive's boring picture (and to know what MichaelG uses to do the conversion).

PS The OP would do well to read GHT on this topic generally.

Edited By ega on 24/07/2021 14:34:37

Andrew

I must have missed that thread.

I was told about tapered bores 40 years ago by an apparently authorititive source and believed him without thinking it through. Once you spelled it out there is clearly no reason for it to happen.

Whenever I have bored a hole and then checked it I congratulated myself on my technique!

I still think it worthwhile to try to have a stiff boring bar though as this allows a decent depth of cut and speeds up thr job, and also reduces or eliminates chatter and one gets a good/ better surface finish.

Thanks for correcting my error on the reduction in stiffness caused by length. I got my 2* 3 and my 3* 2 mixed up, for which I can only blame old age and the heat.

Rod

To be fair about the tapered bore issue the operative words are, as Andrew states :-

"If the cutting conditions remain constant …"

Which can be a little tricky for the home worker using a lighter machine to achieve. Especially when you factor any inexperience in both tool setting and grinding into the equation. At the small end of the scale its quite easy to create a rub prone tool shape that tends to ride away from the cut as boring proceeds due to requiring more stiffness than the tool possesses to hold it into cut. The tool may well appear to cut correct to size as it enters the work but, as the cut moves deeper, will jack itself out of cut until the tool shank stiffness is sufficient to generate enough force to stabilise things.

Generally such is an unfortunate combination of tip radius and clearance angles accentuated by a lack of sharpness. Hand honing a tool on the machine so "its really sharp for the last pass" can be counter productive. Its awfully easy, especially with a small tool, to get the honing angle just a touch off and turn a bluntish, but still cutting, tool into something whose properties are more akin to a scrubbing brush. Something I discovered to my mortification and Big Dens' great amusement more years ago than I care to admit. Big Den believed that the most effective teaching technique, once the basics had been shown, was to let folk get it wrong then show them how it should be done. He reckoned shame and embarrassment great motivators.

Then there was the time when an unfriendly piece of alloy produced a truly spectacular built up edge on the last cut which wasn't good for holding size.

Maybe there was some excuse with the piece of "found, just the right size and hard too" steel that had a soft core with thick hardened outer. Naturally muggins wanted a bore about 20 thou into the hard stuff which took great delight in blunting the tool as the final cut proceeded. Its dangerous to assume a hard outer on a soft core is a simple thin case hardened skin.

Personally I dislike spring passes. Its near impossible to hold real sharpness and exact shape for the last tenth or five with home ground tools so cutting conditions tend towards some indeterminate combination of cutting and rubbing. I suspect only the super-sharp carbide inserts wisely and frequently suggested for use with aluminium and its alloys are capable of proper, repeatable, cutting under such conditions on smaller machines. But with such tooling and a lathe with any reasonable pretensions to accuracy spring cuts won't be needed as the cut will be the size dialed in.

Needing spring cuts is invariably an indication of certain infelicities in the process. Whether machine, tooling or operator. I'm fortunate in that both my lathes are capable of both hefty cuts and holding tenths given half a chance.

Which rather cuts down on my margins of excuse when things go wrong!

Aiming to finish off with a proper cut rather than a half whisker shaving works best for me. If I have extra material I'm more likely to just bin a job thats half a thou(ish) out and make the necessary corrections on the finish cut dial position rather than futz around.

Most folk, especially those with lighter, often elderly, machines generally don't have that option but learning to work to size directly off the dials rather than cut, measure and skim is always worth while. If nothing else it saves a lot of time. Even if you do still have to spring pass occasionally.

Clive

It's unfortunate that in many areas (not just model engineering) there are statements that may have been true, but the specific circumstances have been lost in the mists of time and they then become generic. I was aware of the tapered bore issue, but was always puzzled that I never seemed to produce one, even with significant overhang. Chatter, yes, but not tapered.

I'd agree that it makes sense to use the largest diameter and stiffest boring bar available in any given situation.

Andrew

You don't have to mount a big bar in your tool post, a simple length of square stock bored for the bar and split can be clamped directly to the top of the cross slide. Or in my case I did out the 4-way that came with the lathe and use one of the tool slots as a "vee block" and clamp the tool shank to that.

If you need to arrange a holder for a large boring bar and have welding facilities maybe the one I made to mount a Coventry Die Head in a Dickson S2 holder will be helpful inspiration.

Simply a piece of tube welded to a substantial lump of square steel to clamp in the holder with clamp screw bosses welded on to hold the head shank, boring bar in your case, in. The only tricky bit is getting the mounting lump dead parallel to the tube bore. Careful clamping prior to welding did it for me.

Or get it close and clean up in situ with a drill held in the chuck if you have the means to do so. Rotabroach in the lathe chuck is the obvious cutter for that sot of thing. If you know a man who will lend you one!

I made the clamp bosses one piece, tapped half way and clearance drilled the rest. Used a milling cutter to make seats as deep as I could reasonably go before clamping and welding. Cleaned up the tops flat for cap head screw seats using the mill before slitting tube and clamp bosses. Slitting saw does a nicer job but hack or band saw is just fine. Had to sort some minor details by hand filing and deepen one of the clearance holes to go past the slot but it all went straightforward.

Worth the effort for a proper job. I have a morse taper ended bar made a snug fit in the bore so I can align it central using the tailstock.

With this sort of thing I've learned to be wary of quick'n dirty as opposed to taking the time do do a proper job. Quick'n dirty using the old stuff I have that might do always seems to take much longer than it should by the time I have got it performing well. So the time saving is never what I expect. Unless it turns into a total one time job I generally end up embarking on a proper job do-over that "just works".

Clive

William, if you want to use the toolpost then a 10mm dia bar will be about the largest you could use without cutting flats onto it, you can make or buy these blocks which the round tool fits into and then the whole lot goes into the toolpost.

Alternatively similar blocks can be used but clamped directly to the topslide with suitable packing which should allow you to get upto about 22mm dia.

Similar to Clive's photo you could make a holder like the one shown in this video ( scaled down a bit)or look up the "Gibraltar" type toolpost which replaces the top slide and gives a more rigid setup.