Between centres boring bar bit grinding

[Nigel Graham 2Participant @nigelgraham2]

Useful information in that Eurocode site, though its other tabs suggest it is aimed mainly at structural engineers so some of the calculations may not be strictly relevant or useful for machine design.

It almost certainly won't work for fine-scale model-building where edge distances and pitch or centres, (which it clumsily calls "center [sic]-to-center distance" )  are dictated by the prototype's practice, and sometimes by accessibility. On the original, that was matched to the available fasteners and spanners; so may fail in some scale applications.

It explains those numbers, such as 8.8, embossed on bolt-heads; encoding the bolt's strength, but the effects of scale on strength have long been a topic discussed in model-engineering, with the point emerging that if anything the scale components are proportionately stronger than their full-size originals in similar metals.

The caveat being that our strength is not a scale version of the Victorian or Edwardian fitter's; when spanner proportions were established that related them to both fastener strength and male arm strength. They had no torque-spanners but were 'ard, in them days!

I am not sure taking things to the Euronorm depth need worry us as model-engineers too much, though it could help anyone designing to metric dimensions, critical parts like cylinder cover studs and more so, boilers held together with bolted flanges (e.g., Hindley, Merryweather, Sentinel). Their originals would have used BSW bolts and nuts.

You do though need know the actual material you are using, and for some applications including studs, that is not necessarily a grade listed in those tables made for engineers designing big, heavily-stressed steelwork to surprisingly low factors of safety. And of course the document does not cover non-ferrous metals, such as bronze, at all.

The upshot really is that we need consider what is fit for purpose, and provided it is reasonably stiff for its working range, there is nothing especially critical about a mild-steel boring-bar with a small HSS bit gripped and adjusted by ordinary commercial screws. After all, if use a single-point boring-tool from the tool-post, we know that even a high-quality item from an industrial-rated manufacturer may flex slightly, so we often need take a careful spring cut at intervals.

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Odd that despite the American spellings and apparently being a commercial site (it asks for donations) it shows the EU flag as if some sort of EU-published document.

Edited By Nigel Graham 2 on 23/05/2022 10:50:35

[HopperParticipant @hopper]

Posted by Michael Gilligan on 23/05/2022 06:54:11:…

…

Note the introduction of two new terms

Yield strength

Ultimate tensile strength

… both of which differ between various classes of bolt.

Which, getting back to the original issue, ie silver steel making a stouter boring bar than mild steel per GH Thomas, would seem to indicate that the issue of boring bar flex may not be down to Young's modulus alone. I am pretty sure from experience that it's easier to bend a piece of 1/8" mild steel fencing wire than a piece of 1/8" silver steel rod. Purely anecdotal of course.

[Martin KyteParticipant @martinkyte99762]

Posted by Hopper on 23/05/2022 10:54:30:

Posted by Michael Gilligan on 23/05/2022 06:54:11:…

…

Note the introduction of two new terms

Yield strength

Ultimate tensile strength

… both of which differ between various classes of bolt.

Which, getting back to the original issue, ie silver steel making a stouter boring bar than mild steel per GH Thomas, would seem to indicate that the issue of boring bar flex may not be down to Young's modulus alone. I am pretty sure from experience that it's easier to bend a piece of 1/8" mild steel fencing wire than a piece of 1/8" silver steel rod. Purely anecdotal of course.

That's why yield strength has been mentioned. To bend rather than elestically deflect you have to exceed the yield strength and in Silver steel that is higher than in mild. Youngs modulus is practically the same so the deflection will be similar.

regards Martin

[egaParticipant @ega]

This thread reminds me that GHT began his discussion of boring tools with the confession that, having re-appraised the subject, he had abandoned some pre-conceptions.

His Workshop Manual should be required reading.

[BazParticipant @baz89810]

I will go further and say that anything written by GHT should be required reading.

[Jon LawesParticipant @jonlawes51698]

Sounds interesting, who is GHT? (Although I suspect I know the answer…)

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Nigel Graham 2 on 23/05/2022 10:41:39:

Useful information in that Eurocode site, though its other tabs suggest it is aimed mainly at structural engineers so some of the calculations may not be strictly relevant or useful for machine design.

It almost certainly won't work for fine-scale model-building […]

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An interesting ‘critical review’ of the Eurocode website, Nigel … especially considering that I only linked one page, for the specific purpose of helping answer Hopper’s question.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Jon Lawes on 23/05/2022 12:22:27:

Sounds interesting, who is GHT? (Although I suspect I know the answer…)

.

Geo. H. Thomas

This one: http://modelenginenews.org/meng/upt/index.html

MichaelG.

Edited By Michael Gilligan on 23/05/2022 13:42:48

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Kyte on 23/05/2022 11:20:18:
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That's why yield strength has been mentioned. […]

.

Thank you, Martin

MichaelG.

[Jon LawesParticipant @jonlawes51698]

Thank you

[Martin KyteParticipant @martinkyte99762]

Posted by Michael Gilligan on 23/05/2022 13:46:28:

Posted by Martin Kyte on 23/05/2022 11:20:18:
.

That's why yield strength has been mentioned. […]

.

Thank you, Martin

MichaelG.

You could say I bent over backwards to help, and as my stress level was not great, I must have a low yield strength.

regards Martin

[duncan webster 1Participant @duncanwebster1]

Posted by Hopper on 23/05/2022 10:54:30:

Posted by Michael Gilligan on 23/05/2022 06:54:11:..

Which, getting back to the original issue, ie silver steel making a stouter boring bar than mild steel per GH Thomas, would seem to indicate that the issue of boring bar flex may not be down to Young's modulus alone. I am pretty sure from experience that it's easier to bend a piece of 1/8" mild steel fencing wire than a piece of 1/8" silver steel rod. Purely anecdotal of course.

As long as you don't exceed the elastic limit, deflection is determined by load, geometry and young modulus, yield strength is irrelevant. It is easier to put a permanent bend in mild steel fencing wire because the yield strength is lower, not relevant to boring bars. Just make them as big and short as you can. If possible grip one end in a chuck, if you are going to have to take it out before completing the job, mark which jaw it is aligned with so you won't change the cutter radius.

[SillyOldDufferModerator @sillyoldduffer]

All steels are about the same, and mild-steel is a reasonable choice for a bar because boring is unlikely to get anywhere near the metal's elastic limit. (250Mpa, roughly 35000psi)

It is possible to do better though. High end boring bars are often made of Tungsten Carbide because Its Young's Modulus is two to four times higher than steel. And posh bars are likely to be pre-tensioned, balanced, and otherwise carefully designed to reduce resonances. To keep vibration down, the best bars can actively adjust mechanically to cutting conditions, and might even report to a computer that moves weights inside the bar, alters tensions, and electromagnetically tunes out chatter. Price on Application, which usually means they're unaffordable.

Dave

[GmanParticipant @thomasgman46877]

I didn't mean to stir a hornet's nest here. I found the diagram for cutting geometry helpful and figured I would say thanks! lol.

[HopperParticipant @hopper]

Posted by Fowlers Fury on 27/09/2017 20:57:53:

…

…I thought that might be the explanation for my non-parallel bores and with only about 10 thou left to machine, I made his angled tool design from a wider silver steel bar which he claimed would not only flex less but also make using a micrometer for measuring tool point radius much easier. It certainly wasn't a five minute job !

I don’t decry the use of 90 deg radial tool b/bars; but merely offer that GT’s design did work much better in my particular case and has produced perfectly parallel bores without a problem many times since.

I think we have all been reading Fowlers Fury's post wrongly. Fowlers says he made his boring bar from silver steel but a quick check reveals GH Thomas didn't recommend silver steel for the job (P92 MEWM). GHT says specifically that he made his between-centres boring bar of FCMS (Free Cutting Mild Steel). The claim of less flex by GHT is due to the angled hole for the toolbit, not the material the bar is made of. I think Fowler's use of silver steel was incidental.Probably just what he had on hand. But five year old post so we will probably never know.

GHT did, a few pages earlier (P84 MEWM) say he preferred silver steel for the more usual toolpost-clamped boring bars, because "it is no stiffer than mild steel but is harder and stronger and so better able to stand up to the hurly-burly at the bottom of the hole — especially a blind one".

So it turns out GHT did know what he was talking about after all. Fancy that.

Edited By Hopper on 23/05/2022 23:38:38

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Hopper on 23/05/2022 23:35:43:
.

I think we have all been reading Fowlers Fury's post wrongly. […]

So it turns out GHT did know what he was talking about after all. Fancy that.

.

I don’t think I had read Fowlers Fury’s post at all … until you quoted it.

But yes, I agree with your reading of GHT

MichaelG.

[Brian O’ConnorParticipant @brianoconnor49474]

Amazing where a question on cutting tool geometry can lead!

[bernard towersParticipant @bernardtowers37738]

I'm with fowlers fury, have just bored a redwing cal for a friend with angle plate on crosslide and did it with 21mm b/c boring bar as per GHT and worked perfectly . well worth the time and effort.

[HopperParticipant @hopper]

I bored out the body of my Versatile Dividing Head with GHT's recommended boring bar with the angled tool bit and it also worked perfectly. I could see when I made it how having the straight hole and the grub screw hole all in one plane would make a very weak point. Plus I could measure the tool position with a standard mike and get it right where I wanted it.

Edited By Hopper on 25/05/2022 10:28:00

[Nigel Graham 2Participant @nigelgraham2]

Since our hobby is emulating industrial engineering practices at smaller scales, it's often worth looking back at both trade and model-engineering text-books.

Doing so I found that boring-bars generally hold the tool-bit at about 45º to the axis, close to the end so it extends a little beyond that; allowing use in blind or stepped bores. It also gives more support to the bit.

Sometimes the grub-screw gripping the bit enters from a corresponding chamfer on the bar end, so is correctly perpendicular to the tool shank. In others it enters concentrically from the end, particularly for screw-cutting bits more easily arranged at right angles to the axis.

Another form uses a tubular holder in which a long, close-fitting draw-bar cross-drilled to match the bar and bit, is tightened by a nut on its threaded end beyond the lathe's tool-holder.

The latter pattern can be used in between-centres mode if the draw-bar itself is suitably centre-drilled at both ends Its advantage is that the boring-bar is cross-drilled with only the bit-hole.

A boring-bar can be double-ended: angled bit at one end, right-angle at the other. I think I have a specimen of this pattern somewhere.

'

E.T. Westbury suggests in his Lathe Accessories – how to make and use them, that a hollow boring-bar bar be made from steel tube, which would have to be fully-drawn not ERW, with the tube and closely-fitting draw-bar as mutual stiffeners. (Drilling such a length, many times drill diameter, concentrically for full length, is feasible but not easy. I wouldn't risk it!)

Sometimes the work allowed the boring bar to have a smaller-diameter spigot as a pilot in a hole ahead of the main bore. Particularly suiting chuck and faceplate work, where it may be possible for the pilot to run through a bush in the lathe spindle, such a bar can of course be centre-drilled also for use between-centres.

The above comments also apply to internal recessing, grooving and screw-cutting tools in which the cutter itself is a small bit in a cross-drilled holder. Really, this is the HSS-bit version of the now-familiar carbide-tip tooling; and I can't think of any reason why pilot and between-centres type internal-cutting tools of larger diameters cannot be made for carbide tips. Fit a small, straight-shank indexing-tool through the boring-bar itself.

For setting the tool's height from the boring-bar, if you envisage a lot of such machining or a particularly critical operation, it could be worth making a suitable depth-micrometer or micrometer-thimble device based on a Vee-block.

For gauging the bore, make a ring-gauge that rides on the bar, perhaps parked on the tailstock nose or centre while the cut is in progress.

Between-centres boring-bars assume the use of a lathe with T-slotted saddle, or a horizontal mill. On the lathe, the principle can be extended to internal recessing and threading as second-operations without disturbing the work-piece setting.

I have seen a horizontal-borer, which is basically a development of the lathe, set to face, bore and internally thread a brake-ejector casting for a full-size locomotive. I don't know, but would think, the cones were separate parts fitted in parallel bores.

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