Boring Bars

Mark, Normally you would drill out to close to finished size then bore to size. The reason being is that a tool than can go down a 13mm hole 80m deep won't be half as rigid as one that can fit into a 24mm hole to open the last mill out.

Even with decent tooling going 80mm deep can be hard and you may have to creep up to size and take the last couple of cuts at the same setting to get the spring out of the bar otherwise you finish up with a tapered hole.

I realised your drill chuck probably limits out at 13mm but you either need stepped shank drills, often called blacksmith's drills, or taper shanked drills. You probably don't have these but I'm afraid that just getting a lathe is only a small deposit on the tooling you need

Oh and by the way a bench grinder is a must have for any lathe, even the cheap DIY shop items is better than no grinder at all.

-- Regards,

John Stevenson Nottingham, England.

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The 12mm Glanze boring bar will do what you need without blinking. I was using one to open out a 120mm long by 22mm dia hole in EN24 steel this afternoon. That was at the usable limit of the tool for length, but with Ally and 80mm depth, you should be able to take a 2mm cut without problems.

You don't _need_ the insert tooling for aluminium but it will come in useful for more recalcitrant materials in the future.

Happy swarf making

Mark Rand RTFM

John, Mark

Many thanks guys, that's been really helpful

Regards

I've seen this stated many times, but I just don't see how it works. The unsupported length of the the boring bar doesn't change as it goes deeper into the hole, nor does the cutting force, so why should the bar deflect more? I'm assuming, of course, that the top slide remains locked.

"Nor does the cutting force" this is the reason the final cuts are run through several times at the same setting, the first pass will take of most of the material and have the largest force. The second cut will have much less cutting force as the depth of cut has not been altered, it will only be cutting what was not removed due to slight flexing of the bar and play in the saddle. The third will be even less and so on.

You will also get more deflection if the tip of the tool is worn or slightly rounded so its best to touch up the tool before the last couple of cuts are taken.

Jason

Sorry, you missed the point - or maybe I didn't make it clear. The actual query was "why should the hole be tapered?"

I can see the point of your question - it is indeed hard to see how the hole would end up tapered. However (and I am speculating somewhat here) I can see how it might turn out bell-mouthed.

Imagine the tip of the boring tool. Most of them have cutting ends which are at right angles to the lathe axis. Thus when they hit the face of the metal they will start out by going straight in. However, a tool with a relatively thin shank will easily deflect sideways, and under the random effects of cutting the first few mm of the bore is quite likely to do so. This will be likely to continue until the restoring force of the deflected cutter bar balances out the sideways thrust created by the metal being cut.

Worse, if you are hand feeding, the sideways thrust from the bore will vary somewhat as the speed of the feed varies, so the tool will be sprung out by a varying amount, giving a poor, ridged finish.

A boring tool with a rounded or V-shaped end is likely to spring inwards faster than a flat ended one, but the general effect could be similar.

Worth bearing in mind that the deflection of a cantilever (a beam supported at one end, which is exactly what a boring bar is) is given by

Where F = applied force at unsupported end, L = unsupported length, E = Young's modulus of the material, B = breadth of beam in direction perpendicular to the applied force, and T = thickness of beam along direction of applied force.

Thus, making the bar shorter, or thicker, will have a *dramatic* effect on the amount of deflection - doubling the thickness (or halving the unsupported end) will reduce deflection by a factor of 8.

Note: this assumes a rectangular bar. For a bar of circular cross section, deflection is 6.8FL^3/ED^4, where D is the diameter of the bar.

David

It isn't.

Nick

In article , David Littlewood writes

Just a quick follow-up thought. Looking at the above formula, it occurs to me that a rectangular boring bar would be more efficient than a square one. Doubling the top to bottom dimension would only halve the deflection and very probably cause interference with the bore; doubling the horizontal dimension would increase stiffness by a factor of 8, and be far less likely to cause such interference.

Never seen that thought set out before, though (knowing how these things are) I'm sure it will have been, somewhere.

David

For a given bore size the stiffest bar you can get inside it is still a round one with as little cutting tip sticking out as possible. Anything square or rectangular would just be wasting space.

My point exactly. I raised the question because John said it would be (tapered), and implied that it was because the hole was deep.

In article , Dave Baker writes

Undoubtedly true, but leaves little room for chip removal. What I was thinking of was, if you have to remove metal to leave room for this, doing so from top and bottom is best.

David

On or around Sat, 07 Apr 2007 22:09:24 GMT, John Stevenson enlightened us thusly:

don't have

it's undoubtedly[1] got a taper in the tailstock, though. Morse 2 will get you up to at least a 3/4" drill. ISTR I couldn't easily get 1" in morse 2, they tend to change to morse 3 about 7/8".

I have a 20mm drill which is morse 2, though; mind you, it's a pain in the arse, it drills holes just under 21mm diamater. initially, I thought this was a combination of poor sharpening (inexcusable on a new drill IMHO) and trying to drill 20mm in one go. I therefore reground it (by hand, admittedly, so I doubt it's perfect) and pre-drilled to 16mm. Still got a noticeably oversize hole. Luckily, I have a 3/4" drill and a 20mm reamer.