Lathe boring tool - top rake?

I've always used carbide insert boring tools but haven't got one small enough for what I want to do now, so I want to grind one from HSS.

All my carbide tip holders set the tip with a negative top rake - I don't know why, but they work. Is this something specific to carbide inserts or does boring in general require a different top rake from external turning?

The tool I want to make will be used to bore mild steel.

Any advice will be appreciated, Robin.

Inserted tools tend to be mounted high with the tip angled down enabling a larger dia bar or the ability to be used in a smaller bore. Same difference. In addition, the tips have small rake angles seen easily by examining one. This means they would foul the bore unless it was very large. Without this they would be very delicate.

Hss tools cut differently and can have a little  rake, Too much will weaken the tool but they can also be set a little above centre height to enable better clearance, which unlike an external tool will improve things

in addition some think it is just a matter of dropping the height of the bar then rotating it upwards to put the tool back on centre height. This is a recipe fir disaster as the tool again will foul

i use a set of penny washers, bored at 3 then 5 mm intervals up to 30 mm . Each has a line scribed by the tool across it to indicate the centre line. It’s then very easy to see which tool will be suitable for a particular bore before it goes onto the machine the clearance of the tool or lack of it will be obvious

hss tools will bore quite well without any top rake

oops typos

Edited By Zan on 18/11/2019 23:50:16

How deep and what diamiter do you want to bore Robin.

Here is a selection of tools that I have made. The one's on the right will go down very small holes but not so deep and others with piece of tool steel held by a grub screw. I tend to make them pointed with clearance so that I can face the bottom of the hole as well as bore. The top rake is around 5 deg and only a very small tip radius stoned on if nessesary.

David

There are positive rake insert ranges, usually for Aluminium and plastics. They work well on Mild Steel and softer steels.

Zan:

Good tip about the penny washers!

Horses for courses.

Negative rake cutters are stronger, last longer and use less power than positive rake types. Negative rake is particularly useful in industry where maximum metal removal at minimum cost is premium. Carbide is much preferred even though optimising rpm, depth-of-cut, and feed-rate for best economy and finish is a science!

Positive rake cutters are weaker, blunt faster and use more power to remove a given amount of metal than negative rate types. But they have important advantages too: less force is required to make cuts, and it's easier to get good finish. They're good for fine delicate work.

Carbide inserts work best when used vigorously at high-speeds. They work well enough at ordinary rates too. HSS is more forgiving of slow machines, does a good job in most circumstances, but has to kept sharp.

A special problem with boring bars is making sure the cutter doesn't foul the inner curve, and - for holes less than 10 to 12mm - keeping the bar stiff enough. It's delicate work. I prefer drills and reaming small holes.

Dave

Err ... power = force x velocity

So less force at the same or lower speed means lower power. Certainly production costs are lower with carbide but that includes machine time and tool sharpening costs, not relevant for most of us.

Russell

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I would be grateful if you could explain that, Dave [my emboldening]

My little brain is screaming “Does not Compute”
... but perhaps it is thinking of saw blades

MichaelG.

I think SoD is getting confused with units. Power per unit volume removed would be a better metric. Intuitively I'd think that negative rake would consume more power per unit volume, but I'm not sure without looking it up. However, it makes sense that less power per unit volume is used as cutting speed increases. That's due to increased heat in the cutting zone, so the metal is hotter, and softer, and shears with less force.

Andrew

Having read up a bit more I think SoD is wrong. Negative rake inserts create higher cutting forces, so for a given cutting speed they consume more power. However, as SoD says, negative rake inserts are stronger than positive rake inserts. So they allow one to increase DOC and feedrate. Of course that requires the machine to be rigid and have plenty of power available. But if one has such a machine then negative rake inserts allow one to utilise the machine to its maximum capacity and thus maximise volume of metal removed per unit time. That's advantageous for industry.

I won't be changing to negative rake inserts as I'm quite capable of stalling my lathe (3hp) using positive rake inserts.

Andrew

Thanks for replies. Looking at David's (impressive!) range of DIY boring tools, and taking Zan's advice that zero rake works OK I made a tool (from silver steel) with zero rake. My need was for a 12mm blind hole 15mm deep with a flat bottom - so it is pretty slender. But it worked!

I notice that my first cuts gave a slightly conical bore, with the narrow end towards the headstock. After taking numerous spring cuts I ended up with a parallel bore, but I don't understand this behaviour. How does the tool know where it is?

Robin

Yes, Andrew explains it better. My use of the word 'force' was in the ordinaru lay sense, not scientific. I do mean 'Power per unit volume removed '

Here's how I think it works. A sharp tool cuts with less pressure than a blunt one, but - counter-intuitively - sharp isn't necessarily the most efficient way of removing metal. Other factors can come into play.

First, Carbide tool tips can run at much higher temperatures than HSS, and less energy is needed to remove heat softened metal.

Secondly, I understand carbide also allows a different cutting mode. Rather than having a sharp edge actually in contact with the metal (as when a knife cuts butter), a carbide cutter can be forced in hard enough to wedge metal off such that the edge isn't in contact at all. The benefit is most of the unwanted heat is carried away with the chips, allowing carbide to be used even more aggressively without itself overheating.

Despite hefty performance advantages carbide isn't universally wonderful. It's more brittle than HSS which means the cutting part needs all the mechanical support it can get. Hence the preference for negative rake carbide, because more material supports the edge compared with a positive geometry.

Ought to emphasise that the power-saving advantages of carbide really only benefit industrial users. Production is most efficient when machines are run continually at full power, minimising stops with automatic tool-changers, work-holding and swarf clearance etc. Is this environment, it's highly likely that inserts and cutting parameters will be carefully selected for optimum performance, not bought from ebay! The tide is strongly against master-craftsmen carefully sharpening HSS cutters!

However, 'optimum performance' in a production sense is rarely needed in a home workshop, or even in professional environments doing development or low-volume work. There's still plenty of room for HSS tooling.

Dave

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... and also [when used appropriately] for the sharp-edged carbide inserts that have become so popular, and are apparently exceptions to the rule.

MichaelG

If there is not much clearence between the wall of the bore and the shank of the tool you get swarf build up which can 'spring' the tool in a little and reduce the cut. It will get worse the deeper the bore because there is more swarf about.

regards Martin

Thanks Martin - that makes sense. My insert tool holders have through coolant, I guess that's why I haven't noticed this phenomenon before. Should have thought of that - my one-track mind was too focused on tool geometry!

Robin

Oh dear, yet again I'm going to disagree with SoD. I expect he's pee'd off with me by now.

I'm not convinced that HSS and insert tooling operate in different ways. Although I can't find a definitive explanation for HSS tools I think both operate by shearing off a ribbon of metal. The main differences are the angle between the work and top of the tool and how the swarf is handled.

To take the butter analogy. If a knife is pushed down on butter, sharp edge first, then the butter is cut. I'm not sure what the correct term for this is, but let's call it cleaving. I don't believe that any metal turning tool operates in this mode. Alternatively we can scrape the knife across the top of the butter with the blade near 90°, plus or minus a small angle. This will create a scroll of butter that is sheared off the butter. This is how I think turning tools work.

I am confident that insert tooling work by shearing, so let's consider HSS tooling. First take turning brass. The recommended side rake is zero, so the cutting edge is at 90° to the work. So the metal must be sheared off, and there's no doubt that it works. For steel the side rake is around 10°. The cutting edge is at 100° to the work, so not really cleaving, but more like shearing. For very soft materials such as aluminium the rake angles are larger, but still nothing like cleaving.

Let battle commence!

Andrew

Not in the slightest! I've been known to get the wrong end of the stick before. Several times! As I like the word 'shearing' in this case, there may not be much of a battle to have! I shall consult my references and counter-attack if it's justified.

Engineering isn't about personal opinion and I don't mind educational corrections at all. Informed challenges are good for everyone whatever the outcome. True confession: mistakes and Model Engineering are rather closely related in my world...

Dave

Ooopsie," let battle commence" was intended to refer to the forum membership on the subject of how tooling cuts metal. Not me versus SoD, two falls, two submissions or a knockout to decide the winner.

Andrew

Edited By Andrew Johnston on 20/11/2019 18:40:57

Robin, the hole is bound to taper if there is any rubbing of the tool below the cutting edge as each bit of the bore taper will provide the starting point for the netxt bit which is thus cut smaller and smaller bore.as the tool is pushed further towards the lathe axis. Your dia 12* 15 deep is not small at all I have today bored one at 9 mm 25 deep without any taper finishing with the exact slide fit I required. I didn’t trust the reamer as they are made to cut to nominal size plus a small manufacturing tolerance so that there is a close fit on a shaft of the same actual nominal diameter.

With small tools, the slightest rounding or poorly executed final honing can cause real problems . Following this boring action I had to produce an o ring groove 20 mm down. I initially had problems with the tool not cutting properly. With a powerful jewellery magnifier, it was traced to rubbing where the edge tip had been given a tiny radius on the edge to provide the correct toot radius to the groove. I used with a diamond lap and it was fouling . I had to make a new tool bit n finished with a lot more care and this performed perfectly this bar was dia 6 with a 3 mm round cutter

so check your tool with great care !

Ding, Ding, Round two!

This would seem to confirm Andrew's comments which I would agree with that a positive rake tool needs less force to cut. Which is probably why the **GT inserts are suggested for smaller lightweight machines.

As for a tapered bore it is something that has been debated for years and seems to be more related to the rigidity of the whole machine that the cutter, though a blunt cutter on a long flimsy bar will get deflected more than a sharp one on a stout bar. I often change to HSS for the final couple of passes where I want the straightest bore on a cylinder.