Nose Radius Question

Hi All,

When putting a nose radius on HSS, do you make it like in the photo below (photo credit mini lathe.com) where the radius gets wider towards the bottom?

If so, what are the advantages to doing it this way?

Cheers,

Jim

Thats a pretty grim sharpening job

My preferred way is to get a good sharp point and then slightly grind the point off to leave about 1mm face to do the cutting.

There are guidelines for the clearance angles and rake etc.

The MAP series of workshop books are really useful and can be bought quite cheaply from usual sources.

Bri

Search Amazon for. Sharpening small tools. I found a book straightaway for a few quid.

B

Cheers Bri.

Usually I just put the radius on by rubbing it on a bench oilstone as used for sharpening knives etc. It only takes a small radius, maybe five thou or so, maybe just a tad more.

The wider radius at the bottom is a consequence of the intersection of the side and front clearance angles. The angle at which the tool is presented to the wheel also gets in on the act.

Something that just happens. Not specifically looked for.

Applying appropriate radii to the cutting edge of lathe tools is something sadly overlooked in most expositions of the subject. Especially doing so without inadvertently reducing the sharpness of the edge.

Realistically the whole lathe tool sharpening process explanations and technique are long overdue for revisiting from first principles for the benefit of the lone worker neophyte who has neither mentor nor relevant experience for guidance.

The vital importance of accurate reproducing of effective, sharp tool profiles in helping the inexperienced to quickly turn out good work is self evident. There are plenty of similar profiles that work with appropriate feeds speeds et al but when you are trying to find out how to drive a lathe it makes life a lot easier if the tool behaves exactly the same after each sharpening. Even if its consistently crap at least you know its wrong. random variations between "hey that was good" and "bleaugh" visiting all points in between after each sharpening produces a slightly different profile needing slightly different operating conditions is less than helpful.

I've spent a fair few hours trying to come up with a simple, mechanically guided, way for the beginner to reliably reproduce tip radii with no success. For the actual clearance angles simply setting the height of the top of the tool relative to the grinding wheel axis to give a slight hollow ground is clearly most appropriate for the neophyte. But combining that method with setting a range of tip radii is a different matter. A holder to do one specified radius and one specified tip clearance angle is practical and may be the best approach. But its inelegant.

Of course the Eccentric Engineering Acute sharpener device will do the job just fine. At a price. In both £ and complexity. Which may not be appropriate to the neophyte.

Clive

Edited By Clive Foster on 03/06/2019 08:58:33

Jim as Bri says thats a pretty poor effort for a tool and its been over heated during grinding ! Regarding the corner radius it should be the same all the way down the corner if its larger at the bottom you have increased the front and side clearance rake on the area of the radius thus making a weak cutting edge.

There are some photos in my albums of hand ground lathe tools but none that depict what you are asking - I'll add some later today or tonight to try to show "how to"

If I need a rad on a tool I grind the front, side & top rake first then for a small rad use just an oil stone, for a larger one rough out on the bench grinder and finish by hand, or I now have a small vertical linisher - really for sharpening woodworking tools but it also great for this type of operation as well.

John

I've used a small hand stone - India oilstone medium - to put a tip radius on for 40 years or so. Because I'm inevitably following the intersection of front and side clearance angles, the radius tends to be similar from top to bottom.

I've also found that using the same stone to polish the edges produces a better tool finish that can be reproduced on the workpiece on final finishing cuts if you cut the plan trail angle of the front edge to a few degrees - by turning the toolpost, not by grinding the tool.

The tool in the picture looks to have been fiercely ground on a coarse wheel. I'd use a lighter touch on a finer wheel to finish with, a bigger plan trail angle on the tool, and I wouldn't normally try to grind a tip radius on the wheel at all, unless it's bigger than about 1/32" or 0,75mm and I'm planning to use it to form a rad on the workpiece.

I've tried to photograph my default knife tool that I use for about 90% of everything, but the macro facility on my little compact distorts angles and won't really show the small tip rad - looking at the pics make me wonder if the tool in your pic isn't actually a bit better than it looks. Mine has *about* 12 degrees side rake and side clearance, and 18 degrees plan trail angle - but all of these were just estimated by eye, not measured whilst sharpening. Those angles are rather high if the tool were to be used for heavy cuts, but they work well for me.

The tool in the photo looks as though it has been over heated while cutting, it seems to have a crater in the top behind the radiused cutting edge.

My usual GP tool is a 1/4" HSS one of similar shape. To put the radius on the nose I just touch the tip on the grinder to take the feather edge off, then with a little oil stone (about 3" long), form a small radius, same from top to bottom. Heavy grinding on the front of the wheel on the grinder, finished with a light touch on the side of the wheel, the polished with the oil stone. Only use the angle grinder on larger tools, and then only if I have to remove a lot of metal.

Ian S C

Thanks Clive, John, Mick & Ian.

Cheers,

Jim

It's an HSS tool and will need a bit more than that discolouration before the hardness is affected.

There is the risk with a small slip stone to smooth out the tip after grinding of tilting it so that the top of the cutting edge gets rounded back towards the top or at least reduces the angle from the intended clearance. (humans physically cannot hold a steady line even though old timers like to boast they sharpen woodworking chisels perfectly without a guide)
It may help to place the stone, even a full size bench stone, on a bit of glass to provide a smooth slide, then put the tool on the glass with something like a small drill bit to lift the front edge to a suitable angle. Keep the tool still and slide the stone around. Use the same action as for filing a curve, ie don't try to follow the curve. (tilt your head on one side and imagine you are filing not stoning)

The relief below the cutting edge is there to stop the tool rubbing and is nowhere near as important as the angle at the top.

Angling it like that will do little harm, it's not enough to weaken the edge significantly. Decent HSS will work virtually red-hot so turning blue won't damage its temper.

Prioritise grinding a neat radius, especially if you need a large round nosed tool, as long as there's some relief below it, it should work fine.

Neil

The tool in the first post looks like it has been ground on a blunt wheel. If you're not dressing your wheel regularly you're missing out on the best of it's capabilities.