Notes on Workshop Techniques



3.5 Sharpening Lathe Tools

How sharp is sharp? how good an edge do you need for the tool work efficiently? Clearly, a razor edge isn't going to last too long, especially if it's mis-treated. If you expect to get a fine finish then generally speaking, the finish on the work will only be as good as the finish on the tool. Rough turning is less critical, but blunt tools require more power to cut, become even more blunt very quickly, and they run hot - this heat is transferred to the work which then also gets hot. The trick is to sharpen lathe tools little and often rather than use it until it resembles a blunt nail before showing it the grinder.

For roughing tools you can probably get away with the finish straight from a fine grind wheel. It's useful to have several small wheels with tool rests preset to cover the majority of cutting angles. If you use only one grinder and one rest you will need to set the angle each time, you'll be unlikely to set it spot on each time so you will end up having to cut more metal off to grind the whole facet. One way out is to leave the rest set dead square and make up some wedge pieces (angled tool holders) to grind the cutting angles. It is still worthwhile rubbing the edges with a slip stone, particularly the top surface because if this is kept smooth the chips will flow across it more freely and there will be less likelihood of welding taking place just behind the cutting edge.

There is also the problem of whether to use the outer edge of the grind wheel (the 'correct' way), or the side face (the 'wrong' way). Personally, I use the wrong way. I think the preference for not using the side of the wheel is that it was never designed to take stress at this point and may shatter. I doubt that a lathe tool is going to seriously stress a grind wheel, given that fine cuts are being made, and anyway, my drill sharpener (a commercial item) puts more stress on it and it's *designed* to work this way. I prefer to work with a flat grinding surface than a spherical one, though I think the often used argument about hollow grinding leaving a weak cutting edge is a bit exaggerated, especially with wheels 6" or larger. Anyway, I've just bought a disk/belt grinder so I might try that to see if it's any easier.

Disk/belt grinder.


A few words on safety with grind wheels. Firstly, do keep within the speed range of the wheel - this is not likely to be a problem with commercial grinders bought off the shelf, but it might be if you rig up a spindle with inappropriate pulley sizes and a 3000 (or faster) motor. The speed rating will be on the side of the grind wheel. Grind wheels are brittle and delicate, if you accidentally drop one from waste height onto a hard floor you might be lucky and it will break - you might be unlucky and it doesn't break but shatters when you try to use it! I'll leave it you to decide which poses the greatest danger. When a grind wheel breaks under spin it literally explodes, believe me, you don't want to be near it when it happens. Wear eye protection always, use the shields if provided and don't remove the wheel covers whilst the wheel is in use. Always use a proper wheel center bush made from nylon or cast lead, always use correctly sized washers to support the wheel, and if the card washers have come off the side make your own up - they are there to protect the wheel from damage. Don't over-tighten the clamp nut, and never use a wheel that is out of balance. I'll end the safety issues with a quote - "the only safe speed for a grind wheel of unknown origin is zero" - Ian Bradley I believe.

After considerable use you will find that the corners of the wheel become rounded, that un-even gouges appear on the surface, and that the grit takes on a shiney appearance and fails to cut properly - it becomes 'glazed'. All of these defects can be corrected by 'dressing' the wheel either with a star-wheel dresser (a free rotating wheel composed of alternate plain and star washers), or an industrial diamond on a stick. The star-wheel dresser usually has a guide cast on the handle so you can obtain a straight pass across the front of the wheel, you may have to make your own guide for the diamond dresser. Don't try dressing the sides of the wheel unless you are very clear what you are doing - if you thin an 8" wheel by a significant amount yet leave it near full diameter it's structural strength will be seriously weakened. Dressing a wheel is a very messy business, you definitely don't want to be doing it next to your prized machinery, or even in the same room if you can avoid it. If you have no choice then make sure everything is covered up well (with plastic sheeting preferably, the grit can then be shaken off onto the floor and vacuumed up).

Another method of shaprening tools that does away with the grinder altogether (apart from initial shaping) is to use a diamond stone. If you can afford a big enough one (say, 6" x 2"), and make suitable holders, this is a good way of sharpening tools - particularly for carbon steel tools as there is no danger of them overheating with loss of temper (the tool's that is). I have a smaller hand-held diamond stone for putting a finishing edge on carbide tips.

Double-ended grinder and tool rests.


My main grinder is one of those inexpensive double-ended 6" types that come with rough and smooth aluminium oxide wheels. I removed the rough grind wheel and replaced it with a green grit (silicon carbide) wheel for grinding tipped tools. I also have a separate spindle and motor driving a buffing wheel with another small fine stone (aluminium oxide) on the other end. HSS blanks are brought to shape quite quickly on these, but for exact profiling of special cutters (e.g., 14.5 degree worm tools) I tend to use the mill/drill fitted up with a grind disk and a universal vice as I can control the angles more accurately.

For finishing tools you need to spend a bit more effort and hone the edge - not just to get it sharper but to remove any irregularities along the edge. It is just the cutting edge we are concerned with here - not the entire facet of the tool. If you take a critical look under a magnifier at that apparently clean cutting edge straight from the grinder you'll see the actual cutting edge resembles the teeth of a saw blade, and the one that sticks out the furthest is going to do all the cutting until it gets blunted! You need to stone across the cutting edge to remove these 'teeth' to leave a smooth sharp edge. Again, try taking a look at the edge under a magnifier, run the slip across it a couple of times and look again, what you thought was a smooth flat surface turns out to be anything but as the stone begins to shave off the high spots and the irregularities show up. Slips come in various sorts, but I find the hard Arkansas stones the best. They are superior to carborundum types (which are soft and lose their shape easily) and last a very long time. You must remove the tool from its holder and stone the edge by holding the slip in one hand and the tool in the other. Try to maintain full contact with the whole edge to avoid rounding, and use a clean, light mineral oil to lubricate the stone and clear the metal particles.

The diamond hone is probably the only way of getting even a reasonably serviceable edge on a carbide tipped tool. If looked at under a magnifier you will see (straight from the green-grit grinder) that the edge consists of a series of chips, like a flint axe. The diamond hone just makes this chipped edge a bit finer, but it's still not great. Good enough for most turning work but not good for a fine finishing tool. The proper way of sharpening these edges is to use fine sapphire dust and a little water on a steel plate to form a lap, but if a good edge on carbide tools is that important to you either buy a new one or invest in a replaceable insert type tool.

One really lazy way of honing tools that I confess to using is to stick a fine India stone (2" x 1" x 1/4") onto the face of an orbital sander, with the motor speed greatly reduced with a mains controller. With this held in the vice a few seconds is all it takes to run the lathe tool across the stone for a perfect finish. The method worked great until the stone fell off the sander and broke, the double-sided tape really didn't make it. Take the trouble to mount the stone securely on a thin piece of plywood held on by clips.

Using a small shaped wedge on the grindstone table to maintain correct cutting angles.


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(c) Chris Heapy 1996.

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