There have been many modifications in each the steels utilized in woodturning instruments and within the grinding wheels we use to sharpen them. These modifications have created confusion amongst woodturners. We marvel why we typically don’t get the outcomes we anticipate from costly new instruments. Following is a abstract of how instrument steels have modified and what it means for woodturners.
Evolving Software Steels
For a few years, M2 has been probably the most broadly used and profitable standard high-speed metal (HSS) used for woodturning instruments. M2 revolutionized woodturning by offering instruments that have been rather more proof against each put on and warmth than earlier instruments. Extra lately, now we have seen progress within the reputation of M42, one other standard HSS. We have now additionally seen the introduction of particle-metal instruments (additionally referred to as powder-metal instruments). Instruments with steels equivalent to M4 and A11 (Crucible Industries’ model of which is named CPM 10V) have turn out to be broadly obtainable and are gaining in reputation.
What are the variations between standard high-speed steels equivalent to M2 and M42, and particle metals equivalent to M4 and CPM 10V (A11)? That are the higher selections? It helps to grasp how they’re made.
In manufacturing standard HSS, iron and carbon are melted in a furnace, and alloys are added to the molten metal. The molten metal is then poured into molds and allowed to chill. The alloys make the metal extra proof against abrasion (put on) and to wreck by extreme warmth, in addition to more durable and extra proof against breakage. Since alloys in metal make it higher, logically we want to add extra. However there’s a restrict to the proportion of alloys we will add to standard HSS. Because the metal cools, atoms of a number of the alloys chemically bond with atoms of carbon, forming what are referred to as carbides. Carbides contribute positively to abrasion resistance, warmth resistance, and toughness and are a lot tougher than the encircling metal. However the larger the proportion of carbide-producing alloys, the bigger the carbides turn out to be. Excessively massive carbides create two issues: they’re too onerous to be floor (sharpened) with most standard grinding wheels and so they rob the encircling metal of their advantages.
What we’d like is metal with very small, very onerous, abrasion-resistant carbides which are plentiful and well-distributed. The carbides of vanadium are significantly onerous and contribute very positively to abrasion resistance. That’s why steels excessive in vanadium keep sharp longer. The carbides of molybdenum contribute to wear-resistance, and molybdenum contributes to toughness. Carbides of chromium and tungsten contribute positively, too. How can we add all of the alloy we wish and on the similar time keep away from excessively massive carbides? Enter particle-metal expertise.
Within the manufacture of particle metals, iron and carbon are melted in a furnace, and the alloys are added. However as a substitute of pouring the molten metal into molds and permitting excessively massive carbides to kind because the molten metal cools, the molten metal is blown by means of a nozzle, creating microscopic droplets of metal. The droplets are on the order of 6 to 10 microns in diameter, and the carbides throughout the droplets are 2 to three microns in dimension. One micron is one-millionth of a meter. The metal powder is then put right into a steel canister and subjected to extraordinarily excessive stress, at forging temperatures, in a course of referred to as sizzling isostatic urgent (HIP), remodeling the powder into strong metal. The particles bond collectively so totally that the ensuing metal could be very dense, with nearly no porosity. We now have strong particle steel with very small, very onerous, well-distributed carbides. Particle metals have revolutionized woodturning instruments.
A Nearer Look
The above chart reveals the chemical composition of M2 and M42 standard HSSs, as outlined by the American Iron and Metal Institute (AISI). M2 incorporates vanadium, molybdenum, tungsten, and chromium, all of which contribute to the formation of carbides, making M2 fairly abrasion-resistant and difficult. M42 additionally incorporates molybdenum and chromium, and smaller percentages of vanadium and tungsten. To compensate for its decrease proportion of vanadium, whose carbides are very abrasion-resistant, M42 has 8% cobalt, which contributes to hardness. What M42 lacks in vanadium carbides, it compensates for with better hardness. M2 could be very broadly utilized in woodturning instruments. M42 is much less broadly used, however its reputation is rising.
This chart reveals the chemical composition of M4 and CPM 10V (AISI A11) particle metals. M4 is a wonderful particle steel however has been adopted by a comparatively small variety of instrument producers. CPM 10V (AISI A11) has been adopted by extra instrument producers, and its market acceptance is rising. There are different particle metals, however for functions of brevity, we’ll restrict our dialogue to those two glorious examples.
Whereas particle metals carry out exceptionally properly, the carbides inside them have created new challenges. Most standard grinding wheels are unable to sharpen particle-metal instruments to a eager edge. That’s why standard knowledge says we can’t get particle- steel instruments as sharp as HSS instruments. However that’s not true—enter cubic boron nitride (CBN) grinding wheels.
CBN abrasive is nearly as onerous as pure diamond and is very proof against abrasion; it maintains its sharp edges so it cuts shortly and effectively. The CBN abrasive is usually bonded to the floor of a metal or aluminum grinding wheel, so the abrasive sits happy with the wheel. When grinding with a CBN wheel, simply the abrasive touches the instrument, slicing quicker and cooler. The abrasive in a traditional, vitrified solid wheel, then again, is combined in with a bonding matrix (referred to as the bond) that holds the wheel collectively. Since each the bond and the abrasive rub on the instrument as it’s being floor, standard wheels don’t minimize as shortly, and so they get the instrument hotter than do CBN wheels.
Because the abrasive in most standard wheels isn’t onerous or sharp sufficient to chop by means of the carbides, it both shatters the carbides or knocks. them out of the innovative, leaving a microscopically ragged innovative. It merely isn’t attainable to get a particle-metal instrument as sharp with most standard wheels as it’s with CBN. And CBN wheels work splendidly on M2 and M42 in addition to on particle-metal instruments, getting each steels a lot sharper.
CBN wheels have three different nice benefits. First, they’re much higher balanced than standard wheels, in order that they run extra easily. Though standard wheels could be balanced, as quickly as they’re dressed, the stability modifications. It’s nearly unattainable to maintain standard wheels balanced, and the vibration makes it harder to grind precisely. Easy-running CBN wheels facilitate higher grinds. Second, CBN wheels by no means have to be dressed, so their diameter by no means modifications. For individuals who use jigs when grinding, the slicing angles on instruments change each time a traditional wheel is dressed as a result of the diameter of the wheel is lowered. Except the jig is readjusted incessantly, slicing angles are altering. With standard wheels, your jig’s grinds might not be practically as repeatable and constant as you suppose. With CBN wheels, the angles don’t change. Third, CBN wheels produce far much less mud than standard wheels. Respiratory the mud produced by dressing standard wheels and by grinding instruments on them is usually a well being hazard.
One different distinction with CBN wheels is that they’re available in a lot finer grits than standard wheels, together with 80, 180, 350, 600, and 1000 grits. When a instrument is floor on an 80-grit standard wheel, the scratch sample on the bevel produced by the abrasive within the wheel creates a collection of “enamel” alongside the slicing fringe of the instrument. These enamel are clearly seen beneath a microscope. If the identical instrument is floor on a 600-grit CBN wheel, the enamel are sharper and extra uniform, and there are lots of extra enamel throughout the innovative. The ideas of the enamel do the slicing and subsequently put on down because the instrument cuts wooden and regularly turns into boring. With many extra enamel throughout the innovative, the instrument sharpened on a really tremendous CBN wheel is not going to solely be sharper, it’ll boring extra slowly. That provides a fine-grit CBN wheel a giant benefit over an 80-grit standard wheel.
Now take into account that with particle-metal instruments, every tooth has tons of of carbides inside it which are finer, tougher, and extra evenly distributed. Typical HSS instruments even have many carbides inside every tooth, however the carbides are bigger and never as properly distributed throughout the steel. Due to this fact, although there is a bonus in grinding each standard and particle-metal instruments on tremendous grit CBN wheels, the particle-metal instrument is the winner.
What grits do I take advantage of in my workshop? I’ve two 8″ (200mm), 1725- rpm grinders with two CBN wheels on every: an 80- and 180-grit on one grinder, and a 350- and 600-grit on the opposite. I take advantage of the 80-grit for rough-shaping instruments. It cuts quick and funky. I take advantage of the 180-grit primarily for common sharpening and to sharpen my negative-rake scrapers. The 180- grit wheel produces a burr that’s good for negative-rake scraping. I take advantage of my 350-grit wheel to sharpen gouges used for rough-shaping platters and bowls, and I take advantage of the 600-grit wheel for sharpening gouges used for ending cuts, the place a clean floor with no tearout is crucial. If I had just one grinder, I might select 180- and 600- grit CBN wheels.
A Observe About Warmth Therapy of Metal
All woodturning instruments should be heat-treated as a way to absolutely develop the microstructure of the metal to yield its desired traits of hardness, toughness, and resistance to put on. Throughout warmth therapy, instruments are heated in a furnace to a exact temperature, then quenched to quickly decrease the temperature of the metal to make it onerous. Cryogenic therapy is typically employed within the quenching course of. Instruments are then tempered to scale back stresses within the hardened metal, enhancing toughness and resistance to shock. Regardless of how good the metal, if instruments will not be heat-treated utilizing precisely managed processes, instruments is not going to carry out successfully. A full description of warmth therapy processes is kind of concerned and too prolonged for inclusion on this article.
When you’ve got M2 instruments, proceed to make use of them proudly. M2 has been probably the most profitable metal for woodturning instruments for a few years. However when it’s time to purchase new instruments, reap the benefits of developments in fashionable instrument steels. And with CBN grinding wheels, you may get particle-metal instruments each bit as sharp as standard HSS instruments, and so they keep sharp longer. Put money into CBN grinding wheels as quickly as your finances permits. And take into account getting at the least one very fine-grit CBN wheel to grind gouges used for ending cuts. A boring instrument will at all times produce tearout, at all times! The sharper the instrument, the higher it’ll carry out. Advantageous-grit CBN wheels produce superior grinds. Fashionable woodturners ought to reap the benefits of evolving applied sciences in instrument steels and grinders.
Tom Wirsing is a physicist who has had a lifelong curiosity in metallurgy and woodturning. He enjoys instructing woodturning and has taught at regional and nationwide symposia, in addition to at quite a few AAW chapter occasions throughout the U.S. and Canada. Tom is a previous president of the AAW. For extra, go to thomaswirsing.com.