Model Tools – End Notes
Guys, I’ve given you just about everything I know about tool grinding. I’m sure I didn’t catch it all so if I missed something, please tell me so I can fix it. Some of what is in this thread may differ from what I’ve written in the past; this is because like you, I am still learning and still trying to improve. What I posted in this thread is my current practice, for whatever that is worth.
I want to briefly mention the various kinds of tool blanks available. I’m including this because when I started, I had no idea what was what, which was better for a given application or if cheap Chinese stuff was okay.
My personal practice is to use 3/8” blanks for everything. Keep in mind that I only have a Sherline and an Emco 11” lathe. I would use 3/8” tools up to a 13” lathe, then step up to ½” tooling on larger lathes. This has less to do with stiffness; these tools are damned stiff. It has to do with grinding times. ½” tools take me twice as long to grind and they aren’t any better performance wise; call me lazy.
For a budding tool grinder, I believe the first and best steel to learn with is mild steel keystock from the hardware store that is cut to the length of a tool bit. It is going to take a while for your brain and hands to coordinate their efforts to produce a lathe tool shape that actually looks like a lathe tool. My advice is to stay with keystock until you are totally comfortable that you can grind the shape and angles you want without any problems, then move on to M2 HSS. HSS requires more pressure to grind, and cobalt or tungsten steels will be even harder so don’t be in a rush. Grinding keystock will pay dividends, believe me. Okay, let’s move on to HSS.
I think of HSS as M2 or cobalt. M2 is your basic HSS without cobalt in the alloy. It is the cheapest tool steel, easiest to find and to grind and is the most impact resistant of the tool steels. M2 will handle almost all the common materials we use in the hobby shop – mild and medium carbon steels, aluminum, brass, plastics or wood. It is fairly abrasion-resistant and holds a keen edge. This should be your daily driver unless you're working with hard stuff.
When working with higher carbon steels, semi-hardened steels, stainless steels and other more unusual materials (Titanium), cobalt or tungsten HSS may be a better choice. I say “may” because most of these harder turning materials are usually machined with deep cuts at higher speeds and feeds. While these alloyed steel tool bits retain their hardness and edges at the higher temperatures these cutting conditions impose, whether your small hobby lathe has the power or rigidity to make the cuts required is another thing. Moreover, the higher temperatures that result from machining these materials can, and often does, result in work hardening and this can make taking accurate sizing or finishing cuts a challenge. This is why inserted carbide tooling is usually chosen when working with harder materials. So, why not just use carbide tooling for these materials? You can, if you have the speed and rigidity those tools require.
This is where being able to grind a custom tool comes into play. If you run a small lathe, the cutting forces and cutting temperatures will be quite high when machining harder materials if you use standard tip geometry, but if you were to grind a tool that reduces those forces and temperatures then your lathe will often perform much better. So, when we think of working with harder materials we do need to think about which tool material to use but we must also attend to the tip geometry of that tool; the smaller the lathe, the more important this becomes. Moving on …
In the non-M2 class, we have the Tungsten steels (the T-series of HSS) and the Molybdenum steels (the M-series HSS). Note that beyond the M2 steels, cobalt is added to the M-series alloys to provide its heat and abrasion resistant properties, while Tungsten is the key alloying material in Tungsten bits (they also have cobalt in them). There are actually 7 tungsten steels and 17 molybdenum/cobalt steels but most of them are specialty steels that you won’t see in the form of a square lathe tool bit. Between the two, the M-series is far more available.
Tungsten
The common grades you will see will be T4, T8 and T15 but rather than the alloy, they are usually labelled with some catchy name. Crucible was the main supplier of Tungsten HSS:
· Rex AAA = T4
· Rex 95 = T8
· CPM Rex T15 = T15
· Teledyne used to make the Vasco Supreme = T15; this is an outstanding tool bit if you can find them.
· I’m sure there are others that I don’t know about.
Molybdenum and Cobalt
· Cleveland Mo-max = M2. This is the highest quality M2 tool bit I know of. Most are made in Mexico now but older stock was made in the US and can still be found on eBay. The country of origin is printed on these bits and both work well.
· ETM HSS – M2 of consistent high quality. One of my favorites.
· Armstrong, Morse, Chicago-Latrobe are all good M2 HSS.
· Cleveland Mo-max cobalt = M35 cobalt or 5% cobalt
· Cleveland Super-Mo-Max = M42 cobalt or 8% cobalt
Some generic bits will be labelled “HSS-Co”. I do not know the content of these bits but they contain cobalt; could be 5%, 8%, whatever. I have used all those mentioned above and all are high quality. If I had to choose only one, I would choose the USA-made Mo-max M2 HSS for its consistent quality. For cobalt bits, I prefer the Super-Mo-Max bits but also like the tungsten-bearing Vasco Supreme and anything from Crucible.
So, what about tool bits from China, India, Poland, Japan or Israel? I have used some very high quality bits from Japan and Israel (TTC, the house brand from Travers) and they work great. Chinese bits vary in quality and most of them are not labelled; this makes it difficult to assess them. However, if I was a budding tool modifying monster, I would use Chinese import bits until I hit on a winner design, then make it from a known high quality blank.
Honing
I suggest using diamond stones to hone your tools with. I prefer the mono-crystalline stones from DMT but the poly-crystalline stones from EZ-Lap are okay. For most honing, I use the credit card sized stones in coarse, fine and extra-fine grits. You can use water as a lubricant. I recently discovered the wetting agent from Accu-finish and it works really well and stays on the stone longer.
I hone my tools after every use and oil them before storing. Sharp is good for turning tools and its nice when I pull a tool out and know that is honed and ready to go. Treated this way, a good HSS tool will last for well over a decade.
Let me end this by saying that a well-ground HSS tool is a joy to use. However, it is not the only tool to use. There are times when a brazed or inserted carbide tool will be better and you need to find out when that is. I would encourage you to grind conventional tools and then compare it to your carbide tooling and your modified HSS tools to see which are better for your needs. Keep an open mind about tooling; all are useful but only one will be the best for the job at hand – use that one.
Thanks for following along. I hope the model tools and the information in this thread is useful to you. Please post to this thread about your models or your own tools and experiences. I, for one, would like to see them. Again, if I missed anything or glossed over something that needs clarification, please let me know.
EDIT: I forgot to mention that when stoning on your nose radius, be sure you maintain the angle of the tool at the front. Stone a flat from top to bottom, then carefully round and blend it into the side and end faces. For your threading tools, you only need a flat; you do not need to round it. When deciding on nose radii, it is better to go small unless you need a better surface finish; don't forget that the bigger the nose radius, the higher the radial forces and deflection will be. By small, I mean 1/64" to 1/32". Look at a radius gauge in these sizes to get an idea of what you need and then just estimate it as you stone it on. You do not need to be precise in this but try to get close.
Mike