Three Views of the Basic Left-Handed Lathe Cutting Bit
In order to best visualize the relationship of the various angles of surfaces and how they relate to real cutting tools, just examine the photos with matching numbers. These are 1/4" square HSS lathe bits ground to three different cutting profiles.
I like to grind my bits on a 60 grit gray aluminum oxide wheel for all the initial rough grinds. You do not need to waste time on a fine wheel that barely takes off metal. Finished surfaces are ground on a white aluminum oxide 100 grit wheel. This finer wheel puts on such a good finish that you almost do not need to hone your edges. As you machine, you might have to touch up the various edges avery once in a while. Many times you simply can get away with a light honing with a fine stone and some oil. Heavy nicks and other defects will require a pass through the 100 grit wheel.
Angles of the clearances will vary from almost zero if you are going to machine brass, to 10 -20 degrees. These angles do not need to be exact. Just get it close and it will work just fine. Certain materials like aluminum will need much steeper clearances or they will not machine cleanly. I like to put around 20 degrees of clearance or "rake" on all my bits that will be cutting aluminum. Steels and other ferrous metals will do well on angles between 10 and 15 degrees. You will have to experiment to see what works best.
Those of you who scrounge in metal scrap yards will no doubt find plenty of good and cheap material to work with. The problem is that many times you may not be able to accurately identify the particular alloy you bring home. Aluminum and stainless steels may be of unknown parentage and will display different machining characteristics. You will have to experiment to see what works best.
Though the diagrams above depict the cutting point as a very sharp one, you should always hone a very slight radius to break the sharp point to a gentler one. This will result in much smoother finishes all of your cuts.
Do not ever grind with such pressure and for such lengths of time that you see the metal turn blue. This is the quickest way to ruin a bit. Keep a bucket filled with cold water and quench that tool frequently. How often you ask? My rule of thumb is if you are holding the tool about an inch from the end you are grinding, and it begins to burn you, you need to quench!!!!
Photos of Tool Bits
Cutter for brass -- No top rake! Slightly damaged from over-heating during grinding. Note the discoloration.
60-degree threading bit
Right-hand round nose tool
Carbide insert tool holder 1/4" shank
Diamond hone -- great for touch-ups!
A video covering the subject of tool bit grinding will be available this Spring 2003. It will be offered in VHS and DVD+R for $28 (VHS) and $33 (DVD) + $3.95 Priority Mail. Look for news on this new video addition coming SOON!!!!