Sharpening lathe tools.

[GK1918]

Have to aggree with Ron. You will get it, if it helps I use plain ole red majic marker for a rough guide.
Try to hold the angle you want on the horizon (level) moving back and forth against better judgement
I do use a tool holder some blanks are too small to hold on to, cooling every few passes. Just remember
you need to know your toolholder, some -0- back rake and ones with built in back rake. As far as
facets, I dont want to get you in the grinder business, but although I have three, one is hands off with
a fine and finer wheels. Typically my big lathe uses 1/2 HSS blanks, those I will vise up and rough out
with a 4" angle grinder to save my grinder wheels then to the fine stone. Did anyone see the Utube
vid of a 1/2 drill bit stuck in a shaper doing its work. When stuck I have even ground allen wrenches
for 60* threading. See, no rules, not written in stone. Can be done precise with a tool post grinder which
I avoid.

 

[tkollen]

I thought for a long time that I was pretty good at free hand grinding my tools until one day I decided to measure and make the angles exactly according to Machinist Handbook and I was surprised at how much it improved cutting performance on especially D2 tool steel. That's why I decided to get a South Bend grinding jig (Alisam copy).

At first I was surprised that cutting performance of the tool bits was worse than before I started to use the jig but the problem became quickly apparent . The problem is the jig's 10 degree heel angle intended for grinding front clearance of the tool. It seems to me that the SB jig was designed for grinding against the side of a grinding wheel with the grinding surface perpendicular to the jig. This would produce 10 degree front clearance. However recommended front clearances for most material (Aluminum, brass, steel etc) is 8 degrees. Copper and is the only material I know of that has a 10 degree recommended angle, so a 10 degree heel is too much to begin with.

I use a tool rest which is level and 1" below center of my 8" grinding wheel. This means that with the SB jig level straight (not on its heel) the front clearance becomes zero degrees at the center of the tool bit. This is perfect for grinding side angles and top rakes etc. However with the jig on its heel and the tool bit protruding 1", then the tool top (3/8" bit) is raised to 0.56" over the grinding wheel's center line at which the curvature of the wheel gives the top of the tool bit an extra 8 degrees. The end result is that front clearance becomes 18 degrees rather that 10. Of course this can be corrected by lowering the tool rest when grinding this surface but this is a hassle I want to avoid by using a jig.

Using math and trigonometry I determined that by making the heel 3.4 degrees (rather than 10) an 8 degrees front clearance will be produced on my 8" grinder (A 6" grinder would require about 3 degree heel). A low cost solution was to make a thin beveled aluminum wedge to be screwed on to the heel by using two screws and two holes drilled into the heel. The wedge that gives me the proper 8 degree clearance is about 4 mm thick in the back.

I attach an Excel-Tool I made for simulating the front clearance angle as a function of the geometry of the tool holder, tool and grinding wheel. This can be used for calculating any type of jig when grinding against the front of the wheel.

View attachment SB - Grinding Jig - Geometry.xlsx

 

[SE18]

I'm one of the dumb ones who doesn't know trigonometry (but might give it a try if needed). One thing I noticed is the chinese tool posts are at a straight angle and the lantern style are at an up right angle.

anyway, if u account for that the grind angles and have atest bar, you can see what turns nicely and what doesn't i.e. trial and error. If you want to be really certain, learn trig and read the tables!!!!!! LOL (I was daydreaming in classs)