View Full Version : Lathe Cutting Tools (Grinding)
Purchased a 7x10 metal lathe from Homier Distributing. Am having a problem grinding my cutting tools from hss bar stock. Any good advice. Also what is the rule as far as what speed should be used for cutting different materials. I was thinking the harder the metal the slower the turning, but that didn't seem to work for me. Any and all help would be apreciated. I'm not a machinist by trade, but enjoy in making and fabrcating things that I need.
10-24-2001, 12:11 PM
Lots of books out there that show how to grind tools, Southbend's How to run a lathe being the most common.
But, the illustrations are for tools held in an Armstrong holder and a rocker tool post. An Armstrong highspeed toolsteel holder holds the tools at an angle. You probably have a tool block that holds the tools at level.
Have you any brazed carbide tools, observe the clearance angles that they have, you have to have these clearance angles. Start grinding a high speed bit in similar way, then start modifying it, with high speed tools you can add back rake, end rake, side rake, etc. Grind in a chip breaker groove and more.
Would be easier to show you a few variations in person than to try to type an explaination. Nose around some old machinist's shop, look how he grinds his tools.
Speeds, it is all surface speed, not a given RPM. With high speed tooling I start out at 50 SFPM and work up, especially with mystery steel.
2 quick rules of thumb. Surface feet per minute X 4 divided by diameter gives desired RPM. This is approximate, 4 is easier to remember than 3point whatever, I'd have to look up or figure out exact number.
Another one I can remember easy in my head is that for 50sfpm 1.00 inch is 200 rpm, so 1/2 would be 400, 1/4=800 and 2.00 would be 100rpm. And I drilled a batch of 1/8 diameter holes tonight at 1600 rpm.
Of course one can run higher surface speeds with high speed steel, but 50 is a safe starting point to work up from.
Clear as mud, right. Good luck, remember Rome wasn't built overnight.
10-24-2001, 08:01 PM
There are a ton of old trade school manuals for sale on ebay. Buy the one that is least expensive and read. All will have correct methods but some will be more clear than others so post your questions here. Remember if you give 10 machinists the same job you will get 11 solutions and if they are good machinists all will work. Point is... you will be hit with a lot of rules of thumb, they are just that, you may have to alter to fit your situation. The only rule written in stone is be safe.
10-24-2001, 08:36 PM
How the tool bit is ground makes a world of difference. Keep trying til you get it right. Just remember to grind that clearance. Stoning a small radius on the END gives a much better finish too. Set the tip of the tool at the center of the workpiece. Work close to the chuck and keep the tool overhang short to minimize chatter. Like most everything, it takes some time and practice to get the hang of it. This is where some good help by an experienced machinist can really shorten the learning curve.
By bar stock I assume you mean the square toolbits. If it's really bar stock, it's probably soft. And as a general rule, harder materials require a slower speed. Bigger diameter means bigger circumference which means more metal removed per revolution, so slow down for bigger stuff too.
One more thing. Safety first as thrud says. Never run a lathe without safety glasses. Never leave a wrench in the chuck. Watch those shirt tails and sleeves. Get rid of any jewelry. Long hair is a major no no. Think about what's going to happen before you turn the spindle on. Is that tool tight in the holder? What's your other hand resting on? Not the chuck or the workpiece I hope. Lots of stuff can happen if you're careless or even if you're not!
10-25-2001, 12:52 AM
What they said.
I would ad that a very good book on sharpening (more slanted to wood working) is Leonard Lee's "The complete guide to Sharpening" (Taunton Books & Videos) or from Lee Valley Tools. It tells you how to sharpen everything from pocket knives to shaper knives.
The only thing I would add is to use a open structure soft bond (H in a Norton wheel) grinding wheel. This wheel will cut cooler and cut faster than the crappy wheels sold with bench grinders.
I use a 1"x42" belt grinder with 3M 15 micron SiC mylar backed belts (1000 grit) then change to Chromium Oxide Belts (~8000 grit) for honing the edge.
Safety first - be careful!
Get yourself a protractor with a pivoting arm and measure the the angles you're grinding. Hardly anybody (might as well say nobody) can get the angles truly correct by eye, and the correct angles matter.
Also see if you can find a copy of "The Design and Use of Cutting Tools," by Leo J. St. Clair. It's 300+ pages of details about single-point (i.e. lathe) tools. Along the way he gives some excellent information about sharpening. The book is out of print, but worth hunting up.
10-25-2001, 10:04 AM
As an end note...When sharpening, you do not want the end to turn blue (burn). Every few seconds dip the part into water. Patience, Patience, Patience.
Another hint: buy M-2 or higher cobalt tools. Half again pricier but, they don't loose temper if overheated. We do a lot of thin grooving bits and special two and three operation toolbits often shaped by cut off wheels and Dremel cutoff wheels;saves time and the tool contours are easily changed. Even scorched they hold temper. Thank ( ) because we have some fellers that could destroy anything! One bench grinder at work and at home is set up with a 1/32 cut off wheel on one end. Wonderful for trimming, shaping cutters. and a lot of other cut-off operations, no chop saw needed for the shop.
10-25-2001, 11:04 PM
Toff is right, the M-xx alloy bits make quite the difference.
T-15 Endmills & Bits are better, but really tough to sharpen unless you have CBN wheels. I buy 2" endmills made in Isreal of this stuff from the per/Lb. bin at the surplus store and cut the shank up as I need it with diamond wheels.
It was mentioned not to let the end turn blue. I hold the bits by hand, when it gets very warm, I let the bit cool - it should never change to any colour! This applies to drill bits too!