How "sharp" should HSS lathe tools be?

[zippyslug31]

^^^ Yeah, this is where my head is at the moment.

I appreciate everyone's links and details on how to grind, but that's just not where my skill level is yet so I'm trying to avoid tackling that right now... thus my buying a pre-ground set. I was only asking about sharpness due to my questioning exactly how good my pre-ground set is and the very small alteration I did on the radius.

 

[projectnut]

Like many others I don’t hone the side very often. It is however important to have the proper radius on the cutting nose. Rule of thumb for nose radius is a minimum of 1-1/2 times the distance the tool moves on every revolution of the spindle. As an example if the tool moves .004” with each revolution of the spindle the minimum nose radius of the tool should be .006”. The tool will then partially overlap the area covered on the previous revolution an eliminate lines on the newly cut surface

 

[Nutfarmer]

If the premade high speed steel tool bit are anything like the brazed carbide or the import boring bars , you are fighting a losing battle not being able to grind your own. It really isn’t hard if you have just a standard shop grinder. With a little trial and error you will make a better tool than the premade ones. The South Bend Book How to Run aLathe has a good explanation that is simple and only a few pages long. The book is even a free down load with a little searching. Above all don’t give up. Show some pictures of what the problem is and some pictures of the tooling. There are some good people here that will help.

 

[mickri]

The basics get covered on pages 10 and 11 in that thread.

I highly recommend using a belt sander to grind tool bits. I went from using a hand held grinder to a bench grinder to finally a belt sander. The belt sander made a world of difference. I could now easily and quickly grind whatever shape I wanted on the belt sander. My belt sander is a Delta 1x42. I tilt the table to 15* and use a combo square for the other angles. I start with a fairly rough grit and then finish with a 300 grit. Only use a hone to occasionally touch up the edge.

 

[rwdenney]

I think this was Adam Booth’s video on grinding HSS cutters.

He does hone the edge, but that is made easier by using the curve of the grinding wheel to hollow-grind the tool first.

Rick “the radius is added during honing” Denney

 

[Janderso]

If the premade high speed steel tool bit are anything like the brazed carbide or the import boring bars , you are fighting a losing battle not being able to grind your own. It really isn’t hard if you have just a standard shop grinder. With a little trial and error you will make a better tool than the premade ones. The South Bend Book How to Run aLathe has a good explanation that is simple and only a few pages long. The book is even a free down load with a little searching. Above all don’t give up. Show some pictures of what the problem is and some pictures of the tooling. There are some good people here that will help.

They aren’t very sharp but they have the correct geometry for beginners to learn on. Imho.

 

[homebrewed]

I would start by just grinding the tool with a flat top, a small relief angle on the 2 cutting edges and turn some aluminum 500 RPM, 0.1 mm depth of cut and a slow feedrate 0.05 mm/rev. The tool should cut fine with a poor surface finish.
Than grind a small nose radius on the tip. Just by slightly rotating the tip against the grinding wheel with very very very little pressure once.
Do another turning pass, the surface finish should be better. If so, you could grind the nose radius larger for a better surface finish. Just repeat the above step once and try again. That will give you an idea about the size of the nose radius. You don't need more than 0.5 mm nose radius.
Once turning aluminum is going OK, you could try brass and after that some mild steel. When you start turning steel, you should start worrying about feed and speed settings.
I don't bother to grind a chip breaker or lap the cutting edges and use a €10,-- "diamond disk" for grinding. The tools perform well and are easy to (re)grind.
Lapping the tools could make them really good or ruin the cutting edges.
I use these kind of holders to (re)grind my HSS tools

View attachment 437735

I have a setup that uses the same approach. But yours is a lot more flexible than mine. Pardon me while I steal adapt your idea .

 

[erikmannie]

I'm dealing with pretty poor surface finishes and am fighting this on several fronts: lack of overall experience, initial machine setup, possible machine problems, etc. On some advice, I didn't add to my learning curve by also trying to learn grinding geometry right away so I picked up a "ready to go" HSS set similar to this.

I've since learned that one of my problems is the tool I'm trying to use doesn't have any nose radius so I added a small radius which helped a little. One would think that these would work right from the supplier, but this was foolishly optimistic apparently.

I've been reading a ton of posts here and there's a lot of confusing information of exactly how sharp the edges are supposed to be when doing the final honing of the cutting edge. Some say "razor sharp", yet others say it's a relative term and it's not even "knife sharp". I'd say that all of the tooling that came in my set have "crisp corners", but you would be hard pressed to slice skin.

Can anybody help to clear this one topic for me, or maybe this is one of those topics where everybody has a varying opinion?
Thanks in advance!

I bought a lot of those preground tool bits in many sizes, & I have used them a lot (mostly on mild steel) in a 10” as well as 16” lathe. Here are some of my observations:

(1) the HSS is pretty soft! If you use them on mild steel, they will dull very quickly, leaving you to grind your own cutting tip.
(2) they work **really well for parting and grooving** (!), but will snap easily. You will get to see how a perfectly ground parting tool works for awhile.
(3) the turning & facing tool bits are really sharp, right? These would be great for plastics & maybe soft metals.
(4) the threading ones work great until they dull.
(5) boring bars dull quickly in mild steel.

It is too bad that these manufacturers didn’t start with a higher quality HSS, but these are nice for somebody just getting started, what with being able to work with tools bits with proper geometry for some period of time.

To be honest, I don’t have a whole lot of free time, so I don’t take time to grind my own HSS, unless absolutely necessary. If you are in this situation, the world of carbide inserts awaits you, with perfect geometry at every turn. A lazy person can replace a worn or broken cutting edge with a *perfect* one in a few minutes. If you are using carbide & you need to hit a certain dimension, you will have to learn & use “balanced cutting”, *or* use carbide up until the last .008” or so & sneak up on those last few thou with HSS.

If you ever do decide to grind your own HSS, you would save time if you started off with high quality HSS.

Finally, many machinists will use Emery cloth to improve the surface finish or remove the last few tenth thousandths. If I want something to look really good, it usually gets lathe files, Emery cloth and/or Scotch-Brite at the end.

 

[GT-6 Racer]

I'm dealing with pretty poor surface finishes and am fighting this on several fronts: lack of overall experience, initial machine setup, possible machine problems, etc. On some advice, I didn't add to my learning curve by also trying to learn grinding geometry right away so I picked up a "ready to go" HSS set similar to this.

I've since learned that one of my problems is the tool I'm trying to use doesn't have any nose radius so I added a small radius which helped a little. One would think that these would work right from the supplier, but this was foolishly optimistic apparently.

I've been reading a ton of posts here and there's a lot of confusing information of exactly how sharp the edges are supposed to be when doing the final honing of the cutting edge. Some say "razor sharp", yet others say it's a relative term and it's not even "knife sharp". I'd say that all of the tooling that came in my set have "crisp corners", but you would be hard pressed to slice skin.

Can anybody help to clear this one topic for me, or maybe this is one of those topics where everybody has a varying opinion?
Thanks in advance!

This is a deep subject we all struggle with. If you are turning low carbon steel, hot or cold rolled it can be really tough to get good finish. Too fast you get edge welding. I grind a 10-10-10 degree tool, hone to shave a fingernail, 030-060 nose radius set exactly on center. Then try various speeds and feeds with different oils/coolants depending on the steel. Once you find the combo for your lathe you have it. That tool also works great on aluminum . Defiantly not for heavy cuts but great for finishing or light turning on a hobby lathe. I have buth left and right handed versions I use regularly on my 10EE when turning thin parts to skimming a thou or two When carbide just can’t work hard enough. Look at the old books like the SB manual, machinery’s handbook. There is some useful advice here. Ignore the comments about carbide.

Mild steel machining dilemma

Hi All, I've had this problem in machining mild steels when it comes to the surface finish. There will always come these stripes like material get dragged between the tool and part making deeper grooves and rough finish. No matter if I use carbide insert or sharp HSS tool. Harder steels like...

www.practicalmachinist.com

keep trying !

 

[tailstock4]

I thought I would show a few pictures of the high speed steel I use and how I use it. In this case I’m using a piece of 1/2” square M2 high speed steel from Griggs. I’ve also included a picture of a chart of the high speed steel that they sell and their properties. Stay away from the cheap Chinese or Indian high speed steel as they won’t hold up and will lead to frustration.


Source: https://www.griggssteel.com/resources/properties-2/

I ground this piece of high speed steel from a new blank free hand. It took me about 10 minutes. I’ve also included a picture of the grinder I use as I think this is important. It has a 12” wheel 2” wide. This allows for a better view when you are grinding. It also imparts a very slight radius to the different sides of the tool bit.



I hone my high speed steel cutting tools. I’m not sure how much this improves the finish, but I do know it improves the durability. The reason for this is all the little scratch marks and points from the grinding are heated when the tool is cutting. The resulting transfer of heat leads to a breakdown of the cutting edge which leads to more heat. Honing helps slow this process and adds to the durability of the cutting edge. The slight arc that the grinder puts on the tool makes honing quick and easy. There are some pictures of how the leading and trailing edges of the tool are honed and smoothed. I also hone the nose radius to final size. This arc makes for quick and easy resharpening as you are only honing the periphery.




I decided to use my Southbend Heavy 10 as I thought it might be a more common size for hobbyist. It is equipped with an invertor for variable speed. All the cuts were made in open belt – no back gear. I used a piece of 1 1/8” 1018 cold roll steel. The first cut is about .002 feed, 160 rpm and .400 depth of cut. The second cut is of the same stock at .0062 feed, about 470 rpm at .015 depth of cut and using same tool bit. The following pictures are of those cuts and the resulting finishes with the above parameters.