Models for grinding HSS Lathe Tools

[mikey]

I don't know what the other guys think but I can give you my take on a flycutter tool. I used to grind a left hand roughing shape with a fairly generous nose radius - slightly larger than 1/32". I wasn't too deep into tool modifying back then but I wasn't too thrilled about tool life, although the finish was really good. When doing harder materials, I could go through three tool bits just squaring a work piece made of stainless steel. Then I discovered Sherline's inserted carbide flycutter that had a much longer work life and never looked back.

Knowing what I know now, here is what I would try if I were to grind a flycutter bit:

• Almost every cut will be an interrupted cut so I would only use M2 HSS; cheaper, more impact-resistant, fast to grind.
  
• For a shape, I would grind a roughing tool shape or possibly a general purpose shape like our square tool. Depending on depth of cut, cutting loads may be very high.
  
• Flycutters run at high speed, which lowers cutting forces so we can afford to keep relief angles near baseline levels for strength.
  
• Since the relief angles are stronger, we could afford to increase side rake to boost chip clearance and lower forces more so I would go maybe 4 degrees above baseline.
  
• Almost all cutting is at the tip so I would boost back rake by 5 degrees and this will put all the cutting action at the tip.
  
• Then I would grind a nose radius just a tad bigger than 1/32".

I'm pretty convinced that a LH Square tool would work pretty good if you gave it a slightly larger nose radius. The only way to know is to try it. Of course, you would change the side cutting edge angle of the tool to be sure the nose was the contact point but that's pretty simple to do.

Dunno' ... what do you think? Why not make a LH square tool and try it, then report back and we can brainstorm on it.

 

[tweinke]

Next time I'm in the shop with a fly cutting tool job I will definitely give that a try!

 

[IanT]

Hi Mikey,

A very interesting set of posts but I do have a question about 'relief' angles (I tend to call them clearance). My understanding has always been that the relief angles are exactly that - they provide sufficient 'clearance' to stop the tool rubbing on the work. I also understand that if the relief angle is increased the 'lip' angle is decreased and the cutting edge thereby weakened. But I'm not sure how changing the relief angle improves the 'sharpness' of the tool - that to my mind is a matter of the amount of rake applied (be that side and/or back).

I routinely use the wheel periphery to give clearance to my tools - and the slight curve imparted also helps when stoning the top edge. I do vary the rake(s) used, dependant on material to be cut but not the clearances. I have a mental picture of how this all works but cannot quite see how more relief makes for better cutting? Surely there is either sufficient clearance or there is not?

Regards,

IanT

 

[mikey]

Hi Mikey,

A very interesting set of posts but I do have a question about 'relief' angles (I tend to call them clearance). My understanding has always been that the relief angles are exactly that - they provide sufficient 'clearance' to stop the tool rubbing on the work. I also understand that if the relief angle is increased the 'lip' angle is decreased and the cutting edge thereby weakened. But I'm not sure how changing the relief angle improves the 'sharpness' of the tool - that to my mind is a matter of the amount of rake applied (be that side and/or back).

I routinely use the wheel periphery to give clearance to my tools - and the slight curve imparted also helps when stoning the top edge. I do vary the rake(s) used, dependant on material to be cut but not the clearances. I have a mental picture of how this all works but cannot quite see how more relief makes for better cutting? Surely there is either sufficient clearance or there is not?

Regards,

IanT

The side cutting edge is an intersection between two planes, the relief angle and the side rake angle. This forms an included angle defined by those two planes, right? You can reduce the included angle by increasing either the rake or relief angles and the result would be the same; I assume this is what you mean by "sharpness". The downside to increasing the relief angles is that you lose some support under the cutting edge so tool life tends to go down but a little bit goes a long way.

But here's the thing. I used to think that once you have clearance then you have it. That was until I measured motor load and varied the relief angles and found a reduction in motor load as the relief angles increased. This can be due to a reduction in cutting forces (can't tell without a strain gauge) or that there is more rubbing than the published angles would lead us to believe or both. Regardless of which it is, a small increase in relief angle does reduce motor load and, by inference, cutting forces.

Hope that helps.

 

[mikey]

Wanted to post a follow up on the ceramic belts that ate my keystock. They are excellent for grinding HSS and cobalt HSS. I used a 35 grit ceramic belt that shaped a tool about twice as fast as an Aluminum Oxide 24 grit belt. It also cuts much cooler. Normally, an AO belt turns the HSS a straw color due to the heat but the ceramic belt cut it faster and with no color change to the steel. The ceramic is a finer grit so I expected it to grind with finer grind marks but it is much finer than I thought it would be.

The 80 grit ceramic belt cuts much faster than its AO counterpart but cooler and faster, too.

Bottom line - I found a new kind of belt. Bye bye, AO! Amazon carries them for a good price.

 

[Fallriverbryan]

Wanted to post a follow up on the ceramic belts that ate my keystock. They are excellent for grinding HSS and cobalt HSS. I used a 35 grit ceramic belt that shaped a tool about twice as fast as an Aluminum Oxide 24 grit belt. It also cuts much cooler. Normally, an AO belt turns the HSS a straw color due to the heat but the ceramic belt cut it faster and with no color change to the steel. The ceramic is a finer grit so I expected it to grind with finer grind marks but it is much finer than I thought it would be.

The 80 grit ceramic belt cuts much faster than its AO counterpart but cooler and faster, too.

Bottom line - I found a new kind of belt. Bye bye, AO! Amazon carries them for a good price.

Hey Mikey
I have a medium coarse diamond wheel for sharpening carbide. Have you even used one to sharpen HSS? I've heard it is not advisable but I have done it while applying coolant during the sharpening process.

Sent from my SM-G950U using Tapatalk

 

[mikey]

Hey Mikey
I have a medium coarse diamond wheel for sharpening carbide. Have you even used one to sharpen HSS? I've heard it is not advisable but I have done it while applying coolant during the sharpening process.

Sent from my SM-G950U using Tapatalk

No, I don't have any experience with that kind of wheel. I do have an Accu-finish that turns at 300 rpm and at that speed, finish grinding HSS is no problem. Using diamonds for steel is only an issue at high speeds; my limited understanding is that the heat can weaken the bonding agent that holds the diamonds on. I guess using coolant like you're doing would keep the heat down nicely, Bryan, so I'm not surprised it works.

 

[mikey]

I forgot to mention that these ceramic belts seem to last longer than the AO belts. I ground 9 model tools, a HSS tool and two Rex AAA (5% cobalt) and the belt seems to be as sharp after all of that as it was in the beginning. It took me just a few seconds longer to grind each face of a cobalt tool than it did to grind them on mild steel keystock.

I am really impressed with these belts!

 

[ttabbal]

3dp examples came out good, they aren't 100% accurate but the labels turned out readable and give a good idea of what the finished product should look like

well they would be at least if I didnt print them in black, hah.

That's great! I have a 3D printer and would be willing to print some up for group members when the models are ready.

Been on vacation and catching up with the thread. Great stuff mikey! Thanks! Now I need to come up with something to grind them with.

 

[DHarris]

OK all, I just received Mikey's model tool in the mail (Monday @ 12:00 noon Calif. time). Started to run right out to the garage when wife grabbed me by the ear and said "eat lunch first because I know I won't see you till after dinner time". So, dutifully obeying - I choked down lunch and THEN ran out to the garage!

Note - I'm a complete newbie on lathe tool grinding!

First thing I did was to cut and pasted all of Mikey's write-ups into one single MS Word document (all 23 pages of write ups! Hell of a task and job well done by Mikey, I might add!). Printed it out and into the garage I went. Started with the basic RH cutting tool. Followed the instructions step - by - step and compared my tool to the Sample Tool. It was amazing, even on my old rickety belt sander it took less than 20 minutes to grind. And, this is with me going from 36 grit, to polish with 80, and then just for S%its & Giggle's 220 grit. finished off with honing it. IT CAME OUT GREAT!!!!! and the bloody thing is SHARP!

Apparently, for me at least, what has been missing was a simple step - by - step walk thru to make it clear and easy to do!

Other things i discovered while grinding the 3 sample tools - -
1) the platen on my belt grinder is not rigid enough.
2) Belt grinder motor is a wimp - it's 30 years old & from harbor freight
3) Table is definitely not rigid enough - flexes down from left to right under pressure (is
only held on left side by one bolt.
4) No easy, accurate & repeatable way to place angles on table

So, basically, belt sander is crap - - good for general roughing & / or wood sanding in garage. OH BOY, VALID EXCUSE TO BUY A NEW TOOL!!

Attached is a photo of the tool model (bright shiny tool on right) and my grind (dull color on left). Note: I had not put the slight rounding of the nose on my tool when this photo was taken.



A final note - - after grinding the 3 samples tools in key stock - I duplicated this RH tool in a cheap Chicom metal HSS tool blank I had laying around. Put it in the tool holder in my Sherline lathe & started making cuts I never would have thought possible on this small lathe.

THANK YOU MIKEY FOR TAKING THE TIME & PUTTING IN THE EFFORT TO HELP US NEWBIE'S LEARN & GROW IN THIS HOBBY! (and yes, I did blame you for needing the new belt sander when I told the wife!)

Dave Harris

ps, It's now 2:30 Calif. time & I'm prepping the 3 sample tools to send up to Canada tomorrow! So, in roughly 2 hours I cut the 3 sample tools, one HSS tool and made some cuts!