Models for grinding HSS Lathe Tools

devils4ever said:
If your mini-lathe is similar to mine, the feed rate is 1/256 of the spindle speed. That is about 0.004"/rev. This is calculated as (20/80) * (20/80) * (1/16) = 1/256. The 20/80 reduction is from the gear train and the 1/16 is from the lead screw pitch. Beware that this is for my Micro-Mark Tru-Inch mini-lathe, yours might be slightly different. But, you can do the same calculation.

I converted to Clough42's ELS system. Now, I can dial in 0.001"/rev up to 0.040"/rev. The ELS gives you RPMs as well. You might want to consider doing this conversion as you can change the feed rate as you are turning. This makes it easy to turn to a shoulder by slowing the feed rate as the cut nears the end. Also, you get metric threading for free!

I'm still learning to grind my HSS lathe bits. I found the belt grinder orders of magnitude easier than a bench grinder. It's vastly faster and cooler and you get flat faces. My turning improved a lot by getting and keeping the faces polished by honing. It's makes a big difference. I can take take 0.040" DOC in steel now. Before I was taking between 0.005" and 0.010" DOC.
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I'm in awe of your work. You have done some pretty cool stuff. It would be awesome to convert to ELS. My mini-lathe has the same identical gearing and lead screw.

Agree that using a belt grinder is lots easier. My belt grinder is sub par, so I'll end up making one of my own. It's on the "projects I'd like to do this year" list. I have limited access on one side of the belt which makes it hard. So for my new grinder I need to make a table that is omni-directional.

I'd love to be able to do 0.040 DOC in steel, but I'm not there yet.
 
OK... so let's assume 650 RPM. With a 1/2" work piece, that's 85 SFM. That should be in the range of what a HSS tool can handle in mild steel easy enough. Let's stick to known material for now, once you start getting into unknownium, you could be dealing with Iconnel for all we know. :) Say 12L14 for steel and 6061 aluminum.

Your tool pics look pretty good, so the basic angles look alright. Don't go too nuts with the tip radius. 1/64" or so is plenty. If you don't have a radius gauge to compare against, go for "barely not pointy". :) Then try to get the edges honed as well as possible.

I've found that the things I kill edges with are RPM vs hardness, chatter, and crashing. I'm going to assume you didn't crash, so chatter. Can you try a 0.01" DOC with everything but the carriage locked? The compound can be a big contributor here. Maybe just start with aluminum and see how the tool and part feel. If we're close, it shouldn't have heated much. Double check the tool tip height as well, being off center can cause issues as well.
 
No inconel! A bit too exotic for me and my wallet. Haven't done any unknownium either. Didn't crash.

My mini-lathe doesn't have a way to lock the slide, or the compound. Don't know of any mini-lathe with that ability. I could "add" locking to mine, but in general, these mini-lathes are not maxi-featured. The gibs could be further tightened, although they are on the stiff side already. Good point on the tooltip height, I will make sure of that. I'll give it a try tonight.
 
Ah, I have not used one, I didn't realize it didn't have locks. I guess you could tighten the gib, but then you would lose your adjustment. Not ideal.
 
So, my first go at trepaning didn't work so well, lots of chatter. I ground it mostly like a parting tool, but with a bit more relief on the outside to avoid rubbing, which does seem to be good. I suspect it's just a bit wide for my lathe, at about 0.2". Material is 6061 aluminum, but will be cast iron eventually. But I have seen small lathes run larger form tools, so I'm scratching my head a bit.
 
ttabbal said:
So, my first go at trepaning didn't work so well, lots of chatter. I ground it mostly like a parting tool, but with a bit more relief on the outside to avoid rubbing, which does seem to be good. I suspect it's just a bit wide for my lathe, at about 0.2". Material is 6061 aluminum, but will be cast iron eventually. But I have seen small lathes run larger form tools, so I'm scratching my head a bit.
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You don't say what machine you are using and what is the trepanning dia., what is the material thickness etc. a pic might help. I suspect 0.2 might be little wide for the tool. How much overhang do you have on the tool?
 
Downunder Bob said:
You don't say what machine you are using and what is the trepanning dia., what is the material thickness etc. a pic might help. I suspect 0.2 might be little wide for the tool. How much overhang do you have on the tool?
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It's a PM1127. About 2" diameter, I'm going for a groove about 0.316" wide and deep. Tool has about 1" overhang, 3/8" M2 HSS. I measured the angles after thinking about it and I suspect I need to add some more clearance angle on the sides. Other suggestions are welcome.
 
About a year ago, I made a trepanning tool. Just measured it at 0.130" wide. I had to have the cross-section be a triangle in order to get enough clearance. The bottom off the tool is sharp, not because it is a cutting edge, but because I needed to get the relief. Used mine to make a spindle light out of 6061. Used it to cut out the center, as well as to make slots. Diameter of slot was on the order of two inches, like your application.

I only have a mini-lathe, and it cut fairly well. Used about a 3/4" overhang from the tip. 3/8" M2. Try reducing overhang and add more relief. The front of the tool should define the cut. There should be slight relief towards the base/back of the tool. Length of cutting tip is about 0.425"
PXL_20210920_145615673.jpg
Still use the tool to make grooves and widening slots.
 
I'll get a pic later, it might be helpful to others. I ended up a little rounded on the outside edge, a sharper angle would have worked just as well though. I think my sides are more straight than I was aiming for, so I'm going to start there, I do have some rake on the top, so I think it's good there, but I'll go over it again with fresh eyes. I'm close, and get a decent chip, I just want to get the chatter under control. It's an interesting operation I've never tried before.
 
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