Tool Height On A Lathe

Bottoms out on the compound table.
 
[Mikey slipped]

Wreck, it bottoms on the compound slide surface, which is even with the bottom of QCTP bottom. The problem is the place where a QCTP mounts is too high.

On some lathes is it possible to mill some off the top of the compound slide, or mill some off the turn table seat, which is between the compound slide and the cross slide.

I had the same problem trying to mount an AXA QCTP on my Grizzly G4000. Milling down the top of the compound wasn't much of an option because the tool post bolt was more or less permanently pressed in and was a giant hassle to remove and reinstall. I had to have .045" ground off the bottoms of the tool holders — at great expense. They were very hard and my best carbide end mill would only polish them in a shower of feathered sparks. It didn't occur to me to take some off the bottoms of the insert shanks.
 
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While we are on the subject of Quick Change Tool Posts...

What's your preference? Wedge, or Piston?

Don't forget to say why.
 
JimDawson said:
I agree with you about cutoff tools, but I'm going to have to disagree with you on turning tool height. I have found that under some circumstances, going way below center works very well. I'm still not sure why it works and flys in the face of what we have been taught.
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Jim, I've seen threads where guys have gone negative on tool height but nobody came up with a reason why they thought it worked, either. It seems to me that the only reason for doing it would be to improve front relief and reduce rubbing; this would improve finishes I would guess. Otherwise I'm having some trouble with why this works better than staying on center line, mainly because I don't understand how the cutting forces would be affected by modern tool geometry with the tool set so low. I'm not trying to put you on the spot; I'll just have to try it and see if I can sort this out for myself.

Lots of guys, especially the real old timers, also set tool height a bit above center, usually quoting How To Run a Lathe as the reason. South Bend recommended setting the cutting tip 5 degrees above center for straight tuning and this is like the bible for some guys. The problem with this is that they used lantern tool holders that held the bit at an upward angle and ground the front relief with up to 15 degrees to allow the tool to cut. With modern cutters and tool holders I'm not sure this recommendation still applies but this is why I said the cutter should be on center height ... but it may not be.
 
mikey said:
Jim, I've seen threads where guys have gone negative on tool height but nobody came up with a reason why they thought it worked, either. It seems to me that the only reason for doing it would be to improve front relief and reduce rubbing; this would improve finishes I would guess. Otherwise I'm having some trouble with why this works better than staying on center line, mainly because I don't understand how the cutting forces would be affected by modern tool geometry with the tool set so low. I'm not trying to put you on the spot; I'll just have to try it and see if I can sort this out for myself.

Lots of guys, especially the real old timers, also set tool height a bit above center, usually quoting How To Run a Lathe as the reason. South Bend recommended setting the cutting tip 5 degrees above center for straight tuning and this is like the bible for some guys. The problem with this is that they used lantern tool holders that held the bit at an upward angle and ground the front relief with up to 15 degrees to allow the tool to cut. With modern cutters and tool holders I'm not sure this recommendation still applies but this is why I said the cutter should be on center height ... but it may not be.
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Yup, and I'm one of those old timers and went by How To Run A Lathe also, that and my 70 year old shop teacher in high school. That's been a few years ago. ;) I discovered this trick by accident one day when I inadvertently allowed the height nut to back up on my tool holder. It was cutting just fine and no spring, but I wanted to touch up the tool for another operation and on putting it back on the tool post, I reset it to the normal ''just above center'', the old problems came back, complete with tool spring and the normal issues.

Since I was just taking roughing cuts, I started thinking, what happens when I lower the tool? I started lowering the tool on each pass until I got no spring and was getting a nice cut. When I finished that operation, the tool was actually about an 1/8 below center. I measured after each pass and it was taking exactly what I dialed in with no spring at all. In this case the part final dimension was 2.700, I have also lowered the tool when cutting smaller (down to about 0.500) diameter. If you went too low on a very small diameter I expect the the work would try to climb over the tool. I have no idea what is optimal as far as height, but just experimenting until it feels right is probably the best way.

I still can't explain why it works, but it seems to work. Just try it on a piece of scrap and see what your experience is.

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As far as Piston vs Wedge type tool holders, I have used both. I currently have a piston type Enco branded tool post. It seems to work fine, I've not experienced any problems with stability. I used to run another lathe quite a bit that had an Aloris wedge type, and it seem to work fine also. The wedge action would mechanically seem to be a more stable system, but both seem to work OK.
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JimDawson said:
Yup, and I'm one of those old timers and went by How To Run A Lathe also, that and my 70 year old shop teacher in high school. That's been a few years ago. ;) I discovered this trick by accident one day when I inadvertently allowed the height nut to back up on my tool holder. It was cutting just fine and no spring, but I wanted to touch up the tool for another operation and on putting it back on the tool post, I reset it to the normal ''just above center'', the old problems came back, complete with tool spring and the normal issues.

Since I was just taking roughing cuts, I started thinking, what happens when I lower the tool? I started lowering the tool on each pass until I got no spring and was getting a nice cut. When I finished that operation, the tool was actually about an 1/8 below center. I measured after each pass and it was taking exactly what I dialed in with no spring at all. In this case the part final dimension was 2.700, I have also lowered the tool when cutting smaller (down to about 0.500) diameter. If you went too low on a very small diameter I expect the the work would try to climb over the tool. I have no idea what is optimal as far as height, but just experimenting until it feels right is probably the best way.

I still can't explain why it works, but it seems to work. Just try it on a piece of scrap and see what your experience is.
Click to expand...

I will certainly give this a try to see how it works because it clearly does work, whether the reason is apparent or not. Just goes to show you that convention isn't set in stone.
 
"The wedge action would mechanically seem to be a more stable system, but both seem to work OK."
My understanding was the chief advantage of the wedge was repeatability, since the various degrees of freedom are more thoroughly constrained, but that both were sufficiently rigid to cut properly (just think of how delicate the old lantern types are as far as mass & contact area, but they seem to have been popular for a long while, so I assume they work)
 
As I've never been trained in the wondrous art of machining I dont have anything set in stone to fall back on.
I'm machining a large brass casting at the moment and keep wearing the tool tip away very fast due I believe to the sand caught in the first layer of brass and then it rubs.
Setting the tip lower definitely helps but I havnt tried as far as 1/8"
The best results so far have been with the sharp edge of a woodwork router bit (carbide) set just below centre line.
From a logical perspective I would have thought that if the tip was above the centre line then it would be easier to catch and dig in than if it were below the centre line where it would push it down further and slide over it.
I did find that on small dia (1/8" and below) it was very easy for it to climb over the tool if set below centre.
 
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