...and this is exactly why I need a QCTP instead of having to apply shims under the tool holder...
- Thread starter Beanwood
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Hmm - I need some help understanding this one please... if it's removing the centre nib, surely that's the height it needs to be - otherwise it would cut over the centre height and leave a nib. I thought for the outer (Faster and heavier?) cuts, it could run slightly low.Then raise it a tiny amount to take into account the far heavier load, compared to near zero cutting speed at the centre nib, imposed on the tool tip when actually cutting full width and at possibly higher surface cutting speed and with extra stick-out.
not done it yet
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Oh, so your lathe is perfectly rigid?
No ? But if it cuts clean to the centre why do I need to raise the tool?
RobCox
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Because during parting the workpiece exerts a large downward pressure on the tool and will force it down slightly. If it goes below centre height it will reach a point in the cut where it could get sucked under and then, depending on the power of your lathe, either the motor will stall (thinking minilathe here) or there will be a loud bang (my M300) and you'll be disappointed.
Agroshield
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With the test, or nib cut, the tool is under no significant load, so its end is only being acted upon by gravity.
When you are using it in anger, it will deflect downwards due to cutting load. The initial slight overheight is a rational way to account for this deflection and a logical way to attempt to counteract it. Set it a bit high and when it bends, it will be correct.
Kind of a parallel example is when you scrape in a lathe tailstock. You would aim for the tailstock to be a little high, at the plus end of the tolerance, when you have finished. Then, over the first ten years of use, it wears downwards until it is in the middle of the tolerance. Then after another ten years, it will have worn lower still until it is at the minus end of the tolerance, at which time it needs refurbishing.
On steel buildings, they pre-camber the beams so that when first installed, they curve upwards. Then when you add the metal decking, the concrete, the raised flooring, desks and computers they end up flat rather than bowed downwards.
Since I have switched to a 1/8" HSS part off tool I haven't had a single blade exploding issue. Amazingly I have had it jam and stall the lathe without braking. I was slightly amazed (13" leblond). Used to have blade shattering events a lot for me with thin blades on a smaller lath.
Stu
Morris
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How much down force is being applied as the centre point is approached? Very little would be my (well informed) guess.
RobCox
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What happens at the centre isn't the problem. It's part way in, when the diameter is large enough to exert a big enough downward force and the curvature is small enough for the tool to dive downwards. By the time you get close to the centre your troubles are behind you!
Morris
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if I'm understanding the logic here, allow for the downforce at the start of the cut to enable it to cut on CL. So by the time the centre nib is reached its cutting above the CL. Or am I missing something? Not trying to argue, I just can't see how you can satisfy both conditions.
not done it yet
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That is why I wrote “Then raise it a tiny amount”.
The actual amount will depend on several factors, depending on the size of the lathe and item being parted off and the quality of the lathe (cheap chinese or a tool-room lathe in perfect condition.
My post was a real-world reply. I have an old lathe (over 50 years old). I know that the rear tool post, fitted to the cross slide, parts off far easier than when poking out from the top slide and with a chinese QCTP.
That's possibly why it's worked OK for me so far - my precision fitting probably has it cuting EVER so slightly high - perhaps more by luck than skill - but I'll take the win, and ensure it stays that way going forwards.
