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Compound question
- Thread starter Wheat.Millington
- Start date
Is the secret for quick swap time a very accurate (identical) height of the plinth and the compound so all the tools fit the QCTP without any height adjustments? Is there anything else?
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- Dec 26, 2015
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I wouldn’t call it a secret so much as a fact - that if you want to exchange a solid tool post mount with the compound, you will want the top surfaces of both to precisely agree. If they do not, you will be adjusting tool heights when you swap one for another.
I refer to the following post.
1340GT/1440GT Solid Tool Post
This post will be part five of the thread that I started here: https://www.hobby-machinist.com/threads/1440gt-upgrades-and-additions.96854/ When I ordered my 1340GT back in March of '21 I knew that I wanted to incorporate a solid tool post. And in June when I acquired my RF-45 mill it became...
www.hobby-machinist.com
Having implemented solid tool post platforms (based on a reinterpretation of the prototypes by Robin Renzeitti and Stefan Gotteswinter), I can confirm that on a typical mid-sized lathe (under 1000 pound), the solid tool post provides considerable rigidity improvements. The degree of rigidity improvement can be substantial. It all depends on the specific lathe and how far cranked-out you typically position the compound.
On most medium-sized lathes, the compound is the least rigid structure of the machine, and the more the compound is cantilevered outward, the more flexible and spongy the tool mount becomes. On my PM-1340GT for instance, using the same cutting tool and observing the deflection with an indicator, the compound will flex downward in different amounts depending on how far the compound is extended outward.
With the compound pulled in, and the tool post directly over the center of the compound angular rotation, I measure the deflection of a 2.5mm wide parting tool to be ~0.002”. If the same tool is cranked out via the compound to a cantilevered position 1.500” off the center of rotation, the downward deflection approximates 0.010”. These measurements are based on having blued and precisely filleted the compound gib and engaging the compound locking screw and parting 1018 steel. That’s enough deflection to cause parting operations to fail spectacularly in a difficult material like 304 stainless.
A solid tool post, properly fitted, can decrease the downward deflection to under 0.0005”. Indeed, the solid tool post on my 1340 has been totally liberating in my pursuit of “parting nirvana.” In practice, I have found this increased rigidity eliminates the need to drive threading tools into the material at 30-degree offsets, even with difficult materials - instead, I plunge straight into the material being threaded. And parting failures are extremely rare.
I do swap out the solid tool post for the compound, but in the past 4 years, only for cutting taper profiles. 95% of the time I have the solid tool post on my machine.
I have posted details, along with dimensioned drawings for a BXA-sized solid tool post mount for the PM1340 and 1440 lathe using either Aloris or Dorian QCTPs. The drawings are explicit that the solid tool post height agree precisely to the compound height to ensure tool-height consistency between the two platforms. The details are posted here. Dimensioned drawings in PDF format are here.
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- Nov 25, 2015
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The solid plinth assumes you use carbide / insert holders with their geometry already facing correctly in the holder, if you are using HSS this is of less value since you will be swinging the head a lot. Am I correct?
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- Sep 2, 2021
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In watching Robin's video, I was impressed with how clearly he presented. It held my attention, which is not trivial.
I am curious about something I noticed. He was very careful to show how he ground the back and the front of his tool holder to be certain of remounting parallelism. He does not dress the faces of his tool post similarly which raised another question. If he doesn't trust the mfr of the holder, why does he trust the mfr of the post? Then I remembered why the dovetail is there. On my Yuasa, a known good provider of QCTP, the lock is a broad piston that pushes the tool holder against the wings of the dovetail, away from the tool post. This negates having the back of the tool holder ground because that surface is pushed away from the post. Does his QCTP work differently, ie expand the tail pulling the wings against the TP? Even then, does the face of the tail engage the tool holder cavity face before the back of the wings do? Being a nube, I've not given this much thought because my setup was always more solid than I pushed it. For the tolerances in my hobby work, the repeatability was generally adequate.
It would seem to me that repeatability comes largely from the individual fettling (woodworking term for tramming?) of each tool in it's corresponding holder. Each holder could have a unique position when secured to the post, but it doesn't matter if it is unique because that same holder will return to the same position each time it's mounted and locked.
Another note of curiosity. He doesn't seem concerned in the least about deflection of the comparatively flimsy tool holder compared to the massive block which he was careful to point out distorted under the pressures of holding the QCTP. What am I missing?
DanK
I am curious about something I noticed. He was very careful to show how he ground the back and the front of his tool holder to be certain of remounting parallelism. He does not dress the faces of his tool post similarly which raised another question. If he doesn't trust the mfr of the holder, why does he trust the mfr of the post? Then I remembered why the dovetail is there. On my Yuasa, a known good provider of QCTP, the lock is a broad piston that pushes the tool holder against the wings of the dovetail, away from the tool post. This negates having the back of the tool holder ground because that surface is pushed away from the post. Does his QCTP work differently, ie expand the tail pulling the wings against the TP? Even then, does the face of the tail engage the tool holder cavity face before the back of the wings do? Being a nube, I've not given this much thought because my setup was always more solid than I pushed it. For the tolerances in my hobby work, the repeatability was generally adequate.
It would seem to me that repeatability comes largely from the individual fettling (woodworking term for tramming?) of each tool in it's corresponding holder. Each holder could have a unique position when secured to the post, but it doesn't matter if it is unique because that same holder will return to the same position each time it's mounted and locked.
Another note of curiosity. He doesn't seem concerned in the least about deflection of the comparatively flimsy tool holder compared to the massive block which he was careful to point out distorted under the pressures of holding the QCTP. What am I missing?
DanK
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- Nov 25, 2015
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- 8,273
yes, the wedge type pulls to the holder, not away. I haven't viewed that video in a few years. I didn't remember him doing that.
For him necessary, for us... well I'm a hobbiest... it's above my pay grade.
Why do you think the tool holder is distorting. I am sure it is, but how much. The Aloris holders we have had discussions about before, and seem to be hardened. So lets assume they have less distortion than something a little lower on the hardness scale.
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- Aug 22, 2012
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Heres how I made my plinth
www.hobby-machinist.com
I thread at 90' and never found a problem.
I use predominately HSS tool bits.
The compound is only replaced when I want a small taper or to cut multi-start threads.
Plinth For Qctp On 9 X 20 Lathe
In my never ceasing search for more rigidity in my lathe I decided to make a plinth for the qctp. In its normal configuration I can often see the complete top slide assembly flex slightly. (I think I punish my lathe a bit) After seeing Bill G's plinth I felt it was time to go. I forgot to take...
www.hobby-machinist.com
I use predominately HSS tool bits.
The compound is only replaced when I want a small taper or to cut multi-start threads.
I'm guessing Renzetti would say one needs the ability to change tooling in a consistent and repeatable manner and, after the final measurement, everything is rubber.
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- Mar 31, 2015
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Yes, heights are critical. If the plinth and the compound have different heights you would have to adjust the tool heights when switched. For extra speed I keep a set of bolts and t-nuts installed on the plinth. Mine uses 4 t-nuts and slides on the the cross slide and slides on quickly.
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I made a solid plinth/riser for my 7x12 and it made a huge difference in its rigidity. After machining it, I scraped the mating surfaces. Now parting off, typically the most challenging task for these small lathes, is drama-free. Even when the gib screws are tightened down, the compound's small footprint just can't provide the same stability as the plinth. It really doesn't take very long to remove the compound and install the plinth
Prior to making the riser, I had added a DRO to the Z axis of my lathe. It has turned out to be extremely handy when the riser is installed. But just like many others I won't get rid of my compound slide -- it gives me capabilities I just don't want to give up.
Prior to making the riser, I had added a DRO to the Z axis of my lathe. It has turned out to be extremely handy when the riser is installed. But just like many others I won't get rid of my compound slide -- it gives me capabilities I just don't want to give up.