Rear Mounted tool post question

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BadDog
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Re: Rear Mounted tool post question

Post by BadDog »

Magicniner wrote:
I was in complete agreement with you until I tried one, when I'm making a batch of components and it can save me a tool change on each one ;-)

- Nick
I completely understand that one, and meant that to be part of the "production" exception but looking back didn't make that part clear. Anyway, what I intended to express was my dismay at the frequent explanation and/or expectation that rear/inverted parting is superior on it's own merits. I think the production benefits are unlikely to be justified for a home machinist (even one doing work for pay) vs something like a QC tool post on the front. For something like a 9x20 or one of the small Atlas/Craftsman lathes the potential for offsetting machine deficiencies where the trade-off could be the difference in being able to part or not, I understand that too. But I just don't get all the rear parting tools I see on otherwise capable machines that could part successfully in normal orientation without applying forces to lift the rear of the cross and saddle while trying to part. Sometimes from the same people who talk about needing to remove the rotary base on a milling vise due to rigidity concerns, and in one case I recall from a fellow who also was quite vocal about removing the compound to install a solid block in the quest for more rigidity...
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Re: Rear Mounted tool post question

Post by spro »

It depends on the lathe. Different machines. I'll take whatever you say but can't be all these people. Because when the lathe is run reverse, with a rear mounted tool, its pretty stable. The question is "why" I merely responded to the action before me.
Magicniner
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Re: Rear Mounted tool post question

Post by Magicniner »

BadDog wrote: I think the production benefits are unlikely to be justified for a home machinist (even one doing work for pay) vs something like a QC tool post on the front.
You really should try it, my rear tool post is the same model of quick change tool post as the front set to the same height, on some jobs you can have parting and facing tools at the rear (part length and OD dictate if this is possible) or a boring bar in the rear and a turning tool at the front leaving chuck swapping in the tail stock as the only tool changes required. Add a DRO with tool offsets and the workflow is a joy!

- Nick
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BadDog
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Re: Rear Mounted tool post question

Post by BadDog »

I've considered upgrades along those lines. Specifically a turret that would replace the compound. I saw where someone adapted a Hardinge 8 position turret that looked like it would be fantastic. But then reality set in and I considered that I already have quick change with lots of blocks, and I single point threads often enough that giving up my compound or frequently swapping it would not be a good thing. My lathe is big enough that I think it could be adapted on top of the compound, but even at that I just don't see it paying off even in the non-realistic payback model of the home shop. In the end, the VAST majority of what I make is one off, and I tend to use a fair number of different tools, so swapping a quick change block on the single tool post is likely about as good as it gets for me.

But again, it's not the production improvements of adding immediately accessible tooling that I object to. It's the often claimed superiority of the rear tool post. I've heard some passionate support for rear parting setups, discussions that involved moment arms and how in the end the forces trying to lift the stack don't come close to actually lifting the stack. But then there is all the discussion about needing to remove the vise rotary base, and how "everything flexes", and often extreme measures taken to increase rigidity. But somehow the opposite of ideal rear parting forces don't seem to matter? <shrug/> Maybe they don't, but I use a rotary base on my milling vise without noted issue, and I don't feel the urge to test the reported superiority of rear parting because my lathe parts without issue now...
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Re: Rear Mounted tool post question

Post by Magicniner »

BadDog wrote:I've heard some passionate support for rear parting setups
I think guys with wobbly lathes believe rear parting tools help.
I believe more tools help ;-)
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BadDog
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Re: Rear Mounted tool post question

Post by BadDog »

You'll get no argument from me there!
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Re: Rear Mounted tool post question

Post by Magicniner »

I've finally started to print and affix labels for all of my QC tool holders so that they can be associated with a specific zero in my DRO, in doing this I've realised that most tools for use with their shank at 90 degrees to the work (for turning) in the front QCTP can be dropped on the rear QCTP parallel to the work (for facing) and vice-versa, provided you can run in reverse a rear QCTP can effectively double up your tooling, which is nice ;-)

- Nick
pete
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Re: Rear Mounted tool post question

Post by pete »

Yep I sure agree,it can be an even better modification if the lathe has a DRO and the ability to store multiple tools in it's memory as Nick pointed out. Yes it's used a lot on lighter lathes for the reasons mentioned as well. When I was looking for my last lathe a tee slotted cross slide wasn't optional imo. Between centers boring and a rear mounted tool post were the two reasons. Mounting a cut off tool to a large solid block fixed directly to the cross slide is a lot more rigid. Put a little over hang on the top slide, put an indicator tip on it then take a cut. I think most would be surprised at how much flex a top side has at the tool tip unless it's on a massive lathe. G.H. Thomas was a professional machinist an engineer as well I think and wrote quite a bit about why a rear mounted cut off tool works better with light lathes and he sure knew far more than I ever will. On heavier lathes then it still speeds up the machining a bit. Better for multiple parts, but there is still some time saving and that better rigidity for one offs. When I look at how rigid the new CNC's are and see mentions about how much better carbide works with that rigidity I have to think anything were likely to be using other than maybe a DS&G, HLV, or Monarch EE then most others can use all the help they can get. They don't go through all the hassel of using holders that are heat shrunk onto the tool shanks for no good reason. It's better accuracy and much more rigidity. But I've been wrong before though and will be again.

I dunno, if the OP has a mill and he wants this bad enough I'd seriously consider building a new top half of the cross slide out of a chunk of Durabar, some surface grinding and/or hand scraping to get things true to the lower half and not have to modify what the OEM put on it. If he ever sold it an unmodified cross slide will bring more money. Having a seperate top half already slotted maybe even a bit more. There's at least one casting kit to do exactly that for some of the South Bends. Worst case lets say there's an unknown casting flaw in what he already has. Adding those threaded holes might be just enough to have a bad day with a crash or really heavy cut. That's something we all know can't be checked for with the usual home shop equipment. My personal opinion is that all lathes should come with tee slotted cross slides. But it does weaken them a bit unless it's been designed in from the start.
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