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QCTP Base Question

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q20v

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#1
Hey guys,

I wanted to run something by you to get a second opinion on this. I bought a QCTP recently and the base has green material, as in it's been supplied without any machining done to it.

I don't have a milling machine to mill it to the T-shape required for my compound, but I obviously have the lathe.

My old turret base essentially looks like:



I can easily make the new QCTP base to look like this:



And in the compound:



I can't foresee any issues with the round boss in the base, rather than rectangular. Am I missing something?

Thanks,

Barry
 

q20v

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#3
Thanks for the quick reply, Frank. I appreciate the input.
 

francist

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#4
And as with any T-nut modification, just make sure your bolt or stud, whatever, doesn't protrude past the bottom of your T-nut.

-frank
 

Bob Korves

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#5
Also make sure the cylinder portion does not clamp to the bottom of the tool post when tightened. Make sure you can see light between them.
 

q20v

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#6
Thanks for the reminders, guys. Weather permitting, hopefully by the end of the week I'll have the QCTP ready. I've been looking forward to this day for a long time!
 

Tozguy

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#7
Just a thought but the square style would be more rigid than the round one.
 

JR49

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#8
Just a thought but the square style would be more rigid than the round one.
I respectfully disagree, the bottom of the toolpost should not even touch the top of the round (or rectangular) nut. It should tighten down against the flat top surface of the componnd, just as Bob discribes in post #5. JR49
 

Tozguy

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#9
Hunh?

JR49: My comment refers to the square style T nut shown in the top photo of q20v's post being more rigid than the round one in the lower photo. How does that conflict with what Bob or you wrote?
It has already been written above so it should go without saying again that either T nut style must not bottom against the tool post or it would not clamp to the compound.
As so for what Frank wrote about the bolt not protruding beyond the nut.
Finally, either style will work. But if we are looking for a difference between the two, the square one is more rigid.
 
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q20v

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#10
The Autodesk Fusion software I used to model the two styles does come with an FEA (Finite Element Analysis) package. Tonight after the kids are put to bed I'll apply a load to simulate the tool post being tightened, and measure deflection in the center. I can compare the two designs this way.

Once the tool post handle is tightened and the tool post is clamped between the handle and compound, friction between the mating parts prevents the tool post from sliding / rotating when cutting forces are introduced. Provided the 'round' design doesn't flex to the point that contact is made between the base and tool post when the handle is tightened (ultimately limiting the amount of friction holding everything in place), then I suppose both designs would be okay from a functional standpoint.

Worst case, if the round design does end up flexing a bit more than the square design, I can machine a bit more off the overall thickness to ensure a gap is present between the base / tool post once the handle is tightened. I can also check the stress levels in the FEA results to make sure the base isn't being over stressed. Definitely way way overkill for something like this, but it exercises the brain and is fun.

Thanks again for everyone's input. I'll be certain to share the results of the analysis when I'm done.
 

hman

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#11
I'd be willing to agree (with no evidence other than my own visceral perception [gut feel]) that the rectangular (square) style of T-nut is indeed a bit more rigid than the round. But come on, guys ... we're not talking "farmer tight" here. I've never had to tighten the toolpost nut beyond "good and snug." I seriously doubt that the tool post nut will be the part that flexes when the lathe tool is under load ... especially for the types of lathes many of us have. It's an interesting discussion, but I don't think the OP has anything to worry about.
 

Tozguy

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#12
q20v

Thanks for your positive response. Just to be clear I am not saying that there is a problem with either style of T nut.
However, when I faced the same situation of mounting a new QCTP without the capability of milling the blank, I opted to use the existing round T nut and save the new square blank for posterity. I did not see the point of having two round T nuts.
It did require that a new bolt of the proper dimensions be threaded to fit the old round T nut and that was a fun exercise in itself.

It turned out to be relatively easy to do a bit of milling on the lathe. So I have both style nuts now.

Looking forward to seeing the results of your analysis.
IMG_0118.JPG
 

TomS

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#14
I don't have either style. My setup uses a rectangular piece of steel about 1/2" x 2-1/2" x 3-1/2" with a drilled and tapped hole in the middle for the stud that fits my compound. Been using it for years and the tool post has never moved.

Just saying.

Tom S.
 

q20v

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#15
Hey guys,

I ran two simulations in Fusion representing both base designs.

Objective
The objective was to obtain a comparison between base designs, not precise deflections of a particular setup; any global model / simulation inaccuracies are consistent in both scenarios.
This analysis is strictly limited to the tool post Base to determine how it reacts to the tool post being tightened.

Assumptions:
- The tool post block was not modelled or represented in any way, in other words any added rigidity this may add to the compound is not included.
- Input torque on the tool post handle is 20 ftlbs. I didn't measure this but figure it's a decent ball park. Again, it's the same in both cases so for comparisons sake it can be any value, really.
- Material is steel for both the tool post base and compound block.

Load Case
The load case was identical for both scenarios: an applied load of 8,450 N vertically, or about 1900 lbs of force resulting from a torque of 20 ftlbs (T=kFD). This was applied to the inside cylindrical surface.

Model Constraints
The constraints allowed sliding between base / compound t-slot surfaces, with the bottom of the Compound block being fixed.

Results
Round design
Max deflection = 0.015 mm (0.0006 in)
Von Mises field stress = <40 MPa (5,800 psi)

Note: deflections have been multiplied for visual effect only. Ghost lines show original shape.




Square design
Max deflection = 0.012 mm (0.0004 in)
Von Mises field stress = <35 MPa (5,075 psi)





Interpretation
- Without breaking down the individual stress components, it looks like bending stress in the base in both directions (parallel and perpendicular to the t-slot) is reduced with the Square design. This means less deflection.
- Square base design is stiffer.
- Both are stressed very little (assume AISI 1018, safety factor is around 10 against yield strength).
- There would be no impact on the overall tool post rigidity when cutting forces are introduced with either base design, since the clamping force holding the tool post in place is the same in both cases (this assumes adequate clearance is present between the base and tool post).
- Will either work - yes. Like hman said, nothing to worry about.

Anyway, I think we all knew the answer without me having to do all that, but it gave me an excuse to try the Fusion simulation software if nothing else..

Barry
 

hman

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#17
Barry -

That's indeed slick! And it's a great representation of what I do: I relieve the inner part of the QCTP mount just a bit, so that it only makes contact on the periphery, increasing friction as far out as possible, thereby resisting rotation on the post.
kkHPIM4140.jpg
Wonder if/how the analysis would differ if the QCTP had a perfectly flat bottom and pressed down against the top of the compound, keeping it from bowing upward. My guess would be that most owners of decent lathes have not needed to relieve the bottoms of their QCTP mounts.
 

Tozguy

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#18
Barry, thank you for going to all that trouble for us. That is very enlightening. Much appreciated. If you need more excuses to run stuff through that software I am sure we can help :)

Would you have an opinion or data on which style of T nut would be 'friendlier' to the dovetail on the compound? Compounds are often made of cast iron which flexes differently than steel. For a given torque value on the bolt, which style of T nut would spread the stress on the dovetail more evenly?

hman, do I understand correctly that the recess you cut in the bottom of the tool post leaves part of the lips of the dovetail free to deflect upwards?
 
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hman

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#19
It does. The recess I cut was something like .010" or .015" deep, but that's plenty to allow the deflection that Barry's analysis indicated (.0006" for the round option, .0004" for the square). It's an "unintended consequences" type of trade-off to making the QCTP less subject to rotation. But as I'd mentioned previously, I don't make the retaining nut up "farmer tight". I use a wrench that's cut down to about 6" length.
 

Tozguy

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#20
I see. Thanks. Are you concerned about what cycling through that amount of deflection will do to the cast iron over time?

Your idea kinda has me imagining a cupped or arched T nut that would reduce the concentration of deflection in the area surrounding the bolt.
 
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hman

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#21
I see. Thanks. Are you concerned about what cycling through that amount of deflection will do to the cast iron over time?
Not at all. Even cast iron has some toughness/strength. Think of all the bridges (and myriad other items) made of cast iron over the years. But especially the type of bridges that use the "chain bar" type of construction (ie, lengths of iron bar linked together with pins, into catenary "chains".) The load on a bridge varies a lot, at irregular intervals, under large temperature variations. I've read about a failure or two, but all of them appeared to be due to very unusual conditions or to construction mistakes.
Your idea kinda has me imagining a cupped or arched T nut that would reduce the concentration of deflection in the area surrounding the bolt.
That could indeed address the issue. Just don't see where it would be worth the effort. And if you ever had to mount the QCTP off center (to reach into somewhere?), you might be on part of the cupped surface.
 

epanzella

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#22
I may have missed this in all the NASA type theories flying around here but make sure your threaded post can't screw down thru the T-nut and contact the compound. The threaded post can act like a screw jack and pick the T-nut up braking a lip off the compound.
 

q20v

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#23
Way cool! Thanks for doing that, Barry. Most interesting.
-frank
Frank,

No problem! I enjoy that kind of stuff.

Barry -
That's indeed slick! And it's a great representation of what I do: I relieve the inner part of the QCTP mount just a bit, so that it only makes contact on the periphery, increasing friction as far out as possible, thereby resisting rotation on the post.
Wonder if/how the analysis would differ if the QCTP had a perfectly flat bottom and pressed down against the top of the compound, keeping it from bowing upward. My guess would be that most owners of decent lathes have not needed to relieve the bottoms of their QCTP mounts.
hman,

I see your logic in moving the friction force as far out from the centre of rotation - tool post centre - as possible. This creates a better lever arm to react the tool post torque (a result of cutting forces trying to rotate the tool post).
The thought of including the tool post on top of the compound had crossed my mind, but for analyzing the base designs ("Which is stiffer?") it makes no difference. Another way to look at it, I could have 'fixed' the t-slot (i.e. made it 100% rigid) and the overall deflections would have been less, but there would still be a difference between the two Bases. Maybe I'll give it a go, though...

Barry, thank you for going to all that trouble for us. That is very enlightening. Much appreciated. If you need more excuses to run stuff through that software I am sure we can help :)

Would you have an opinion or data on which style of T nut would be 'friendlier' to the dovetail on the compound? Compounds are often made of cast iron which flexes differently than steel. For a given torque value on the bolt, which style of T nut would spread the stress on the dovetail more evenly?

hman, do I understand correctly that the recess you cut in the bottom of the tool post leaves part of the lips of the dovetail free to deflect upwards?
Tozguy,

Again, not a problem at all. Any other modelling (not that kind) or stress analysis stuff, I'd love to help!

An easy way to evaluate how parts interact is to follow the load path then look at the mating surfaces between the parts. In this case, it's easy to see the Base is being pulled 'up' out of the t-slot. There are two surfaces of the t-slot holding the Base where it should be. The contact stress on these two surfaces can be calculated knowing the applied Force and Area (Stress = Force/Area). So, to keep stress on the surface low, either lower the Force or increase the Area (assuming all other variables are held constant, like t-slot and Base dimensions). If the full area of those two surfaces is used, there's really nothing else to do to improve it. This assumes the base of the t-nut is as thick, long, and wide as it can be in the dovetail, if you know what I mean.

You posted a picture above with two base designs, one square and the other round. The round one would not use the full bearing area of the t-slot and is therefore inferior. That being said, we're not talking about highly stressed parts here, so I really doubt there are any issues whatsoever (similar to my comparison above, yes one is better, but by such a small amount).

And you're correct about cast iron vs plain carbon steel flexing differently, as this is related to the modulus of elasticity which is different between the two. Not only different, but steel has a linear elastic region of the stress-strain curve while cast iron is non-linear and calculating the modulus of elasticity is a bit more involved... to keep it simple, let's say the Tool Post Base is steel and the Compound is aluminum (to make it easier to visualize), you can now appreciate how the Tool Post sitting on top (clamped in place) helps prevent deflection of the less stiff Compound. In terms of optimizing t-nut (Base) shape considering the difference in modulus of elasticities, I would say apply the same logic as the previous paragraph. The stress levels are so low and the difference in stiffnesses is not worth losing sleep over.

I see. Thanks. Are you concerned about what cycling through that amount of deflection will do to the cast iron over time?
Pretty much what hman said. At these loads the amount of deflection and stress is so low.

I may have missed this in all the NASA type theories flying around here but make sure your threaded post can't screw down thru the T-nut and contact the compound. The threaded post can act like a screw jack and pick the T-nut up braking a lip off the compound.
:p it was mentioned above, good point though!

Barry
 

Tozguy

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#24
Barry, thanks for your comments. I enjoyed the exercise out of curiousity even if there may have been no problem to start with.
Ever since epanzella shared his experience with us on making a milling attachment I have been keen on better understanding the forces that are put on parts of the cross slide and compound. Epanzella may have actually saved me from inadvertently wrecking my cross slide. Beats learning the hard way.
 
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BigMo

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#25
Wow thats really awesome. Autodesk fusion you say? And wats a ballpark cost for that software??
 

q20v

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#26
Barry, thanks for your comments. I enjoyed the exercise out of curiousity even if there may have been no problem to start with.
Ever since epanzella shared his experience with us on making a milling attachment I have been keen on better understanding the forces that are put on parts of the cross slide and compound. Epanzella may have actually saved me from inadvertently wrecking my cross slide. Beats learning the hard way.
Your welcome. I'm tempted to model the tool post sitting on the compound to see what the impact is. Maybe in the next few days if I get the time I'll give it a go.

Wow thats really awesome. Autodesk fusion you say? And wats a ballpark cost for that software??
Surprisingly, it's free for hobby users. Once you've downloaded / installed it, the first time you run it it asks you to register. Select hobby user and it's free, easy as that. I just started my second year with it, although I don' t use it too often. The Autodesk site seems to be acting up right now, but search for Autodesk Fusion 360 Free in google and it should come up.
 

Bob Korves

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#27
(snip)Once you've downloaded / installed it, the first time you run it it asks you to register. Select hobby user and it's free, easy as that.(snip)
I registered, but it is only open to students and educators and wants the institution and attendance dates. Can't find anything for hobbyist. Any more ideas?
 

q20v

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#28
I registered, but it is only open to students and educators and wants the institution and attendance dates. Can't find anything for hobbyist. Any more ideas?
Hi Bob,

Interesting... I renewed my subscription over the Christmas holidays if I recall correctly and had no issues. I just found this link online (coincidentally the thread was started by a guy named Barry):

https://forums.autodesk.com/t5/desi...-indefinitely-if-your-a-hobbyist/td-p/6280628

When I get home tonight I'll check my subscription status to see what it's listed as.

Barry
 

brino

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#29
interesting.....I found two other Autodesk Fusion pages linked from that one above.

The "Entitlements" page:
http://www.autodesk.com/company/leg...b-services/autodesk-web-services-entitlements
talks about the categories "Start-up" and "Educational", and within the second "Student", "Faculty" and "Institution".

The "How to activate Start-up, Student or Educational licensing for Fusion 360" page here:
https://knowledge.autodesk.com/supp...-or-educational-licensing-for-Fusion-360.html
seems to lump "Enthusiast" into the "Start-up" category. Their own screen shot of the activation dialog box on that page shows:

upload_2017-3-9_13-1-25.png
That clinches it for me!
-brino
 

Bob Korves

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#30
interesting.....I found two other Autodesk Fusion pages linked from that one above.

The "Entitlements" page:
http://www.autodesk.com/company/leg...b-services/autodesk-web-services-entitlements
talks about the categories "Start-up" and "Educational", and within the second "Student", "Faculty" and "Institution".

The "How to activate Start-up, Student or Educational licensing for Fusion 360" page here:
https://knowledge.autodesk.com/supp...-or-educational-licensing-for-Fusion-360.html
seems to lump "Enthusiast" into the "Start-up" category. Their own screen shot of the activation dialog box on that page shows:

View attachment 228268
That clinches it for me!
-brino
Thanks, Brino and Barry. I will try again later, hope I have not screwed myself forever by starting on 'educational.'
 
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