Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

[Chipper5783]

People overtightening fasteners drives me nuts (pun intended). The point made well by others is that threaded fasteners hold - they really don't need to be refe'd down hard. I have made T-bolts for most of my vices - then it is easy to relieve the center portion of the head near the bolt shaft. I have also made T-nuts for each of my machines that: A. fit nicely, which distributes the load better (the standard T-nuts are very loosey goosey, every table is a bit different) and B. I make the web portion as tall as possible so there is some decent thread depth (when using standard bolts to secure that special component it is handy to have several nuts of different sizes and as long a female thread as possible so you don't have to cut the bolt to just the right length). Of course, the nut is not threaded quite through, so as to prevent "jacking" the table slots and breaking out a section. The taller web and better fitting nut/bolt will mitigate some of the coning effect that Richard is talking about. The nuts I have been making are about 50% longer than the standard nuts in a clamp kit. I can agree that 3" long would be even better, but even 50% longer is still a significant reduction in point loading.

Another of my huge pet peaves is when someone just grinds down a hex head bolt, or a carriage bolt and claims that is fine for a T-slot .

Shown here is a selection of table nuts and bolts from 6 of my various T-slot tables. Five of them nominally would use the regular 1/2” clamp kit - they are all a bit different (does not seem to be any real standard), not a big deal when you simply make your own.

 

[Richard King 2]

Tonight I wrote my friend and retired MIT Engineering Professor Alex Slocum to comment. He has several papers and books on Machine Tool Design. When he replies I will cut and paste it here.

I have seen some Bridgeport table with very little bend and I suppose that owner didn't use a pipe cheater to tighten the nuts. Those tables sometimes had broken T-Slots. Hopefully Professor Slocum will have some info so we can all learn a bit more.

 

[Richard King 2]

He answered:

From: Alex Slocum _ Retired MIT Engineering Professor

Hi Richard

if the nut is longer it will spread the stress and deform things less locally.

I think it also depends why the T-nut is pulling up against

thread is 1/2-13
issue is whats the strength of the iron?
https://www.pentictonfoundry.com/news/differences-between-ductile-iron-and-gray-iron-castings-ductile-vs-gray-iron/#:~:text=There are many grades of,is 20,000 psi – 60,000 psi

The Differences Between Ductile Iron and Gray Iron Castings | Ductile vs Gray Iron. Ductile iron and gray iron are two different kinds of cast iron with specific applications. Read this post today to understand the differences. www.pentictonfoundry.com

.

but it certainly seems likely that the material could be yielded (compressed) so as to force the top of the table to go convex--here is the analysis:



Threads per inch (1/2-13)​	13​
Lever arm (in)​	12​
force (lb)​	50​
efficiency thread​	0.5​
force generated (lbf)​	24,504​
T nut effective width​	1​
T nut length​	1​	3​
area​	1​	3​
stress (psi)​	24,504​	8,168​
Grey Cast iron yield strength (psi)​
low​	20,000​
high​	60,000​


RK

 

[Richard King 2]

MECHE PEOPLE: Alexander Slocum | MIT Department of Mechanical Engineering

MIT's Department of Mechanical Engineering (MechE) offers a world-class education that combines thorough analysis with hands-on discovery. One of the original six courses offered when MIT was founded in 1865, MechE's faculty and students conduct research that pushes boundaries and provides...

meche.mit.edu

 

[Richard King 2]

Some more info he wrote this morning.
Hi Richard
It will depend on thread lubrication and how hard one yanks on the wrench--to summarize (and fwd)

1). what is the force on the T nut?

Boltforce.xls
To determine force from a bolt
By Alex Slocum, last modified 2023.03.12 by Alex Slocum
Be consistent in units (lbf in or N mm)
Enter numbers in BOLD, output in RED
Units:	lb in
Force on end of wrench	50​
Length of wrench​	12​
Torque​	600​
Bolt diameter​	0.50​
Pitch (Lead)​	0.0769​
Coefficient of friction, mu​	0.2​
Screw pitch diameter, dscrew (mm)​	0.42​
Thrust bearing diameter, dthrust (mm)​	0.56​
Thread angle (deg), alpha (rad)​	30​	0.524​
Thread root stress concentration, scf​	1.5​
Beta​	0.182​
Thrust bearing efficiency, etathrust​	52%​
Backdriveable?​	NO
Thread efficiency, et​	20%
Total system efficiency, eta​	10%
Force, thrust​	5,077​
Estimated torsional stress, tau​	40,352​
Tensile stress, sig​	44,588​
Mises equivalent stress, sigma​	124,355​

if well lubed threads... force might be 2x, and maybe the person yanks for an impulse of force because wants to make sure no slip of the vice being bolted down:

Threads per inch (1/2-13)​	13​
Lever arm (in)​	12​
force (lb)​	100​
efficiency bolt thread and head​	20.0%​
force generated (lbf)​	19,604​
T nut effective width​	1​
T nut length​	1​	3​
area​	1​	3​
stress (psi)​	19,604​	6,535​
Grey Cast iron yield strength (psi)​
low​	20,000​
high​	60,000​

this compressive force squeeze the cast iron which causes it to bulge out (Poisson affect) thereby "expanding" the cast iron around the T slot... so a number of bolts doing this in a number of different places... voila!

 

[gard]

Very nice analysis.
Any chance he would be willing to share his spreadsheet?
I think this analysis suggests a longer T nut would pretty much eliminate the problem of bowing of the table. This assumes a uniform pressure distribution along a 3 inch T nut and enough effective surface area between the vise and table. I am not sure how beefy the T nut would need to be to accomplish this. Perhaps one long T nut with 2 holes the same spacing as the vise?

My go to for a quick look at bolts is the Holo-Krome catalog for socket head cap screws, the table lists induced tension of 15,760 lb at 1800 in-
lb, for 1/2-13 so not too far off from the above calculation.

 

[Richard King 2]

Very nice analysis.
Any chance he would be willing to share his spreadsheet?
I think this analysis suggests a longer T nut would pretty much eliminate the problem of bowing of the table. This assumes a uniform pressure distribution along a 3 inch T nut and enough effective surface area between the vise and table. I am not sure how beefy the T nut would need to be to accomplish this. Perhaps one long T nut with 2 holes the same spacing as the vise?

My go to for a quick look at bolts is the Holo-Krome catalog for socket head cap screws, the table lists induced tension of 15,760 lb at 1800 in-
lb, for 1/2-13 so not too far off from the above calculation.

If you email me, I can forward his email to me. The spread sheet is on excel I think. Richard@handscraping.com

 

[Richard King 2]

The way I met Alex is when the SME asked me to be part of a Seminar during an IMTS show in Chicago 20 years ago or so. They asked me to give a 20 minute talk on scraping. Other people who talked was Drew Devitt of Moglice, A Tech from Fanuc, Leadscrews International, SKF and a few others. There were about 100 people in the audience . After the Seminar people were coming up to us in the front of the hall and He can up to me and introduced himself. Since then I have asked him a few times on things other engineers have asked me. One of his assistant professors attended a class I had in Boston too. I've helped him locate some new machines that MIT needed .

 

[Provincial]

Some more info he wrote this morning.
Hi Richard
It will depend on thread lubrication and how hard one yanks on the wrench--to summarize (and fwd)

1). what is the force on the T nut?

Boltforce.xls
To determine force from a bolt
By Alex Slocum, last modified 2023.03.12 by Alex Slocum
Be consistent in units (lbf in or N mm)
Enter numbers in BOLD, output in RED
Units:	lb in
Force on end of wrench	50​
Length of wrench​	12​
Torque​	600​
Bolt diameter​	0.50​
Pitch (Lead)​	0.0769​
Coefficient of friction, mu​	0.2​
Screw pitch diameter, dscrew (mm)​	0.42​
Thrust bearing diameter, dthrust (mm)​	0.56​
Thread angle (deg), alpha (rad)​	30​	0.524​
Thread root stress concentration, scf​	1.5​
Beta​	0.182​
Thrust bearing efficiency, etathrust​	52%​
Backdriveable?​	NO
Thread efficiency, et​	20%
Total system efficiency, eta​	10%
Force, thrust​	5,077​
Estimated torsional stress, tau​	40,352​
Tensile stress, sig​	44,588​
Mises equivalent stress, sigma​	124,355​

if well lubed threads... force might be 2x, and maybe the person yanks for an impulse of force because wants to make sure no slip of the vice being bolted down:

Threads per inch (1/2-13)​	13​
Lever arm (in)​	12​
force (lb)​	100​
efficiency bolt thread and head​	20.0%​
force generated (lbf)​	19,604​
T nut effective width​	1​
T nut length​	1​	3​
area​	1​	3​
stress (psi)​	19,604​	6,535​
Grey Cast iron yield strength (psi)​
low​	20,000​
high​	60,000​

this compressive force squeeze the cast iron which causes it to bulge out (Poisson affect) thereby "expanding" the cast iron around the T slot... so a number of bolts doing this in a number of different places... voila!

Richard, thanks for your efforts!

One thing I like about this input is that it gives hope that the bending is due to abuse, rather than design limitations. The discussion of torque and length of T-nuts points to ways of avoiding the issue. One solution is design of the T-nuts and the other is establishing a torque specification. The third option is more fasteners spread over a larger area while limiting torque.

Someone with a junk Bridgeport table could provide a sample for compressive strength testing. That could narrow the Yield Strength input for the spreadsheet.

These sorts of problems are now being attacked by volunteers (often with little training, resources, or equipment) by pooling information and working out ways to pin down the problem. It gets much easier when someone with training, experience, and smarts helps out!

Kudos to Dr. (I assume) Slocum!!! Helping people like us when retired is a noble task, and well-appreciated.

Richard, I would appreciate it if you would pass my thanks on to Dr. Slocum.

 

[jwmelvin]

[mention]Richard King 2 [/mention] Tell Prof. Slocum that Jason Melvin questions whether tripling the T-nut length will cut the contact pressure to one third, due to elastic deformation in the T-nut. That’s the only point in question here.