What is the ideal shape of the moving jaw bar? Round - rectangular or square?

I do prefer the art deco look of a round tube to guide the moving jaw.
In practice what are the advantages or disadvantages of a round tube compared to a rectangular tube?
Ease of manufacture is clear with a round tubular bar & the sliding fit easily controlled with standard workshop equipment. One size to measure & no corner squareness geometry to contend with either.There are no corners where cracks can propagate.
The smooth 100% engagement around the round tube perimeter gives additional vertical support on the sides when subjected to a downward load. Whereas a rectangular bar offers no support from its disengaged vertical faces.

The option to swivel both jaws 360 degrees is easily accomplished with a round tube & looks appealing. However perhaps this feature is overselling the idea as it will rarely be used at varying angles. Maybe restricting the swivel to increments of 90 degrees will satisfy the need, especially with bespoke jaws on the opposite side.
A rectangular section is often used to give additional stiffness without any extra material, accomplished by thinner section vertical sides. In this application, perhaps a square tube with a round bore will achieve the required stiffness. The moving jaw and fixed jaw could be quickly removed & re-inserted in four 90 degree positions.

Or. A round tube with both jaws keyed to it with a square location spigot protruding behind the far jaw that locates in a square recess in the jaw body. A screw would hold the far jaw in the square recess with the square spigot taking any forces. To swivel the whole moving assembly including the far jaw the screw is released. Then the far jaw is pulled out a few millimetres to disengage from the recess, then the whole jaw assembly swivelled to one of the 4 positions – pushed back & clamped. If eight 45 degree positions are desirable, then an 8 pointed star shaped female recess could be machined in the body.

My first thought is that rectangular is the best shape because the vast majority of the stress it sees is in one direction created by the force of the jaws clamping together. It will see loads in other directions, but very few will be anywhere near as great as what the jaws produce.

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I wonder if keeping the mass and cross sectional area in all 3 shapes will still be within the specification to resist downward bending.
For example using these assumptions: To clear the nut, a hole 2" in diameter would be ok.

The rectangular section looks to be 1"x3" on the top and bottom & 1/2"x3" on the sides, making the area 9"sq. With a flat support base of 3"

For a square section with a 2" diameter hole - 3.5"x3.5" would be about 9"sq with a flat support base of 3.5".

For a round tube 4" diameter with a 2" diameter hole would be about 9.5"sq. with a semicircular support base of 6".

Except it’s not just cross sectional thickness that determines strength, there’s also the distance from the stress plane, and orientation.

It’s why Cannondale tried gigantic but thin aluminum tubes in the 90’s, and I beams are taller than they are wide.

Round is great if expecting loads from potentially any direction, and for pivoting, but not great if the loads are predictable and can be engineered for.

I used similar areas as a rough comparison to determine equivalent sizes for different shapes of the same mass. The point was to see if the alternatives still fell within the original designed stress limits. Bearing in mind this section is not the weakest link in the chain. The nut failed in the test & I would assume it would still fail well before either of the alternative round or square options.

Would a round tube be less prone to distortion when subjected to twisting loads compared to a rectangular section?
Would it stand up better than a rectangular tube if subjected to side loads from clamping a part in one side of the vise?
It would be great if someone could test the deflection using FEA of all 3 sections and post the results here.

A round tube only has a diameter, which means if you want it taller it also needs to be wider.

If a round tube distorts even a little, it’s not going to fit back in.

I get the geometry of a circle. If I wanted more strength I would use the next size up tube. That is easier than making another casting pattern. Having slightly more width is not a major issue, if at all.
The difference in area between a 4" tube and the fail area on the nut is huge. This massive difference in strength always allows the nut to fail before any damage to the tube.

Only if you’re talking about breaking from the clamping force. Side loading the jaws has no effect on the nuts likelyhood of shearing, and where the stronger horizontal plane of a box section comes into its own.

Edited because I wasn’t very clear with my intention here. I was trying to imply a square or rectangular box section would have a stronger resistance to breaking if side loading the jaws when it comes to the jaw to box section connection and resisting deflection in the moving section

I assumed side loading would be from a part clamped in one side of the vise with the force through the nut.

The side loading from that force is more like trying to twist the moving jaw off the box section (or cylinder in your design case) and wouldn’t really cause any additional load to the nut compared to clamping in the center. The box section or cylinder fit to the “ways” of the vise limits the deflection to a small enough amount that the nut isn’t providing any support against side loading, or under any real stress from it.

I can only see side loading from having a piece in one side of the vise. Any forces holding to the part have to be delivered via the nut which puts it under tension until it fails.
Is there another case I should consider where significant side loading occurs without the nut being subject to any stress?
My point is that whatever size & shape the box section is, it is still far stronger than the head of the nut. So damage to the box section is always limited to far below its capabilities.

I can’t think of any scenario that would sideload the nut short of clamping something off center with the vise past max extension so the box section isn’t fully supported and can deflect a lot more than it ever should. Otherwise all loads the nut will see in use will only be tensile loads.

The chances of the nut face shearing off would be reduced if you use a larger thread diameter and nut, but that also reduces leverage and would require a longer handle to achieve the same clamping force by hand.

Another downside would be that you may move the vises failure mode to components less easily replaced. If Fireball goes out of business tomorrow, you could have someone machine you a new nut and screw assembly if you grenaded yours with a cheater bar, but if you crack the casting that may be a lot harder problem to solve.

Sometimes you design the weak link into the system on purpose to improve repairability if something does break. Sometimes they’re features, not flaws.

I agree. I did not intend to suggest the nut was subjected to side loads, always just tensile load. My point regarding the nut head shearing was intended to suggest that this is ideal, as it protects the box section and the body whatever shape it is. I guess a suitable round tube would perform better in a fully supported round hole with no corners to propogate cracks from.

Ahh, but there’s going to be a corner where the moving jaw connects to the tube, so that’ll still be the most likely failure mode in side loading.

True but the shear nut is designed to protect the far beefier tube well before enough force is applied to damage the tube. I won’t attempt guess how many tons will be needed to permanently bend a 4" diameter tube with a 1" wall thickness cantilevered out 14". I bet it is substantially more than the force required to shear the nut head. A round tube will not be cast iron, so will have better resistance to cracking. This situation will never arrive as the shear nut will always break well before then.

Just curious, have you looked at the cost of a 4" OD tube with a 1" wall? I was kind of shocked what they get per foot when I just looked it up.

A piece large enough for the hardtail vise is $500+ on Speedy Metals, and even though they’re a bit more expensive than bulk ordering, it’s still got the potential to be crazy expensive.

I just wanted to throw out that “ideal” is a loaded term when it comes to a design like this. By changing the moving jaw bar shape you may optimize it for another function, but you WILL decrease its overall strength or increase its size if you go to a shape other than rectangular.

You can try to minimize the effect, but until someone proves me wrong there is no stronger shape for the sliding feature of the hardtail as it’s currently designed (fixed plane sliding jaw with swivel base) than rectangular for a given size and amount of material used. You need the vertical axis taller than the horizontal axis is wide, period, because the deflection from the clamping force of the jaws is by far the greatest source of load the vise will ever see unless abused. If it’s too strong for the nut and that bothers you, make it smaller and get more clearance for parts clamped in the jaws, or make the nut larger if you just want the vise to be able to clamp parts even more firmly.

Improving designs like this make you really think about the angle and types of forces involved in something.

If you want to change the shape of the moving jar bar to create additional functionality, that’s fine, but you will be creating other compromises in the process, and you have to decide if they’re worth the tradeoff.

I think I need to put some numbers to this to make it real to you how much stronger a rectangle is. I had to dig out my machinist handbook for this one, so I hope it was worth the time.

Section modulus of a few different structural sections I calculated out:

4" round with 1" wall : 11.78097
4x3x.75 rectangle: 14.04688
4x4x.75: 18.078
5x3x.75 rectangle: 25.8906

Ready to have your mind blown? Every section is lighter than the 4" tube despite being a little to a lot stiffer, even the 5x3. (not quite true, the 5x3 is very slightly heavier)

4" round cross section: 11.78in² (correct number is actually 9.42in²)
5x3x.75: 9.75in²

Doesn’t matter if the materials are steel or ductile cast iron, the yield and tensile strengths are similar enough to be inconsequential.

these numbers represent the square and rectangle cross sections as having sharp corners. They would lose a small fraction of their strength from the minor radii or chamfer involved in a cast section external corner, and a minimal amount of weight gain from any internal chamfer or radii. Section modulus calculations are for the stronger bending plane, if you want I can calculate the section modulus for the minor axis too, but I hope the point is clear that even a 3" width on the rectangular section is going to provide a lot of strength because it’s in line with the major force planes on that axis

That’s great, thanks for chewing over the numbers. As I don’t have a vise here, I tried to scale the sizes from the website images. I guessed the rectangle was 1/2" at the sides & 1" top + bottom giving 9"sq. To match that I calculated the area of 4" x 1" wall tube to be 9.4"sq whereas your figure is 11.78"sq so maybe the round tube is not the heaviest section?
I suppose that providing the section is sufficiently strong enough to sustain more than the maximum load the nut head can take, then any section will be ok. Knowing the tested failure load I guess the minimum size sections could be worked back from that. Looking at the difference in size between the jaw section and the nut head it seems the rectangular section currently used is far larger than it could be? Now they would be interesting figures.

Although I prefer the look of the round tube, the pivoting bracket as you suggested in the other thread, negates the need for the jaws to swivel in the body.

My posts try to concentrate on my ideal vise using the hardtail dimensions with most of its parts modified that I see could be better. Designs evolve & some of my suggestions could be considered in a MK2. I still like the idea of pulling the rear jaw a few mm out of a square register rotating it in 90 degree steps & popping it back in again using a round tube. No need for a separate base. Unfortunately that is a completely new design.

A need that has been mentioned is the need to swivel in another axis & your bracket is almost there. It is easy to make, robust, small & a simple retrofit at an inexpensive price. If you already own a vise then you could probably make your own. I hope the alternative clamps in in the other thread help with the design.

Thank for catching that error, you’re right, I used 1/2" radius for the inside of the tube cross section, not 1". I think I had 3" on the brain because of the 4x3 and 5x3 calculations.

Pi x R² - pi x r² = area of tube cross section

Pi x 2² - pi x 1²= 12.566-3.14 = 9.42 in²

You’re correct that the 4x1 round tube is marginally lighter than the 5x3x.75, but the 5x3 is more than twice as stiff, so the rest of the rectangle VS round sliding section is still valid.

I agree, the pivoting brace bracket added to the pivot design in the other thread will definitely help lock the angle in better, I just need to cut some parts and see how big they’d need to be to really lock it down. 3/8" bolts are what I have drawn in that design, and what I’d start with for the side angle brackets for the 5" forged steel vise. The hardtail vise might require 3/4" bolts and brackets, and possibly separate hole stops at specific angles instead of a slot to hold it firmly enough.

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