[keen]

Hi all. Another video on stand and machine stresses, accuracy and three foot design.

Cheers .Keen

https://www.youtube.com/watch?v=z_Cz3DSwA1E

[SilverWings]

Perhaps there's a good reason milling machines have rigid support under them.

Have you considered or analyzed what will happen to the 3 leg base with applied transverse loading due to X-axis table movement? The center mounted single foot will act as a pivot point and will allow the machine/base to flex with asymmetric lateral loads.

[MichaelHenry]

In the video, Keen mentions that he's got an additional two feet in the back that are about 1 mm off the floor as a safety in case the mill tips for some reason.

[keen]

Perhaps there's a good reason milling machines have rigid support under them.

Have you considered or analyzed what will happen to the 3 leg base with applied transverse loading due to X-axis table movement? The center mounted single foot will act as a pivot point and will allow the machine/base to flex with asymmetric lateral loads.

Hi . Good to hear you are thinking deeply about this. To reply I need to go into this in more depth. First of all my video is about movable machine tool stand designs. A really solid stand is grouted into the concrete and just always stays exactly there, and so the concrete floor is part of the rigid structure of the machine.

With movable machines the floor is more of a hindrance than a help, because moving the stand slightly changes the stresses in the stand and machine. Most movable machines have flexible contact with the floor to reduce vibrations. I believe the Tormach feet have urethane inserts underneath. This means the machine is sitting on stiff springs. I am currently using plywood, but it does the same kind of job.

OK so with that in mind what happens when a machine with 3 or 4 feet has the X table travel to extremes with a heavy 4th axis weight down one end... is that the whole machine leans over to the left and right because of the shear force of gravity. I have measured the movement at the feet and this occurs under each corner whether a three or four foot design is used.

OK so given that is happening, your question is I suppose, in addition to the leaning over of the machine as the feet compress, does the machine stand and base also flex, and does it flex more with a 3 foot design than a 4 foot design. Well I have in the past tried to measure this, and the amounts of machine twist are small and difficult to measure. Mainly the whole machine leans over, but some flex or twist does also accur. It is doubtful it would be enough to affect the real world accuracy and fit of the Y slideway - but in theory there would be some minute errors introduced.

As MichaelHenry has alluded, or prompted me to say, I can wind down the two safety feet until they are in contact just in case there is more stand flex with the three foot design. But I would need to remember to do that each time I shifted the machine.*

In summary, with 3 or 4 foot design, even 3 or 4 feet 'on springs' the heavy weight of the machine and stand causes the whole assembly to flex to a stressed resting position. This first resting position is when the base is , (or should be) set up and shimmed on that stand, in order to achieve the machines accuracy potential. Now if the machine is moved to another section of floor, it is unlikely to have the same 'flatness' - and so different stresses are likely to affect the new stressed resting position.( even with 'sprung' feet, the 'springs' are loaded to take their share of the weight, and any change in load just transfers back into the stand) With a 3 foot design, the new position cannot introduce different stresses from different floor positions..

* I wish I had thought to point out in the video that understanding of this can help with the set up also of a four foot design. A temporary eg block of wood could be placed in the back or front centre to produce the initial 3 point contact each time the machine is shifted. Then the two outer feet can be brought into even contact with the floor. (Even load bearing contact via eg an exact 1/4 turn would probably be better than nothing).This has the same effect and means a standard machine stand can shifted via this settling down method - and be used as is.

Thanks for your question. Keen

[SilverWings]

Thanks for your thoughtful reply. Let us know how it works out!

[philbur]

.....Mainly the whole machine leans over, but some flex or twist does also accur. It is doubtful it would be enough to affect the real world accuracy and fit of the Y slideway - but in theory there would be some minute errors introduced.

What is the basis for saying it is doubtful and which theory have you applied to prove they are only minute errors. The effectiveness of your concept appears to be based on pure conjecture.

If the machine leans over when you move the table then you have relieved some of the load on one of the front feet and increased it on the other - the stand will twist, by how much you have not determined.

With 4 feet it is quite easy to assess the load on each foot by the force on the wrench when adjust the foot. No risk of falling over and no twisting stand when you move the table from x - to x+.

Phil

PS: You stand looks to be rather narrow across the front.

[keen]

Hi Phil

Thanks for your challenge. You often are a watchgaurd to keep posters honest and logical, I think that is an important role you play.
I am open to challenge and always prepared to stand corrected and change my mind.

In this case you have quoted me selectively and not quoted the previous sentence which was:

"OK so given that is happening, your question is I suppose, in addition to the leaning over of the machine as the feet compress, does the machine stand and base also flex, and does it flex more with a 3 foot design than a 4 foot design. Well I have in the past tried to measure this, and the amounts of machine twist are small and difficult to measure".

You can see this is not pure conjecture as you claim. My basis is: As stated I have tried to measure the twist (with a dial indicator set on the floor measuring the movement of the free to flex back corners with the foot clear of the floor when the table with a high load at one end is X traversed to the extremes) and the amounts of stand distortion are so small that it is difficult to establish if any machine base twist occurs, if it does it is a minute amount. This is not pure conjecture but dial indicator readings.

You state it is quite easy to assess the load on each foot via the wrench - well i think the trouble with that approach is that there is variable friction between feet - rotation friction of the screw and screw bearing point, that varies according to lube, point of movement, surface finish etc. However as mentioned a wooden block or packer can be installed temporarily, the feet screwed down to contact, and a equal rotation down from there would be reasonably accurate way to establish even stand stress.

To be on the safe side I think I will actually use this method and set the two outrigger feet down to take load - so my three feet design will really only be used as a initial stand adjustment procedure, each time my machine is moved.

Phil the main purpose of my video is to get folk thinking about stand and more importantly machine base distortion, as this affects whether the machines accuracy potential is realised, and wear, especially regarding the Y slideway. If that is better understood, more accurate machine settings and wear life can result. Whether a temporary packing block is used, or a welded in centre foot, the same end result is possible. It is the same process that is used to set the base and shim it down on the stand, in order to assure it is set down on all for corners in a non distorted state.

Yes I agree my finished stand is quite narrow - My current build is a wider stand, but the pros and cons of that are complex and another whole subject!

Keen

[philbur]

You state it is quite easy to assess the load on each foot via the wrench - well i think the trouble with that approach is that there is variable friction between feet - rotation friction of the screw and screw bearing point, that varies according to lube, point of movement, surface finish etc.

Yes but if they are all the same materials, all in the same state, the loads are relatively small and you place the feet on pieces of sheet steel then some differences in bolt tension will occur but it is doubtful it would be enough to produce meaningful differences - but in theory there would be some small differences.

To be on the safe side I think I will actually use this method and set the two outrigger feet down to take load.

Doesn't this make it effectively a 4 feet arrangement. How will you measure and adjust the load on each of the out rigger feet?

So my three feet design will really only be used as a initial stand adjustment procedure, each time my machine is moved.

But there is none need for adjustment with a 3 feet design?

Phil

[keen]

Hi Phil - If you slow down read my previous reply more fully you will find I have already answered most of your latest questions - or I am in agreement with most of what you are saying.

Re your first point. The trouble is slight differences eg the effective lubrication on each screw, has quite an impact on the torque to downward force applied. In the past I have done tests on the impact of friction and slight variations in lube and it is quite significant. I accept that with care, your approach will often work. The method I outline is almost a quick anyway.

Keen