Would this structure collapse or not? (Bridgeport mill base)

I worry about:

- vibration, depending on what you put in the Bridgeport

- getting electricity to it, and

- I don't see any lock on those casters. Do any of those wheels lock? Should have at least two that lock.

With that "groove" you mention in the floor, it would not take much to generate 4g's and fail the bearings in the casters.

I guess based on your description that you don't intend to leave the casters installed during operation. What does the concrete bearing surface look like? Have you created a wedge suitable for concrete demolition?

David A. Smith

They do not lock, however, I have moved a lot of stuff on casters and always use chocks.

I will not run the mill while it is on casters, casters are mounted on bars which are removable. (other than just trying it for a couple of times). It will be powered via a VFD, which I already have and set up for 1HP.

Good point on the groove -- I will try to either fill it or insert a suitable round rod into the groove. The groove is about 1/2" wide.

My question is, mainly, do you think that the frame will hold up under described use.

I am not sure I understand your question regarding concrete demolition, the mill will sit on the shown base, which in turn will be sitting on top of 2x4s lying flat. To remove or insert 2x4s, I will use a car jack, most likely.

i

Dear Ignoramus27175:

"Ignoramus27175" wrote in message news:A_GdnQEpUewW7vnbnZ2dnUVZ snipped-for-privacy@giganews.com... ...

Does the bridgeport straddle the "box", or sit down inside it? I worry about those unreinforced joints at the bottom of the box if the latter, and the lack of diagonal bracing (or at least corner bracing) if the former.

You could build the 2x4s into the frame bottom. If the frame does not cut the 2x4, then you have no issues.

David A. Smith

The box is approximately 1/2" wider in both dimensions than are the dimensions of the base. So, if the base is placed in the center, there would be approximately 1/4" distance between the base and the walls of the box. That's why I hope that the bottom bars will not bend too much, as they have nowhere to go.

Note though that I will appreciate opinions of real engineers (I am a "software engineer" a.k.a. computer programmer, and not a mechanical eingineer).

Well, but I like the flexibility of being able to install or remove

2x4s. i

Dear Ignoramus27175:

"Ignoramus27175" wrote in message news:S_qdnYJfHKU2SfnbnZ2dnUVZ snipped-for-privacy@giganews.com...

Oh. Well, if the welds don't break right off, the "flat bar" members will probably never clear the floor. They will arc down, trying to get necessary support from the floor. I would guess you will need 3 or more inches lift before the center of the flat bars clear the floor.

Where the "1/4 x 4 inch" flat bars cross the bottom of the box... is this where the normal support feet of the bridgeport are located?

Is the base of the bridgeport hollow, such that away (4" or so from outer walls) from the box edges, the flat bars could be made thicker / taller?

I forgive you. ;>)

You would similarly turn your nose up at my coding style...

I love the idea of looking for sacrificial 2x4s, after having left them behind where the machine came from. It is your labor. "Let each man lay his dead according to his own fashion."

Seems like a lag bolt shot through a hole in the base would work just fine. And enable future replacement without difficulty.

David A. Smith

A Bridgeport mill does not have feet, it has a flat rectangular base. So, I hope that the flat 1/4x5 inch bars will support it on the outside perimeter of the base.

I think that it is not hollow.

i

I don't see bending the bottom of the "box" itself to be any problem. Putting the flat pieces diagonally at the corners might have been better. If you put your weight on the flats about 4" from the edge that will cause about as much bending as the weight of the mill at 1/2" from the edge. If I understand your design the weight of the mill is supported on the

4 bolts that go thru the square tubes, That looks to me to be the weakest point of the design. At the very least you should have some big heavy washer under the nuts to spread the load a little. Seems to me the whole "box" thing is superfluous. Usually the base of a mill will have drilled holes so that the mill can be bolted to the floor or set on adjustable feet for leveling. Your 2 pieces with the square tube and castors could be bolted directly to the mill base instead of to the box.

-jim

I agree. Note though that the flat bottom pieces, also have "lips" that connect them to the front and rear of the box, that, to me, sort of acts more like a corner.

OK, that's exactly what I will do, will place some thin plywood flats along the sides so that the mill's foundation applies its weight close to the sides of the box.

4 bolts that go through steel angle.

OK. I used 1/2" bolts. I will indeed use such washers as you say, thanks.

The purpose of this setup is to make the mill movable around my garage, and the casters detachable so that they are not a trip hazard, dop not flatten out from weight etc. For regular operation, casters will be removed and the square box set on 2x4s.

i

[I posted this article to sci.engr.mech. With apologies for double posting, I am also posting it here to solicit second opinions, so to speak. If that is not appropriate, please accept my sincerest apologies] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I built a mobile base for a Bridgeport mill. (2,200 lbs)

It is shown here, along with copious comments (pasted here at the bottom).

This base will be used for slowly moving the 2,200 lbs mill on concrete floor (garage), with one groove in the floor.

Do you think that it is adequate?

====================================================================== Pictured here is a homemade base for a Bridgeport mill that runs on casters. Design Objectives

• A failure of the base should not make the mill fall (hence it needed small floor clearance) * The bars with casters should be removable so as not to take valuable floor space and not to be a tripping hazard * During operation, the mill would stand on 2x4s beneath the base, with casters removed * Bars with Casters should be easy to mount again if the mill needs to be moved.

Some notes

The base is made from 1/4" flat stock (1/4" by 5" steel bar) and 1/4" angle iron. The bars holding casters are 3/16" thick 1.25" steel square tubing.

The casters are something to behold, these are NOT the usual Harbor Freight "heavy duty" casters. I bought them on eBay for $40, Here's the auction screenshot. They are rated for 2,400 lbs each, all swivel, and are made from very heavy plate. (1/2" and 3/8")

The welded-on T-nut with a bolt in it (seen in front) was attached to provide attachment point for an eyebolt in case if the mill needs to be pulled. It is threaded for 1/2"-13 NC. The bolt is there just to show it better, it will be replaced with an eyebolt.

Enjoy the video of this base being gently kicked.

The internal dimensions of the base are 1/2" more than the dimensions of the bridgeport base.

Credits -- the PDF file showing Bridgeport mill base dimensions was created by Richard Kinch (see his shop). As a thank you, here's an SEO friendly organic link to his site: TeX for Windows.

The base weighs 160 lbs.

Sorry for that spurious second post, ignore it.

i

Well I don't think that is really necessary but probably won't hurt either. The weight will naturally rest on the outer edge of the cast base. As soon as the flat pieces bend even the tiniest amount all the weight will be on the outer edge of the mill base (which won't bend). If the welds can hold the weight there shouldn't be a problem.

Yes each 1/2 in. bolt should support several tons, but will the square tube support the weight concentrated at one point? That point being also the weakest spot due to the drilled hole. The angle iron or flat iron underneath doesn't really do anything to keep the square tube from bending or collapsing.

Yes I understand. You are hanging the thing from 4 bolts. Alternatively, the 4 bolts could be coming up thru the holes in the mill base itself. In theory it could be designed so you had 4 bolts or studs sticking up thru the base. You would slide one of your pieces with castors attached over 2 of the bolts and tighten down the 2 nuts and it would lift the mill off the floor on one side. Do the same for the other side and the mill is completely off the floor and can be moved. Lifting it 1/4" should be enough if the floor is very flat. Ultimately it still comes down to making the cross member (what is now a square tube) sufficiently strong and rigid. You also should consider twisting forces on the square tube. When the castors are at right angles to the tube there will be quite a bit of force twisting the tube. The tube may be able to handle that but the bolts may not and bend and eventually break.

-jim

Thanks. (like I said, even if the base fails in any manner, no danger to life will result due to low ground clearance, just an embarrassment). I am happy that you think that it should hold.

The welds are quite solid, made with dry good E 7018 electrodes, I have full faith that they are at least half strong as solid steel of the same cross section.

OK, I see your point now.

A couple of days ago, I used the metalgeek calculator here:

and entered 1.25" square tube, supported on both ends, 18" long (twise the bending radius of 9 inches), 0.188" thick, with 1,200 lbs applied in the center.

The calculated deflection came out to be 0.0082".

That IGNORED the drilled bolt hole, so the calculation is not definitive, however, it gives some hope.

You have a great point on twisting, which I am not competent at all to even look at and was just going by the seat of my pants.

Worst case is, if this tubing fails, I will unbolt it and buy bigger tubing.

Yes.

I am hoping, personally, that if the bolt is tightened strongly, then the bending force would not be great. Would you think that it is not warranted?

i

The diagonal cross member is not doing much good. That's for reinforcing against shear loads and most of your loading will be normal to the frame (downward). Under load it could deform downward a fair amount. Short diagonal members or triangular pieces in the corners would be better, I think.

You can test it out and see if the diagonal member comes close to touching the floor and redesign as needed then. Good luck.

Jeff

>

Note, check out my later thread saying that the mill is installed in the base and things look OK.

I agree. I only tacked it for the purposes of making this thing rigid before welding.

I removed the diagonal member prior to installing the mill in the base.

There are little lips that connect the middle of the crossmembers, to the front and back sides of the frame, which, I hope, are helping to keep this structure more rigid.

See the mill in casters here:

i