Can I mill a 6ft length of box steel on a 4ft milling table?

[Apples]

Hi all.

If I had a miliing maching that had a 3-4ft travel, and I wanted to mill/ fly cut a length of say 3" box section that is 6ft long, is this possible.

I will get the 3 or 4 ft milled, but after that I'd have to stop slide the steel up and start again. When I restart will the cut be spot on etc?

Trying to find out if a knee mill is capable of milling say 6ft lengths of steel so that I can get them parrallel and use it as a rail for a cnc machine.

Just wondering If I could do that with a knee mill at home, rather than have to send it off to an engineering shop that has a long travle mill e

[Geof]

Originally Posted by Apples ....I will get the 3 or 4 ft milled, but after that I'd have to stop slide the steel up and start again. When I restart will the cut be spot on etc?....

It depends entirely on you.

You want to mill 6 feet on a machine with 4 feet of travel; you machine the 4 feet, then reposition the part.

Because you need a total of 6 feet and already have done 4 feet you have 2 feet of the previously machined surfaces to use for aligning the part so that the remaining 2 feet is true to the first 4 feet. You will need to rig up dial gauges on big supports so you can dial off your previously machined surfaces to get the part aligned for the final 2 feet.

I know it can be done, becausde I did it; machining about 5 feet on a mill with 3 feet of travel. And getting a letter of commendation in my personnel file because I did it.

[Apples]

huh...you make it sound easy....lol

If I was to just mill the first 4ft then just slide it up and do the last 2ft, and did not gauge it up after I slid it along.

Once I had milled the full length, be it 5,6,7,8ft etc. Would it be good enought to be grinded flat? I mean how much can or is a surface grinder expected to take off?

Peter

[VWSatOz]

I once had to machine inside 4 pieces of 8metre long 12" by 3" channel to create a seating prox 40 by 8mm for a brass strip. I fitted thin wood board to the Bridgeport table to protect it from the rough steel that was to be dragged through, & rear fence block to locate the work. I made 4 roller support stands to take the weight of the long channel to the floor & level. I suspended the whole Bridgeport on a mobile gantry so it dangled level off the concrete floor! So the whole machine was then able to self align to the workpiece! The Power Feed of the X table pulled, while the work was manually encouraged through along the 4 roller stands. The cut was made with a 2" diam tipped cutter in a right angle head attachment on the quill in one pass. We had selected channel from the steel supplier that was as straight as possible as this affected the end result as we were using the outside of the channel for our reference straight.
This method would not be accurate enough for beds for an NC mill, but if you are going to have them surface ground after it is a way you can machine bars of any length if you put your mind to it. It is the weight of the overhanging length that is the problem where you need to support this, but in a way so the machine slides are not stressed too much.
If you are going to get them surface ground by an outside company then ask them 1st if they need to be machined at all. If its a big grinder able to grind the length in one hit then 1mm to remove off each face should be not too much. Holding the work on the magnetic chuck requires shims etc so the job is not pulled out of straight or twisted, but with care and grinding and flipping a few times to grind both sides it is the best and cheapest way to get accurate bed rails/frames for linear rails to bolt to.

[Geof]

Originally Posted by Apples huh...you make it sound easy....lol...Peter

Well in one sense it is easy, you know what you need to do; machine, slide along, align, machine again.

What I did was clamp the workpiece onto an even longer length of flat bar and then slid the whole assembly along, but I had levelling screws in the longet bar so I could tweak the alignment. I was cutting guide ways so the alignment had to be within about .001" per foot.

You may get away with just sliding it along the table and leaving a larger grinding allowance to cover the milling inaccuracies. This will be a play-off between you spending more time on precise alignment or spending more money getting the extra allowance ground off.

[Apples]

Yeah thats about it Geof. Time money and accuracy are all relative.

VWSatOZ. Well,, that sounds like it would work. Pretty full on mind you, none the less it worked!

[BobWarfield]

Originally Posted by Geof You will need to rig up dial gauges on big supports so you can dial off your previously machined surfaces to get the part aligned for the final 2 feet.

This sort of thing is where a picture is worth 1,000 words.

BW

[Geof]

Originally Posted by BobWarfield This sort of thing is where a picture is worth 1,000 words. BW

This is so correct. I did that job back in 1969 well before digital cameras where available; now I would have pictures documenting a setup such as that.

The dial gauge mounts where long pieces of cold rolled flat bar fastened to the body of the machine.

[BobWarfield]

Originally Posted by Geof This is so correct. I did that job back in 1969 well before digital cameras where available; now I would have pictures documenting a setup such as that. The dial gauge mounts where long pieces of cold rolled flat bar fastened to the body of the machine.

I'm trying to envision how I'd approach this with your comments in mind.

First, I'm thinking 2 Kurt vises to hold the stock. For the initial segment, I would position them on opposite ends of the table, and square away the initial 4 feet of travel.

Next, I am ready to get into slide+cut mode. I want to continue using the vises, but my tendency is to want to grip the workpiece only on finished edges to help keep things aligned. Because of this, I will move the lefthand vise inward by however much I want a pass on the remaining unfinished stock to be. For the 4 foot table, let's same I'm going to do the remainder 1 foot at a time--I'm chicken to do the whole 2 feet only holding onto 2 feet between the vises, but maybe that'd be okay too. I'd be suspicious of the rigidity.

So I'm going to move the leftmost vise, and dial it in so that for the 3 feet of travel the jaws are precisely aligned with the rightmost vise jaws. When I grip 3 feet of stock, I'm gripping 3 feet that's already square too. So I slide that vise, and I slide my stock down so the uncut portion is just shy of the left side of the leftmost vise.

Seems to me I can dial off the finished portion of the stock real quick just to be sure everything is good, and then I can cut a foot of stock to the left of the left vise. Probably should support that end with a machinist's jack too. If I had a third vise, I expect I'd use it too!

I cut that foot, open the vises, slide down another foot, clamp down, double check with the indicator, and keep going like that until I'm done.

I think I can do the whole thing off an Indicol, but I suppose it'd be faster and easier to have the bar you mention and 2 indicators--one riding on top of the workpiece and one on the side closest to the machine column. Just need to make darned sure my vises don't contact that lower indicator!

Does that work?

Cheers,

BW

[Geof]

Look at my post further up, I mention a longer bar that the work piece was attached to; I did not use vises. I also mention levelling screws.

The workpiece was fastened to what you consider a sub-table, and it was not moved relative to this sub-table. The sub-table was clamped to the machine table for the first cut then slid along and everything aligned from the previously machined surface and reclamped for the second cut. The levelling screws where to help with the re-alignment.

There were six of these to do along with a whole bunch of other parts. The long ways actually had tee slots machined in the sides of a vee groove and hardened and ground stainless strips where pushed into the slots. A carriage ran along these strips on precision balls and a gimbal assembly supporting a large gyrocompass was on the carriage. These went on some icebreakers being built for the Saint Lawrence Seaway.

[BobWarfield]

First, what's your opinion of the vise approach?

Second, why a subtable? Was it just the shape of the workpiece that made more sense to do that way?

What work had to be done to make the subtable? Was it also machined true?

Having to build a subtable as large or larger than the main workpiece seems the machine equivalent of begging the question, LOL. If, OTOH, the subtable was any arbitrary piece of metal, not particularly true, and largely just there to provide a reasonably smooth sliding surface and a place to mount your leveling screws, that's another matter.

Cheers,

BW

[Geof]

Originally Posted by BobWarfield First, what's your opinion of the vise approach? Second, why a subtable? Was it just the shape of the workpiece that made more sense to do that way? What work had to be done to make the subtable? Was it also machined true? Having to build a subtable as large or larger than the main workpiece seems the machine equivalent of begging the question, LOL. If, OTOH, the subtable was any arbitrary piece of metal, not particularly true, and largely just there to provide a reasonably smooth sliding surface and a place to mount your leveling screws, that's another matter. Cheers, BW

It was a length of flat bar, I did say you could consider it a sub-table.

Your OTOH explains its purpose exactly.

The reason to not use vises was we didn't have any big enough, the part was eight inches wide. Also with vises the accuracy of repositioning is going to be limited by the accuracy of the outer edges of the part.