Driving the Knee? What do you think? and Why?

[handlewanker]

Hi Jackall re post 59, whenever I've had to use a knee type I religiously lock the knee for all machining ops.

This means that the table will always be in a fixed position after it is positioned.

I rarely use the knee to increase the depth of Z travel while the machine is running, except of course on a horizontal mill, and I can't remember when I last used one of them, and even then the lock is half on to make sure the table is pulled up tight to the slides.

The fact is you need to have the knee slack enough to be able to fast traverse up or down, or you end up wearing the drive mechanism out if the knee is tightened up to get the slack out.

Anybody who machines with the knee in the unlocked position is asking for innacurate results.

I tried your method on my Ajax turret mill, (similar to Bridgeport), with the quill at it's uppermost position and locked.

By this I mean if the quill is in the top most position and the knee is raised and lowered for clocking, then you must lock the knee at each position before testing the bearing bore, if you don't the knee will just be in a position that it happens to get to, due to the slackness required for traversing, when down or up, but the lock will pull it against the opposite dovetail etc.

Some years ago I worked for a firm that bought a new Mass horizontal mill with the vertical bolt on head.

From the beginning it never gave accurate results, untill they stopped using GREASE in the knee slides and used oil as the manufacturers stated.

When they used the grease the slides had to be slacked off to get the drive motor to raise and lower the knee, and this led to all sorts of problems, especially in winter.

Which leads on to anyone who is going to use the knee for Z axis travel must decide if the tightness of the knee slides can be overcome when positioning and fast traversing without affecting the positional accuracy due to the slackness required.

Here's a test for you, position the table in it's central position in the X axis and mid way in the Y axis, and lock both X and Y.

Now position a dial indicator to read at the end of the table, ( I swung the head around for this test).

Try to lift the table ends by placing a short plank of wood under each end alternately, and using a piece of wood as a lever to exert some pressure.

This will give you some idea of the amount of movement you will have when the knee is allowed to be left unlocked.

Try this method with the knee locked and unlocked.
Ian.

[Jackal66]

Yeah, that is what I've run into. I've been repairing the bearing fit in a gear box or end plate of a rotary blower. After indicating the locating register or dowels, bore it out for a sleeve.

When the sleeve arrives, you find out they want a different style seal in the end cap also.

This means you must crank the table down, maybe 1/2", and use different boring bar or head.

After moving the table down with all 3 axis' locked you re-indicate the dowels or locator register and find out the table moved .002 and you have a tolerance of .0004 . Time to relocate the part, and that doesn't count having to tram the head to the face now because it has moved, too. You have to keep the quill as short as possible for ridgity, yet at the same time allow enough for the stroke. Some of the knees on these mill even have 2 locks.

I have seen some of the bridgeports with air gap ways for smoother travel They were even more sloppy than the standard ones.

It gets real frustrating, especially when they expect you to do a job like this in 3 hrs total.

Some of the machines in this area are so worn that if you try checking the play on the ends of the table it would read .020+ with everything locked. The gibs are bad or the dovetail is so worn in one area.

I have checked some new Birmingham, Kent, and Supermax mills that had a quite a bit of play in the gibs and dovetails. The shops in this area tell guys applying for jobs " Anybody can make parts on a perfect machine, you have to know how to sweet talk this kind of junk". They don't know how true the word "Junk" is.

Jackal

[chich2]

I take the angle of if it woks.......Use it. For example, Look at any "real" CNC like a vertical machining center. X and Y are rigid in a base and Z is a big heavy head that slides up and down a column. It simply has a counterweight. Works perfect. That's another reason I "Z" my knee. Quill's deflect too much. Adjust the Z knee properly (just like a real CNC machine is adjusted) and anjoy the rigidity. I'd also like to say that knee mills are good......BUT...... there's a reason real CNC manufacturers don't design them in a knee mill configuration.

My 2 cents
Cheers
Chich

[handlewanker]

Hi all, I think there are two possibilities that can drive you, one is an old mill that is well past it's use by date, and a new mill that is just not "bang for the buck".

The moment you put the dollar value as the first priority, then you get what we had in the 70's the "Taiwanese Terrors", those horible little bench mills with round columns that couldn't mill to save their lives, but didn't cost much as they were glorified drills anyway.

I deeply sympathise with anyone who does not understand the phrase that "you only get what you pay for" in it's fullness, and are wowed by the shine of a first time novice buyer's latest aquisition.

If you don't know what machinery is then you will fall into the trap everytime you go to an auction and grab the "cheapies".

The bottom line is if you bought a Bridgeport, then you only get what you pay for, that is if it's worn out but looks a million dollars, don't expect it to perform like a new machine with all the certificates of trueness that the makers build into a new machine.

This goes even more so for a new Chinese piece of crap, or for that matter any other country that produces cheap machinery, and if the buyer bought strictly on the price, more fool them.

I would go so far as to state that I'd rather buy a cheap worn out Bridgeport and rebuild it, than a new foreign piece of crap that no matter how you adjust it just won't "cut the muster" no matter how you struggle with it.

OK, so it's going to cost you a bundle, if you dive into the realms of Bridgeport rebuilding, but at the end of the day you get more "bang for your buck" than any new cheap forign junk, and will outlast you given proper care and maintainence.

I bought a cheap Bridgeport, A$2,800 in '97, last century, part of our factorie's down sizing plan, and lived with the worn out bits that in the end didn't really affect the accuracy of the work I was doing and could be relied on to get things square and flat, which is all I asked of it anyway.

Second hand machinery is not for the novice or newbie, unless you have a fair degree of machine knowledge, or someone else's you can tap into, and can do the allignment checks that will determine if you have a "treasure" or a boat anchor.

As far as "sweat talking" old junk, I worked for some of the places that had "old junk" as part of their main armoury, only for a short time at that, at the very least it wasn't a well paying situation, and the work they turnd out was never of the highest standard too.
Ian.

[Jackal66]

That is really different. This is one of the better paying shops around here, between $16 to $20 an hour. The shops with all of the new equipment usually pay from $8 to $11 an hour. The claim is: with the paying for new machines, there isn't any money left over for wages . Besides that, it is their belief that anyone can make parts on good equipment. If you go in one of those shops you see high schools kids out for the summer, cutting keyways and such. The foreman will tell you "On this good equipment it's easy to be a boss, tell 'em to dial in .500 and it will take .500, no problem."


If I have something that needs real rigid machining I'll take it to the Haas VF3 or the Mori-Seiki. Just the little stuff on the Bridgeport Boss, 10-32 tapping , 1/4-20 and a lot of small aluminum stuff.

JAckal

[handlewanker]

Ha Ha, I get your drift, bit like going to a society wedding in a $2 suit from the pawnbrokers.

It comes down to how intelligent the business executive is when he tools up to compete on a world platform, some don't know what day it is let alone the time.

You have to be pretty desperate or cluless to take wages as low as that.

I worked a few places like that when I finished my apprenticeship, but then I was still learning and learned fast.
Ian.