How tight is too tight on machine adjustments?

[Your Old Dog]

Sorry for the basic question but I have no "Elmer" as Ham Radio operators call'em. Don't know any machinist and I'm not over my head but up to my shoulders in questions.

It occured to me one day while tightening up the bolt to lock my carriage so I could make a facing cut that I might be damageing my lathe by over-tightening things when they don't need to be. In my field, photography, we tell the rookies if you have to apply a heavy hand to a lens then something is wrong and you are likely causing more damage.

Would I be better served by taking a lighter touch on the chuck and wait to see if the work ever spins in it so that I could develop a feel. Or, should I just go ahead and bind her down tight? Does it matter. Is it a big deal?

I've read on this board that rookies tend to turn too slow. I did to. Now I'm wondering about this over-tightening thing.

As always, thanks for all the great help available on this board.

Ray.........

[SteveM]

Most of the time when if I stop and ask myself,"did I get that too tight" then usually yes I did.

[ricksplace]

Hi Ray.

As a photographer, I hear ya when you talk about too much force. I enjoy using retina folders for 35mm work. They don't respond well at all to force. The old speed graphics seem to be pretty much bulletproof.

As for tightening the lathe carriage, enough so the carriage doesn't move is just enough.

As for the chuck, if you don't tighten enough, and the work slips, you could mess up your chuck jaws and have to grind them.

It's my understanding that a good chuck will have a well hardened scroll plate (if a three jaw). I have a Polish chuck on my lathe and tighten work pretty tight in it. It still holds to less than a thou.

My .02

Rick

[precisionworks]

Sort of like the question "how long do the table legs need to be? Long enough to reach the floor <LOL>"

The short answer is, it varies. Soft materials like 6061 or 304 are easily deformed in a 3-jaw or 4-jaw. If you measure the tiny contact area of each jaw (assume that you're gripping the OD of the work) you may have 1/8 of one square inch of contact per jaw. So it's really easy to generate hundreds of pounds of pressure, especially if you've extended the wrench handles as I've done.

Harder materials like 4140 pre-hard, W-1, anything case-hardened or through hardened, need more pressure or the jaws will slip, ruining the workpiece. No problem IF it can be replaced

On softer work, you'll develop a feel for what it takes. If you take light cuts, low pressures will work.



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Barry Milton

[SGW]

I'd err on the side of not tight enough, until I developed a feel for it.

[wierdscience]

Machine features like your carrage lock are usually designed to work with minimal strain.
Usually I run things down finger tight and then 1/4 turn with a wrench and that's all that is needed.

Tooling is another story,usually a bolt torque guide for the particular size fastner will give you a ballpark number.

[C. Tate]

Hard to give specifics on this subject. Generally I have found most people overtighten things. The fact that you have some concern tells me you probably do not. If you use any type of cheater or you put one foot on the head stock while chucking work in the lathe you are overdoing it. If any chuck or other work holding device needs more force than can be exerted using included tools and normal human input you need to examine the set up.

[PSD KEN]

Rule of thumb is to tighten til' the threads strip, then back-off a 1/4 turn.

[nheng]

If a part is small and light weight, you can err on the side of too light.

If the part can take part of your body with it when it lets go, err on the side of too tight (i.e. leave jaw marks on it). Den

[J Tiers]

Gosh, that depends on the part. You have to know why you have it in the chuck, how much force will come on it, if it will deform, if it will creep under pressure like plastic........

You pretty much need to be able to judge that from the part and work to be done. Not rules.

A rule will mess you up if followed blindly.

My rule is tighter than looser, since I find that the marks from spinning the part are usually worse than the jaw marks. You can always put softer stuff under the jaws to save the work.

Spinning the part also can foul up the jaws, and that gets expensive.