Need Help!- Piece clamping on lathe

[sebastian_dogar]

Hello everyone. Is there a formula for calculating the extra material you need to have for a piece, on a normal lathe, depending on the diameter and lenght?
For example, if I have a piece 40 mm long and 60 mm in diameter, how long should the material for the piece be (on what lenght should I clamp the piece), before machining?

[Geof]

There is not really any formula as far as I know but as a rule of thumb I try to grip a length equal to the diameter. Of course this does work for very small or very large diameters so you have to be flexible.

It depends mostly on how heavy your cut will be. For very small cuts it is possible to get by just gripping about 4mm length but it can be risky if you do take a cut that is too large because the part will probably come out of the chuck.

This is the kind of thing you learn mostly by experience.

[fizzissist]

Originally Posted by sebastian_dogar Hello everyone. Is there a formula for calculating the extra material you need to have for a piece, on a normal lathe, depending on the diameter and lenght? For example, if I have a piece 40 mm long and 60 mm in diameter, how long should the material for the piece be (on what lenght should I clamp the piece), before machining?

Your first problem is that you're using millimeters. If you convert to inches, it'll work.

We'll assume you're using a 3-jaw chuck or the like, and not a collet. A collet is intrinsically more secure because it clamps the entire periphery....

The very first thing you have to worry about is the engagement of the clamping surface....is it gripping enough along the length of the part so that the part is held securely? See for yourself by clamping just a tiny, tiny bit, then turn the lathe on slowly and see if the part runs true. If it wobbles, then it's not clamped securely. Soon as you start to take a cut it can shift in the chuck and the whole thing gets real ugly real fast. Sometimes tapping the part with a hammer and re-tightening will make you more comfortable.

But the issue is really safety, and in this case it boils down to the clamping forces being greater than the cutting forces. Each situation is different, and like Geoff says....all it takes is experience. Or some good calculus. And strain gages.

If I've got enough material, I'd generally like to chuck up on at least 15mm or so to be comfortable for the sizes you're using...depending on the material too. Less if I'm just doing some light turning/facing, more if I'm going to be doing some heavy drilling or cutting off. Heavy Grooving/cutting off is where things can get nasty.

The reality is, we learn from our mistakes...Stupid system, I know, but that's why we make the big bucks.