Limitation in self centering jaw chuck

[woodchucker]

@NC Rick he is not talking about boing out the hole. He is showing you the limit of the jaws.

@Alberto-sp You have exceeded what the jaws can handle on the 4 jaw. That's not safe. 3 things can happen:
1 you break the chuck because there's not enough support there.
2 you have a jaw fly out while it's spinning.
3 you hit the carriage with that jaw out that far.

First, your jaws can be turned around on the 4 jaw, when you need to go out that far, yes it will only be grabbing by a smaller amount, but it will be safer.

Your lack of space on your 3 jaw: I see you have another jaw there. You should have 3, those are for grabbing a big part, you put those in, the opposite way of the current jaws and you can grab a bigger part, the same as the 4 jaw.

The entire jaw will not grab, you need to use the landing. But it is safer, if it's a flat part you are good to go, if it's a long part, you need to use the tailstock or steady rest to hold the rest of it.

 

[woodchucker]

being that this is a 7" 180mm lathe, a 4" 100mm chuck is the right size 3 jaw. You could go one size up, but you are not going to gain more than 1" 25.4 mm diameter, or 1/2" 12.7mm radius, which is still short of your goal.

I know that's not going to help, but if you really need to turn big stuff, you need to get a bigger lathe, for multiple reasons.
capability, power, The smaller lathe doesn't have the rigidity for bigger parts.

 

[RJSakowski]

As a rule of thumb, I never extend my chuck jaws more than the first step past the outer diameter of the chuck. As a result the maximum safe diameter of work held by the inside jaws is the chuck diameter less twice the distance from the first step to the inside of the jaw. The maximum safe working diameter of my 4' chuck is 4"-2*1.1"= 1.8"(46mm). For my 5" chuck, it is 5" -2*1.4"=2.2" (56mm). While not having a 6" chuck, I would guess that the distance from the first step to the inside of the jaw would be more like 1.7" so the maximum work that could be safely held would be 6"-2*1.7"=2.7"(67mm)

However, the maximum possible work diameter that could be held with the inside jaws is the swing of the lathe less twice the length of the jaws. The OP's lathe has a 180mm ( 7") swing. Guessing that the overall jaw length of a 6" chuck is something like 2.4", the maximum work diameter without the jaws hitting the bed of the lathe would be 7"-2*2.4"= 2.2" (56mm)

From this analysis, there is little or nothing to be gained in terms of work diameter held by the inner jaws by going from a 5" chuck to a 6" chuck. There is some advantage in having a longer grip length with the 6" chuck but this has to be balanced against swinging a greater mass. My 5" chuck and back plate are about double the weight of mt 4" chuck and back plate. I would guess that a 6" chuck would weight close to double my 5" chuck. That extra weight will icrease the stress on the motor and on the spindle bearings possibly leading to an early failure. Additionally, there will be some spindle sag caused by the additional weight which could cause problems when machining.

 

[mikey]

I know that's not going to help, but if you really need to turn big stuff, you need to get a bigger lathe ...

I've been following this thread and I have to agree with woodchucker. If you are going to regularly work with materials that require a larger chuck then you really need to step up to a larger lathe. Your spindle and spindle bearings are not meant to handle the weight of a larger chuck or the weight of the work it will have to carry plus the cutting loads imparted by the tool.

 

[Alberto-sp]

Thanks for all your answers. So now it´s clear, I´ll have to use the other 3 jaws in combination with the tailstock.

But I don´t open the jaws of any of the chucks so big. That was only for show you how far can go each one of them.

@NC Rick I wish you the best for you and your wife

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[woodchucker]

Thanks for all your answers. So now it´s clear, I´ll have to use the other 3 jaws in combination with the tailstock. Good

But I don´t open the jaws of any of the chucks so big. That was only for show you how far can go each one of them. Ok, that's good too. I Think you were trying to express that it is the opening you were referring to then? Correct?

@NC Rick I wish you the best for you and your wife

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[Alberto-sp]

Yes.

One question off topic. I know a nylon hammer is useful in the shop. But I have pieces of delrin at home. Can I make that hammer with delrin instead nylon?

 

[RJSakowski]

Yes.

One question off topic. I know a nylon hammer is useful in the shop. But I have pieces of delrin at home. Can I make that hammer with delrin instead nylon?

Delrin will work perfectly well for a soft blow hammer.

 

[Alberto-sp]

Thank you!