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How deep should I bore soft jaws?

alan speyrer

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Feb 15, 2009
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My cnc lathe has a 6” hydraulic chuck and hydraulic tailstock. I’m trying to machine a part, stock material size is 2.75” OD 14.50” long solid 1045 steel bar. My question is how deep should I bore the chuck jaws to hold this material?, I will be using the tailstock and live center.
 
So you think I only need to grab .150” in the chuck and support the other end with the tailstock? I was thinking I need to bore the jaws 1” deep. This material weighs 20lbs.
 
My cnc lathe has a 6” hydraulic chuck and hydraulic tailstock. I’m trying to machine a part, stock material size is 2.75” OD 14.50” long solid 1045 steel bar. My question is how deep should I bore the chuck jaws to hold this material?, I will be using the tailstock and live center.
This depends on the part profile. How much of it are you going to turn?
For me, on a part that long, I'd like to hold by about 1." in properly bored jaws. But again, it depends on the profile.
Heavy hogging? Heavy grooving?
Or simple profile turning?
 
Already center drilled
Since it is already centered if you hold very deep at all chances are the part will be warped after turning.

For parts like this use a backstop that hits the center of the stock. A steel ball in the stop works great. Don't shoulder the part in the jaws and chuck very shallow so the jaws don't try to hold the part misaligned with the center. You want the part to hinge in the jaws.

OTOH, if you are chucking, center drilling, and bringing the center up you can chuck as deep as you want. The center should run true, although if the residual stress in the stock is not symmetrical about the center the part might be crooked after turning.
 
It’s being held concentric w/the tailstock. Where’s it gonna go?
I was worried about chatter, I’m removing a lot of material, big end of shaft is 2.370, small end is 1.875, several diameters and a short threaded section. The part get turned between centers on a grinder for a 16 finish, customer told be to leave those diameters.019” larger for finish grinding.
 
If you decide to chuck 1" deep, indicate the center hole. It will be eccentric. If you turn the shaft in this condition the finished part will not run true on the grinder centers.
 
So if you are turning both ends, why are using soft jaws on this? I must be missing something?
 
If you decide to chuck 1" deep, indicate the center hole. It will be eccentric. If you turn the shaft in this condition the finished part will not run true on the grinder centers.
i planned on using a dial indicator and a deadblow hammer to try and eliminate runout on the tail end of the material, before engaging the tailstock. i never machined any parts this long before.
 
Already center drilled
More reason for only gripping the chuck end of the bar by a short length, a one inch long grip will control the point aspect of the bar too much if the Centre Dilled feature is not concentric with the spindle within a small tolerance. I would be inclined to face it in a lathe where the bar can be held short, then in the CNC Lathe, hold it by at least a 1/2" length, tap the Tailstock end true and centre drill the part in the CNC Lathe.

Regards,

Bill
 
We could try starting with the basics : cold finished or hot rolled, and how did you face and center the far end ?

Actually, with .020" grind stock on this diameter, I don't think he has much to worry about. Unless the bars were really bent or the center was put in really bad, it should be all right ...

Also he didn't say how many pieces ...
 
We could try starting with the basics : cold finished or hot rolled, and how did you face and center the far end ?

Actually, with .020" grind stock on this diameter, I don't think he has much to worry about. Unless the bars were really bent or the center was put in really bad, it should be all right ...

Also he didn't say how many pieces ...
Lots of grind stock but what about the short threaded section? Maybe not good for it to be eccentric to the grinding.

i planned on using a dial indicator and a deadblow hammer to try and eliminate runout on the tail end of the material, before engaging the tailstock. i never machined any parts this long before.
You can certainly so this, but you may not be able to get the center true, it is time consuming, and less than friendly to your test indicator unless you back it out each time.

We made a 1" square shank to hold a center drill like you woukd a turning tool. So the axial length was maybe 1.5". We could chuck the part however deep we wanted, face, fit the center drill between the part and the tip of the center and drill the center then bring the center in without moving the tailstock. It saves an operation and the center runs as true as you can centerdrill it.
 
We could try starting with the basics : cold finished or hot rolled, and how did you face and center the far end ?

Actually, with .020" grind stock on this diameter, I don't think he has much to worry about. Unless the bars were really bent or the center was put in really bad, it should be all right ...

Also he didn't say how many pieces ...
10 to 20 parts at a time, there also some keyways to be milled after turning in the cnc
 








 
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