Author Topic: Bell mouthed chuck  (Read 641 times)

Offline Brian Rupnow

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Bell mouthed chuck
« on: January 29, 2022, 06:30:10 PM »
The  3 jaw chuck on my lathe has gone bell mouthed. That's not surprising, because the lathe gets almost daily use, and the parts I make are short, they do not extend full length of the chuck jaws. A new 6" chuck will cost me $350.  I can not buy replacement jaws, so must buy an entire new chuck. Is there any GOOD way of fixing these bell mouthed chuck jaws? I don't think there is, but just in case there is, I'd love to hear about it. I know about locking the jaws in place and regrinding them, but as I understand it, the chuck is then only "accurate" at whatever diameter it was locked at before regrinding.---Brian

Offline Jo

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Re: Bell mouthed chuck
« Reply #1 on: January 29, 2022, 07:10:56 PM »
Where is the wear?

If it is on the gripping face of the jaws then it sounds like the jaws are not that hard so you could grind/cut them.

If it is on the slots or top face inside of the jaws in the slots then time to invest in a better chuck.


A 6" chuck that costs $320 should last a life time and will have spare jaws available for it - both hard and soft jaws.

Try using pot chucks to hold short pieces rather than over stressing the chuck jaws & its body.

Jo
Enjoyment is more important than achievement.

Offline internal_fire

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Re: Bell mouthed chuck
« Reply #2 on: January 29, 2022, 09:58:18 PM »
I know about locking the jaws in place and regrinding them, but as I understand it, the chuck is then only "accurate" at whatever diameter it was locked at before regrinding.---Brian

Unless you hit an unusual spot on the scroll the accuracy should not be significantly worse than what already exists. I have done this operation with some success in the past.

I recall that you have a brand-new tool post grinder. Sounds like a good time to test it.

Gene

Offline simplyloco

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Re: Bell mouthed chuck
« Reply #3 on: January 29, 2022, 10:21:33 PM »
I invested in a very cheap ER20 stub chuck with a set of collets  that is now held in the jaws of the 3 jaw chuck. No more bell mouth problems with small stuff.
In fact I don't know how I ever did without one!
« Last Edit: January 29, 2022, 10:49:53 PM by simplyloco »

Offline Jasonb

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Re: Bell mouthed chuck
« Reply #4 on: January 30, 2022, 07:05:24 AM »
I think Brian means short not small dia as holding long small diameters won't bell mouth a chuck, also not good to use ER collets for short items without a back plug.

Would be worth investing in some soft jaws for the existing chuck or get them with the new one as they are ideal for short items or holding  a minimal amount of the part

You have nothing to loose trying your tool post grinder out, even if it's a failure you will be faced with buying a new chuck anyway. Make sure you just spark off the jaws with the grinder , then a few minimal depth cuts are the way to do it not a big heavy one that will wear the grinding wheel and result in a bell mouth.

Offline john mills

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Re: Bell mouthed chuck
« Reply #5 on: January 30, 2022, 08:46:20 AM »
when you remachine jaws it is important how you clamp the jaws  while you re cut the clamping faces the same as machining soft jaws
we machine lots of jaws for cnc machines clamp to take up the slack and machine so the part fits correctly when clamped.
often by the time the 3 jaw chuck is worn out of shape it is time for a new one.if using soft jaw chucks you have to re machine the jaws often
and have lots of jaws .with cnc machines it can be quite a problem to store and keep jaws for regular jobs .
  john

Offline Brian Rupnow

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Re: Bell mouthed chuck
« Reply #6 on: February 15, 2022, 10:12:00 PM »
Today I bought my new 6" chuck. It didn't have any backplate, and I thought I might have to make one. Surprise--Surprise--the backplate off my old chuck bolted tight up to it. No muss, No fuss. I chucked up a piece of 1" hardened and ground shaft and turned the lathe on.--No visible wiggle. I mounted my dial indicator on it, and depending on which of the three "key ports" I used to tighten it, I got 0.005" total indicated runout, 0.005" total indicated runout, and 0.0035" total indicated runout. I marked the "key port" that gave 0.0035" total indicated runout. There are three studs that hold the backplate to the spindle, which gives three possible ways to mount the chuck, and I will repeat my tests in all three positions to see if the runout gets any better. I tried to slide a 0.0015" feeler gauge in between each individual jaw and the piece of steel they were holding, and it would not slide in any place. On my old chuck the feeler gauge would slide in under one jaw about 1 1/4" and under the second jaw about 1/2" and wouldn't slide in under the third jaw.

Offline Admiral_dk

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Re: Bell mouthed chuck
« Reply #7 on: February 15, 2022, 10:57:50 PM »
Nice surprise, to a great addition to your shop - you gotta Love it, when things pan out that way  :cheers: