Lathe chuck or spindle issue?

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1923Touring
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Lathe chuck or spindle issue?

Post by 1923Touring » Sat Feb 04, 2023 12:46 pm

While this question does pertain to model t parts, I am really trying to figure out what I should expect from my lathe. I was going to try and chuck a model t transmission shaft up and true the flange like Mike Bender did in the model t tips video. I figured I would first indicate it in, and if that worked out, then I would buy a steady rest and continue with the job. Having indicated the shaft in, I decided I wanted to know how firmly in place it was, so I decided to push the end of the shaft near the indicator and attained 0.003'' movement. Wanting to know how far this would go, I climbed up over the ways and lifted as hard as I could and saw 0.008''
I am using a 16'' south bend lathe from 1950, so it is not new, but I don't think it is shot either.
My first though was to check the spindle bearings clearance. The book said to put the indicator on the nose of the spindle, and with a 2 foot bar passed through the spindle and lifting with 75 lbs the bearing clearance should be between 0.0007 and 0.001''. Mine was 0.0009.


My question is, is this somewhat reasonable for a part of this size? Is the problem in my 4-jaw chuck, and if so how might I remedy this? My understanding is that a steady rest is supposed to help steady the work under load, but if I can move it with my hands is it worth messing with a steady rest?
I ended up buying a new transmission shaft, but I would still like to know how much I can trust my machine, or if I can make it better.

-Joshua


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Re: Lathe chuck or spindle issue?

Post by Scott_Conger » Sat Feb 04, 2023 12:55 pm

so much of what you're asking requires much more information to assess whether it is an infirm chuck, worn plain bearings in the headstock, or just what the issue is

if you're gripping 2" of a 10" long bar and heave-ho on it, it's going to deflect some no matter what

a steady rest is an almost mandatory accessory and when properly used either with an accurate chuck or tied firmly to a dead center and drive plate at the headstock, (if the part has a really TRUE center) it will support the end of a long lever-arm perfectly to dress off things like the flange on your shaft.

It was a lot of years before I bought my first steady rest and once I did, I very quickly outfitted all of my lathes with one. They're indispensable.
Scott Conger

Tyranny under the guise of law is still Tyranny

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1923Touring
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Re: Lathe chuck or spindle issue?

Post by 1923Touring » Sat Feb 04, 2023 1:36 pm

Scott, thank you for your reply. Compared to your experience with this type of machinery, I am just beginning to learn.

My chuck has slop in the jaws, but that is removed when they are tightened. Additionally the chuck threads on to the spindle nose. I have cleaned the threads and insured that the chuck threads on all they way. The chuck did not come with the lathe, it is a Cushman chuck but "South Bend Lathe Service Shop" is stamped into the side of it.

I opened the bearings a few years ago to inspect them, flush the oil reservoir and replace the felts. I did this because there was a slight bearing knock developing under heavy load, and because they had not been cleaned in this century. The bronze showed signs of heat in a couple spots, but no scoring on the spindle or sleeve. I would presume that if the bearings have clearance within specifications that they are not worn, but this could be due to my lack of knowledge. There are shims still under the caps.

I had the part chucked just as shown in this picture. How much would you think a shaft like this would deflect with 150 lb on the end? I figured I needed to pull as hard as I could because there is so much more introduced when the tool bit makes even a light pass.

While in this specific application, yes I could use the tailstock center, but if I wanted to bore the bushings in my brake drum, then I have the same issue. (unless you would suggest a different way to bore the brake drum).

If more information is needed I can obtain it. I just need to know what you need.


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Re: Lathe chuck or spindle issue?

Post by Scott_Conger » Sat Feb 04, 2023 2:23 pm

something is dreadfully wrong if you have 150# pressure on a part when cutting

For FWIW, you can put a rod of any diameter, say 1", and let it stick out 3"...put a tenths indicating indicator on it and press moderately with your thumb...the needle will deflect...it's just a fact

a dead sharp tool, the part being turned has proper support (no deflection) and the correct material speed it will cut cleanly with no undue force

regardless of the size of the lathe, you should be working within 2-3" extension of the part from the jaws, max. Anything more and you're going to get harmonic rippling. For FWIW, you can put a rod of any diameter, say 1", and let it stick out 3"...put a tenths indicating indicator on it and press moderately with your thumb...the needle will deflect...it's just a fact. You're running a plain bearing lathe which rides on a .001" (or so) film of lube. When stationary, with a long enough rod in the chuck you can make it appear to bend all over the place. When turning, that .001" of lube becomes as rigid as a ball bearing and will spin perfectly true to whatever alignment the headstock has relative to the bed (but that is an entirely different subject)

I suspect your chuck is shot to the extent that repeatable work can not be performed from one installation to the next...pretty much all old chucks are worn to the point that they're great for rough work but not so much for precision or repeatable work. Thus my admonition earlier. A trick you can use is to use a marker to "spot" the chuck at 1 jaw and then "spot" the work immediately opposite the spot on the chuck. Work must ALWAYS be placed back into the chuck as it came out. This is good for parts that will be parted off but not so much for something that is going to be put into service using the entire length of the rod as the part turned will not be centered with the part in the chuck (unless it was a 4 jaw chuck carefully indicated and the far end held in a live center).

To use a so-so chuck:

Put some stock in the chuck and then turn to the exact diameter of the shaft. Bring the steady up to the turned stub and adjust for a gentle rub with all 3 fingers and slide out of the way.

install the about 1" of the shaft in a 4 jaw chuck and lay the flange end in the steady cradle (pad the shaft with brass shim stock to avoid damaging the shaft). Dial indicate the shaft at the chuck end to .0000" while supporting the flange end with the steady rest. Put a dab of high pressure assembly lube on the shaft at the steady rest and close it down. You're now set with the shaft running perfectly true and the cutting range is now within 1" of the steady and with a sharp tool and proper speed, it will cut like butter.

this is the lube I use for steady rest work: https://www.zoro.com/permatex-permatex- ... W9EALw_wcB

Finally, precision work can be done even on a clapped-out treadle lathe...you just have to be creative. If it is at all possible on your budget, treat yourself to a new chuck. I upgraded all of my vintage stuff years ago to new chucks, though I still pull out an old-timer periodically for particular jobs. Chucks are insanely expensive now and often will eclipse the purchase price of the lathe if it's more than 20 years old. Still, they are a wonderful investment. I planned my lathe purchases such that they all take the same chucks/tooling and that keeps expenses within reason when I can amortize any single tool or chuck across 3 different lathes.
Scott Conger

Tyranny under the guise of law is still Tyranny

NH Full Flow Float Valves
Obsolete carburetor parts manufactured


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Re: Lathe chuck or spindle issue?

Post by Scott_Conger » Sat Feb 04, 2023 2:56 pm

Let me follow up my own post with another as I think this is important and should not be missed:

If you are getting advice from someone with less than 20 years of turning experience, understand that their experience is almost surely limited to extraordinarily ridged CNC machines turning at furious speeds using carbide or ceramic tooling. Almost nothing they have experienced will have any connection to you or your machine whatsoever. You are turning at hundreds of RPM's at the most while they are turning at 1,000's of RPMs with tremendous rates of feed compared to you...now THAT can induce huge loads on the part(s). You're not that guy. Your machine was made to use HSS cutting tools and very low SFM with low feed rates. Here's a tool that you may benefit from for forming and sharpening tool bits...I have no connection: https://www.ebay.com/itm/361625308285?h ... R-7BysmsYQ

Myself, I do use carbide indexable tooling for much of my work because I am lazy. However, carbide tooling DOES require higher forces to use but still nowhere near what I think you are saying. HSS tools kept sharp will cut like a surgeon with very little force and is almost mandatory for very delicate work and and/or extremely tight tolerances (because you can keep part deflection to almost "zero"). I can make carbide repeat to .0002" on my machines because I know them and have developed techniques to do so. Lots of folks are happy to repeat to .002". I am not.
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Re: Lathe chuck or spindle issue?

Post by Jerry VanOoteghem » Sat Feb 04, 2023 9:58 pm

My chuck has slop in the jaws, but that is removed when they are tightened.
Not really. What happens is, the jaws will get skewed in the chuck body such that most/all of the clamping force is concentrated at the back of the jaws, while the outer tips of the jaws may even be gapped away from the work piece. That would explain why you can move the workpiece in a tightened chuck**. In other words, you do not have the benefit of having the whole length of the jaw supporting the workpiece.

**Aside from whatever flexure you induce while pushing/pulling on the workpiece. As Scott points out, you can always move the workpiece at least a tiny bit, but that kind of movement usually springs back when you remove the force. A faulty chuck will allow the workpiece to move, and stay moved.


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Re: Lathe chuck or spindle issue?

Post by TXGOAT2 » Sun Feb 05, 2023 4:21 am

A shop teacher once said, "It's easier to run a new machine. It takes a machinist to get good work out of a worn machine".


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Re: Lathe chuck or spindle issue?

Post by 1923Touring » Sun Feb 05, 2023 7:57 pm

From the sounds of it, I think I will start with a steady rest and most likely a new chuck too. Scott, the advice I have been receiving is limited to a pre-CNC era machinist, and whatever books and videos I can find on the subject. While I am at it, do you think that it is worth getting a new back plate as well? Do they wear out much, or is the "slop" created by thread clearance supposed to be taken up when the chuck is threaded on?

Jerry, what you are saying does make sense. One thing I should have mentioned, but neglected to, is that after I am done pushing on the part, the indicator does revert back to zero. I would presume that the jaws are then reverting to their former position?

Thank you all for the advice,
Joshua


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Re: Lathe chuck or spindle issue?

Post by Scott_Conger » Sun Feb 05, 2023 9:12 pm

Joshua

the spindle has a "register" on it which is the unthreaded part before the threads. This should be a dead fit to the backplate and that is what makes the chuck run true to the lathe axis. Easy to check the lathe spindle and easy to check the initial part of the backplate as that is what just barely slips over the spindle register immediately before seating fully

A good chuck made for your lathe should have all of that correct, but maybe not. I do know I would not be in a hurry to remove a factory backplate from a $1500 chuck! At least not without good cause. Thinking about it, your 16" lathe is going to need to have a backplate purchased because the new chuck won't have one...so yes, you'd need a new backplate and you'll have to fit it to your spindle and face it true/perpendicular to the headstock axis.

Lots of South Bend info out there and with luck and persistence you may find the drawing for your spindle and determine if your register is OK or if it has been tampered with...that's where I'd start. If you don't find info, probably OK as you're going to match the new backplate to whatever is there anyway, so the need for a drawing is probably moot. With a 2 3/8-6 spindle thread, I'm guessing the register is 2.3745" or so.
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Re: Lathe chuck or spindle issue?

Post by Kevin Pharis » Mon Feb 06, 2023 1:32 am

Chuck’s are usually replaced due to traumatic events, not wear and tear. Cushman chucks were high quality, and I’ll bet it’s still in decent shape. Back plates (if equipped) are sometimes threaded by the end user, and can be faced to correct axial runout caused by traumatic events. If you are pulling on a 1” diameter shaft extended 8” out of your chuck and seeing .003” deflection… you have a tight lathe and chuck! The indicator returning to “0” after the test shows that the part is not moving in the jaws

All chucks require jaw clearance to operate. This clearance will cause jaw deflection, and so will chuck body distortion due to clamping pressure. The only way to work around this distortion is to bore soft top jaws while clamped on a boring ring (or similar) to ensure that the jaws are bored while the chuck is in the deflected state.

There are plenty of folks of all ages out there doing so-so work on all lathes, both knob twisters and CNC. If you don’t understand how to set up a part, you are going to struggle to make a good part. You must level your lathe properly to ensure there is no twist in the bed, and the spindle is aligned. If using a steady rest, you must indicate the part for alignment along the top and front of that part. Only then will that part be faced perpendicular to its centerline


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Re: Lathe chuck or spindle issue?

Post by 1923Touring » Thu Feb 16, 2023 1:53 pm

Follow up:

Following this discussion, I purchased a brass fingered steady rest, and I also looked at the spindle register for my chucks and found the register on them to be 0.030 larger than the register on the spindle. It is to my understanding that the difference should really be about 0.001, and I would need to machine a new plate to fit.

In the end I was able to true the transmission flange, and the shaft end now has 0.001 run-out when fitted to the crankshaft (which I think is due to the transmission shaft being bent).

It really is more enjoyable to fix the original parts than buy new, and now I am thinking that while the end of the shaft is 0.001 larger than the flange end, this would mean I would need to bore the brake drum bushings out by an extra 0.001. Would that be an acceptable procedure, or is the shaft hardened deep enough to facilitate regrinding it?

Thank you,
Joshua


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Re: Lathe chuck or spindle issue?

Post by speedytinc » Thu Feb 16, 2023 2:47 pm

1923Touring wrote:
Thu Feb 16, 2023 1:53 pm
Follow up:

Following this discussion, I purchased a brass fingered steady rest, and I also looked at the spindle register for my chucks and found the register on them to be 0.030 larger than the register on the spindle. It is to my understanding that the difference should really be about 0.001, and I would need to machine a new plate to fit.

In the end I was able to true the transmission flange, and the shaft end now has 0.001 run-out when fitted to the crankshaft (which I think is due to the transmission shaft being bent).

It really is more enjoyable to fix the original parts than buy new, and now I am thinking that while the end of the shaft is 0.001 larger than the flange end, this would mean I would need to bore the brake drum bushings out by an extra 0.001. Would that be an acceptable procedure, or is the shaft hardened deep enough to facilitate regrinding it?

Thank you,
Joshua
I have an 11" logan with quick change gear box. I purchased a European 8" 3 jaw revesrable jaw chuck. It runs out .0005" after I tuned it. I can live with this in a repeating 3 jaw chuck. The chuck cost more than I paid for the whole lathe. well worth the money in time saving to not have to indicate in parts.

I found truing up a transmission shaft with an interrupted cut from it being off, recut it off. My answer is to square up with a tool post grinder. That may account for yours ending up still off .001". Confirming being actually bent can be confirmed between balance rollers, or by indicating the shaft @ the end as the shaft is turned in an accurate chuck with the part just in, in half way & mostly in.

Are you sure your crank flange is not off a bit? Are you indicating the shaft @ the top + the drive plate bushing surface as well as @ the bottom, near the flange when bolted together? I have found the drive plate bushing surface off enough to need a clean up grind on occasion.
I wouldnt/dont grind the main shaft other than for the output plate bushing.

If .001 run out is the best you can do, its acceptable, Remember, your bushing clearance is/should be around .004".(bored finish) Adding extra clearance isnt necessary.


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Re: Lathe chuck or spindle issue?

Post by 1923Touring » Thu Feb 16, 2023 5:06 pm

John, I am pretty sure that the shaft is slightly bent, as when it is rotated with the indicator on it, the high spots are 180 degrees from each other from the main shaft tip to the main shaft base. Additionally, when the brake drum is slid on, it binds a little before it seats all the way. This does not occur on my new shaft.

As regarding my crankshaft and run-out: The crank shaft flange has a little less than 0.00025 run-out. The run-out I measured regarding the engine was at the end of the transmission shaft, not the driven plate. I considered the number I recieved after machining good, because I took the same measurement with a new shaft and had 0.0005, which after the driven plate was added, resulted in a total of 0.005 run-out.

-Again, the purpose of this post was to learn how to successfully machine one of these shafts so that in the future I can repair my own.


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Re: Lathe chuck or spindle issue?

Post by speedytinc » Thu Feb 16, 2023 5:11 pm

Sounds like you are on track. Good job.
I find it quite satisfying to do a blueprint job restoring transmissions.

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