Simple problem yet confused result

[panofish]

I am a novice machinist. I am using a sherline lathe to set a diameter to a piece of aluminum round stock. My problem is a simple one, yet I am confounded as to what is happening. I am using the sherline 4 jaw chuck with tommy bars. I put a piece of aluminum rod in the chuck about 1 inch in diameter and 6 inches long. I cut along the side to make it perfectly round for the last 1-2 inches and also face the end. Now I unchuck and flip the aluminum and chuck the end I just dimensioned into the 4 jaw chuck. Now I repeat and round the end of the stock.

The problem is... no matter how many times I flip and round the ends and rechuck... the other end wobbles and is not centered?

My cuts are very light so I can't imagine part deflection accounting for the amount of wobble I am seeing.

Am I correct in assuming that you should be able to flip the rod and expect to have little or no wobble on the end after machining both ends?

 

[Cogsy]

A couple of things: a 1" bar with 5 or so inches protruding from the chuck will deflect very easily (way more than you'd expect). The rule of thumb is 3 times the diameter maximum for unsupported work, it's especially important for small diameter materials.

Is it a 4 jaw self-centering chuck or independent? If it's independent then you need to dial in on the machined end when you flip it, then you should be able to eliminate the 'wobble'. If it's self centering then there will be some runout and that's why independent 4 jaw chucks are so handy.

 

[panofish]

This is what I've learned so far. Please add your commentary and feedback and let me know if you agree with my analysis. Thanks.

Lesson#1: It would appear that it is difficult to rechuck a 6" rod so that the end of the rod will be accurately centered as the chucked end is. If you need to rechuck or flip a rod and maintain precision... you must use a center drill and a live center on the tail stock.

Lesson#2: A 6 inch length of 1 inch diameter aluminum will deflect an almost imperceptible amount while being cut on the side. Such that, the deflection decreases as you get closer to the chucked end of the rod. The end result will be a very small taper to the aluminum rod. The diameter nearer the chuck will be smaller than the end of the rod. The amount of deflection is proportional to the distance from the chuck, the thickness of the aluminum, and how aggressive the cut is. Again.. this is another reason to use a live center.

Lesson#3: Unlike wood, sanding an aluminum rod on the lathe (320 paper) has very very small impact on the dimension of the rod, but it will smooth a rough cut surface nicely. Doing the same with a rod made of wood on a lathe will quickly change the diameter of the wood where ever you sand. When I measured the diameter of the aluminum rod before and after aggressive 320 sanding, the difference was not measurable with my digital calipers (not even a thousandth of an inch).

 

[Herbiev]

Centre drill and live centre should fix your problem.

 

[panofish]

Now the reasoning behind the concept of "turning between centers" and using a dog becomes clear to me.

 

[Tin Falcon]

Now the reasoning behind the concept of "turning between centers" and using a dog becomes clear to me.

Looks like you are getting it .
But be aware you must insure the tailstock is on center left to right or you will still geta taper when turning on centers . One way to turn tapers is purposely offset the tails stock.

I recommend getting a copy of army TCTO 9-524

It is a public domain document available for download as a pdf.

As far as 6 inches of a 1" rod sticking out unsupported it should not be done.
rule of thumb is 2.5 times diameter. Some lathes you may be OK with 3 on a sherline more like 2.

Tin

 

[panofish]

Yeah... the pain of turning 3 parts that are junk and spending the better part of my Sunday on it. Mistakes and confused results like that are worth more to learning than reading from a manual.

Sure wish I had some machine shop training. Perhaps some day I may try my local community college, but timing is not doable as it conflicts with my day job.

What is the accurate technique for measuring and insuring the tailstock is centered left to right?

 

[Cogsy]

Not sure it's the best way, but what I do is put a center in both the tailstock and chuck so that the points come together. Place something nice and thin between them, like a blade from a feeler gauge or a razor blade, and bring the points into contact with it. The deflection of the thin material shows you the misalignment of the tailstock. Adjust it and keep repeating until the points hold the thin material straight between them. That's close enough for what I do and doesn't need any special equipment.

 

[Brian Rupnow]

Panofish--I'm in a good position to answer this because I am going through that exercise with my lathe this week. Although there are a number of videos on youtube showing two different methods of doing it with dial indicators, and although you can spend $100 for a pre-machined hardened and ground "test bar", my local machine shop guy who has been running a lathe for fourty years said this to me today. The quickest, surest way is to put a center in the chuck end of the spindle and bring the tailstock up close and check it by eye from directly above. If the centers look to be aligned (no visible offset), then put the chuck back on and with about 1/2" of material extending out from the chuck center drill a 60 degree "center hole" in the end of the material. Then flip the material end for end and put a "center hole" in the opposite end. Then support the material between a center in the tailstock and a center in the spindle end, put a lathe dog on the driven end to transmit torque to the part, and take a .005" to .010" cut about 1/2" long on one end of the material. Record the setting on your cross feed dial, then back the tool out, run your carriage down to the other end of the mill, turn the cross feed back in to the same setting and machine about 1/2" of the other end of the material. Now use a micrometer to measure both ends. If the ends are the same diameter, then the tailstock is aligned perfectly. If the diameters are different, then your tailstock is out of alignment. Put a dial indicator against the side of the tailstock housing and offset it by the amount of "taper" to correct the problem. Then take two more cuts and measure again to make sure you moved the tailstock the correct direction. If the stock measures smaller at the tailstock end, then the tailstock has to move away from you. If the stock is larger at the tailstock end, then the tailstock has to move towards you.

 

[portlandron]

I would like to add to Brian's statement, about checking center alignment, bring them together with a piece of thin flat stock between them. If the centers are in good condition and they are aligned the flat stock should stay at 90 degrees from the center line of the lathe. If the centers are not aligned it will be at an angle other then 90 degrees.

 

[barnesrickw]

He method Mr. Rupnow mentions is the exact method shown on an instructional video I just watched.

 

[10K Pete]

There is one small source of error in the method described to Brian. That
is moving the tool and getting it back in exactly the same place. This
variable can be eliminated entirely by using a bar with each end a bit
larger in diameter than the body. That way you don't have to move the
tool on the cross slide. Just run the carriage to the other end and make
the second cut without resetting the tool. Known as the two collar
method. Same test, you just don't move the cross slide.

I loctite an aluminum collar on each end of a steel test bar I have to do
this test. And if the collars are ever (?) cut down too much, I'll heat the
aluminum a bit to remove them and put new ones on! Plus I'll always
have the undamaged steel test bar.

Pete

 

[abby]

Whilst the comments are informative at no point has it been stated whether the 4 jaw is self centering or independent. It is almost certainly an independent chuck as few model engineers have enough use for a 4 jaw self centering chuck to waste precious funds on such a beast.
So panofish why are you putting round stock in an independent 4 jaw ? do you not have a 3 jaw ? if not I would suggest that should be your very next purchase .
Setting work accurately in an independent 4 jaw requires some experience and ideally a dial indicator.
Although the procedure is fairly obvious ie. slack off a jaw and tighten it's opposite number , when getting down to a thou or less quite a small pressure can move the part a long way and a beginner my resort to pulling hair out.
in the case of a 1" bar 6" long you should be able to accomplish an accurate result without the need for a centre as only just over half its length needs to be projecting from the chuck and it will not deflect unless your lathe has a badly worn or mal-adjusted headstock.

 

[panofish]

Thanks abby, I've tried my 4 jaw scroll self centering chuck and 3 jaw scroll self centering chuck and a 4 jaw independent chuck with dial indicator. I got similar results with all 3. My conclusion is not that the deflection was as much a problem as was the fact that you can only depend on the chucked end of the part being centered. The opposite end of the part can not be reliably centered without using the tailstock. And you shouldn't use the tailstock center to force that end to center unless you use a dead center in the spindle as well. I was also trying to machine one end of the part, then unchuck it, flip it around and rechuck it to machine the other end. This is how I discovered that is not a good idea if absolute accuracy and concentricity of both machined ends is important.

I have now decided to machine between centers and use a dog. I will machine both ends to 2 different diameters without flipping or removing the part. I expect good precision and concentricity of the 2 diameters on each end.

I also double checked the alignment of my headstock and tailstock. The sherline is very accurate from the factory, however the headstock has a key to accurately align it. There is an extremely small amount of play in the headstock alignment with the key. I tweaked a small misalignment of the headstock and now the spindle and tailstock are perfectly aligned.

 

[abby]

How will you machine the end that is being driven by the dog ?

 

[panofish]

I will not machine that end... I will cut it off.
So, I will have to use a little longer piece of stock and throw away the extra.

 

[10K Pete]

How will you machine the end that is being driven by the dog ?

Usually one moves the dog to the other end....

Pete

 

[panofish]

Move the dog to the other end?
On my sherline lathe, the dog is driven by the faceplate which is on the spindle.
Not sure how I could move the dog.

 

[10K Pete]

Move the dog to the other end?
On my sherline lathe, the dog is driven by the faceplate which is on the spindle.
Not sure how I could move the dog.

You're turning between centers, right? So take the part out of the lathe,
move the dog to the other end of the part, and put the part back between
centers!

Pete

 

[pkastagehand]

If the headstock is not aligned with the bed properly, he'll have problems turning the part around in chucks. If the jaws of the chucks aren't parallel he will also have problems when turning the stock around. Didn't seem like anyone mentioned these two conditions.