What is "Work Tolerance"?

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dml66
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What is "Work Tolerance"?

Post by dml66 »

The manual my lathe came with states "Work Tolerance" is 0.005".

What exactly does that mean?

After meticulously aligning my tailstock, as best I can, I center drilled the end of a piece of 9/16" aluminum rod with a 2.25* projection from the chuck. Using a live center, I turned it down to the point where 100% of the surface was cleaned up using a carbide insert tool. The taper is right at 0.001" per inch of length measured with a Mitutoyo 0.0001" micrometer.

I pulled back the live center and turned an additional 0.0005" off, unsupported; still 0.001" per inch taper.

I don't really know what to expect as far as deflection goes but it seems odd to me the deflection would just so happen to agree with any residual tailstock offset. Which made me think of "Work Tolerance".
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Bill Shields
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Re: What is "Work Tolerance"?

Post by Bill Shields »

What is the native language of the writer of the document from which you are quoting the phrase?

Translated to English, that phrase can mean something entirely different to someone from Western Europe than someone from Asia.

At it's worst case, it can mean the closest repeatable tolerance that should be expected from the machine.
Too many things going on to bother listing them.
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Harold_V
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Re: What is "Work Tolerance"?

Post by Harold_V »

Some things to ponder.
Gripping material with a chuck or collet and following with a center isn't recommended. I make mention although it's something I do all the time. The reason is simple. The tailstock, in spite of being carefully aligned with the headstock, is rarely, if ever, truly concentric. When one uses a center, it forces the stock to rotate with a constant bend, or it causes the tailstock quill to be displaced. Which condition that happens is determined by the rigidity of the machine and its condition in regards to wear, as well as the size of the material being machined. Small diameters held in rigid machines simply bend without materially affecting the tailstock.

If a center is not drilled in situ, it isn't likely to run true, thus the tailstock must force the part to run true, or the eccentric center hole causes the tailstock to oscillate. Neiter condition is acceptable. Large diameter material in that scenario will cause the tailstock to oscillate.

Rule of thumb for lathe work is that it should NOT be extended more than three times its diameter, otherwise you'll experience deflection. You have violated that rules, so the results are not to be trusted. Please note that I am not trying to be rude, nor am I trying to be critical of what you've done.

If you'd like to determine how straight your lathe can turn, it should be done without the use of a center, and the material should be known to be free machining. Make the test piece much larger in diameter, so you can extend it less than three diameters, yet achieve a reasonably long cut. If you have some 1½" diameter aluminum, that would be acceptable. Once the material is rounded, the cut to determine straightness should be relatively shallow with a sharp tool, so cutting pressure is at a minimum. If your choice is something like 2024-T351 or T-4, a couple thou depth of cut will be adequate, and it will cut without skipping or tearing. The cut can be taken dry, but a small amount of kerosene or WD-40 would assure no tool buildup. The shallow depth of cut will eliminate deflection as a problem.

It is important that the bed of your lathe not have twist. That's usually the source of taper when turning without a center, although a well worn machine can be troublesome in that regard, too.

Turning between centers may demand that the tailstock NOT be in alignment with the headstock, so don't obsess with that bit of geometry. The reason is that the tailstock can be offset such that it compensates for error, yielding a straight turn when it is not in dead alignment with the headstock. If your machine is not pristine, you'll likely come to understand that each time you turn between centers, minor adjustment will be required if you're trying to establish a straight turn.

I can't address the .005" mentioned, as it's not clear what it meant, but even one poorly trained should be able to do work much closer than .005". It's not uncommon to turn to tenths on a lathe, although it does require one's undivided attention and an understanding of machining.

I might see things differently, especially with my comments below, if I knew more about your lathe. It would be very helpful for you to make mention of the machine--swing, horse power and weight, plus a report on its condition. A picture would be nice as well.

I took note of your use of an insert carbide tool. If it is negative rake, the results you achieve are pretty much useless to you in the way of determining how well your lathe performs, as negative rake demands greater rigidity than positive rake. That raises cutting pressure substantially. If you're using insert tooling on a fractional horse power lathe, I strongly recommend you rethink what you're doing. HSS for light duty machines is by far superior unless you are machining either hardened material or abrasive materials.

H
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atunguyd
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Re: What is "Work Tolerance"?

Post by atunguyd »

And here I am thought "work tolerance" was how much workload you can absorb relative to your remuneration before quitting!



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Bill Shields
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Re: What is "Work Tolerance"?

Post by Bill Shields »

Nah...that is bulls**t tolerance
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dml66
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Re: What is "Work Tolerance"?

Post by dml66 »

Thanks for the replies.

The machine is a Central Machinery 7" x 10" mini lathe. I bought it used, the only things I can see that were added to it is a QCTP and a tachometer. I have very limited shop space, less than half a 2-car garage so, it'll have to do for now.

As far as carbide insert tools go, it's all I have on hand right now, I do have a HSS set on order. In this case I used an E tool perpendicular to the workpiece. The inserts are triangles, I think that implies a 30 degree negative rake. Using an AR tool, I tried both a zero degree rake, and a modest positive rake, I really couldn't tell if the machine ran any differently, nor could I see any improvement in the workpiece surface.

In order to perform the free machining test I had to resort to a piece of 1.25" aluminum stock, I cut it so that 3-5/8" extends beyond the chuck jaws. I took off just enough material so the entire surface is clean, then made a light cut, about 0.001 if the vernier on the cross slide is to be believed.

This led me to another problem: my micrometer has a 1" maximum opening. I had to use my Mitutoyo caliper which reads to 0.001", it has a 4th digit but that's either 0 or 5.

Being as careful as I could be, and repeating the caliper measurements, the result is 0.003" taper over 3", so 0.001" taper per inch.

I'll keep the piece of aluminum and re-try the test when I have the HSS tools.
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Harold_V
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Re: What is "Work Tolerance"?

Post by Harold_V »

dml66 wrote: Tue May 24, 2022 3:17 pm I do have a HSS set on order.
Blank HSS is all you need. Don't rely on pre-ground toolbits. They rarely have the configuration that is required for your needs, and are most certainly less than sharp. They also may be ground for use with a lantern toolpost, so the relief and rake angles would be wrong for other use.

I can think of nothing in the home shop that is less rewarding than not being able to grind cutting tools for a lathe. If you take the time to learn the purpose of proper angles and gain the ability to fashion your own tools, you'll be rewarded with total freedom. It takes very little on your behalf---just a small pedestal or bench grinder and the proper wheel(s). The typical wheel that is supplied with commercial grinders of that description border on being useless, as they are too hard for the task at hand.
The inserts are triangles, I think that implies a 30 degree negative rake.
The shape of an insert does not determine its rake. Triangular inserts are made in both positive and negative. 30° negative would be quite extreme, likely to perform extremely poorly, even on robust machines.

Negative rake on the machine you described would be considered way out of the ballpark. It lacks the power and rigidity to benefit. Negative rake, unlike positive rake, doesn't cut the same way. The tip of a negative rake doesn't contact the workpiece (in theory) and demands a reasonable depth of cut in order to perform as designed. When you use negative rake with a shallow cut, that isn't true, so cutting pressure is elevated due to the metal being pushed off instead of being cut. That raises cutting pressure to an unreasonable level. When the proper depth of cut is taken, accompanied by an acceptable feed rate (generally quite fast) on a rigid machine with ample power, the cut is quite nice, although displays a coarse feed, even with a reasonable radius on the tip. None of that is true with lesser conditions.
Being as careful as I could be, and repeating the caliper measurements, the result is 0.003" taper over 3", so 0.001" taper per inch.
You didn't mention which way. I suspect it's larger at the end towards the tailstock, but is it? That would indicate that the machine lacks rigidity and is deflecting, or that there's twist in the bed. If it was smaller at the tailstock end, I would then suggest that the bed has twist.
It isn't beyond possibility that the headstock isn't aligned with the ways. Little effort is expended in those tiny machines, which are notorious for lacking rigidity and quality. Again, I'm not trying to be rude---just hoping to see you gain an understanding of what you can expect from your lathe.

No need for me to tell you that readings from a caliper are not reliable. Doesn't matter what type---it's the nature of the tool. In this case, however, while you can't say with certainty that you have a thou taper per inch, it's clear that the machine is not generating a straight cut. Now to determine why.

H
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dml66
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Re: What is "Work Tolerance"?

Post by dml66 »

I clearly need to gain a better understanding of rake angle, let me ask: If I point a triangular insert's cutting point directly at the workpiece, then I rotate the compound by 5 degrees, doesn't matter which way, does the rake angle change by 5 degrees?

I do have a fairly nice bench grinder, 8" with variable speed. It's outfitted with the stock wheels now, I mostly use it to to sharpen my lawnmower blade, lol. I believe it has a 1/2" arbor. What would be the best wheel for making my own HSS cutting tools?

With respect to the observed taper, the workpiece is wider near the chuck.

I tried another test which is more go, no-go style than one blessed with sensitivity and accuracy. Using a chucked, focusable laser, I mounted a reflective target on the right side of the tool post. Even with a collimator, it's difficult to achieve a true dot with a laser but this one is pretty good, it's designed for tool alignment. Rather than lining up a dot with a dot, I arranged the target dot to sit on top of the laser dot, like a figure 8. Running the carriage from left to right, the target dot clearly moves to the left of the laser dot, not by much, but enough that even my aged eyes can see it. Since the target dot represents the cutter tip, it confirms the direction of the taper, and that there appears to be a lack of parallelism.
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Re: What is "Work Tolerance"?

Post by John Evans »

2 things.
First get ahold of on line the south Bend how to run a lathe book,it will show you the various angles on a HSS tool bit.
Second , your problem may well in in the construction and alignment of your lathe. Lets see a picture of your lathe. Some of the Chinese lathes have adjustments for the alignment of the head stock .
But if you are getting a indication with your lazer you may have a bed twist issue. With out seeing your lathe and what it is and how it is mounted makes trouble shooting rather hard! I use a lot of carbide insert tooling on my 12X36 Clausing but it weighs about 1000 lbs. And most of my carbide inserts are postive rake with a chip breaker !! Taper getting smaller away fom the chuck sounds like deflection either of the material or the tool. How tight are your compound and cross slide gibs ? Any play there will cause issues. I personaly keep the compound locked using the screw on the side of the gib made for that purpose, not all lathe however have that locking screw.
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Harold_V
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Re: What is "Work Tolerance"?

Post by Harold_V »

dml66 wrote: Tue May 24, 2022 5:31 pm I clearly need to gain a better understanding of rake angle, let me ask: If I point a triangular insert's cutting point directly at the workpiece, then I rotate the compound by 5 degrees, doesn't matter which way, does the rake angle change by 5 degrees?
I am no mathematician, but I can say that there would be a minor change in the angle, but it isn't the 5° you mentioned. Rake is the angle on the top face of the tool from the tip of the tool towards the shank end of the tool, or from the tip of the tool to the side of the tool. All other features are relief.
I do have a fairly nice bench grinder, 8" with variable speed. It's outfitted with the stock wheels now, I mostly use it to to sharpen my lawnmower blade, lol. I believe it has a 1/2" arbor. What would be the best wheel for making my own HSS cutting tools?
I have posted on this subject previously. It would be much easier for you to check the threads than for me to answer questions that have already been answered, along with reasons for the comments made. If you'd like to read my comments, please follow these links:

viewtopic.php?f=44&t=76065
viewtopic.php?f=44&t=75985
viewtopic.php?f=44&t=75969

In those threads you can find the answers to your questions and much more, including how to mount wheels that are far better suited to grinding HSS. The greatest wheel selection is with 7" diameter wheels, ½" wide (more than adequate for grinding HSS tools) with a 1¼" bore.
With respect to the observed taper, the workpiece is wider near the chuck.
That is not normal. You have issues beyond tool deflection, and, with respect to John's comments, you should NOT experience that issue with a loose cross slide or compound rest. Any pressure applied via the cut taken should cause deflection, which would alter size, but not influence taper. Taper such as you described is generally a function of poor headstock alignment as compared to the carriage ways. The headstock may not be properly aligned with the carriage ways, or the bed is badly twisted. The amount of twist would most likely be difficult to determine with a common level. Even a Starrett model 98 may not provide the needed resolution. Start with the level condition of your lathe to try to determine if the bed is straight, or not. Level isn't really important--what is important is that any readings should be identical at each testing position. Making it level makes it easier, but it isn't a requirement for the lathe to turn straight.

H
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dml66
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Re: What is "Work Tolerance"?

Post by dml66 »

I decided to try and gain some insight into the headstock and the bed ways. After removing the headstock and cleaning up, the ways look fine to the naked eye, the two bolt holes used to secure the headstock to the V look clean, no burrs.

The headstock is another matter. The front milled (I use that word reservedly) V looks terrible, as if the part was whisked from the mill while still being worked. It's virtually impossible for me to know if this might be the cause of the taper but, if nothing else, I'm more inclined to suspect the headstock versus the bed.

Replacement parts are readily available and not terribly expensive, I suppose there's no guarantee such a part may have the same, or worse problems.

I've attached a picture of the bottom of the headstock.

Thanks.
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Harold_V
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Re: What is "Work Tolerance"?

Post by Harold_V »

It's clear that no effort was expended in assuring alignment. You are most likely correct that the fit isn't proper, and the error may manifest itself in both directions. A close inspection could reveal that condition.

The problem can be corrected, but with considerable effort. That isn't likely a solution unless you don't mind spending endless time scraping and inspecting. In the process you would have to ensure that the end result is that the spindle centerline is parallel to the ways both vertically and horizontally. That may not be the case now, judging from the results you've achieved.

Note that any alteration of the bottom of the headstock will result in the lowering of the spindle centerline. The tailstock may then not be on center vertically, although that condition, too, can be addressed.

That said, what I'd suggest to you is to gain experience with the machine and ignore the error as best you can. If you find you have a continued interest in machining, assuming it is within your ability to do so, step up to a more capable lathe. Beginners have a way of doing stupid things, often resulting in damage to the machine (I was no different). You can look at what you have as the price of education, sparing damage to the upgrade. When you come to realize you've outgrown what you have, you'll be far better equipped to judge one that would be adequate for your needs.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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