Many small lathes quote impressive but incomplete accuracy numbers in
their specs. For example, the Prazi SD400 (see
http://www.emachinetool.com/new/cata...?ProductID=180)
claims a "spindle accuracy of 0.0002". What would be a reasonable
expectation of total accuracy? Say the variation in diameter along 12"
between centres?

(Ken Strauss) wrote in message om...
Many small lathes quote impressive but incomplete accuracy numbers in
their specs. For example, the Prazi SD400 (see
http://www.emachinetool.com/new/cata...?ProductID=180)
claims a "spindle accuracy of 0.0002". What would be a reasonable
expectation of total accuracy? Say the variation in diameter along 12"
between centres?

I don't think .0002" is necessarily exceptional for spindle runout. My
Chinese 12 x 36 is at least that good. The question of variation in
12" between centers would probably depend more on how well your
tailstock center is aligned with your headstock and how accurate your
centers are. You should be able to turn something between centers to
the accuracy of your spindle TIR which in the case of the Prazi should
be .0002".

"Phil Teague" wrote in message
om...
(Ken Strauss) wrote in message
om...
Many small lathes quote impressive but incomplete accuracy numbers in
their specs. For example, the Prazi SD400 (see
http://www.emachinetool.com/new/cata...?ProductID=180)
claims a "spindle accuracy of 0.0002". What would be a reasonable
expectation of total accuracy? Say the variation in diameter along 12"
between centres?

I don't think .0002" is necessarily exceptional for spindle runout. My
Chinese 12 x 36 is at least that good. The question of variation in
12" between centers would probably depend more on how well your
tailstock center is aligned with your headstock and how accurate your
centers are. You should be able to turn something between centers to
the accuracy of your spindle TIR which in the case of the Prazi should
be .0002".

Center line-up probably is the biggest practical factor, although it
shouldn't be hard to get them perfectly lined up, assuming the lathe has
normal tailstock adjustments.

In a cheap lathe another factor is straightness of the bedways. A little sag
or vertical arch to the bed will have little effect, but a curve in the
X-axis will have a pronounced effect. This is easy to detect by turning a
sample piece and charting its actual diameter every inch or so along its
length. Straight taper is a tailstock alignment problem; anything else is a
bedway problem.

Ed Huntress

I hear this question every so often, at work and here.

I don't think I've ever seen a lathe incapable of producing accurate results
(such as a lathe should be able to produce). I think they only time this
really comes up is when the left end of the ways are worn due to the
carriage staying near the chuck.

Other than that, I think it's a foolish question. My standard answer at work
is something to the extent of, "it will make parts as accurately as you are
capable of making them".

Regards,

Robin


"Ken Strauss" wrote in message
m...
Many small lathes quote impressive but incomplete accuracy numbers in
their specs. For example, the Prazi SD400 (see
http://www.emachinetool.com/new/cata...?ProductID=180)
claims a "spindle accuracy of 0.0002". What would be a reasonable
expectation of total accuracy? Say the variation in diameter along 12"
between centres?

In article , Phil Teague wrote:
(Ken Strauss) wrote in message om...
Many small lathes quote impressive but incomplete accuracy numbers in
their specs. For example, the Prazi SD400 (see
http://www.emachinetool.com/new/cata...?ProductID=180)
claims a "spindle accuracy of 0.0002". What would be a reasonable
expectation of total accuracy? Say the variation in diameter along 12"
between centres?

I don't think .0002" is necessarily exceptional for spindle runout. My
Chinese 12 x 36 is at least that good. The question of variation in

as is my 4015 3n1. well, close, i could only guess, it was less than
1/2 a thou. what impressed me was the the perimeter of the chuck wasn't
much worse. now, i cannot actually remember the 3 jaw accuracy, but it
was better than i expected, certainly better than the rigidity expected
from the lathe. but that is moot, right? i did not have any really
"round" stock for testing the chuck.

12" between centers would probably depend more on how well your
tailstock center is aligned with your headstock and how accurate your
centers are. You should be able to turn something between centers to
the accuracy of your spindle TIR which in the case of the Prazi should
be .0002".

what, steel? rigidity seems to be the limit on my Grizzly 4015, which
weights 380lbs or so. the lathe base is approx the same size as the 9x20
chinese imports. that Prazi does look nice, pricey little devil. --Loren

"Robin S." wrote in message
...
I hear this question every so often, at work and here.

I don't think I've ever seen a lathe incapable of producing accurate
results
(such as a lathe should be able to produce). I think they only time this
really comes up is when the left end of the ways are worn due to the
carriage staying near the chuck.

Other than that, I think it's a foolish question. My standard answer at
work
is something to the extent of, "it will make parts as accurately as you
are
capable of making them".

Regards,

Robin


"Ken Strauss" wrote in message
m...
Many small lathes quote impressive but incomplete accuracy numbers in
their specs. For example, the Prazi SD400 (see
http://www.emachinetool.com/new/cata...?ProductID=180)
claims a "spindle accuracy of 0.0002". What would be a reasonable
expectation of total accuracy? Say the variation in diameter along 12"
between centres?



This is one of those cases where hobbyists have an advantage over commercial
machinists. For 100 years, it's been traditional in home shops with limited
lathes to lap parts that require extremely accurate diameters. Commercial
machinists don't have the time for that kind of stuff. With lapping, you can
produce a consistent diameter of 0.0001 in. or better.

And, today, we have those nifty lapping films that make it so much easier.

Ed Huntress

That specification is the spindle runout. They are claiming the spindle
runout is .0002 or less. That doesn't mean you can turn with that accuracy.

Factors that would affect your accuracy in real life conditions would be
tooling and setting thereof, hardness of the work, adjustment of the
machine, feed and speed (does a prazi have power feed?), & coolant use.

Typically when I need to make gauges on my Southbend I use a toolpost
grinder because it is too difficult to take off .0001 with a single point
tool, and its hard to hit the dimension dead on the first time.

Even in my production days we were expected to turn to tenths with a
Hardinge turret machine, and it took a lot of tinkering and adjustment. With
turret tooling you make you adjustments one time and lock it in. Plus we had
to let the machine run for awhile and let the bearings reach operating
temperature.


--
Tony


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"Ken Strauss" wrote in message
m...
Many small lathes quote impressive but incomplete accuracy numbers in
their specs. For example, the Prazi SD400 (see
http://www.emachinetool.com/new/cata...?ProductID=180)
claims a "spindle accuracy of 0.0002". What would be a reasonable
expectation of total accuracy? Say the variation in diameter along 12"
between centres?

Ahh yes the warm-up period. When I worked at an aerospace contractor all the
union guys wanted to work in grinding or the toolroom. They would get to
work at 6 am, turn on the spindles and go have coffee for 45 minutes so
their machines could "warm up". I think they pushed it just a bit.

ds

Even in my production days we were expected to turn to tenths with a
Hardinge turret machine, and it took a lot of tinkering and adjustment.
With
turret tooling you make you adjustments one time and lock it in. Plus we
had
to let the machine run for awhile and let the bearings reach operating
temperature.

"das" wrote in message
k.net...
Ahh yes the warm-up period. When I worked at an aerospace contractor all
the
union guys wanted to work in grinding or the toolroom. They would get to
work at 6 am, turn on the spindles and go have coffee for 45 minutes so
their machines could "warm up". I think they pushed it just a bit.

It's very possible they did, but modern machines that work in the region
around one tenth or less often need a lot more time than that to warm up.
IIRC, Wasino's S-mu lathe settled down at around two hours, which is
considered very fast at that level of accuracy (guarenteed +/- 20
millionths). And a C-frame mill of high-accuracy pretensions, such as the
Makino extreme toolroom machines of a decade or so ago, can require hours
more.

Ultimate toolroom machines, like the discontinued Wasino Wing Ace $300,000+
surface grinder, should be left running all the time.

Ed Huntress

In article , Tony says...

Even in my production days we were expected to turn to tenths with a
Hardinge turret machine, and it took a lot of tinkering and adjustment.

The spindle and spindle bearings are of course going to be
vastly different between the Prazi-in-question, and your
hardinge example! That right there makes up most of the
difference in behavior.

I think the Prazi spec is basically quoting what a dial
gage will read, if it clocks the OD of the spindle - which
of course is a nonsense number. The real number is, how
much does the dial move when you put a broomstick into
the spindle hole, and push on it with 30 lbs of force?

The hardinge turret tooling sure does come in 'tenths'
micro-adjustable versions - one sees it all the time on
ebay.

Jim

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12" between centers would probably depend more on how well your
tailstock center is aligned with your headstock and how accurate your
centers are. You should be able to turn something between centers to
the accuracy of your spindle TIR which in the case of the Prazi should
be .0002".

Turning a 12 inch long shaft with 2 tenths taper on a small machine
is a pretty tough job. I highly doubt I could achieve this with my
myford or my rockwell. Especially if the shaft has a small diameter.

chuck

"Other than that, I think it's a foolish question. My standard answer at work
is something to the extent of, "it will make parts as accurately as you are
capable of making them"."


One thing often overlooked is that an beginning machinist trying to
make accurate parts needs a lathe with more intrinsic accuracy than
some old fart who has learned to live with the inaccuracies of his
lathe and still make good parts.

brownnsharp

"das" wrote in message nk.net...
Ahh yes the warm-up period. When I worked at an aerospace contractor all the
union guys wanted to work in grinding or the toolroom. They would get to
work at 6 am, turn on the spindles and go have coffee for 45 minutes so
their machines could "warm up". I think they pushed it just a bit.

Ummm, no. When I was grinding, the machine wouldn't settle down
sometimes for as long as a couple of hours. Not much of a factor on
production, but in a tool room situation where tolerances were
tighter, (much) the variation would have been intolerable. Most
times, minutes saved turned into hours wasted.