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Prazi,
the short word for precision in German, is back. This year,
at the April Names show, I met John Szot, distributor and
importer of Prazi products in North and South America. I
was impressed by the Prazi SD 300 metal lathe and the accessory
BF 400 milling head, which attaches on a column to the back
of the lathe. This combination of lathe and mill-drill head
makes for a very versatile machine.
I
picked up the lathe and milling head at International Sales
and Marketing Group in Huntington Beach, CA. Also, I brought
home most of the accessories available for them. Everything
was checked against the packing list to see that all the
items were included with the shipment. Nothing was missing,
so I continued to unpack the lathe and set it up on my heavy
beech woodworking bench. The weight of this lathe is 100
lbs., which gives it the heft that makes the machine really
perform. However, one does not need special lifting equipment
to carry this lathe home and set it up on the bench.
Next,
I went inside and read the manual. I am reminded of another
manual that came with German machine. It translated: "Achtung!
Better to read the manual first." There was a little
cartoon showing nuts and bolts flying from the lathe for
those who didn't follow this advice. The manual is well
written and the black and white pictures are very clear.
The
lathe is almost ready to be used when received. I mounted
the gear cover, safety shield and installed all of the handles.
Next,
I installed the milling head support that is attached to
the back of the lathe bed. I found that the hole used to
fit a locating pin which protrudes from the milling column
support casting was a little undersize. I suspect it was
supposed to be reamed and this step was left out when the
lathe was manufactured. I carefully enlarged the hole with
a rattail file and mounted the part. Four mounting bolts
hold this part onto the base of the lathe. Also, four bolts
with locking nuts are used to align the column with the
ways. The 68-lb. milling head was heavy, but I managed to
get it into position by myself. One nice thing about this
size machine is that it can still be manhandled by one person.
I then installed the quill handle and the handle that moves
the head up and down on the column. I found no problems
with this operation.
After
removing the top cross slide, I proceeded to mount the milling
table accessory. It is made of aluminum and measures 4.9
x 6.6". It is held onto the cross slide with four socket
head cap screws, which thread into T-nuts. The manual specifies
that the socket head cap screws should have washers underneath
each screw head. I found that if they are left out, the
end of the screws will foul the cross slide. The information
for this step is clearly stated in the manual that came
with the milling machine head, but I got in a hurry and
didn't look at this manual. See, I didn't follow the warning
about reading the manual!
The
lathe has four spindle speeds selected by changing the belts
on the pulleys. These speeds are 300 through 2,400 rpm.
The pulley and gear cover must be first lifted and then
rotated to gain access to the interior. These gears are
made of plastic material - probably nylon - and when the
machine is operated, little noise results from the gears
as compared with metal gears. One nice feature is the spring
loading on the drive belt system. It provides automatic
tension for the belt drives.
In
most lathes, a half nut engages the lead screw to move the
carriage along the ways. The SD 300 is a little different.
To engage the lead screw, a lever is lifted and moved to
the left. This would be done with the operator's left hand.
When it becomes time to disengage the lead screw, the lever
is pushed to the right without having to lift it again.
The carriage can be cranked back to its original position
or powered back by reversing the lathe motor. The reverse
switch should never be operated while the lathe motor is
in the "on" position.
Threading
on this lathe requires a little different technique than
that of most larger lathes. The lead screw engaging lever
is used to control the start and ending of a thread cut
operation. It is never disengaged at the end of a cut. The
lathe is stopped and then the tool bit is backed off from
the workpiece by moving the handle of the cross slide. The
lathe motor is reversed and the on button is pressed. The
carriage is then returned to its original position for the
next cut, if required.
As
received, the cross slide and carriage gibs were adjusted
for a nice sliding fit. They weren't too loose, and both
move very freely without excessive play.
CROSS
SLIDE AND SADDLE HANDWHEELS
The manual specifies that the maximum play of the cross
slide and the saddle handwheels should be two divisions,
which would be .002". This was checked and found to
be within specification. If necessary, this can easily be
adjusted to bring the backlash within tolerance.
HEADSTOCK
AND MAIN SPINDLE
The headstock is a robust aluminum alloy casting of
.625" thickness in some places. The back is open to
accommodate the motor mounting plate.
The
main spindle has two precision bearings. One is an angular
contact ball bearing and the other is a radial ball bearing.
Provisions are made to adjust the axial play in the spindle
be using adjustment nuts. The spindle has a Morse taper
MK2 on it. The inside diameter is .472". The three-jaw
chuck that comes with the lathe has three studs protruding
from the back. When installed on the spindle flange, the
nuts hold the chuck securely in place so the chuck can be
run in either direction without fear of it becoming loose.
The one drawback to this design is that the lathe operator
will not win a speed contest in changing chucks.
MOTOR
The lathe motor has a rating of 475 watts. It is designed
to operate on 110V AC at 60 Hz. The output of the motor
is specified to be 250 watts, which would be 1/3 horsepower.
BED
The bed of the lathe is a high-grade continuous casting.
It is different than a lot of lathes in that it is round
shape with a flat surface on the top. The length is 19.44"
and the diameter is 1.58" with a 1.38" flat on
the top. This way is ground on the carriage engaging surfaces.
There was no evidence that it would easily deflect. The
way length allows about an 11.25" length bar to be
held between the face of the three-jaw chuck and the center
placed in the tailstock. A longer bar could be turned, if
it protrudes into the chuck or if it is turned between centers
using the Turning-Between-Centers accessory attachment.
The lathe is specified to turn a 12" long bar and I
am sure this could be done if the Turning-Between-Centers
attachment is set up on the lathe.
TAILSTOCK
The tailstock has a Morse WT1 taper in it and a travel of
1.57". The easily red dial on the handwheel is 1.57"
in diameter and has 50 divisions. The dial is on a sliding
ring, so it can be zeroed and will lock in this or any position.
One turn of the handwheel advances the quill .059".
This works out to be a little over .001" per division.
Although I didn't take the tailstock apart to verify the
pitch of the quill feed, my calculations indicate a pitch
of 1.5mm. There are no graduations on the quill. Also, no
provisions are made to set the tailstock over for taper
turning or the purpose of alignment. The feed of the handwheel
has a nice feel when it is turned.
COMPOUND
SLIDE
The compound slide is made of high-duty cast iron. Both
the cross slide and the compound slides have dovetails and
there are gimbals to adjust the play. The compound has a
protractor that allows radial movement of 45°. It pivots
about a little pin and is locked in position with four screws.
The pivoting compound allows for taper turning of short
workpieces and bevels. Not obvious in the photos is a socket
head cap screw located on the backside of the carriage.
This is used to lock the carriage to the ways when performing
operations like facing and cutoff functions. Perhaps this
is not as conveniently located as the locking device in
some lathes, but I found it was not hard to operate with
the hex wrench supplied with the lathe.
Mounted
on the top of the compound is a toolholder that can hold
four tool bits. They are held in place with two socket head
cap screws on each side. A lever in the center allows the
tool post to be rotated to any position and then locked
in place.
The
handles on the slides are the same design as those on the
tailstock. They also have locking sliding ring type dial
scales.
LEAD
SCREW
A sliding and thrust bearing are located in the headstock
that supports the lead screw. At the handwheel end, a hexagon
nut allows for adjusting the axial play of the lead screw.
A wear-resistant bronze nut is used in the carriage to transfer
the lead screw motion to the carriage. The lead screw is
driven through a dog clutch attached to the lever that engages
the feed. This is located on the headstock end.
SCREW
THREAD CUTTING
The chart on the front of the lathe allows for cutting
a quantity of ten threads from 11 to 26 tpi. There are 18
metric threads the lathe can also cut. However, after a
call to the friendly people at International Sales and Marketing
Group, I found out the lathe can cut many more inch threads.
They can supply a copy of this chart and plan to include
it in future manuals. The finest thread on their list is
60 tpi. I was very pleased to discover this fact. I believe
a lathe with maximum diameter capacity of 5" should
have the capability to cut fine threads since it is going
to be used for smaller projects.
BF
400 MILLING HEAD ACCESSORY
The milling head accessory is powered with an electric
motor with a nominal output of 180W. Four speeds are available,
ranging from 365 to 1800 rpm. The speeds are selected by
changing a belt accessible by loosening two knurled thumbscrews
on the side cover and rotating towards the top of the housing.
Two hex bolts are loosened on the top so the motor can pivot
a little to adjust the belt tension. The belt is then moved
to the selected pulley groove position. The housing is also
made from an aluminum alloy, and is of a heavy construction
and quite solid. The milling head can also be mounted on
a table, which is another accessory.
The
spindle has a stroke of 1.97" and has an MT2 taper.
An arm moves the spindle for drilling holes on the machine.
Also, the milling head can be fed up and down by turning
a crank on the top of the column.
A
special feature Fine Feed accessory is available and it
is attached directly over the top of the spindle. It has
a handle and dial at the top for feeding the spindle. This
item is easily removed for access to a drawbar accessory.
The
head also can be tilted in the long axis of the table and
a degree scale is located on the side. The scale has a range
of plus or minus 60° with increments of 1°.
LATHE
TESTS
A .625" diameter round bar was placed in the three-jaw
chuck and the other end supported by a center in the tailstock.
A cut was taken at both ends using the same setting. The
headstock reading was .6222" in diameter and the tailstock
reading was .6228". A difference of only .0006".
Not bad at all. I was impressed.
A
tenths dial indicator was placed against the spindle. The
TIR was found to be around .0002".
Next,
I machined a flywheel of 3.25" diameter to see how
the lathe machines a larger diameter. I was quite pleased
with the performance of the lathe. A slow speed adapter
is also available that further reduces the spindle rpm of
the lathe.
I
thought it would be interesting to cut a thread using the
method required to make threads on this lathe. I have cut
many threads on a lathe that uses the conventional half
nut engagement on the carriage. I found stopping the lathe
and reversing the motor to be a little different for cutting
threads but not at all difficult. The thread I cut was easily
made and turned out quite well.
Cutting
off a bar is a good test of the solidness of a lathe. I
decided to cut off a piece of oil hardening tool steel in
its non-hardened state. I compared this to the same operation
on my 10" lathe. The cutoff operation went along very
smoothly, although it didn't feel quite as solid as the
larger lathe. I didn't sense and amount of chatter.
Overall,
I was very impressed with the quality and operating characteristics
of the Prazi lathe and milling head. It will be a real asset
in my shop for the smaller projects for which it was designed.
It will also light up my shop since it is canary yellow
- not obvious in the black and whit photos.
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