Untitled - Home Model Engine Machinist

Untitled - Home Model Engine Machinist
Components required for the jackshaft assembly. Two skate bearings, shaft, spacer, key,
timing belt, custom driven pulley (41 tooth), and new motor pulley. In addition to these
parts, there are two bearing blocks, one back plate and two stiffener bars, all made of
Aluminum, 6061-T6 alloy. The bearing blocks were sawn from a piece of 3/4" thick plate,
then mounted on the lathe in the 4-jaw chuck to be squared to dimension. The center of the
bearing bore was laid out and scribed on a coating of layout blue lacquer. The center point
was dimpled with a prick punch (60 degree angle point). With the marked-out block remounted in the lathe chuck, the dimple was brought to center using a dial indicator in
contact with a round bar that has a 60 degree point on one end and a center-drilled dimple
in the opposite end. The bar is installed with the point in the marked-out dimple, and the other
end is supported by the Tail Stock center. As the work is rotated slowly (and the bar prevented
from rotating in case the outside surface is not concentric with the centers), The chuck jaws can
be adjusted to bring the relative motion of the workpiece center mark to a perfect zero point.
After the jaws are secured, machining of the through hole and the counterbore for the bearing
may proceed. Be very sure that the face of the work is exactly perpendicular to the lathe axis.
The bearing bore diameter must provide a light press fit for the outside diameter of the bearing.
If the bore is too loose, consider using Loctite Bearing Locker solution at the time of assembly.
After the machined components of the bearing housing are together, mount the assembly in the
lathe with the shaft ends adjusted to center. Now, the contour on the backplate (which
simulates the shape of the motor) can be machined to the specified radius. The distance from
the middle of the shaft to the cutting point can be measured with a Dial Caliper (and add the
shaft radius) to determine when the final radius of the contoru has been reached. In the photo,
the left end of the assembly is held directly in the chuck. It can also be setup with no chuck
mounted, and a 3MT center inserted in the Spindle bore to support the shaft by it's center
dimples. A 1/4" diameter bolt about 3 inches long with two nuts can be mounted in a Spindle
flange hole to provide a drive pin for the free-spinning Aluminum parts. Stop rattles with a
strong rubber band to hold the part against the drive pin. Otherwise, it will surely bang hard
every time the interrupted cut is entered by the lathe bit. If a dead TS center is used, be sure it is
properly lubricated to prevent wear and (horrors) galling of the center-drilled dimple. Make
sure that there is no interference anywhere of the rotating parts with any portion of the lathe.
Note that the tool holder on the Compound of the lathe is mounted on a round post and is fitted
with a brass height-adjusting screw. This arrangement allows the tool cutting point to be
adjusted to the exact center-height of the spindle, and the holder can be rotated on the post to
make the angle of attack of the cutting edge at the best possible position.
plans can be found at http://www.savefile.com/projects/808585083
Machining of the contour on the backplate has begun. This is the second roughing pass at a
depth of approximately .020" (did not keep exact notes, as usual) running from the middle of
the backplate toward the left end of the assembly. When a nearly full radius had been
achieved, a left-hand cutting tool was substituted in the holder to remove the material from the
middle toward the right end of the assembly. The final clean-up cuts were made with a parting
tool mounted at a right angle to the work, which left a nice finish from end-to-end and cleaned
up the corners where the contour ended. The chuck jaws should have been padded with shims
to avoid leaving tracks on the finished part. Tracks on a finished part are embarrassing!
Keep a constant eye on the bearing point at the TS center, lube as needed.
The interrupted cut makes lots of very small chips that fly about at high speed. Be sure to wear
good eye protection to avoid an injury emergency!
As the machining nears completion, do a trial fit of the motor-mount flag against the new cut to
make sure that you have achieved a good fit. If the radius is too large, there will be a gap
between the parts at the centerline, requiring the mounting bolts to spring the flag when fully
tightened. This arrangement will be subject to loosening of the bolts because of vibration of the
parts when in service on the lathe.
Here is the 7x10 250 watt motor assembled with the motor mount weldment as furnished by
Little Machine Shop in the 14 inch bed conversion kit. The two M6 flathead screws are really
short, and engage the motor by only about 4 threads. Note that the shaft of the weldment has
been modified to place the flag vertical behind the motor when installed, not horizontal under
the motor as intended by the maker of the kit. This alteration was done by me in a fit of stupid,
I did not study the instructions well enough to understand what was supposed to be done. This
alteration did make the new jackshaft design easier to adjust the belt tension, I think, than it
would have been if done as directed by the kit instructions. On the bottom of the bed casting
you can see the lower belt adjusting screw and locknut, ready for insertion of the motor parts.
I placed the hex key (Allan)under the motor to stop it from rolling to an angle not suited for the
photograph, but, got in a hurry and did not position it out of sight for the actual snapshot. Grrr.
Note that the lathe does not have the standard chip pan under it. What you see is a large baking
pan from the restaurant supply isle of the local Smart and Final box store. The pan is round
about 18 x 26 inches by 1 inch deep, made of Aluminum sheet with a steel reinforcing rod
rolled into the edges. The first one we had was used as an oil catch pan for occasions when
something containing fluids had to be disassembled for repair. Then, the wife wanted one for
drying flower seeds and stuff. It is hard to keep one loose for shop needs.
The assembled jackshaft and motor components are test-mounted in the lathe bed to check for
the range of adjustments needed for final operation. The upper belt-tension adjusting screw and
locknut are installed in the lathe bed, and the jackshaft is in the approximate working position.
Since the threaded holes for the adjustment screws are included in the 14" bed kit, it was easiest
to make the stop bars for the top and bottom of the jackshaft assembly rather than try to drill
and tap new holes for the adjustment screws. The addition of the side bars also stiffen the
assembly and reduce the possibility of racking if the belt tension in the various directions is too
great for conditions. One less than perfect feature of this geometry is the fact that one of the
motor brush ports ends up on the underside of the motor. Servicing the brush will require
dismounting the motor, then having to re-do the belt tension. Not really a problem, though.
Note that this lathe has had the Change-Gear arrangement modified according to the plan posted
by Paul Hackathorn. The B/C idler gear shaft and bushing have been replaced with one that is
24mm long vs the 16mm standard shaft. The shaft is seen in the photo with two gears and a
spacer installed to accommodate the speed reducing scheme designed by Paul. Also, note that
there is no nut and washer on the Banjo standoff that mounts to the end of the lathe bed.
Instead, there is a spacer that has an M6 hole drilled and tapped for a clamping screw. An extra
thick washer with 82 degree countersink is used with an M6 flathead hex socket screw. Now,
the banjo can be adjusted for mesh of the C/D gear pair using a 4mm ball end hex wrench with a
T-handle. Much better than trying to tighten a nut with an open-end wrench.
Another view of the Speed Reducer components installed. Note that there is a fair amount of
adjustment available at the motor pivot point that threads through the side of the casting.
Visible is the motor power cord that passes through the opening in the bed to reach the
attachment points within the controller space. The jacketed cord is a little short of going all the
way to the controller terminal strip. Male and female connectors similar to those in use were
found to make extensions for the 3 conductors. Bad part - there should be some proctective
jacketing around the loose wires, such as a piece of shrink tubing.
The motor and jackshaft components are nice and rigid with this mounting system, and
adjustments are quite easy to make. The motor is adjusted from side to side by loosening the
two hex nuts under the mount with a 10mm open end wrench. In this view, the clumsy position
of the underside motor brush holder can be seen.
Also note that the long lathe bed provides a lot of extra length between the end of the 250w
motor and the end of the motor cover that could be used to mount an auxiliary cooling fan. I
have not checked, but there is probably an 120vac fan of about 3" diameter that would do a fine
job of improving motor cooling when doing long turning sessions at less than optimum motor
speed. If only computer fans of 5vdc or 12vdc were available, a "wall wart" power supply
could be pressed into service to operate a fan at safe voltages. A short piece of extension cord
with a female connector could be wired directly into the controller terminal strip, to accept the
plug-in transformer.
Here is a view of the jackshaft installation from behind the TS end of the lathe. The cover has
been completed and has the AC power cord installed. The cable clamp was salvaged from the
old motor cover by drilling out the two spot welds. The strain relief sleeve is included.
Both the upper and lower (sort of) jackshaft belt adjusting screws can be seen supporting the
jackshaft assembly in the desired location. Access is not too difficult, although the final
adjustment is easiest done with the motor set aside. The motor and it's mount are secured to the
foot-bar under the lathe by a single M6 button head cap screw. The hex in this size fastener is
4mm, as are most of the adjusting screws now used on this lathe. The issue rubber foot is
installed on the underside of the foot (Unistrut rail), in accordance with the drawing details.
Between the rubber foot and the open side of the rail is a large (1.5" OD) extra thick fender
washer, happily available in onesies and twosies from the shelf of my local Industrial Hardware
store. They stock every fastener I have ever wanted to use in the Minilathe shop, in every
quantity from 1 to 1000 and up. Of course, small quantities of fastener are more expensive than
buying by the box.
Here, it is possible to see the bumper piece that was added to the bottom edge of the motor
cover, needed so that the cover is supported by the bed casting, instead of hanging out in space
to vibrate and rattle while the lathe is operated. I can see, as these words are written, that the
left end of the cover should have been as long as the bumper extension, to more completely
protect the moving parts from wayward chips that would get into the works.
The motor cover is attached to the lathe bed, with all the moving parts well covered. You can
see the gap left between the cover and the bed, which could invite the entry of chips. The
clamp for the power cord is installed, but the cord is still missing. Final assembly is coming!
The cover tends to vibrate and make noise in this condition because the metal stops clatter
against the lathe bed. Another piece with a rubber nose has stopped all that. The cover was
cut from a handy piece of Galvanized sheet metal that had a former life as an underpan for an
electric cooktop. It is interesting what benefit you can realize by taking long walks around the
neighborhook every morning. The bumps and holes in the shape were features of theoriginal
part. Looks very nice in Minilathe Red. (Rust-Oleum American Accents Colonial Red #7925
Satin Aerosol Paint)
Coupled with adjustments made to the three potientiometers on the controller board, this lathe
modification (which requires no changes to the original hardware, save the clamp from the old
motor cover) has greatly expanded the versatility of the machine, provides lots of torque at the
slowest speeds and runs fast enough for all the jobs we have experiences.
Marty, of the 7x12minilathe list, has expressed concerns about overloading the various gears
between the spindle and the motor, but, no symptoms have appeared in several months of
usage. Hopefully it will continue to perform for a long time, probably longer than the operator
has before him. Look back to the photo on pages 19 and 23. Study the pulley arrangement.
You may have noticed that the large pulley on the jackshaft is made of clear polycarbonate
plastic sheet. The available sheet was .375, not the standard .394 inches thick. Works, too!
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The author and/or Manufacturers mentioned herein assume no liability
for damages, injury and/or loss of life from performing
the procedures mentioned herein.
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