© all rights reserved G W Howe 2017 - 2020
Projects
Project to design a belt drive fine feed mechanism on the BH600G lathe.
This project started some time ago following the making of a Screw Cutting Clutch unit designed by Gray Meek and based on
that used on a Hardinge lathe. The BH600G lathe originally had a standard tumbler reversing arrangement to select forward
and reverse feed or screw cutting direction. With the introduction of the screw cutting clutch the tumbler unit was removed as
it already incorporated forward and reverse operation.
Yet another version has now been implemented.
The original design was basically very simple and easy to make in a couple of days So why change it? With time on my hands
plus a desire to alter the design to provide feed in both directions. This has been nagging me for some time even though I
rarely use feed towards the tailstock but when needed my original design failed this requirement. The original design follows
on below but you can jump ahead to the new design article here.
In its original configuration the BH600G lathe provided forward and reverse
operation using the tumbler mechanism for both screw cutting and fine
feed operation. In the case of fine feed operation, the gear train provided
the power to the gearbox input gear. With the completion of the screw
cutting clutch I decided that my next project would be to attempt to reduce
the running noise which emanated from the gears in the gear train which is
ever present as power feeding availability is always needed. Despite several
efforts to reduce this noise using oils and even special open gear additives
the noise was still unacceptable especially as the machine workshop is
immediately below the main living room! This became a major issue as the
screw cutting clutch had additional gears and this would slightly increase the
noise but more importantly the gears and bearings in the clutch would wear
quicker because the mechanism would be operational all the time due to
the need for a fine feed drive. I had already thought along these lines when
discussing the initial design requirements with Gray and asked that he
incorporated a lever to easily disengage the clutch gears from the rest of the
gear train. Of course with the lever disengaging the clutch it also
disengaged the drive to the feed shaft and removed any fine feed!
I prefer to accept a compromise solution when it comes to problem solving rather than build in complex designs which
inevitably out-weigh the main user need. In my case the screw cutting clutch, which normally operates at slow speeds,
provides the perfect solution for screw cutting needs and the resulting noise issues are minimal because the speed is low. This
then left the need to design a solution to provide a quite drive for the fine feed especially as it operates all of the time and at
faster spindle speeds. Initially this problem was solved by introducing a simple belt drive from the spindle direct to a pulley on
the gearbox input shaft bypassing the gear train and this worked incredibly well, it was quiet and effective. but I was concerned
it may cause wear on the gearbox input shaft bearing. The only other negative was the this new arrangement would provide
fine feed in the forward only direction. This was not a major problem as most times I need forward direction and in the event I
should have to use the reverse direction then this could still be accomplished by reverting back to the conventional gear train
including the noise implications but for me, this is rarely needed.
The direction issue could be sorted by introducing another tumbler mechanism into the mechanism but this would over
complicate things considerably and also introduce more noise. In practice I rarely use reverse direction for feed but in the
event it became essential then the screw cutting clutch can provide this even if somewhat noisily! The other remaining issue
was the coarse feed rate which using the original set up with gears and tumbler mechanism gave ae bottom-end feed rate of
0.0047” and the new belt drive method provided an opportunity to resolve this issue.
After some experimenting a final design evolved. The new arrangement is much the same as before with a belt being driven
from a pulley fixed to the lathe spindle driving another pulley. Attached to the lower pulley spindle is a small 25t gear (20PA,
1.25MOD) which drives another 25t gear and this finally meshes to a 50t gear on the gearbox input shaft. The lower pulley
and gear mechanism is fixed to a sliding arm which enables the gears to be quickly and precisely engaged to the 50t gear by
way of an adjustable stop to restrict meshing movement .
There are several advantages to this new mechanism:
•
it provides a much slower input to the gearbox shaft providing a range of
fine feed rates, the slowest is 0.0017”
compared to
the original 0.0047”
•
noise level is very low
•
load on the gearbox input shaft is almost eliminated
•
the design is simple, elegant and easily engaged at correct mesh
•
quick and easy to remove giving full access to the main gear train
A free set of drawings in A4 PDF format are available on request
Latest revised design (May 2020)
The original design was basically very simple and easy to make in a
couple of days but whilst it worked without problems it did have some
short comings. It could only provide feed towards the headstock; it was
time consuming to change over to screw cutting as the fine feed unit
needed to be removed; it required the gear door to be opened to make
adjustments; there was no intermediate setting for no feed.
These negatives are very minor but I always felt that the fine feed unit
should replicate that originally provided with the stock machine. The
reason it was changed from the stock version was initially to incorporate
the screw cutting clutch mechanism and to provide a finer feed rate plus
of course reduce gear noise. With the stock version all gears in the train
from the spindle gear to the gearbox were in motion at all times and unfortunately the large 120/127 gears were very noisy
having a ringing sound which I found annoying and impossible to resolve.
The new revised fine feed unit builds on the simple one direction
version and adds another gear which enables forward and reverse
operation plus a neutral mid position. The belt drive is still used as this
eliminates all the noise and now has a jockey pulley with a spring to
maintain tension. The spring has a tubing surrounding it to prevent any
noise. In practice very little belt tension is needed as the gearbox
provides gear speed reduction to the final fine feed main drive but in
the event of a crash situation the belt would minimise the resulting
damage by slipping unlike the direct gear drive which is unforgiving as it
has no clutch build in. Belt tension is adjustable. The new design is
fitted once and remains in position however if access is needed to make
up a special screw cutting train the complete unit is removed by undoing
a single nut centre) and 2 M6 screws that connect to the lever
extension(lower centre). The fine feed assembly is mounted on a shaft
which is fixed to a base plate which in turn is fixed to the lathe
headstock base casting. The base plate arrangement enables
adjustment of the shaft for alignment as the casting may not be square.
In addition the new assembly also incorporates an adjustable centre of
rotation, again to facilitate adjustment of meshing gears but also its
design provides extra bearing surfaces from front to back on the shaft
maintaining squareness. This bearing enables the rotation of the
assembly and gears to stay in alignment to the shaft axis with no lateral
movement.
Operation of the unit is done without having to open the end gear
casing door and incorporates a similar lever handle in exactly the same
way as the levers are used for the gearbox selection of speeds and
feeds. The fine feed lever can be changed whilst the lathe spindle is
turning though this is optimal when the spindle speed is not running at
high speed. The reverse direction, towards the tailstock or in the case of
the cross slide towards the operator makes use of a 30 tooth gear unlike
the forward which has 25 teeth. This does mean that the feeds are not
identical but both considerably finer than that originally available. In
use the belt drive and gears run almost silently needing occasional oil
lubrication. If needed the complete mechanism can be quickly removed
and replacement requires no special alignment process as the lever
operation determines the position and meshing of the gears.
When it comes to wanting to set up screw cutting operations the fine feed driver 50t gear to the gearbox is removed (single M6
screw) and exchanged for the appropriate gear, commonly a 40t gear plus M6 fixing screw, which covers most screw cutting
threads. The fine feed assemble can remain in situation as it is clear of the main screw cutting gear train. If the casing door is
removed then if needed the fine feed drive belt can easily be removed by moving the jockey pulley to slacken the belt tension.
Originally I planned for the index plate to be mounted on the gear cover
door but in practice found this resulted in significant additional noise
being amplified by the door being made from thin sheet steel in a box
form. To get round this I added a new index plate which is attached to
its own stand away from the door, which in turn is mounted to the lathe
base tray. This tray as supplied is folded at the sides to improve rigidity
and conveniently provides a base on which the new stand is fitted using
a a steel square fitted under the folded ridge. This arrangement
ensures the index plate is clear of the door and so no additional
resonance is created and the door can be opened easily. The base plate
of the stand is fixed with two M6 bolts to the top of the front fold with a
sturdy square bar under and also has a third M6 bolt which provides a
simple jacking method of adjusting the vertical alignment of the stand and index plate so that whilst close to the door it has
about 1mm of clearance and the door can be opened unhindered by the stand. This fixing arrangement has proved to be very
effective and the stand is completely rigid allowing the selector lever to latch as needed.
Making the unit is easy but takes more time than the previous version as there are more parts to make.
If conventional screw cutting is to be done using the 40 tooth gear as a direct input to the gearbox then the existing 50t fine
feed gear can be easily swapped over by undoing the M6 gear attachment screw. This process is done easily and quickly with
the belt drive left in place (unlike the previous version) or if preferred the belt can be removed but this is not essential.
GWH Engineering
creative engineering in a home workshop