© all rights reserved G W Howe 2017 - 2020
Projects
Project to fit a DRO on the BH600G lathe.
This project was a natural addition to the BH600
lathe and, although for me a luxury, it provides an
accurate use of the ‘X’ and ‘Z’ axis.
I decided to opt for a general kit supplied by
MachineDRO UK, which utilizes magnetic scales
rather than the optical ones. The main advantage
was the magnetic scales and housing is a better fit
to the cross-slide and also the magnetic scale can
be easily cut to length required.
I decided to mount the DRO display unit bracket
arm to the back wall behind the lathe swarf guard
and towards the tailstock end. Although not the
conventional way normally over the headstock
because as a right handed person it made sense
to make adjustments with my right hand, plus the control unit for the lathe start, speed etc. is also, in my case, fitted towards the right
end of the the back swarf guard.
The lathe is fitted close to a rear wall in my workshop and I was not wanting to move it, so fitting the rear scale to the lathe rear side of
the bed was a bit of a challenge. That said, although the bed fitting surface was as cast and not machined flat, fitting the aluminium
backing strip was relatively easy. The scale backing strip is an extruded shape so that the magnetic scale slides in to the centre part
and has a thin stainless steel cover. The assembly is quite rigid and to make life easier I only used two mounting screws, one at each
end plus spacing washers. This made the alignment task very
simple and since the read head is not in contact with the scale
then this fitting was more than adequate. The fitting of the read
head was again no problem though I found the brackets
supplied less than ideal but this was a minor task to alter. A
protection top cover keeps the scale secure and protected from
dirt, suds etc.
Fitting the DRO scale to the cross slide was very straight forward
as it was fitted to a machined surface. Once again, the supplied
kit parts for the read head attachment were totally unsuitable.
This was not a major problem and instead of using the supplied
aluminium castings a steel plate arrangement was made and
this included a ‘stop’ bolt to prevent the tailstock crashing into
the DRO scale.
The photo shows the cross-slide scale fitted and also the quick retract mechanism on the top -slide.
There was however a major consequence of fitting this new DRO scale to the rear of the lathe bed and the arrangement I had used to
operate the trip mechanism for the Screw Cutting Clutch was no longer practicable. For some time I had wanted to move the trip
mechanism away from the rear of the bed as setting the trips was inconvenient but now with the DRO scales close by I did not want to
be making adjustments and accidentally damaging the DRO unit. No matter, this was a problem I always knew would at some time
need resolving.
With the DRO successfully fitted and working as expected I now needed to resolve the Screw Cutting Clutch trip mechanism. I decided
that although it could be modified to remain at the rear of the lathe bed this was far from ideal and I never liked having to reach over
to make adjustments to the trips blindly. The better solution was plain to see and was to move the mechanism to the front of the lathe
bed.
Fortunately as it turned out, since I had previously
fitted a speed control inverter which had a remote
control unit I no longer used the lathe start /stop,
forward /reverse control handle fitted to a fixed
longitudinal rail just below the feed drive shaft.
However, since this rail was non functional it could
now be incorporated into the new design. So, the
screw cutting clutch lever now has a horizontal rod
attached with pivot pins at both ends and the RH
end attaches to the top of a vertical ‘rocking’ arm,
This vertical ‘rocker’ arm can move about a mid
position (set to give a 3:2 top:bottom)
approximate ratio. At the bottom of the rocking
arm is another horizontal rail pivoted with a pin
and at the other end slides in a block housing
which is now fixed to the unused lathe switch rail near the tailstock end. Two trip disks (shaped to pass under the apron) are clamped
as needed along the sliding rail, one for forward control and the other for return. The activating trip arm is a simple lever attached at
one end to the lathe apron and rotated into position as needed.
The photo shows the trip lever (just left of the hand wheel) set
horizontally in the ‘not in use’ position. Also an M4 screw fixes
to the apron on which the trip lever rests, this ensures the
lever does not accidentally rotate down into the active
position.
Operationally, when the trip arm is rotated to point down it
will make contact with the forward and reverse trip disks and
then on contact push the sliding rail forward or backwards as
appropriate when the carriage unit moves. The sliding rail
then causes a vertical rocking arm to rotate a small distance
which in turn causes the top rail to move and rotate the main
screw cutting clutch handle arm until the clutch disengages.
(note, the rocking lever ‘magnifies the carriage movement and this causes the screw cutting clutch to disengage more accurately).
From previous practical experience there is always a chance to forget to rotate the trip lever out of engagement after a screw cutting
task is completed. This obviously is essential otherwise it will
cause a lock-up situation and potential damage. A simple yet
very effective solution has now been incorporated as follows.
The trip lever can only rotate downwards after the ‘safety’ M4
screw has been removed from the apron (just left of the hand
wheel). and is then repositioned to a bracket attached to the
gearbox end (just above the chuck key). On screwing the M4
screw into the bracket it, via an attached micro switch, causes
an LED light to illuminate next to the lathe start control button.
Thus if all screw cutting is finished and the LED remains
illuminated then it is obvious the trip lever needs to also be
rotated back and the bolt re-attached to the apron. The bolt in
the apron also acts as an additional safety precaution since the
trip lever even if knocked cannot drop down unless the screw is
removed.
If the trip lever is set in the horizontal position, an LED indicator light fitted above the
start/stop control is not illuminated, however when the M4 screw is removed to enable the
trip lever to rotate to the down (engage} position and is transferred to a ‘keep’ plate fitted to
the end of the screw cutting gearbox it operates a micro switch which illuminates the LED as
shown above the Inverter control panel in the second image.
Since the operator has to press the green motor start button to start the lathe, this provides
an immediate indication as to the trip lever position and if not doing screw cutting, then the
trip lever needs to be rotated upwards and the M4 cap screw removed from the ‘keep’ bracket
and replaced into the apron. Whilst this may seem over the top, when I previously hat the trip
mechanism fitted to the rear of the lathe bed on more than one occasion I started the lathe
motor and on engaging normal cutting feed feed suddenly realised the trip lever was still
active. if the trip lever engaged a trip the carriage could now only travel a short distance and
it would lock up resulting in something the has to give! Fortunately, I never previously
experienced a major problem but on several occasions it was a close call!
GWH Engineering
creative engineering in a home workshop