Please excuse my mistake in yesterday's posting on this topic.
The two quarter-inch rods should be 34 inches long instead of 33. I
copied the dimensions from my original design. If you have already
cut the rods to length, you can do what I had to do: Cut two one-
inch-long pieces of quarter-inch rod, drill through with a #19 drill
and use 1 and 1/2 inch long 8-32 screws to extend the top of the
rods. The 33 inch rods restrict the quill travel by about 1/2 inch.
The corrected information is listed below.
I have really enjoyed my dovetail mill_drill from Wholesale
Tool, except for one failing. The quill could drop up to .034"
without warning. Needless to say this can cause disaster.
This problem occurs on many types of vertical mills, and is due
to slop in the rack (which is cut into the quill) and its matching
pinion. The pinion is on a shaft which rotates and lifts the quill
up, assisted by a helical spring. When you reverse the shaft, it
turns several degrees before it begins to push the quill down.
However, only friction holds the quill from dropping to take up the
slack in the pinion-rack. The make-do solution is to keep tension on
the quill lock, but if the lock is loosened a bit, the quill may or
may not drop suddenly.
My solution is to use a gas spring to hold the quill up. I bought
two (always buy a spare if it is surplus) gas springs from
www.surplus center.com This is item 1-1676 at $2.99 each, and there
are 3290 of them left. It has 30 lbs of push, and has a 5/16 cross
hole at each end for mounting.
Ideally you might want a traction gas spring, but these seem not
to be available in surplus, and are very expensive when made to
order in Germany.
The solution is to mount the gas spring vertically and use it to
push two 1/4" rods up. These rods are 2.125" apart, center to
center, and are fastened
via a 1.25" x 3" x .125" steel plate to the cast iron clamp which
circles the quill. Cut out the plate to clear the depth guage nut
and fasten with four machine screws.
The two 34" long rods are tapped on each end, and are guided by a
1" x 3" x 3/32" thick plate attached to the top of the gear box.
This same plate holds a 5/16"D x 1.5" rod which threads through the
bottom of the gas spring cylinder. The tops of the two rods are
fastened together with a 5/16"D x 3" rod which has two #19 holes
drilled through it 2.125 inches apart. This rod also threads through
the end of the gas spring rod and has steel spacers to hold the gas
spring rod centered.
All pieces are held together with 8-32 machine screws.
One of the two tension rods also operates the sensor on my
Chinese DRO vertical scale, via a milled flat and a 4-40 machine
I removed a bit of the tension on the helical spring to make the
quill feel more sensitive.
The result is great. No quill drop even with a heavy 4" diameter
mill attached to the quill.