Construction Notes For mill/drill Depth-stop

Mill/drill depth-stop - For most imported mill/drills. (DXF and PS) - ready soon!

The standard depth-stop is a sorry affair, and probably rarely used by most folk. Not only is it very slow to set, but the seating for the screw adjuster on my mill/drill was left as-cast which meant the threaded rings wobbled instead of stopping firmly at the required position. This is a pity because the depth-stop is actually a very useful accessory. Definitely a case for a re-think.

What's needed is something that can be set in a couple of seconds, can be moved back to the top with equal facility, and is positive in action. I decided to replace the threaded rod with a plain rod, and designed a clamp using a cotter which has sufficient grip to resist the considerable force that can be applied by the quill lever arm. A scale was added together with the ability to use spacers of fixed length, a useful way of working to measured depths differentially.

The new bush cemented in place.

First job is to remove the original threaded rod, which turned out to be 39/64" diameter (or 15.4mm, take your pick - neither are standard) with some unidentified thread. The rod was a loose fit in a hole fractionally under 3/4" diameter which was in the cast lug on the mill/drill head. I decided to make a gunmetal bush for this hole to provide a flat seating, and to make a new stop rod from 5/8" silver steel. The hole was first reamed 3/4" by hand, an easy job from the underside once the quill clamp ring is removed. The bush was made from 1" dia material, drilled 5/8" (not reamed - you want a running fit) and the outside turned to 3/4" diameter for a length of 3/4" leaving a head the full 1" diameter and 1/8" thick. The bush was cemented in place using Araldite.

The original threaded rod had a length of 12 mm thread on the end which screwed into the quill ring, and this was reproduced on the end of the 5/8" rod. You'll not have much luck doing this in the lathe with a die as the rod will likely slip in the chuck. If you can at least get the die started in the lathe you can then transfer to the bench vice to finish the threading (use some copper packing to protect the workpiece). A better alternative is to screw-cut if you have a metric machine, it's important this thread is straight otherwise the rod will not seat squarely.

The clamp body is made from two round pieces of mild steel bar silver soldered together. One piece is 1" diameter and 1-1/2" long, the other 5/8" diameter and 1-1/8" long. Drill and ream the 5/8" piece 3/8" right through, then set up in the milling vice holding by the ends. Use either a 1" diameter endmill, or a small fly-cutter set to cut 1" radius, to cut a semi-circular recess 3/8" deep and central to the bar. This piece should now sit on the 1" bar. Place it so that it is 1/16" from one end, apply flux round the joint and heat to dull red. Touch a little silver solder wire to the joint and it will flow right around filling any gaps. Allow to cool and then dunk in pickle for a short while to loosen the flux residue. Clean up using a fine wire mop mounted on drill or grinder.

Now a tricky operation. It's necessary to cut out from the 3/8" hole that part of the 1" bar which now protrudes into it. I find the easiest way of tackling this job is to use a slot-drill, a drill would be pushed to one side and in the worst case might jam. The slot-drill will cut on it's end face and will not wander off-line. A slot drill of 5/16" or (better) 11/32" will do the job, cut as much metal away as you can - use several passes to shave close to the original 3/8" bore. Finally, pass the 3/8" reamer through again.

I think if I were to do this job again I might consider using a center drill to just mark the end of the 3/8", then silver solder the two parts together before drilling and reaming 3/8" using the center mark as a guide. Either option works but you might find the second method a little easier. Just make sure the 5/8" diameter part is truly vertical before attempting to drill it as there will be no hole to guide the drill.

Turn a piece of 1/2" brass rod down to 3/8" diameter for a length of 1.115", and drill No.3 and thread 6 mm right through (I found this the best size). Part off leaving a head 1/2" diameter and 1/8" thick. This piece will be used to plug the 3/8" bore whilst the 5/8" hole is being bored. Later it will form the clamp cotter.

The 1" diameter bar was made 1/2" longer than it's finished size so that you can now hold it in the lathe to drill the 5/8" hole, this extra length will be milled off later. With the brass plug held in place by a 6 mm bolt, drill and bore to 5/8" diameter, this will shave the brass plug to depth of 1/16". Remove from the chuck and take out the brass plug, turn the 1/2" diameter head to 3/8" diameter then use a 1/16" parting blade or fine saw to cut it in half. One half (the longer bit that had the 1/2" head) should be re-chucked and drilled right through 6 mm clearance, the other end is left threaded. Now you can mill off the extra 1/2" length of the 1" bar.

A small ball-handle will look nice to use as a clamping handle, mine used 5/8" and 1/2" diameter balls, and the total length was 2-3/8". Turn a flat on the 5/8" ball 15/32" diameter and drill and tap 6 mm. Make a 6 mm threaded rod 1-9/16" long and loctite this in place in the ball handle.

Now assemble the clamp body onto the 5/8" stop rod and try out the clamping action. You should find when tightened that the cotter clamps very securely with little effort, it might take a little time to bed-in. If the handle ends up in an inconvenient position it's a simple job to shave a little off the plain-drilled end of the cotter to move it round to a better position. Each .039" will represent one full turn so if you need it moved around 1/2 turn take off about 20 thou.

The scale can be a home-made affair using a piece of 1/4" x 1/16" bms, the scale can be engraved using the lathe cross-slide to measure the distances between the marks. If you are lucky, you might be able to pick up a scrap 6" scale from an old depth measure. I acquired 3 at one time (thought they might come in handy for something!), having a 6" long imperial scale on one side and metric scale on the other, engraved on a 1/4" wide piece of steel. It's a simple job with a 1/4" endmill to put a groove in the stop rod to inlay the scale, it can be Araldited in place.

The clamp body can be loosened and moved with a flick of the wrist, which offers another facility in the form of fixed length spacers that can be slotted onto the 5/8" stop bar. Such spacers could be made from 1" bar drilled through 5/8" then milled down the sides to form a 'U' groove. A selection of these can be parted off to length to cover 1/16" steps for common sizes. Say you wanted to drill (or mill) to exactly 1/2" depth, just run the drill or mill down to touch the work, insert a 1/2" spacer under the clamp body and tighten it, then remove the spacer. Easy.

Another quick way of setting is to use the inside jaws of your callipers. Set to the desired depth and lock them, move the drill/mill to touch the work and insert the callipers between the bush and clamp body and then tighten the handle.

One tip (in retrospect), it would be better to make the stop rod first before the bush, screw it in place and check that the clearance aound the 3/4" hole in the cast lug is even. It may turn out that the hole does not quite line up with the threaded hole in the quill ring (no wonder they leave plenty of clearance!) To get around this, measure the amount by which the 5/8" stop rod is off-center, then turn up your bush with an eccentric hole to compensate (just use the 4-jaw to hold the bush for drilling).

Addendum: After using the stop for a while I decided that it would help to key the clamp body to the stop rod. Having decided the best position for the locking handle I was not entirely happy about it being able to move around at will. A second slot was machined the length of the stop rod 1/8" wide and 1/16" deep, and a hex grubscrew (with it's end turned to a 1/8" diameter stub) used to locate in the slot.

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