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Proxxon MB200 Drill Stand Modification

Add an adjustment screw to control height more carefully than by using the handle!

I have a mini drill setup made by Proxxon, comprising an IBS/E drill, an MB200 drill stand, a KT70 compound table etc. which is quite convenient for milling the occasional small thing. One of its greatest shortcomings, however, is its lack of a proper adjustment screw in the z direction, i.e. height or depth. The graduated depth scale is absolutely useless e.g. if you want to mill a groove to a given depth with any precision.

I saw this beautiful modification by Kris Avonts, and started thinking about doing something similar. I came up with a plan which seemed somewhat simpler than Kris's. I use a plain old M6 screw instead of a micrometer head, and instead of only pressing downwards to move the drill, I can both push and pull it, downwards or upwards (though to maintain any accuracy, the height should be locked before milling annyway). The limitation of my system is that it only has some 20 mm of travel at one time, although the depth can be coarsely changed to place that 20 mm working range wherever you like.

The materials needed are a tiny length of aluminum angle profile (40×40 or 50×50 mm or similar), a couple of M4 machine screws, and an M6 bolt and lock nuts. You'll need drill bits and taps to make 4 mm and 6 mm holes and M4 and M6 threads. Finally, you'll want some kind of knob for the vertical adjustment, which you can either make yourself, or just use some ready potentiometer knob or similar.

First the drill holder needs to be disassembled somewhat. To remove the return spring which pulls the drill upwards, first remove the screw at its lower end. Then grab the square end of the spring with pliers, pull it downwards until you have a length of straight steel wire visible, and tease it out sideways, out of the slot in the moving drill holder (the yellow part). That part should be a couple of centimeters lower than its highest position, so there's room for the spring to bend outwards. Once the lower end of the spring is loose, pry up and remove the knob holding the spring at the top. It is simply hooked onto the spring. Then remove the spring entirely. (You can also find youtube videos of how the spring is attached, for example this one.)
Finally the black, countersunk Allen screw which holds together the drill holder and the rack and pinion's toothed bar needs to be removed, as well as the the plastic height scale indicator / limiter assembly. Now the moving drill holder will come off, sliding downwards out of the stationary green assembly.

I cut a small piece of aluminum angle profile. Mine was 40×40 mm in its dimensions, with a material thickness of 3 mm. (I would have preferred 50×50×5 mm profile, but had none in shelf, and it's only sold in 2 or 3 meter lengths, if at all. Damn.) I drilled two 4.2 mm holes at one side near the end, about 10 mm apart. (4.5 mm holes would do just fine, but I had a 4.2 mm bit available, and thought the tighter tolerance for the M4 screws can't hurt.) Both holes are countersunk as well on the profile's "outer" side.
But I didn't drill both immediately. I first drilled the one further from the end. I then bolted the piece onto the movable drill holder (the yellow part), and drilled the second hole first with a 3.2 mm bit, and all the way through the drill holder part as well. After removing the angle piece, I drilled its hole to 4.2 mm as well, and I threaded the new hole in the drill holder with an M4 tap. I went to all this trouble to ensure that the holes are the exact same distance apart both in the angle piece and in the drill holder part.

Next I drilled a 5 mm hole at the top side of the green assembly. I then threaded it with an M6 tap. I put the hole along the centerline of the assembly, quite close to the front. This place is not ideal, as the future adjustment screw will only have some 20 mm of downwards travel before it hits the rack and pinion's gear. You could get a longer length of travel if you locate the hole further back, beyond the gear. This will, however, require a longer horizontal length for the angle piece (I doubt even the 50×50 mm profile would reach far enough), and it will also put quite a bit of torque on it—I'm not at all sure it would have worked very smoothly!
You can now clean up and re-assemble the entire assembly, though you might not want to re-attach the spring just yet. But while it's still open, why not grease up the moving parts of the rack and pinion. You might also check the condition of the sliding dovetail surfaces, perhaps smooth them out with fine sandpaper, and goop them up as well with good quality grease. That will make it move a lot more smoothly!

To go into that M6 threaded hole, I made a thumb screw out an M6 bolt and a piece of 20 mm diameter Nylon rod. With the help of my dividing head, I marked ten divisions on it, like there are on the hand wheels of the compound table. Just like those, the M6 height adjustment screw also has a travel of 1 mm per revolution. I engraved the divisions in the knob's top surface and sides, and also dragged a thin black marker along the grooves. I drilled a hole through its center, and made a recess below the surface level for the bolt's head. I ground the head into a flattish shape, and made the recess similarly flat, so the knob cannot spin freely on the bolt. I attached it onto the bolt with a washer and a lock nut. I placed a couple of additional lock nuts at strategically selected positions. Their purpose will become clear soon.
Oh, man, this thumb screw is ugly! :)

I drilled a 6.5 mm hole in the angle part's top side, at a position that will line up with the M6 threaded hole I made, when the angle piece is installed. I then cut the angle piece to length, and made a slot from the hole to the edge, as shown here. I used a hack saw to make a rough cut, and then milled it more nicely with the Proxxon. The thumb screw fits into the slot as shown in this photo. There's a bit of sideways play, which is intentional, but I adjusted the lock nuts so that the screw just turns freely, but with hardly any play along its axis, i.e. in the vertical direction which it will be adjusting.
Finally I attached the angle piece onto the drill holder. I used two countersunk M6×16 screws, one of which was the perfect length for the upper screw (which reaches all the way through the angle piece, the (yellow) moving drill holder piece, and screws into the threaded hole in the rack and pinion's toothed bar. The other one had to be cut to length some 5 mm shorter. Both will be affected by how far the holes in the angle piece are countersunk. I wasn't clever enough to countersink just the right amount to use the original screw here.

Here's the angle piece and adjustment screw in place. The screw can be attached and removed easily, since the angle piece has a slot for it, rather than just a hole. When attaching the screw, first bring the drill holder down to a suitable height, and insert the bolt into the slot of your choice between lock nuts. This coarse adjustment helps mitigate the limitation of only having about 20 mm travel adjustable by the screw. The lower-most lock nut limits how far the screw can go, so it just does not touch the rack and pinion's gear.
Once this was in place and tested, I re-attached the spring, and was done! If you will only ever use the drill stand for milling and will only ever adjust the bit height with this new thumb screw, you can even leave the spring out.

The verdict? Ugly as hell, but it works! And it is a lot more precise than trying to read off millimeters from the awful depth scale. One revolution is exactly one millimeter, and that's it. I did not bother to put any kind of pointer to indicate the divisions marked on the knob—I generally just eyeball a number at a suitable direction and rotate away. I don't expect this mechanism to be that precise anyway! There is a bit of play, in fact, but unlike the axes of the compound table, the height adjustment is spring loaded, so most of the time even that bit of play is all but removed. But it's still a good idea to tighten the original locking screw on the height axis before starting to mill anything.

A serious word of warning!

Wear eye protection! And stuff. You can hurt yourself with power tools. Once is all it takes sometimes.

Antti J. Niskanen <uuki@iki.fi>

	First the drill holder needs to be disassembled somewhat. To remove the return spring which pulls the drill upwards, first remove the screw at its lower end. Then grab the square end of the spring with pliers, pull it downwards until you have a length of straight steel wire visible, and tease it out sideways, out of the slot in the moving drill holder (the yellow part). That part should be a couple of centimeters lower than its highest position, so there's room for the spring to bend outwards. Once the lower end of the spring is loose, pry up and remove the knob holding the spring at the top. It is simply hooked onto the spring. Then remove the spring entirely. (You can also find youtube videos of how the spring is attached, for example this one.) Finally the black, countersunk Allen screw which holds together the drill holder and the rack and pinion's toothed bar needs to be removed, as well as the the plastic height scale indicator / limiter assembly. Now the moving drill holder will come off, sliding downwards out of the stationary green assembly.
	I cut a small piece of aluminum angle profile. Mine was 40×40 mm in its dimensions, with a material thickness of 3 mm. (I would have preferred 50×50×5 mm profile, but had none in shelf, and it's only sold in 2 or 3 meter lengths, if at all. Damn.) I drilled two 4.2 mm holes at one side near the end, about 10 mm apart. (4.5 mm holes would do just fine, but I had a 4.2 mm bit available, and thought the tighter tolerance for the M4 screws can't hurt.) Both holes are countersunk as well on the profile's "outer" side. But I didn't drill both immediately. I first drilled the one further from the end. I then bolted the piece onto the movable drill holder (the yellow part), and drilled the second hole first with a 3.2 mm bit, and all the way through the drill holder part as well. After removing the angle piece, I drilled its hole to 4.2 mm as well, and I threaded the new hole in the drill holder with an M4 tap. I went to all this trouble to ensure that the holes are the exact same distance apart both in the angle piece and in the drill holder part.
	Next I drilled a 5 mm hole at the top side of the green assembly. I then threaded it with an M6 tap. I put the hole along the centerline of the assembly, quite close to the front. This place is not ideal, as the future adjustment screw will only have some 20 mm of downwards travel before it hits the rack and pinion's gear. You could get a longer length of travel if you locate the hole further back, beyond the gear. This will, however, require a longer horizontal length for the angle piece (I doubt even the 50×50 mm profile would reach far enough), and it will also put quite a bit of torque on it—I'm not at all sure it would have worked very smoothly! You can now clean up and re-assemble the entire assembly, though you might not want to re-attach the spring just yet. But while it's still open, why not grease up the moving parts of the rack and pinion. You might also check the condition of the sliding dovetail surfaces, perhaps smooth them out with fine sandpaper, and goop them up as well with good quality grease. That will make it move a lot more smoothly!
	To go into that M6 threaded hole, I made a thumb screw out an M6 bolt and a piece of 20 mm diameter Nylon rod. With the help of my dividing head, I marked ten divisions on it, like there are on the hand wheels of the compound table. Just like those, the M6 height adjustment screw also has a travel of 1 mm per revolution. I engraved the divisions in the knob's top surface and sides, and also dragged a thin black marker along the grooves. I drilled a hole through its center, and made a recess below the surface level for the bolt's head. I ground the head into a flattish shape, and made the recess similarly flat, so the knob cannot spin freely on the bolt. I attached it onto the bolt with a washer and a lock nut. I placed a couple of additional lock nuts at strategically selected positions. Their purpose will become clear soon. Oh, man, this thumb screw is ugly! :)
	I drilled a 6.5 mm hole in the angle part's top side, at a position that will line up with the M6 threaded hole I made, when the angle piece is installed. I then cut the angle piece to length, and made a slot from the hole to the edge, as shown here. I used a hack saw to make a rough cut, and then milled it more nicely with the Proxxon. The thumb screw fits into the slot as shown in this photo. There's a bit of sideways play, which is intentional, but I adjusted the lock nuts so that the screw just turns freely, but with hardly any play along its axis, i.e. in the vertical direction which it will be adjusting. Finally I attached the angle piece onto the drill holder. I used two countersunk M6×16 screws, one of which was the perfect length for the upper screw (which reaches all the way through the angle piece, the (yellow) moving drill holder piece, and screws into the threaded hole in the rack and pinion's toothed bar. The other one had to be cut to length some 5 mm shorter. Both will be affected by how far the holes in the angle piece are countersunk. I wasn't clever enough to countersink just the right amount to use the original screw here.
	Here's the angle piece and adjustment screw in place. The screw can be attached and removed easily, since the angle piece has a slot for it, rather than just a hole. When attaching the screw, first bring the drill holder down to a suitable height, and insert the bolt into the slot of your choice between lock nuts. This coarse adjustment helps mitigate the limitation of only having about 20 mm travel adjustable by the screw. The lower-most lock nut limits how far the screw can go, so it just does not touch the rack and pinion's gear. Once this was in place and tested, I re-attached the spring, and was done! If you will only ever use the drill stand for milling and will only ever adjust the bit height with this new thumb screw, you can even leave the spring out.