Stand for mini-drill

As soon as you want to drill a printed circuit board, for example, you need a drill stand. If, moreover, you use carbide drill bits, which are much more effective than the high-speed steel ones, but also much more brittle, a very rigid stand must be chosen, which doesn’t bend during use. This also applies if you want to use the mini-drill as a milling machine — for plastic only, do not ask for the impossible!

So, the criteria of choice will be the base thickness, the column size. Then the working distance between the column and the drilling axis, the maximum working height, and so on. It is also necessary that there is the minimum of backlash, but this is difficult to verify on catalogue or in the store! Obviously, the stand must be compatible with the drill collar.

Proxxon MB 140/S stand

Of course, I chose a stand compatible with my Proxxon drill. There are two possible models: MB 140/S and MB 200. I chose the cheapest. We’ll see if I was right or not.

Speaking of stiffness and minimum backlash, it’s OK. The column is a chrome-plated steel cylinder with a 20 mm diameter and a 300 mm length, fixed in a cast aluminium base having a 30 mm maximum thickness, equipped with a 90° guide and fixing holes for vice or XY table. The drilling head slides on high and low spans with a distance of 75 mm and is guided by a chrome-plated rod located 40 mm from the column axis. The distance between the latter and the drilling axis is 150 mm. Finally, the drill axis inclination is adjustable from –90 to + 90°.

This is interesting, but I soon discovered some mechanical weaknesses.

Mechanical weaknesses

First incident: break of the pivot which allows to tilt the drill axis. The cast aluminium did not withstand the screw pressure, without the least washer to distribute the force, due to the too small metal thickness at this place (estimated: 1 mm).

It should be noted that this possibility of tilting the drill axis is not very useful, in any case for drilling, since the travelling axis remains vertical! I used it for example by positioning the drill horizontally, equipped with a saw disc, to make a kind of mitre saw.

Repair: I replaced the original BTR M5 screw with a longer one so that I could insert wide washers leaning on the outside of the part, which has a 10 mm thick crown. The stand tapping is 2 mm thick. It had to hold, and, so far, it held!

Broken drill stand

Repaired drill stand

Second incident: the locking screw on the column no longer tightens, and jumps. On examination, the thread is intact; then is the tapping — in aluminium — messed up? Well no! With a BTR screw in place of the original screw, the tightening is rather good, yet there is only one tenth of a millimetre more on the outside screw diameter.

I have equipped the screw with an Allen key (thank you, Ikea!) glued permanently, but it has to be glued again from time to time! This repair has been lasting for years, despite many manoeuvres.

You can notice on the screw a spacer which is not used as an extension, but which simply compensates for the too long screw length for which I had no choice!

Tightening on column

Latest news: the tapping in the aluminium ended messing up. Therefore, I removed the remaining thread by drilling to ø 5 mm and added a hexagonal nut behind the flange.

There is also a defect on the base square guide: it has backlash, and you can tighten the knurled screw to the maximum, nothing to do! Cause: the trapezoidal cross-section tongue supposed to guide and block the assembly is too thin to get caught in the groove. Remedy: a 2/10 mm thick brass plate drilled and shaped to slightly increase the total section. With this, no more backlash, even with a moderate tightening.

Square guide

For Proxxon equipment, it has many defects, isn’t it? The €70 model is perhaps better, but I cannot say anything about it, except that according to the photo, the guidance is done by trapezoidal slides, thus with adjustable backlash, and that the whole head rotates, which allows oblique drilling. It seems to me that the sliding can also be blocked for milling, but without micrometre adjustment.

Stand enhancements

Height adjustment for milling

Before I had an almost actual milling machine, and since I had installed an XY table on the stand, I wished I was able to adjust the working height to mill plastic parts, such as a wagon chassis to stick an elongation drawbar. It is always possible to use the depth stop, but then you have to keep a hand on the lever, and you are not sure to be able to reproduce the same depth because you have no reference.

So, I installed a long M6 threaded rod (1) (thus, the advance, equal to the pitch, is 1 mm per revolution). This threaded rod is screwed into the fixed part of the stand (2) and leans at the bottom on the movable part, the contact zone of which is protected by a brass or nickel silver plate (3). The rod length, about 180 mm, is sufficient to let you handle it easily without being hindered by the column or the depth stop.

There is not much choice to place this rod, trapped between the return spring that surrounds the column and the depth stop screw. Its axis must be 10 mm from that of the stop screw. A potentiometer knob for ø 6 shaft can be mounted at its upper end.

Naturally, one should not expect a great precision of this device. On the other hand, it must be used in conjunction with the depth stop, otherwise, it happens that the mill bit engages in the material and punctures it (incident experienced!). In other words, the movable part of the stand must be completely fixed.

Milling screw

Adjusting backlash

The guiding of the movable part takes place at two points: on the column on the one hand and on a ø 5 metal rod (1) on the other hand. Column side, there is a not adjustable backlash. But on the other side, the rod slides into a bronze nut (2) which has practically no backlash at first but gradually takes on due to wear.

The remedy is to split this ring (2) to give it some flexibility on its diameter, and to put an M4 screw for backlash adjustment (3) which will press on the ring. The BTR screw (3) must be tightened manually, finding a compromise between the minimum clearance and the good sliding on the guide rod. The professional will find that this assembly lacks elasticity, and that the sliding is not necessarily very regular on the rod not ground to the 1/100 mm. That’s why I replaced it with a ø 5 round stub which is supposed to be ground. This allows to correct a little the backlash which reached at least 1/10 mm before.

Screw for backlash adjustment

Here, the chrome plated guide rod is not yet replaced by the stub. Dimensions of this rod:

Dimensions of the guide rod

Thanks to all these repairs and all these improvements, I still manage to work properly with this stand. But can I honestly advise it?

Micromot drilling stand
MB 140/S ref. 28606
€49.16 — price 2015
at micro-modele.fr

Micromot drilling stand
MB 200 ref. 28600
€70.00 — price 2015
at micro-modele.fr