The variable speed motor controller (VSC) switch of hand held rotary drills are known to fail occasionally. This happened with our Dremel® rotary drilling tool recently. In our case, the failure of a small, inexpensive chip on an extremely simple VSC circuit board inside the tool’s switch housing meant that the tool was limited to ON and OFF options only from the extreme limits of the sliding variable speed switch.
The motor either ran full speed at 35,000 rpm or not at all. Leaving the tool this way would not be healthy over extended periods, as the full speed can quickly lead to overheating of the motor and reduced operating life. The high speed was also not a workable workshop option when using the tool on a mini CNC cutting machine, PCB drilling press, or even for hobby projects and sanding.
We decided to open up and DIY service our hand held Dremel® drill to find the problem ourselves. We also cleaned and serviced all internal parts and bearings while we had the opportunity. After our DIY service, the tool worked like new again!
Our handy workshop rotary drill tool is an aging Dremel® MultiPro (Model 395). They are actually extremely simple tools internally with very few parts. We assume many other models and brands would be relatively similar, so the following information would likely be relevant for servicing them as well
Our particular Dremel® drilling tool has provided us with years of excellent service life. We certainly cannot complain. The tool has been indispensable for a multitude of DIY and hobby projects in our workshop. We admit that it has been used and abused under conditions and heavier duty applications than it was obviously rated or intended for, but still it powered on. It has not failed us until now, many years and operating hours after purchase.
The following is a quick diary entry of our experience of DIY fixing our rotary drilling tool for use by anyone with an interest in the internals of rotary tools.
Taking the Dremel® Rotary Tool Apart
Before starting work on anything, WE UNPLUGGED THE HAND HELD TOOL’S POWER CABLE FROM ANY AND ALL POWER SOURCES! Not doing so would be plainly dangerous and silly!
Ensure you have a safe and convenient working space where you will not lose small parts.
Step 1 – Remove the Motor Brushes
The graphite “brushes” (ie: blocks of graphite on the end of an electrically conducting spring) which contact and transfer current to the rotating motor stator are located under two small blue caps, either side of the tool’s outer casing. The caps were removed with a small flat head screw driver. Care was taken as the brushes are spring loaded and the caps often spring off once released.
Removal of the brushes allowed easier removal of the motor stator shaft later in the strip down process, without the potential for damage to the brittle graphite heads.
TIP: It helps to turn the spring anti-clockwise while removing the brush from the socket. This tightens the spring’s holding tension around the top of graphite block and helps prevent it detaching while being withdrawn. Should the graphite block detach, do not fret, the brush is not broken. Carefully twist the spring lightly in the opposite direction and loosen the springs coils around the top of the graphite again. Handle the brushes with care as they are brittle and crack easily.
Step 2 – Opening Up the Dremel® Rotary Drilling Tool Shell Casing
Four screws keep the two sides of the hardened plastic casing of the tool together. Removal of the four enclosure screws requires a special Torx© head screw driver. (Don’t you just love this modern world of different designs of anti-tamper screw heads?! Grrrrrrrr.)
Once the casing screws are out, the plastic sides of the tool and the internal components tend to (literally) fall apart. (This is where photos come in handy to understand exactly where the parts fit back together after such an event. Of particular interest and caution is perhaps the little silver motor shaft locking pin and associated spring assembly which fall out and get lost easily! The rubber bearing sheath also falls off and rolls away easily.)
Along with the main motor parts, we were surprised that the power cable wire ends also fell out of their normal contact holes extremely easily. The wire ends are usually positioned (and very lightly clamped in by contacts) in the smaller, top holes of the plastic ON / OFF (variable control) switch housing.
Once the main components of the electric motor, shaft, air fan and bearings were removed from the plastic casing as one piece, the large outer stationary core of the motor could be slipped off the inner rotating motor core. The plastic VSC switch assembly remained attached to the end of the removed motor core at this stage.
The inner rotating shaft section remains complete with 2 x bearings, the rotating contact surface for the brushes, the inner motor coil/core/magnets, plastic air fan and the shaft itself.
This was a great time to give the entire set of components a quick clean and blow down with compressed air. We also gave the bearings a quick spray with a penetrating liquid lubricant (eg: CRC, WD40, etc).
Important: If you do a similar service on fast moving parts, DO NOT contemplate using Silicon sprays, as although they are slippery on a surface, they become sticky rather than slippery when subject to speed and associated sheer forces!
Step 3 – Removing the Plastic Variable Speed Control (VSC) Switch Housing
The variable speed ON / OFF switch is contained in a separate plastic housing. The entire housing can be removed from the motor by simply pulling straight out from the end of the motor core. The switch housing assembly is only attached to the motor core by four small brass electrical contacts, so care was needed not to bend or damage the contact pins.
Step 4 – Opening Up the VSC Switch Housing
Once the plastic housing was separated from the motor completely, the switch housing itself was opened to gain access to the mini electronic PCB board and sliding switch arrangement inside. The internal components of the switch housing control the current flow to vary motor speed.
The plastic housing had an internal plastic clip arrangement that cannot be seen from the outside of the housing. The clips held the two sides of the molded plastic together.
From the seam of the plastic, a small screw driver was used to gently wedge the two sides of the plastic apart. Care was taken not to break the fragile clips during separation.
In separating the two sides of the switch arrangement, two internal parts fall out. The top blue plastic switch slider plate which can be seen from the outside of the tool and the smaller black plastic slider that sits underneath it.
Step 5 – Separating Variable Speed Motor Controller PCB board and Replacing TRIAC Chip
Once the switch housing was totally apart, a small white PCB circuit board could be seen. It was gently removed by sliding the part sideways. The PCB board was only lightly clamped into the switch housing by the electrical contact plates that surrounded it.
The part found to be stopping the variable speed control from working was the small black on the white PCB. It was a small chip with four contacts (three small legs on one side and one larger contact on the opposite edge of the chip).
The chip is known as a TRIAC, or a “Triode for Alternating Current”. The triode chip code was BT134W. They are relatively easily available items and (should) cost around a couple of bucks online or from local electrical components suppliers.
We wont go into the ins and outs of the TRIAC and how it works in the Dremel© drill specifically, but as a basic overview, it is a chip that allows large power flows to be triggered in two directions with only small currents at the gate contact. Applying a small trigger pulse at the gate at a controlled phase angle in an AC current cycle allows control of the amount of current which is able to flow to the motor. This subsequently controls the motor speed.
This chip is commonly used for control of low current induction motors and dimmer switches. (If you would like to know more about TRIAC chips in general, start by looking at some of the basics here.)
Once we had our new chip on hand, we gave the board and all contacts in the switch housing a good clean with compressed air, a cloth and/or light abrasive sandpaper.
We then used a little careful soldering to remove the old TRIAC chip and replaced it with the new TRIAC chip. We were careful not to overheat or damage the surrounding components while doing so.
Once done, it was then just a matter of getting the tool back together into one piece before we could test and see the rewards of our successful DIY fix.
Re-Assembly of the Dremel® Rotary Tool
Basically, re-assembly of the tool was the reverse of the strip down outlined above. There were a few little lessons to be learned for anyone attempting similar however.
Power Wires (Special Note)
Careful attention was require for re-attachment of the power cord wire ends to the switch housing on re-assembly. Once the entire motor core and shaft were together and repositioned on one side of the outer plastic case, the power wires were re-inserting into the smaller, upper holes of the switch housing (not the lower holes which have no contacts behind them!).
Since the wires slipped out extremely easily while dis-assembling the drilling tool, we remedied this problem while the switch housing was still open. We bent the internal power contact clips ever so slightly so there was greater clamping pressure on the wires by the internal contacts than beforehand. This helped keep the wires securely in place and stopped them from slipping out once inserted.
Once the wire ends were inserted correctly and securely, the plastic coated wires needed re-bending around the plastic screw posts of the outer casing. The cord stopper also needed positioning on the edge of the casing.
Closing Up the Casing
Many of the components were easier to align and put back together after careful placement in one side of the casing before finally trying to clip/screw the case pieces together.
In particular, this relates to:
- the switch sliders (top and bottom) before snapping the whole switch housing together
- the rubber vibration sheath over the rear bearing
- the overall placement of the main shaft and bearings.
- the placement of the tricky little shaft locking pin and locking pin spring.
(TIP: It is easier to place the entire motor assembly in one side of the case and the locking pin and spring on the other side before joining the case sides. See associated photo below.)
Before closing up completely, we pressed the entire motor and shaft evenly and securely into one side of the casing, otherwise any small mis-alignment of the parts resulted in a knocking sound when the motor restarted, as the air fan hit the side of the casing. Alternatively, the slim clearances in the casing did not allow the motor to turn at all! A dangerous situation with ample opportunity for burning out the motor entirely if the tool was left switched on for more than a few seconds. Care was required.
Once positioned correctly, the outer case was closed completely and the Torx® screws were re-inserted and tightened.
We were done! The Dremel® rotary drilling tool was back to a state which was as good as new!
The DIY repair of our Dremel® variable control switch was successful. The project only took an hour or so to complete; the new chip was cheap and readily available, and overall the job was easy. (We do however, recommend professional servicing for anyone not competent at safely servicing electrical tools themselves.)
It constantly amazes us that many people might consider throwing an almost perfectly good power tool away for the sake of changing out a failed $2 internal component. A little inquisitiveness and DIY nous can save a lot of unnecessary cost in replacing “failed” workshop tools.
Thankfully for us, its back to our Dremel® driven mini CNC milling machine and (hopefully) years more loyal service from our trusty rotary drilling tool!
Disclaimer and Disclosure: We cannot guarantee the accuracy or fitness for purpose of this information for this or any other similar project. Do not act on this information and not attempt repairs on power tools unless you are competent in safely doing so. Seek a recognized and professional servicing agent for advice. We cannot be held liable for any damage, injury, or any consequences associated with or as a result of using this information.
HomeDIYStuff.com has no direct sponsorship from, or affiliation with the makers or suppliers of Dremel®. All thoughts and opinions on this excellent tool are entirely our own and are formed from years of our own experience and use of this brilliantly handy tool for hobby, workshop and DIY projects.