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My grandfather gave me two old drills, as he does not use them anymore. The drills are the AEG 800W Powermatic RL (SB2E-801RL) and the SKIL 1473H2. They suffer from the same problem, although the AEG slightly more than the other. When pulling the trigger, they sometimes give their full speed as supposed to, but other times they start slower, and/or they start rattling, if at all. Then, suddenly (while keeping the trigger pressed) they might step up to a higher speed. It then keeps the speed if it is the maximum speed. If not, it might receive another speed boost a short moment later, until it reaches the maximum speed, but it does not slow down unless releasing the trigger. The behaviour happens more frequently when holding the drill sideways. The frequency of the problem does not seem to be affected by choosing the rotational direction of the drill head, or the instructed velocity. Although, for low commanded velocities, it more frequently remains stuck.

Since they are completely different models while suffering from the same problem, it makes me think that this is a side affect of their old age, which as far as I know is the only property they have in common. But, what exactly causes it, and can it be fixed?

My first guess is that this is caused by old rusted and/or worn down brushes of the motor, but to my understanding this should simply lower the power and efficiency of the drills as a whole, and not induce strange step-up behaviour. So far, I haven't been able to find someone who has experienced the same behaviour with one of their (old) drills.

Update: the answer and comments convinced me to check out the brushes after all, starting with the AEG. The brushes were more easily accessible than I thought. Upon inspectation however I didn't see anything wrong. They didn't seem worn out given that they were spring loaded with at least 1cm more of travel, and their surfaces looked smooth and did not show rust. Nevertheless, I cleaned them from all the dust and carefully polished the tips,ever so lightly (perhaps not enough), with the hope that this would solve the problem. Unfortunately though, it didn't. The performance did not seem to be improved in any way. Below attached some pictures.

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  • Motor bearings and/or worn rotor causing intermittent/incomplete contact with the brushes (also could be worn out brushes).
    – Jimmy Fix-it
    Feb 1, 2021 at 19:04
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    Agree with @JimmyFix-it brushes plus bearings based on my experience.
    – Solar Mike
    Feb 1, 2021 at 19:06
  • I have a hunch as to why he stopped using them and wishes for you to have them. Unless you're willing to fix it yourself you might find it less irritating to just buy a new one.
    – MonkeyZeus
    Feb 1, 2021 at 19:28
  • @MonkeyZeus haha, no, at 90 years and after at least 5 years of not practising his DIY hobby, he decided to pass the content of his DIY barn on to me. There were also functioning tools. Anyway, I get it, the drills are broken and it's likely money better spent buying new ones rather than time spent attempting to fix these.
    – JJM Driessen
    Feb 1, 2021 at 19:34
  • @JJMDriessen Well a broken tool is a great way to learn how to repair a tool. It's already broken so you won't feel bad if you break it more! However, don't rely on such tools if they're mission-critical because a working tool is worth it's weight in gold when you are trying to get a project done; it could be the difference between something taking 2 hours versus 8 hours.
    – MonkeyZeus
    Feb 1, 2021 at 19:40

2 Answers 2

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JJM your understanding of how motor brushes work is lacking some practical experience. For less than 20$ you can probably have both running like new and gain the experience. Holding the motor at an angle may cause speed changes. Bumping the motor can get them going or kill them. I looked for AGE brushes and a video came up , easy to change , 2 screws remove the black grip and the brushes are right there.

Some brushes when the wear through damage the armature. Pulling the brushes is usually not difficult and on some models down right simple so please check them or they will be yet another thing for the trash.

Some skill motors Removing a plastic disk that is screwed in and pulling the brush takes a few seconds other models opening the case and pulling the brush may take a few minutes. Most brush sets can be found for under 12$ and some for as little as 2.50$ for a pair.

I would encourage you to at least try before the armature is damaged and you might find the motor preforms like it should. And you learn a new skill that is rapidly being lost.

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  • You've convinced me to at least check out the brushes, but they didn't seem bad, and a clean-up didn't improve the performance (see update question). Would you still suggest purchasing new brushes, or do you think the source of the issue could be elsewhere?
    – JJM Driessen
    Feb 1, 2021 at 21:49
  • The brushes can look good but be short so there is not enough pressure to hold them tight to the armature. When they start looking bad (pitted or burn marks) they are actually damaging the armature. I cannot see the brush face but they may need to be seated. As I said this is becoming a lost art or skill. I use emery paper or 400 grit paper to seat brushes it can be tough in small tools but improved the performance I put the sandpaper on the armature put the brush in and work it back and forth, if there is no clearance I polish the armature I normally do not do this unless I need to seat brush
    – Ed Beal
    Feb 2, 2021 at 14:35
  • When seating the brush to the armature the abrasive is to the brush, most brushes are carbon it is very soft so use caution. If I can not seat the brush I will polish the com I usually do not do this because the chocolate brown color is actually a conductive oxide layer, if polished it will turn bright copper colored and wear faster but the oxide layer will reform as the brush seats. 1 cm of brush left is not a large amount brushes are carbon the holders are usually brass and the armature or communicator is copper so there should be no rust in this assembly but possibly dust buildup from wear
    – Ed Beal
    Feb 2, 2021 at 14:44
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The value is only sentimental , and some chance of a learning tool . I have a 70 + year old Cummins 1/2" of my fathers that I hate to throw away. My 1/2" Makita ( hammer option) is half the size , a quarter of the weight and at least the same torque. I bought brushes , cleaned the commutator on the Cummins, but the plastic brush holders fell apart . I am leaving it to my sons.

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  • LOL don't saddle your sons with the burden, throw it out now. I still have old bits and braces, and sad irons that were my grandads... worthless but I can't throw them away...
    – Jimmy Fix-it
    Feb 2, 2021 at 0:53
  • It has a cast aluminum frame and weighs 8 lb; you can[t get them like that anymore.
    – blacksmith37
    Feb 4, 2021 at 19:19

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