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I've been looking at tearing out my fiberglass shower, installing Kerdi, and re-tiling the whole thing. During my research into this project, I've come across quite a few forums that have members typing in all caps about not mixing thinset with a cordless drill because the motor will burn out. Is this true? Even for small batches? My shower is only 3' W x 3' D x 6' H.

I've got an older DeWalt 14.4V drill that's on it's way to retirement (its batteries aren't holding a charge well), as well as a new Milwaukee M12 lithium ion 12V drill and an M12 Fuel 1/4" impact driver. Is one of those sufficient or should I go buy a cheap HF corded drill or check Craigslist to add to my growing collection of tools-that-rotate-things?

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No one can predict time to failure of a single trial. Your tools are too light duty for the task. They may survive, or not. You really should use the right tool for the job rather than risk ruining an otherwise functional tool. –  bcworkz Jul 5 '13 at 20:10
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Consider renting a sturdy electric drill for the project. It'll be cheaper than replacing your cordless, and will probably save time as it will be properly sized for the job. –  TomG Jul 7 '13 at 1:56
    
How long is a piece of string? –  Matt Jul 7 '13 at 21:49
    
If you look at the original wording of my question, it was actually less nebulous. –  Doresoom Jul 8 '13 at 11:02
    
@bcworkz: You're right, that's just not the answer I wanted to hear. Oh well, if the worst case scenario is buying another tool, you won't have to twist my arm too hard. :) –  Doresoom Jul 8 '13 at 13:48
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4 Answers 4

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First of all, I have to say that with the volume of work you have you can just mix the thinset by hand. I once had to install about 2.7 square meters of tiling and mixed several small batches of thinset with a spattle - no problems at all, I wasted much more time on applying the thinset and installing the tiles evenly. Totally not worth risking the drill.

Next, you should not use any drill for mixing unless its manual says it's suitable for mixing or that it has overload protection that slows the drill down in case of overload. Drills explicitly suitable for mixing are typically professional drills with very low RPM (500-700 RPM) and so very high torque. The "HF corded drill" you reference is nothing near that. Drills with overload protection are typically professional cordless drills with smart electronics that monitors all the parts for overheating and reduces the output power in case of overload. The Milwaukee M12 you have likely has the right type of overload protection (that's the impression I have from reading the online specs of the drill) and so you likely could use it for this purpose.

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The overload protection is something I hadn't considered. I had been looking at lower-cost name brand drills, and my target price had jumped from ~$30 to ~$90, because I usually gravitate toward quality tools. After hearing your input and Mike's, I think I'll just mix it by hand. –  Doresoom Jul 10 '13 at 12:48
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1) The issue is not so much the total amount of work the drill will need to perform over the course of your project. The issue is more with the load you will be putting on the drill at any given moment. Chucking a small stirrer will (likely) let the drill operate within it's design specs. Chucking a larger stirrer will (likely) force the drill to operate outside it's design specs.

2) For any power tool, if it starts to overheat, then it is best to stop and assess the situation. Unfortunately, there is no answer to what temperature constitutes the threshold for overheating.

3) For any power tool, if the rpm drops significantly when put under load, then it is best to stop and assess the situation. Unfortunately, there is no answer to what rpm drop constitutes an overload.

4) For either overheating or rpm-drop, the inadequacy could be with the tool or could be with the power supply, for example an extension cord whose gauge and length are too small/long for the task.

5) Given that, I suggest going ahead with your existing drills. If you encounter warming or rpm-drop, stop, let it cool, and switch to a smaller stirrer perhaps by cutting off some of the 'paddle'.

2, 3, 4 apply to any tool/project.

I wonder if a variable speed 1/2" router would work?

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This is a reasonable approach if one had no other options. But by the time someone notices a tool overheating, the internal insulation has likely been damaged enough to shorten the tool's useful life. For some tools, it may be worth the gamble. But what if the tool should fail? You have bucket of half mixed thinset waiting to go off, you need a backup plan. Hacking up another functional tool is not a good backup. Much better to have the right tool in the first place. I don't think any router is really designed for high torque applications. You certainly may not use mine for this :) –  bcworkz Jul 6 '13 at 19:35
    
@bcworkz - backup plan: at worse, the loss is half a bucket of thinset. If the OP does not have an electric drill, then it is not likely that they have a stirrer to begin with. Given they are already concerned about burnout, it's not likely they'll be buying an industrial grade/size paddle. –  mike Jul 6 '13 at 20:56
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Fair enough. I think we've presented enough information for the OP to make an informed decision. Your input is appreciated even if I don't agree :) –  bcworkz Jul 6 '13 at 21:28
    
You give some good pointers for determining when the drill is being used beyond the design specifications. I think I'll go with bcworkz's answer in the comments though and just play it safe. Plus, I don't really want to get started on a project and then have to make an hour round trip to pick up a tool I should have just bought prior to starting. –  Doresoom Jul 8 '13 at 13:51
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I use my cordless Makita to mix thinset with a small paddle. I get through one mixing per battery - batteries are 3 years old.

The method I use if I am doing a full bag is about 6 inches of water and slowly pour the powder in. When it gets anywhere near oatmeal consistency, more water. Then I pour more powder. Repeat 1-2 more times. Once all powder is in I am mainly mixing the top 10-12 inches. I take a putty knife and scoop the bottom every 45 secs of stirring. I also stir in reverse and forward almost equal.

Others may have their own method but I found that stirring the very bottom with a paddle is useless. Basically you push down the powder not mix it. Rolling the powder up with a putty knife is a little messy but saves 5-10 mins of mixing. Also when I am using my drill I am hardly ever pegging it. I want it going at a medium pace and I am moving the paddle around the bucket randomly.

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I will say though that I think I got thinset stuck in one of my drills and I cannot get the bit out.......... –  DMoore Jul 8 '13 at 15:25
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Mixing thinset by hand while remodeling a small shower with Kerdi and new tile wasn't a physically taxing process for me, but YMMV.

By the way, the John Bridge forums were the most helpful resource that I found during my research and remodeling job.

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Thanks. I'd already run across the John Bridge forums, and there seems to be a lot of useful information there. Is his Kerdi shower book worth buying in your opinion? –  Doresoom Jul 10 '13 at 12:45
    
I didn't think so, but at the time I had already done a lot of research, so I already knew most of the info in there. Reviewing it now, there is a fair amount of info that could help others. –  Mike Partridge Jul 10 '13 at 13:08
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