Angle Grinder Fix Using Epoxy Resin

My angle grinder is a tool I've bought about 15 years ago, and which I use pretty often (see this instructable for example). Given the wide variety of cutting and grinding wheels one can use, it can handle almost any material, even with a small one like mine (115mm / 4-1/2 inches wheels and 600 Watts)

The problem is that when I wanted to change the wheel earlier this week, it was blocked, and the internal plastic part which maintains the rotor in position to unlock the wheel broke in the process.

If it had been a recent angle grinder, I would have easily found a replacement part for the factory, got it in a few days, and this would have been forgotten as quickly as the fix would have been easy.

But with an old one the game was not that easy : this part is out of stock and declared obsolete by the manufacturer ... this part will never be produces again.

I looked for an old (even broken) angle grinder on used equipment resale site, and even found a very cheap broken one (5€ !), but the seller did not reply ... Others were selling unused ones (same old model), but asked more than the price of a brand new and recent one.

Thus I decided that I would fix mine by creating a replacement part.

Materials :

• Polymer clay
• Wax (from old candle leftovers)
• Epoxy resin
• Kevlar fabric leftovers (like the strands you remove from the borders when you cut a piece of kevlar)

Tools :

• Brushes (a fine one for the wax, and a bigger one for the resin)
• A scale and a container for the resin
• Heat gun (to melt the wax)
• Various tools to shape the polymer clay

Introduction

The broken part

Polymer clay

Wax

Epoxy resin

De-molding

Edit : 3D Modeling and Tough Resin 3D Printing

The broken part is a black plastic part, which has two functions : hold the top needle bearing of the secondary rotor, and allow the rotor locking part to slide up and down.

It is quite complex, and some part are rather thin.

I had three option to create the replacement part :

1. 3D printing (SLA, SLS, FDM, DLP, ... using PLA, ABS, Resin, ...)
2. Resin molding
3. Machining (CNC or other)

At first, one would think that all three options would require creating a 3D model of the part, but it's wrong.

Indeed, 3D printing requires a 3D model. And even if I have a 3D printer (common FDM kind), the part is rather complex and I'm not very good at modeling complex parts. The other drawback of this solution is that FDM does not provide very strong parts, and the complexity of the part would have required a lot of support.

If I had a DLP resin 3D printer, I would have considered this option.

For the third one, A 3D model is required if you use a CNC, but a manual solution would also provide good results. With this solution you can use stronger materials, like aluminum, but the complexity of the piece would have required a 4 axis CNC or very strong machining skills, and I have none of these.

Then came the second solution : resin molding.

I had no need to create a mold using 3D modeling, I already have most of the mold : the angle grinder itself !

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In order to create the new part using the angle grinder as a mold I would have to protect all the moving parts (the gears) and constrain the resin in the desired volume.

This will be achieved using polymer clay (but any plasticine would do the job as I won't cure it).

I applied polymer clay over the gears, and in all the holes which I did not want to fill with resin in order to keep the final part as simple as possible, and as close to the original one as possible. I also had to make sure that the resin would not leak outside of the cavity I was creating, so I pressed both parts of the shell together until I saw deformations of the clay all around the cavity.

At first I used cellophane to wrap the top needle bearing of the secondary rotor, but I was not happy of this solution, so I thought of a much better one : wax.

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I finally removed the cellophane and applied melted wax on the bearing outside so as to get a thin layer of wax. I decided to apply wax everywhere inside the "mold" in order to ease de-molding of the part.

I used a thin brush to apply the wax, and a simple heat gun to melt the wax. Using a heat gun rather than the flame of a candle to get melted wax avoids loosing part of the wax which would burn to maintain the candle flame. The drawback is that you have to heat it periodically to keep it melted.

Sorry, I forgot to take a picture of this step for the angle grinder, but I took one when melting the wax for a 3D printed mold I made to create a new resin handle for a trowel which missed one (when I mix epoxy resin I try to regroup many things to do in order to limit the losses as I always mix too much resin, and as it uses gloves and requires acetone to clean the brush).

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Well I do not have pictures of this step either, but this time it not that I forgot to take pictures, I rather did not want to put resin on my phone, as this step is often rather messy.

The goal here is not to create a N+1 epoxy resin tutorial, as I think there's already enough of the over the net (video, text, pictures, ...), so I will not detail the "resin specific parts", only the specifics for this molding work.

First, I decided not to use only resin, but also some short strands of Kevlar to reinforce the resin a get a much stronger piece. I even cut some strands in very short length, under 1cm (under 1/5 inch) and added them directly to the resin. The longer strands got laid on the bottom of the "mold" and around the bearing before being soaked with resin.

This will make for a very strong part, even better than the old one.

The other difficulty has been to fill the whole mold with resin while the piece was inside the mold. Hopefully the piece also has a part which gets out of the plastic shell of the angle grinder to hold the moving rotor locking part, so I filled one half of the mold as much as possible, put some resin on the other part to "glue" the kevlar strands in position on the second half of the mold, and closed the mold (the angle grinder plastic shell). I was then able to fill (or try to fill) the mold with resin using the top hole, even if after de-molding it turned out that a rather big bubble of air got caught in the mold.

At last I inserted the rotor locking part in it's position, and used some paper scotch to holt it.

I then had to wait for the resin to set.

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I waited about 30 hours before opening the "mold", and get the new part out.

It turned out that things went pretty well, and I got the above part.

Of course it is not an exact replica of the original one, but that was not the goal !

The old plastic part looks longer, but remember that it's broken, and it appears longer than it should be.

The part is functional, even if I had to remove a few parts using a cutter to improve the shape, and it sounds more resistant.

I then had to remove the polymer clay from the plastic shell, and I was then able to test the part in the angle grinder.

Everything worked quite well and I now have an angle grinder back to work !

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I finally found two other angle grinders of the same model (almost) with the same problem ... so I decided to make a model of the part and 3D print the part (online service craftcloud3D, parts printed by Varishapes).

I used Openscad for the 3D design. It may feel repulsive if you are not used to programming, but anyone with programming skills, even basic ones, will be able to make complicated parts using Openscad.

The principle behind openscad is to work directly with the common, basic operations which are used by complex graphical tools.

The advantage is that the files are simple text files, which you can easily share, modify (with you favorite text editor !), take bits from a file and add it to another, comment your files/design, and share the result with any versioning system (even github, though this would be my last choice :( ), easily identifying the differences between two versions.

It has been quite a challenge to design the part as I only had broken parts as models, but I finally got it.

I added the source file (rename "Piece_Disqueuse.txt" to "Piece_Disqueuse.scad" as instructables does not accept .scad files) with the pictures, and also the exported version for 3D printing.

I then printed a test version using my own FDM 3D printer and PLA in order to validate the design (and I've had to modify it once ... and still missed a small mistake, which I fixed in the attached files).

I then decided to test craftcloud3D to print the part as they give access to a huge set of available materials, because they do not print the parts, it's only a 3D printing hub.

I choose tough resin as I think it is a good compromise between price and durability.

I received the parts a few days ago, and tested them yesterday. They feels really strong, and I have been able to fix the two other angle grinders in minutes :)

I'll now post the parts to thingiverse and cults3d for others to be able to fix their own angle grinders if need be :)