Dremel tool/attachment

I have to elongate (sp?) two 1/4+" holes drilled in cast iron approx. 1/2" thick. Is there a Dremel tool grinder attachment that would do this? I looked on the Dremel website and the only thing I found that may possibly work is an attachment for sharpening chainsaws. Does anyone have any experience doing something like this using a Dremel?

What do you mean by elongate? Make the holes bigger? Taper them in some way? I don’t know how cast iron reaacts to being ground down, but a grinding stone would probably do the trick slowly.

By elongate I mean to maintain the diameter but increase the length by approx. 1/8 inch. More like a slot to allow for lateral adjustment.

Look to the Rotozip section of your hardware store. There’s a 1/8" diameter tungsten carbide tile or ‘duracut’ bit that’ll do the job, and fit in your mototool. Be sure to reduce the speed from 30,000 rpm, as that’ll turn the bit white hot and melty quite rapidly.

A friend of mine worked in a custom pump shop, drilling and shaping cast iron housings. He said cast iron is tricky to work with. He ran across hard spots, soft spots, and voids; hitting one of those would deflect a bit from its intended path. I’m not saying you can’t do your slots with a Dremel, I’m just saying it might get weird. Can you get a spare piece, if something goes wrong?

The problem isn’t clear to me. Are the holes drilled all the way through the 1/2" thickness? If so, they can’t be made longer/deeper. If not, the thickness of the cast iron is irrelevant. If you’re talking about increasing the length/depth of a hole without increasing the diameter, why not use a drill bit? This is not elongation, but deepening a hole.

Now, if you’re talking about turning a round hole into an oval one, elongation is the correct term but “maintaining the diameter” doesn’t really make sense - ovals don’t have diameters. But I’m thinking you’re trying to go from, say a 1/4" diameter hole to a 1/4" x 3/8" oval. In this case, a cylindrical grinding stone on a shaft that fits into a drill would probably be the most practical method. It’s likely to be somewhat laborious and tedious. You’d want a stone that is just slightly smaller than the diameter of the existing hole, and if at all possible at least the length of the hole (1/2").

I don’t think a Dremel tool is the best option here. These are designed for craft work, and using one on a 1/2" thick piece of cast iron is like using a hacksaw to cut down 2’ diameter tree. You can get cylindrical grinding stones for it, but only in lengths shorter than 1/2", which means more work than a longer stone. And I would expect a Dremel-type grinding stone to clog and/or wear down rapidly doing this - you could go through a lot of them.

I agree with Gary T I had a cast iron griddle that I needed to cut the handles off of the ends so it would fit on my BBQ. I used a die grinder (think of a Dremel that is on steroids and is air powered.) I thought I was never going to get those two flippin handles cut off.
If you need this done with some accuracy I think a machine shop with a mill is the way to go.

I third the sentiment that a Dremel tool is just too small and underpowered for this sort of work. You can probably get it done with one if you absolutely must, but it’s gonna be a LOT of work. Of course, people managed to work cast iron before power tools were developed, but it was anything but easy.

Sorry if I wasn’t specific enough. Let me try again. The holes go completely through the cast part. I don’t need deeper holes. I need to make a slot, not an oval, with paralell sides the same measurement as the diameter of the hole. A 1/4 inch bolt will go through this slot from underneath and screw into tapped holes in a bearing housing above. Similar to the slots on a electric motor mount that allows lateral movement for alignment of the pulleys.

I need 1/8 inch of lateral play so I can mate the housing to it’s mount. I can’t drill it because the bit will wander off the solid metal and try to follow the existing hole. It could possibly be machined using and end mill but that may be expensive. I can also spend hours filing away with a small round file but that is exactly what I want to avoid.

Not if you drill an 1/8" or smaller pilot off to the side of the original hole. Then switch to the 1/4" bit, and you’ll be left with a couple of nubs to file off. That’s not fun, but it’ll give you nice straight parallel sides on your slot.

Ditto. The Dremel is a very useful tool for what it is, and with patience can take the place of a router for small amounts of work on soft materials, but I wouldn’t try to use it here.

Is there some reason you can’t just use a drill? With a drill guide clamped to the workpiece you should be able to keep it on center. And (upon preview) as Squink says, start with a pilot hole (which is a good idea when drilling, rather than using an end mill, with metal anyway.)


I could use my drill press instead of a drill guide but I can’t lenghten the slot more than the 1/8" needed. That makes it exactly the radius of a 1/4" drill bit so whether I drill a pilot hole or just use a center punch the exact center for a 1/4" bit falls exactly on the edge of the existing hole.

Thanks to all for answering. I think I’ll just have to invest in a little time with a round hand file.

And this probably won’t take as much time as you’d think either. Maybe 10-15 minutes on each hole. You’re only going 1/8 inch right? Maybe to make it a little easier you could take the drill bit (the 1/4 one that drilled the holes) and rock it from side to side while running the bit through the already drilled 1/4 holes.
You know what I mean? IOW, get the drill bit going full speed in the already drilled holes and then lean the drill to the left and then to the right going further each time you do this. This will at least clean out some of the material that you have to get out of the way and make your file job a little easier (or unnecessary).

We do this in the field as a quick and dirty way to slot a hole a small amount.

Reaming the hole had occured to me. I thought of using a slightly undersize bit and then filing to the final size. It needs to be fairly accurate so I need to be careful about removing too much material.


I’m a machinist and if you weren’t 1500 miles away I’d love to help you out. It would take me a minute in a mill to make a slot from a hole.

Offer a guy in a local shop a 12 pack of beer to do it for you.

If that isn’t an option, drill a 3/16" hole at the far edge of where you want the slot to end. A carbide burr tool in a dremel will work to cut out the bit of metal in between the two holes. A round fle will also work, but I’ll always use a power tool whenever I can over doing it by hand. As said a stone will gum up in a minute.

For cast iron try the finest tooth burr tool you can find. Wear a mask, cast makes dust rather than chips when cut.

What’s an approximate optimal rpm for that sort of work? I’d done similar mototool work at ~8Krpm, but wonder if that’s really too fast or slow for efficient cutting.

It’s more my laziness than a problem but I would need to completely disassemble the machine, an 8" longbed jointer, and take it 50km to a shop with a milling machine. I’m replacing the cutterhead and the new one seems to be just a little bit longer than the old one. The bearing housings bolt in place without any way to adjust them. I need to increase the space between the 2 bearing housings on the ends of the shaft by about 3mm in order for the bolt holes to line up.

Are you sure you don’t just need a bigger hammar? :wink:

I finally just took a chainsaw file to it. My hands are a little sore but it didn’t take me as long as I had thought.