[How-To] Make a double D slot?

Dan Krager

Dan Krager

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How does one create a "double D" slot in, say a sprocket hub? For example, the sprocket hub with double D slot slips over a shaft with opposing flats and gets clamped against a flange with a bolt. I've had to do this before (hobby work) and it involved a lot of drilling and filing. I don't have punch capability, nor linear broaching. But I'm wondering if rotary broaching into a 1/2" thick hub would created said hole shape? A rotary broach may be in my future if it can be made to do this. The diameter of the shaft with opposing flats would be 1/2". I would use the rotary broach to "punch" a square hole in another bushing for a different application. A quick search indicates that I can get the three rotary broach pieces, the head and two punches, for under $150. Granted it's not commercial duty quality, but almost affordable.
Perhaps there is a simple method I'm not familiar with to make these non-round holes.
DanK
 
Never heard of a "double D" slot, but I am not career machinist.

A picture, diagram or even a 5th grade quality sketch on a napkin would help me understand what you are needing.
 
There are several ways to go about this. (In no particular order.) not all good ideas......
1. Broach.
2. Rotary Broach
3. mill the hole using a small end mill and a rotary table, then just need to clean up the corners. The smaller the end mill the less the cleanup.
4. Start with a round hole and weld in 2 semi circles to create the flats.
 
I've done similar by cleaning up the last bit using 1/8" HSS endmills and plunging rather than side milling. Run mill as fast as it will go. Going a little past the end of the flat doesn't affect the hold. I don't have good luck with very small carbide end mills.
 
Dan Krager said:
How does one create a "double D" slot in, say a sprocket hub? For example, the sprocket hub with double D slot slips over a shaft with opposing flats and gets clamped against a flange with a bolt. I've had to do this before (hobby work) and it involved a lot of drilling and filing. I don't have punch capability, nor linear broaching. But I'm wondering if rotary broaching into a 1/2" thick hub would created said hole shape? A rotary broach may be in my future if it can be made to do this. The diameter of the shaft with opposing flats would be 1/2". I would use the rotary broach to "punch" a square hole in another bushing for a different application. A quick search indicates that I can get the three rotary broach pieces, the head and two punches, for under $150. Granted it's not commercial duty quality, but almost affordable.
Perhaps there is a simple method I'm not familiar with to make these non-round holes.
DanK
Click to expand...
Wouldn’t an endmill perform that function or am I in the wrong zip code on the question?
 
I have made shapes like this with the die filer. If you're only making one or a few, that might be as fast as anything. Drill or mill out what you can, file out the rest. A round or half round file would be a good match for a double D shape.
 
MyLilMule said:
Never heard of a "double D" slot, but I am not career machinist.

A picture, diagram or even a 5th grade quality sketch on a napkin would help me understand what you are needing.
Click to expand...

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Had to look it up myself. Hadn’t heard it called by a proper name. Rotary table on a mill gets my vote.
 
Thanks guys. Milling out what you can and file the rest is a reasonable way to go even without a filer. This is for hobby prototype fittings on a manual Legacy Ornamental Mill (woodworking), which is not manufactured anymore. So there may be hobbyists that want more of these. I was just thinking of ways to "beat the rush" AND get a new tool!
mmcmdl, you must have been hungry when you read the OP. LOL!
DanK
 
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