Pete.
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- 14,379
- Location
- Kent, UK
I had to make a part in aluminium today with a 12mm hex socket in the end, so I thought it would be a good opportunity to make something I have had in mind - a wobble broach.
First I made the cutter out of an old MT2 drill bit. I have a big pile of 9/16" drills which I only keep for when I need to make a throw-away tool like this. A 12mm hex is 13.6mm across the corners so I turned a piece of drill taper down to that size, stuck it in my dividing head and milled a tapered hex in the end. The taper was achieved by packing up the rear of the dividing head by 10mm.
Then I hardened it by using my DC tig to get it hot then dunking it in water. Gave the cutting edges a lick with the grinder to clean off the scale.
As you can see I melted the edge on one flat but since it's the only place it doesn't really cut I wasn't so bothered.
Next to broach the hole. The part was pre-drilled with a 12mm hole and a 13.6mm shallow counterbore to start the broach central. I had already center-drilled the back of the cutter to hold a ball bearing so I made a plug to fit in my boring head and center-drilled that too.
After that it was a simple matter of dialling in some off-set on my boring head to make the broach cutter wobble. The greased ball bearing acted as a, err, bearing so the top bit turns (whilst also describing a small circle) and the bottom bit just wobbles. This wobble does the cutting by advancing the cutting faces a tiny bit each turn. I used about 1.5mm offset on the boring head.
And that's it, a lot easier than it sounds TBH.
Things I did wrong:
Made the first cutter dead on size leaving nothing for clean-up. It's much better to make it up to half a mm big then grind the face back.
Too much centerbore in the cutting end. I figured it would help with cutting action and make it easier to heat with the tig without melting the edges. In fact it melted to fast on the thin bit AND the cutter broke when extracting it showing a very grainy structure.
As above should have drawn a temper in the cutter. If it was going to have more than one use I would have but it did the job at full hardness.
First I made the cutter out of an old MT2 drill bit. I have a big pile of 9/16" drills which I only keep for when I need to make a throw-away tool like this. A 12mm hex is 13.6mm across the corners so I turned a piece of drill taper down to that size, stuck it in my dividing head and milled a tapered hex in the end. The taper was achieved by packing up the rear of the dividing head by 10mm.
Then I hardened it by using my DC tig to get it hot then dunking it in water. Gave the cutting edges a lick with the grinder to clean off the scale.
As you can see I melted the edge on one flat but since it's the only place it doesn't really cut I wasn't so bothered.
Next to broach the hole. The part was pre-drilled with a 12mm hole and a 13.6mm shallow counterbore to start the broach central. I had already center-drilled the back of the cutter to hold a ball bearing so I made a plug to fit in my boring head and center-drilled that too.
After that it was a simple matter of dialling in some off-set on my boring head to make the broach cutter wobble. The greased ball bearing acted as a, err, bearing so the top bit turns (whilst also describing a small circle) and the bottom bit just wobbles. This wobble does the cutting by advancing the cutting faces a tiny bit each turn. I used about 1.5mm offset on the boring head.
And that's it, a lot easier than it sounds TBH.
Things I did wrong:
Made the first cutter dead on size leaving nothing for clean-up. It's much better to make it up to half a mm big then grind the face back.
Too much centerbore in the cutting end. I figured it would help with cutting action and make it easier to heat with the tig without melting the edges. In fact it melted to fast on the thin bit AND the cutter broke when extracting it showing a very grainy structure.
As above should have drawn a temper in the cutter. If it was going to have more than one use I would have but it did the job at full hardness.
