# Reducing a 1/2" shank to 8mm? Just an idea...



## Abbasalavi (Aug 24, 2012)

Hi everyone!!! I'm fairly new to this forum, and just got into woodworking. have been reading and reading and reading. There's so much info here and in other forums  just makes my mouth water. 

I had a question about an idea I had. I'm designing a tenon jig. I have a black and decker 1100 watt plunge router with a maximum size 8mm collet. 

I've been running around the whole city trying to find a long bit 1 1/4" straight bit with a dia of 1/2". I found several online, but the shipping is kinda out of my budget. So necessity being the mother of invention, I came up with the following idea.

I'd buy a 2" straight bit with 1/2" cutting dia and a 1/2" shank. Go to a lathe machine and precisely lathe the 1/2" shank down to 8mm. That way it would fit the collet size. 

This seems out of the books, so could be unsafe. 

my tenon target length is 1", i have a straight bit which has 1/2" cutting depth. but the plunge depth, the cutting depth and the guide template opening depth do not equate to 1".

Would like everyone's feedback on the idea. Maybe I can try it out and post the results.

Regards,
Ali

PS - The attachment shows the jig I'm making. Its the basic design that I've seen in alot of places. The problem highlighted in the depth of cut. you can see that the 1/2" cutting depth bit i have doesnt come close to the target depth. and the figure represents the bit depth when the router is fully plunged.


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## Abbasalavi (Aug 24, 2012)

*Attachment - Sketchup Model*

Here's the Jig model


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## Chris Curl (Feb 13, 2012)

For me, the biggest worry would be related to making sure it was dead on center of the 1/2" shank. If it is even a TINY bit off, then the bit would be out of balance, and could become dangerous when spun at the typical router speed of 25,000 RPM.

If you have some way to be 100% sure that the new 8mm shank is EXACTLY on center with the original 1/2" shank, then I would think you would be OK.

While that approach might work fine for a straight bit, I would still worry about doing it with a big bit like a lock miter or bottom cleaning bit.


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## Abbasalavi (Aug 24, 2012)

hey chris!!  thank you so much for the feedback. I tried to make sure it was dead center. The lathe machine had a collet for centering the shaft in the middle. every mm cut was checked with a vernier. my concern was also the same, it was a "what if" there's an offset? will try to figure out a way to measure that its perfectly center. i wont be doing it with complex bits. might try it on a rabbet bit if this one succeeds. 

thankyou again for your time. Its very encouraging for a beginner like me 

Regards,
Ali


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## Chris Curl (Feb 13, 2012)

Abbas, you're welcome, and welcome to the forum! They are a great bunch of folks and very helpful.


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## Harrison67 (May 30, 2012)

It's common practice to turn down drill bit shanks, using a metal lathe.:


Turning Down Drill Bit on South Bend Lathe - YouTube




If it wasn't perfectly concentric, at those speeds.....it could whip, shatter, and then impale you with sharp metal. All in about one half second.


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## harrysin (Jan 15, 2007)

I recently turned down this 1/4" shank bit to fit my Dremel and it runs true. With the 1/2" shank in the chuck I see no reason why there would be any eccentricity.


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## stevenrf (Jul 30, 2010)

Yes machining works.


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## Phil P (Jul 25, 2010)

Harrison67 said:


> It's common practice to turn down drill bit shanks, using a metal lathe.





stevenrf said:


> Yes machining works.


Hi Guys

Machining works, BUT, there is an issue of strength. A 1/4in shank (6.35mm) shank cutter has a cross sectional area of around 31.7mm2, an 8mm has a cross sectional area of about 50.3mm2 and a 1/2in (12.7mm) shank a cross sectional area of about 126.7mm2. Reducing the shank diameter from 1/2in to 8mm will considerably weaken the cutter and would be completely inadvisable for some types of cutter (e.g. large dianeter panel raisers). Mortising is a heavy duty application therefore it is probably best done with the largest shank you can get

Regards

Phil


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