1/2" vs 1/4" shaft router bits

Don Baer

Moderator
Staff member
Do 1/2" shaft stay tight better then 1/4" shaft bits. I been cutting dove tails and have hasd several times where the bits have come loose in the middle of cutting dove tails. They are 1/2" 14* bits with 1/4" shaft. The router is a Milwakii 5614-24 and it is almost brand new and I tighten the dickens out of the collet. :huh::dunno: It would make sense to me since you have 4 time the surface area to grip.
 
I have had the same thing happen.

Now I tighten the c##p out of the 1/4 collet - then part way into the dovetail cutting I will retighten again.

I got a 1/2 bit - but it will not fit with the collet, so I couldn't use it.

And based on the laws of physics - the contact area of a larger bit has nothing to do with holding power. The holding power is directly a function of clamping pressure.
 
Might be worth taking the collet out and cleaning it with some mineral spirits. Use something like a .22 gun brush to make sure there's no grease or sawdust in the collet. Give the shaft of the bit a good cleaning too...
 
Couple of issues here, Don.

1/2" before 1/4" if it is available in the profile you want and if it fits the jig you are using it with; not the least of which is the amount of metal that comes in contact with the collet when mated up into the machine.

But your issue is not with the diameter of the shank. It is with the bit creeping out of the collet. Yes?

Bit creep, in my experience, has always been a matter of dirt. The collet, nut, socket, and shank must be operating room clean. All that keeps things together here is a mating of dry, clean metals.

First check the shank. Clean with solvent and allow no residue. Then check for any burrs from bit spinning. If you find them you get to trash two items, the bit and the collet.

Then clean the collet/nut and socket. No left over residue. No lubricant of any nature. Dry, clean mating. This is how it was designed.

Collets are consumable items, not permanent tool fixtures. Over-tightening them will also render them unfit for duty. Over-stressed metal is distorted and applies pressure unevenly.

Dirt and burrs detract from a clean, dry. tight mating of the metal of the shank and collet. A 'tight' collet is loosened when the dirt is smashed further into a pancake due to the high speed spinning of the bit and the lateral stress placed on the cutter when routing. Its not much but it certainly can be enough for things to creep.

Now before you guys line up to shoot me and pepper me with "but" stories, there are exceptions to everything - for a while. My observations have been over the years with many, many students - not just my own experience. And I have a story about a newbie student, a new pair of jeans, and a router bit that came out of the collet at full speed. But I'll save that for my safety demo! Suffice it to say that shorts were stained. And the collet and socket were dirty with dust and pitch.

One more thing with regard to technique; allow the cutter to cut. Do not force the cutter. That problem is more likely with a scraping type cutter in the collet. A cutter than presents its cutting edge to the wood fibers blunt, straight on is a scraping type cutter. A dove tail bit is an example. A shear type cutter presents its cutting edge to wood fiber at an angle, like a spiral bit.

FWIW, a dove tail cutter presents another problem. It is an undercutter. That means the swarf cannot easily get out of the cut. That combines for two things - heat and a small amount of pressure. The heat serves to more rapidly dull the cutter. The pressure is minute but comes from swarf trying to get out of the cut. Slow down the rate rate of travel, but not so slow as to unnecessarily "dwell" in the cut. That overheats the cutter as well.

All these little things can add up to trouble. Big things in life take care of themselves when the little things are carefully attended to.

This is the retired Router Lady breaking her own rule about answering router questions on an open forum. Be kind.
 
And based on the laws of physics - the contact area of a larger bit has nothing to do with holding power. The holding power is directly a function of clamping pressure.

I don't think this is correct. I believe what is happening is the 1/2" dia of the cutter is just to much for that little 1/4" shaft. the radius of the bit 1/4 inch is twice that of the radius of the shaft. The 1/2 collet is 4 times the area to grip the torque that the 1/2 bit transmits to the shaft would be distributed to much larger area. The collet is like brand new, I've only used it for a few practice dovetails. I have the 1/2 collet so I think it's worth a try. The only thing that I can see is the guide won't be necessary sine the jig is designed for a 1/2 dovetail. The contour will change.
 
Could be an undersized bit. I have seen that on some import bits and even on a Whiteside once. :dunno:
After I saw your post I put the mic on it, nope.

After what Carol said I took the collet apart and it looked good but there was some dust in the slits and the shaft was buggered up on the bit so I'm just gonna scrap the bit. I cleaned the collet and will get a new bit. Think I'll go with the 1/2 in shaft though.
 
After I saw your post I put the mic on it, nope.

After what Carol said I took the collet apart and it looked good but there was some dust in the slits and the shaft was buggered up on the bit so I'm just gonna scrap the bit. I cleaned the collet and will get a new bit. Think I'll go with the 1/2 in shaft though.

As Carol said get a new collet too if there are burs on the shaft the the collet is shot also even if you cant see it. As they say TRUST ME :D:D
 
as far as the 1/2" vs 1/4" all i can say is skin friction will always win. the other thing to think about it the type of cutting surface and tip speed. high speed steel and carbide like different speeds. :huh:
 
I don't think this is correct. I believe what is happening is the 1/2" dia of the cutter is just to much for that little 1/4" shaft. the radius of the bit 1/4 inch is twice that of the radius of the shaft. The 1/2 collet is 4 times the area to grip the torque that the 1/2 bit transmits to the shaft would be distributed to much larger area. The collet is like brand new, I've only used it for a few practice dovetails. I have the 1/2 collet so I think it's worth a try. The only thing that I can see is the guide won't be necessary sine the jig is designed for a 1/2 dovetail. The contour will change.

I puzzled at that also. Now, Leo might be some kind of engineering genius. I'm not. But, my gut instincts tell me the more gripping area means more gripping friction and, ergo, better grip. Otherwise, Tod would be running 1/4" shafts on his huge, and I do mean "huge", shaper bits.
 
Sure can't doubt you. But I don't understand why/why not. :huh:

Collets work by drawing up a tapered slotted piece into a tapered shaft, this closes the slots and allows the collet to tighten against the bit. During this process, the bit will move downward a little as the collet has some grip but not quite enough - this allows it to slip further into the tapered hole, gripping the bit even further. Both need to be able to move inward some in order to get maximum holding.

If you bottom the bit out, the collet will gain partial grip on the bit, locking it to the bit prematurely and making it feel like the it's tight when it really isn't. You won't be able to turn the wrenches, so you'll figure all's good 'n tight. But in the end, it's really only got a marginal grip on the bit.

That's my understanding of collets :)
 
I puzzled at that also. Now, Leo might be some kind of engineering genius. I'm not. But, my gut instincts tell me the more gripping area means more gripping friction and, ergo, better grip. Otherwise, Tod would be running 1/4" shafts on his huge, and I do mean "huge", shaper bits.


This one actually turns out to be something lots of folks forget to factor in when increasing surface area. It helps to think of it in terms of only changing a single variable:

Take exactly the same amount of tightening force and apply it to both a 1/4 and 1/2" bit. What you'll find is that while the surface area of a 1/2" bit is 4x more, the force it takes to apply the same amount of force per square inch of that surface as with a 1/4" bit goes up accordingly. I believe that increase is linear relative to the increase in surface area, but I'm not entirely sure.

But in the end, a 1/4" bit should grip just as solidly as a 1/2" bit in the collets designed to hold them. I've had some 1 1/4" diameter cove bits in 1/4" collets that held just fine in my router. It really comes down to GOOD surface contact, as Carol said.
 
Collets work by drawing up a tapered slotted piece into a tapered shaft, this closes the slots and allows the collet to tighten against the bit. During this process, the bit will move downward a little as the collet has some grip but not quite enough - this allows it to slip further into the tapered hole, gripping the bit even further. Both need to be able to move inward some in order to get maximum holding.

If you bottom the bit out, the collet will gain partial grip on the bit, locking it to the bit prematurely and making it feel like the it's tight when it really isn't. You won't be able to turn the wrenches, so you'll figure all's good 'n tight. But in the end, it's really only got a marginal grip on the bit.

That's my understanding of collets :)

On my Milwaukee the the shaft that the collect fits into is hollow for at least 3 inches making it vertualy inpossible to bottom out a bit unless you got a real real long shaft.:rolleyes:
 
Last edited:
This one actually turns out to be something lots of folks forget to factor in when increasing surface area. It helps to think of it in terms of only changing a single variable:

Take exactly the same amount of tightening force and apply it to both a 1/4 and 1/2" bit. What you'll find is that while the surface area of a 1/2" bit is 4x more, the force it takes to apply the same amount of force per square inch of that surface as with a 1/4" bit goes up accordingly. I believe that increase is linear relative to the increase in surface area, but I'm not entirely sure.

But in the end, a 1/4" bit should grip just as solidly as a 1/2" bit in the collets designed to hold them. I've had some 1 1/4" diameter cove bits in 1/4" collets that held just fine in my router. It really comes down to GOOD surface contact, as Carol said.
Jason the circumferance of a 1/4" bit is .196875". The circumferance of a 1/2" bit is .785 " if you apply the same for ove a bigger area wouldn't you have more resistance to force.

Here is an excet from a Physics web site.
Friction and Area of Contact

Part of the standard model of surface friction is the assumption that the frictional resistance force between two surfaces is independent of the area of contact.
fare.gif
here is the web site for credit reason.
http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html#ass

I forgot to mention I majored in Physics in college..
 
Jason the circumferance of a 1/4" bit is .196875". The circumferance of a 1/2" bit is .785 " ...

Using Pi * D as the formula for circumference, I get different numbers, Don. Try 0.785" for a ¼" bit shank and 1.57" for for a ½" bit shank.

So, twice as much surface area.

However, it makes little difference to the job you're doing. Most dovetail jigs use a 7/16" guide collar, and a half inch shanked bit won't fit, so you have to use a quarter inch shanked bit.

From the sound of things, you need both a new bit and an new collet for your router. So...Bite the bullet and Git-R-Done. :D
 
Using Pi * D as the formula for circumference, I get different numbers, Don. Try 0.785" for a ¼" bit shank and 1.57" for for a ½" bit shank.

So, twice as much surface area.

However, it makes little difference to the job you're doing. Most dovetail jigs use a 7/16" guide collar, and a half inch shanked bit won't fit, so you have to use a quarter inch shanked bit.

From the sound of things, you need both a new bit and an new collet for your router. So...Bite the bullet and Git-R-Done. :D
Jim I plan on getting a new collet and bit tomarrow but I was wondering since I am doing 1/2" dove tails then I should be able to cut the dove tails without a collet, it will change the profiles a bit but that should be OK..:huh::dunno:
 
Top