Slot Mortising Feed Direction?

[jsburger]

With 'slot mortising' the concept of climb cutting is unavoidable. As Buckeye stated the resultant force when feeding needs to be towards the rigid fence.

In the example, the edge between the fence and the bit is plow cutting, and the edge outside the bit is climb cutting. More important with slot cutting, is the area ahead of the cutter (end of slot). There the bit forces the workpiece towards the fence.

It is not too often we get a chance to correct this guy!

Actually the edge between the fence and the bit is climb cutting, and the edge outside the bit is plow cutting. No big deal but we want to get this straight.

More important is the area ahead of the cutter, as said in the quote.

Also the edge away from the fence is being pulled slightly toward the fence when the workpiece is moved from left to right as is the example. This force actually helps to keep the workpiece next to the fence, while the climb edge is having less force.

It is a good practice to use a small diameter bit for a shallow first pass. A shallow groove is followed by more passes to get to the desired depth. After the desired depth is reached, move the table backward a little, insert the bit into the slot (on the right side) and cut again. Repeat this until the correct width is reached.

Nay Chuck!

Draw it out.

The bit is rotating counter clockwise as viewed from the 'top'.

The workpiece is fed from left to right again as viewed from the top.

As far as climb/plow, the fence is irrelevant. The relative motion of the bit and workpiece determines which is occurring.

However one wants to use the fence so the forces are under better control.

When the workpiece is moved left to right, the rotation of the bit against the 'inside' work surface will resist the workpiece in the feed direction. That be plowing.

The rotation of the bit against the 'outer' work surface will pull the workpiece in the feed direction. That be climbing.

I did however have the cutting action wrong. The bit will be cutting the 'trailing' end of the cut and thus will be pulling the workpiece AWAY from the fence.

Thus the fence provides a stop for the pressure required to keep the workpiece tight to the fence.

Unless you have a reversing switch on the motor the bit is turning clockwise when viewed from above.

Anyhow, this is much ado about nothing. If you use a standard router table you always feed right to left. Most of the time you are only cutting on one side of the bit which is trying to push the stock away from the fence. We do this all the time with no problem so I don't see how this can be a problem with slot mortising.

[charlese]

Oh My! clockwise and counter clockwise is so confusing. Good thing we mostly use digital clocks.

Pretty soon kids will say HUH..?? When told turn clockwise to tighten.

[wrmnfzy]

To tighten, righty tighty, lefty loosey. Aaaahhhhh that never gets old. :-)

[JPG]

The bit is rotating counter clockwise as viewed from the 'top'.

Ya got yerself some of them rare left-hand router bits, and a headstock-reversing switch there, Red?

Well THAT certainly makes it all add up correctly!

I stand corrected!

Feed L-R, bit rotation clockwise(both as viewed from 'above'), climbing at the workpiece cutting area towards the fence, plowing on the opposite side, and feed motion causes the bit to push the workpiece towards the fence.

Sorry for my getting the rotation bass-ackwards.

[JPG]

To tighten, righty tighty, lefty loosey. Aaaahhhhh that never gets old. :-)

Beware of left handed threads!

[BuckeyeDennis]

... and four pictures are even better.

Here is a drawing of a standard (right-hand) router bit, viewed from the shank end. Trust me, this is what it looks like; I have a real bit sitting right next to me. In drill press mode, this is the top view. As you can see, in order to cut, it has to turn clockwise. Spin it counterclockwise, and it's a "burning tool" at best.

Router bit.JPG (14.42 KiB) Viewed 5461 times

Next up, here is a drawing showing a workpiece centered on the fence, with the router bit in the initial plunge position.

Plunge position.JPG (52.3 KiB) Viewed 5461 times

Now then, here's what happens when you move the workpiece to the right, as recommended by PTWFE.

Slotting.JPG (55.4 KiB) Viewed 5461 times

Which brings us to the good stuff. The next drawing is a closeup of the bit's cutting action as it slots.

Resultant force.JPG (25.35 KiB) Viewed 5461 times

The red crescent represents a chip that will be sheared off of the workpiece as it advances into a cutting edge of the bit.

The thickest part of the chip, requiring the heaviest cutting force, is right on the centerline of the slot. The shearing force that the cutting edge applies to the workpiece to make this part of the cut is represented by the yellow arrow. As you can see, it is pushing the workpiece directly against the fence.

As the cutter is entering and exiting the red crescent, the chip is very thin, and thus the cutting force is light. Plus, as jsburger noted, the entry and exit forces oppose one another, and thus cancel.

There is no opposing force for the yellow vector, because the bit is "cutting air" on the backside of the slot. So the resultant force is directly into that nice rigid fence. PTWFE got it right.

(edited on 5/25/2002 to restore original image placement)

[JPG]

... and four pictures are even better.

Here is a drawing of a standard (right-hand) router bit, viewed from the shank end. Trust me, this is what it looks like; I have a real bit sitting right next to me. In drill press mode, this is the top view. As you can see, in order to cut, it has to turn clockwise. Spin it counterclockwise, and it's a "burning tool" at best.

Router bit.JPG

Next up, here is a drawing showing a workpiece centered on the fence, with the router bit in the initial plunge position.

Plunge position.JPG

Now then, here's what happens when you move the workpiece to the right, as recommended by PTWFE.

Slotting.JPG

Which brings us to the good stuff. The next drawing is a closeup of the bit's cutting action as it slots.

Resultant force.JPG

The red crescent represents a chip that will be sheared off of the workpiece as it advances into a cutting edge of the bit.

The thickest part of the chip, requiring the heaviest cutting force, is right on the centerline of the slot. The shearing force that the cutting edge applies to the workpiece to make this part of the cut is represented by the yellow arrow. As you can see, it is pushing the workpiece directly against the fence.

As the cutter is entering and exiting the red crescent, the chip is very thin, and thus the cutting force is light. Plus, as jsburger noted, the entry and exit forces oppose one another, and thus cancel.

There is no opposing force for the yellow vector, because the bit is "cutting air" on the backside of the slot. So the resultant force is directly into that nice rigid fence. PTWFE got it right.

So did thee!!!!

[edflorence]

wow...nice illustrations! Thanks for taking the time to put it all together...very clear and helpful.

[edflorence]

wow...nice illustrations! Thanks for taking the time to put it all together...very clear and helpful.

[BuckeyeDennis]

wow...nice illustrations! Thanks for taking the time to put it all together...very clear and helpful.

Thanks, Ed. I was itching to try out a set of graphics features that I had recently discovered in Microsoft Visio. Al's thread, plus a dollop of insomnia, provided the perfect opportunity.