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Re: Slot Mortising Feed Direction?

#186597 by dusty » Tue Apr 07, 2015 10:00 am

Those are nice illustrations. There is one missing though. The one that I think would be very beneficial is the one of a work piece trapped between the fence and the blade.

I supposedly learned NOT to do that but I have done it twice.

In case anyone does not know, if you trap a work piece between the fence and the cutter the work piece will be shot across your shop like a bullet. Very, very ill advised. DO NOT DO THIS.

---

"Making Sawdust Safely"
Dusty
Sent from my Dell XPS using Firefox.

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Re: Slot Mortising Feed Direction?

#186826 by BuckeyeDennis » Mon Apr 13, 2015 12:35 am

dusty wrote:Those are nice illustrations. There is one missing though. The one that I think would be very beneficial is the one of a work piece trapped between the fence and the blade.

I supposedly learned NOT to do that but I have done it twice.

In case anyone does not know, if you trap a work piece between the fence and the cutter the work piece will be shot across your shop like a bullet. Very, very ill advised. DO NOT DO THIS.


Dusty, I was a bit stumped when you first brought it up .. it's a pretty challenging thing to illustrate. But I thought about it some more this week, and here's my best shot.

First up, here we have a workpiece being fed between the router bit and the fence, in the same direction that the bit is spinning.

Fence trap entry.JPG
Fence trap entry.JPG (17.8 KiB) Viewed 1885 times


The woodworker probably wants to make a cut like this, with the fence-to-cutter distance set to provide a nice uniform workpiece thickness.

Fence trap objective.JPG
Fence trap objective.JPG (17.28 KiB) Viewed 1885 times


But alas, the bit cutters don't cut -- they instead become sharp cleats that bite into the workpiece, accelerate it up to the cutter speed, and fire it across the shop like a baseball-pitching machine. Lets just hope that the woodworkers fingers don't follow it into the bit! :eek:

Fence trap missile.JPG
Fence trap missile.JPG (18.48 KiB) Viewed 1885 times


Now let's say that's a 3/4" diameter bit in the illustration. So the cutting circumference is Pi * 0.75 = 2.36". My PC 690LR router spins at 27,500 RPM, or 458 rev/sec. Thus the cutter speed would be 2.36 in/rev * 458 rev/s = 1080 in/sec. That's a cutting-edge speed of 90 ft/sec, or 61 mph, folks. And that router has ample kinetic energy to fire a good-sized workpiece at very close to that speed.

So what happens if you feed the workpiece from the opposite direction (opposing the cutter rotation)? I have some theories, but I'd like to hear what other people think.

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Re: Slot Mortising Feed Direction?

#186833 by dusty » Mon Apr 13, 2015 9:03 am

I gotta think on this.

Your illustrations seem to be correct but do not represent what I believe my experiences have shown me. I seem to recall that the work piece was taken away from me and shot across the shop to what would be the left side in these illustrations.

Unfortunately, this is not one of those situations where I am going to run to the shop to get a demonstration.

PS: This discussion gets confusing I believe because I mix up the fine details by discussing under the table routing, free hand routing and overhead routing. At least that is the case for me.

My stated concern was for when working with an under the table router. In that case, bit rotation is in the opposite direction of what is shown in your diagrams and a work piece trapped between the fence and the bit would be shot off to the left.
Last edited by dusty on Mon Apr 13, 2015 9:41 am, edited 2 times in total.

---

"Making Sawdust Safely"
Dusty
Sent from my Dell XPS using Firefox.

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Re: Slot Mortising Feed Direction?

#186838 by BuckeyeDennis » Mon Apr 13, 2015 9:31 am

dusty wrote:I gotta think on this.

Your illustrations seem to be correct but do not represent what I believe my experiences have shown me. I seem to recall that the work piece was taken away from me and shot across the shop to what would be the left side in these illustrations. The walk action of the cutter head does not support that recollection.

Unfortunately, this is not one of those situations where I am going to run to the shop to get a demonstration.


To be consistent with the earlier slot-mortising illustrations, I showed a top view if routing with a Shopsmith in drill-press mode. With an under-table router setup, the bit rotation would be reversed (from the top view), and the workpiece would indeed be fired in the opposite direction.

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Re: Slot Mortising Feed Direction?

#186839 by dusty » Mon Apr 13, 2015 9:40 am

Yes, you are and were absolutely correct. This is another case where I was too quick to comment without really understanding the situation being discussed.

I have to think seriously about this. It may be time for me to begin backing out of the shop. Mistakes like this, made in real time, would be hazardous. Not all of us, unfortunately, can work in the shop safely into our eighties.

---

"Making Sawdust Safely"
Dusty
Sent from my Dell XPS using Firefox.

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Re: Slot Mortising Feed Direction?

#186840 by BuckeyeDennis » Mon Apr 13, 2015 10:07 am

dusty wrote:Yes, you are and were absolutely correct. This is another case where I was too quick to comment without really understanding the situation being discussed.

I have to think seriously about this. It may be time for me to begin backing out of the shop. Mistakes like this, made in real time, would be hazardous. Not all of us, unfortunately, can work in the shop safely into our eighties.


I've never been able to "remember" the routing-direction rules, myself. Or remember a phone number either, for that matter. :( I'm lousy at memorizing stuff. So before cutting, I always give the router a test spin, and note the bit direction as it winds down. Then I make sure to feed against the cutter rotation, instead of with it.

But I learned something from drawing this stuff. If I simply look at the angle of the cutting edges, I can see which way it will rotate, and a test spin is not necessary.

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Re: Slot Mortising Feed Direction?

#186842 by forrestb » Mon Apr 13, 2015 10:44 am

The drawings are excellent in helping us all to understand what's going on. Thanks! :)

My Porter Cable router must be for us old folks that forget things now and then: the base has a little arrow on it showing me which way the bit moves. :D

I plan to make a big arrow on the top of my router table for the SS that shows which way the bit will move. Now all I have to do is keep the PP in the Forward direction. :eek:

Forrest

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Re: Slot Mortising Feed Direction?

#187887 by Mike907 » Sun May 03, 2015 1:20 am

I've been browsing the Shop Tips videos, and here is a video of right vs wrong direction -

http://www.shopsmithacademy.com/Tips_Archives/SST104_Backrouting.htm

Mike

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