Question about router table coping sleds - miter slot and fence alignment

[Michael Jasper]

Hi - Just purchased a woodhaven router coping sled. This one offers the option of either guiding off the miter slot or the fence (although I realized after that if you guide off the fence you risk routing into the sled base since it doesn't have a separate plastic guide as other sleds do.) However, that isn't the question I have.

The instructions say that if you choose to use the miter slot, it's critical that the fence be parallel to the miter slot. This doesn't make sense to me - but perhaps I'm misunderstanding something.

My assumptions:

1) You extend the workpiece past the end of the sled, with a sacrificial backer board to prevent blow out. The amount of extension should be such that the end of the piece is even with the shank of the bit.
2) Once you set that extension, the sled is guided by the miter slot to keep the distance constant as you move across the bit.

Three things in the instructions don't make sense to me.
1) If the fence is only used to set that depth, why is it critical that it's parallel to the miter slot? Obviously if it was completely skewed you'd have a hard time getting the depth right, but once you get that depth, the fence should be completely unnecessary.
2) They recommend using an auxiliary block between the fence and the end of the work piece to set that depth. The logic seems to be that you use this to butt the piece up against, but then as it moves to the bit, it clears this block and prevents jamming against the fence. This makes sense - but once you have the depth set and the piece clamped, why would you need anything there ? In other words, it seems to me that if the miter slot is the guide, you just need to use the fence to get the end of the piece in the right location and then you ought to be able to move the fence or a stop block completely out of the way.
3) If I'm correct about all of this, using the miter slot should not be any more "fussy" than guiding off the fence. If anything, the opposite since it's fixed and you don't have to worry about aligning it at all.

What am I missing?

Thanks!

[Kevin Jenness]

Hi -
What am I missing?


Not a thing.

[Michael Jasper]

Thanks! Then why do most people indicate that it's easier to guide off the fence than the miter slot? Seems to me it's the opposite.

[John Kananis]

If you're guiding off the miter slot, the fence is just a stop. In this case, if the fence isn't parallel to the slot, your stop will be deeper or shallower on one end than the other and your cut will be off on either entry or exit. If you're guiding off the fence, only the correct bit depth is relevant.


Thanks! Then why do most people indicate that it's easier to guide off the fence than the miter slot? Seems to me it's the opposite.

[Michael Jasper]

If you're guiding off the miter slot, the fence is just a stop. In this case, if the fence isn't parallel to the slot, your stop will be deeper or shallower on one end than the other and your cut will be off on either entry or exit. If you're guiding off the fence, only the correct bit depth is relevant.

This is the part I'm not getting. If you're guiding off the miter slot, once you set the depth, how would a non-parallel fence cause what you're describing? I'm assuming the work is clamped down, so the amount extending towards the bit doesn't change. I must not be understanding something because I've seen this expressed in multiple discussions.

[John Kananis]

If you're using the same exact spot on the fence as a reference/stop before you clamp down, you won't have any issues unless the fence is closer to you on the left than it is on the right...


This is the part I'm not getting. If you're guiding off the miter slot, once you set the depth, how would a non-parallel fence cause what you're describing? I'm assuming the work is clamped down, so the amount extending towards the bit doesn't change. I must not be understanding something because I've seen this expressed in multiple discussions.

But then you're not using the fence as a guide any longer, just as a stop. Setting up using the fence as a guide (imho) simplifies everything.

[Michael Jasper]

If you're using the same exact spot on the fence as a reference/stop before you clamp down, you won't have any issues unless the fence is closer to you on the left than it is on the right...



But then you're not using the fence as a guide any longer, just as a stop. Setting up using the fence as a guide (imho) simplifies everything.

Exactly - I'm saying if I guide off the miter slot and just use the fence as a stop, why is it so important that the fence and slot are parallel? I suspect there's something I'm not understanding.

[John Kananis]

Try both and you'll quickly understand. When you use the guide off the fence, just move the guide material out of the way (I'm not familiar with your device but I have to image it has some adjustability) and use a backer (which you should use anyway).

[Mike Cutler]

Michael

I've never seen the coping sled you have, but I looked at the directions on their website and something is wrong.
If you're guiding off the fence, there's no relationship to the miter slot necessary. You don't even need a miter slot.
If you're using the miter slot, the reverse is true. You don't need a fence at all.
On my shaper table, if I am referencing from the miter, I leave small space of maybe 1/32nd to a 1/16th, between the workpiece and the fence, and it is parallel to the miter slot only to facilitate dust collection ,and a sacrificial backstop, if the piece of material on the sled gets loose and cocked in position it won't go flying.
From what I can tell from their 4 pages of instructions, that sled was designed to run in the miter slot and they found a way use it off the fence. However, according to their directions on the website, I would not do it the way they detail. You will end up with the front left edge of the sled unsupported and referenced to nothing. Everything is reference to the front side right, trailing edge.
If you look Infinity's coping sled in this link, https://infinitytools.com/products/improved-coping-crosscut-sleds You will see that there is a piece of Lexan that spans the length of the sled so that the front edge of the sled is always referenced along it's entire length to the fence face. This is the way I know of to reference a cope off the fence, and is the way the ones I have built for my shaper are made. I would personally not do what Woodhaven seems to be detailing in their instructions.
BTW, I do not own that Infinity sled, or am trying "push" them". I only linked to it as an example.
Use your sled in the miter slot, set up the fence for dust collection, and I think you'll be much better off.

[Kevin Jenness]

I use a sled for tenoning/coping on my shaper with a stop screwed to the fence. Once the work clears the short stop it has no contact with the fence and the alignment is not an issue. Trying to keep the typical fence aligned to the miter slot is difficult and pointless. Use one or the other. I use sandpaper and a toggle clamp on the sled to keep the work in position and a backer to control blowout.

[John Kananis]

Kevin speaks the truth here. One way or the other, eliminate something.

[Michael Jasper]

Michael

I've never seen the coping sled you have, but I looked at the directions on their website and something is wrong.
If you're guiding off the fence, there's no relationship to the miter slot necessary. You don't even need a miter slot.
If you're using the miter slot, the reverse is true. You don't need a fence at all.


Mike - thank you! That's exactly what I thought. Good to know my reasoning was sound.
I tried it out tonight, using the miter slot. I used the fence to set the depth and then moved it back enough to stay clear of the piece. Worked perfectly. I don't know why the instructions are written as they are.
It's a nice sled though - very solid. Perhaps I'll add some standoffs and an acrylic guide to use the fence in the future, but working off the miter slot worked smoothly.

[Michael Jasper]

Thanks Kevin. You and Mike have confirmed what I thought.

[Rod Sheridan]

Yes, when I make tenons or copes I use the sliding table and a cross cut fence with a flip stop.

I don’t have a fence on the shaper at all, just a tenon hood

Your jig running in the mitre slot would be similar to the above

I think the jig instructions are incorrect

Regards, Rod

[Mike Cutler]

Mike - thank you! That's exactly what I thought. Good to know my reasoning was sound.
I tried it out tonight, using the miter slot. I used the fence to set the depth and then moved it back enough to stay clear of the piece. Worked perfectly. I don't know why the instructions are written as they are.
It's a nice sled though - very solid. Perhaps I'll add some standoffs and an acrylic guide to use the fence in the future, but working off the miter slot worked smoothly.

Michael
I think you're better off doing as you describe.
There are some operations that are made easier by having the ability to do both. T&G joints can be easier if you're using a combination bit to do off the fence. This way the depth of the tongue and the groove are exactly the same.
If you ever want to achieve perfect lock miter joints, a coping sled(s) in the miter slot makes them perfectly.You can Make razor sharp edges for those miters. Trust me. :(
One common function performed on a shaper, that for some reason has not migrated fully to the router table, is the use of a back fence. If you ever need to make a profiled edge, on multiple lengths of material, exactly the same width, ie baseboard moulding or window trim, the back fence is indispensable.

[Michael Jasper]

Thanks Mike. I do have a lock miter bit which I tried out previously without a sled. Results weren't fabulous, but I was also trying to do it in 1/2" plywood (and not Baltic Birch). So that probably had something to do with it as well.