Cross Cut Sled Fence - Bowed At Kerf

[DerbyDad03]

This just a tip/warning that I thought you folks might want to be aware of. ..

Last weekend I built a cross cut sled, following the techniques used in The Wood Whisperer, Epsiode 146. If you scroll forward to 8 minutes, you will see the portion of the video where Marc attaches the fence and then aligns it using the 5 cut method.

My fence was made from a 4" tall piece of 3/4" melamine covered composite m aterial that my neighbor gave me. He uses scraps he has for sacrificial fen ces since the material is perfectly flat and smooth.

I went through the alignment process and then test cut a few pieces of 1 x

6 poplar. The fence was initially aligned using the left side of the kerf a nd all boards were fed from that side i.e cutting off small strips and smal l ends from the right side of the boards. I was quite satisfied with the sq uareness of the cuts.

Both ends of the fence were screwed to the sled as part of the alignment pr ocess, and then when I was satisfied, I added one more screw just to the ri ght of the kerf, planning to add more screws once I was sure that everythin g was aligned to my compete satisfaction. I'm glad I waited.

Last night, I decided to make a few zero clearance insert blanks from some true 1/4" hardwood plywood left over from some old drawers. Since the panel was ~14" wide, it spanned the kerf in the fence more than anything I had u sed for my test cuts. That's when I noticed that there was a gap between th e fence and the far right side of the panel. As first I thought there was a n issue with the panel, but I decided to check the fence with a straightedg e. There was indeed a slight bow in the fence right at the kerf.

I bent down and looked under the sled and saw the single screw I had placed in the center area of the fence. As soon as I removed that screw, the bow disappeared and the fence is now once again flat across the face.

I went back and watched the video again and I didn't see any mention of Mar c checking the face of fence once it is secured to the sled. My next step w ill be to carefully pre-drill for more screws, starting from right, working towards the kerf and checking the flatness after each screw is secured.

Just something to keep in mind...

[dpb]

On 02/10/2015 10:41 AM, DerbyDad03 wrote: ...

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I think the problem basically is the 3/4" laminate simply isn't stout-enough material for the purpose. I notice in his video it appears to be at a minimum 1+" and guessing 1-1/2" maybe???

I generally use hard maple or similar hardwood of about that thickness for the purpose.

[-MIKE-]

I agree. The fence he uses in the video appears to be built up finish plywood, at least 1-1/4' thick. It should also be checked for straight and flat before attaching. Plywood can warp, too.

I used a simple 2x4 for my fence. My intention was to build a prototype, then make a "good" one using better materials. But heck if that prototype didn't turn out perfect! No need for another.

I passed the 2x4 over the jointer a few times, then through the planer. That made it perfectly flat and straight.

[DerbyDad03]

I assume you didn't watch the entire video - and that's OK...

If you scroll to 3:50, you'll see that the fence he used is made from 1 1/8" HDO plywood with a "resin impregnated fiber" coating. He called it "regular plywood on steroids." At 7:00 Marc does indeed check it for flatness *before* installing it, as did I.

The point I was making is that he doesn't check it for flatness *after* installing it. That was when I found the bow caused by the screw. I'm not sure how the screw caused the bow, but I will be much more careful when I install the new screws tonight.

As I often say: "It's only temporary...unless it works."

...snip...

[-MIKE-]

I don't have to, I've made them myself.

The real point is, you used 3/4" material. I don't think that's stable enough. Screwing into it is enough to move it one way or another ever so slightly. You may consider (even on a thicker fence) screwing it to the base using pocket screws with a washer in an enlarged hole. This way it can be fine tuned when tightened and readjusted for seasonal movement.

I have a ton of "temporary" jigs and tools in my shop that are still going strong. :-)

[dpb]

On 02/10/2015 11:54 AM, DerbyDad03 wrote: ...

Which is at least 3/8" thicker than your 3/4" (and 50% as a percentage)...

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Because he had sufficiently-stout material that moving it wasn't likely to be an issue...it never hurts to check, yes, but imo in your case it's inevitable in that the material simply isn't very rigid owing to it's thinness or lack of bulk.

The "how" is likely that your screw tip hit the edge of one of the plies instead of dead center either in or between and that difference in hardness pushed the material to the side instead of letting the screw penetrate straight in at the starting location.

That's what doesn't happen with the stouter material, or at least not nearly so likely and with clear hardwood isn't nearly the issue as with the ply.

[dadiOH]

Or 2 layers of decent 3/4 ply.

[Dan Coby]

I will add my experience on building a crosscut sled.

I checked my alignment after I added screws to help hold the fence aligned. The alignment was fine before adding the screws but off after adding screws. The process of torquing down the screws took the fence out of alignment. To fix the problem, I finally realigned the fence and alternated gradually tightening the screws to keep the fence square.

Dan

[DerbyDad03]

I originally glued up 2 cutoffs from the 1/2" ply I used for the base to cr eate a 1" fence. It was not perfectly flat, so while chatting with my neigh bor he offered the 3/4" melamine covered option. I was going to buy a piece of hardwood, but decided to try his suggestion first.

If I have anymore issues with the current fence, I'll probably go the hardw ood route.

[Max]

Just like torquing lug nuts or manifold bolts or.....

[DerbyDad03]

I torqued the manifold bolts on the cross cut sled to 30 ft-lbs.

[John McCoy]

DerbyDad03 wrote in news: [e-mail]:

I have seen the same thing happen in various situations, where putting a screw in the center pulls a piece out of line. It's always better to work from one end to the other, or from the center out to both ends, than it is to do the ends and then the center.

John

[Max]

[whit3rd]

If I care about alignment (usually do, in any jig), I predrill and drive one nail as a pivot, then glue the piece into position. When the glue is set, before using the assembly, I predrill and screw along the glue joint so it won't break. Optionally, also remove the nail...

This allows me to get good results even with hotmelt glue (which allows later disassembly for adjustment). Predrilling is key, a bit of woodgrain can c*ck a sheetrock screw badly, and has some effect even on shanked woodscrews.

[Lew Hodgett]

I bent down and looked under the sled and saw the single screw I had placed in the center area of the fence. As soon as I removed that screw, the bow disappeared and the fence is now once again flat across the face.

---------------------------------------------- As others have suggested, too small a pilot hole and 3/4" stock requires screws to be dead nuts on center, which leaves very little for error.

SFWIW, I use 75% of full body dia for pilot drill dia.

Haven't stripped a screw yet.

Lew

[dadiOH]

If/when you need to make a thicker piece of ply by gluing together two thinner ones, cut them from the same sheet and glue face "A" to face "A". If they didn't start out flat - like most all borgply - this will *help* flatten both thereby minimizing the additional work needed for total flatness.

[Swingman]

On 2/10/2015 1:34 PM, DerbyDad03 wrote: ginally glued up 2 cutoffs from the 1/2" ply I used for the base to create a 1" fence. It was not perfectly flat, so while chatting with my neighbor he offered the 3/4" melamine covered option. I was going to buy a piece of hardwood, but decided to try his suggestion first.

Or, laminate both plywood, and hardwood(for the critical face of the fence):

Ran the (scrap) walnut face across the jointer for "flat".

My goto crosscut sled for just about all crosscutting under 12", that puppy has been rock solid for years.

(it has white oak runners, routinely waxed)

[DerbyDad03]

That's what I did and while it *helped*, it still wasn't flat enough for my liking. The best of the 2 was used for the back fence, the other one is in the ever growing scrap pile.

[DerbyDad03]

Good idea.

What are your thoughts on sandpaper on the face of the fence?

[dadiOH]

You didn't ask me but I've finished my work for the day so I'll butt in anyway :)

All it does is make the piece you are going to cut harder to position.