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Problem creating accurate dado when creating a crosscut sled

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Forum topic by cag0331 posted 10-26-2016 10:34 AM 659 views 0 times favorited 8 replies Add to Favorites Watch
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cag0331

13 posts in 415 days


10-26-2016 10:34 AM

Topic tags/keywords: jig crosscut sled table saw dado rabbet top track kreg

Hello,

I’ve been reading this site for a while now and finally decided I should sign up. I’m pretty green when it comes to woodworking, but I’ve put together a decent looking shop and have been trying to put more time into the hobby.

Right now, I’m working on completing a crosscut sled for my table saw. I’ve been using Nick Ferry’s plans (found here ). I have it pretty much done, but I ran into some trouble with the top track and keeping the fence square. I cut the front fence to final size and used 2 passes to create the rabbet for the top track and screwed it on, squared the fence and made the final kerf cut. However, there must have been a bit of a lip in my rabbet because when I made the final kerf cut (and cut through all the remaining wood in the fence) the track must have straightened and pulled the 2 sides of the kerf out of square (one side was sitting about 1/16 proud of the other). So I decided that the best thing to do was try to remake the fence.

This time I decided to cut the rabbet with my dado stack. It seemed like it worked; however when I went to dry fit the top track, and pulled it tight to where it would be screwed, there was some bow in the middle (i.e. it seems like the dado stack cut deeper in the middle of the rabbet). Because of this, I’m nervous to attach the track and complete the build for fear that I will run into the same problem with the screws causing the track to flex and then when I complete the kerf the track straightening and pulling the fence out of square.

Could someone suggest some ideas either on how to make my dado more accurate or to prevent the fence from going out of square when I make the final cut? Any help would be greatly appreciated.

Thanks!


8 replies so far

View Lazyman's profile

Lazyman

1503 posts in 1223 days


#1 posted 10-26-2016 12:13 PM

Note sure I am visualizing the problem exactly. A picture or 2 showing the results might help. When you measure the depth of the rabbet along its length, where does it appear to be deeper or shallower. Here are my thoughts…

1) Did you use Baltic birch plywood like Nick did? Sometimes the cheap plywood you get at the big chains is not flat enough to use for this. Also, when gluing the 2 layers together, it is a good idea to clamp it to a surface you know is perfectly flat so that you don’t wind up with either a bow or some internal stresses glued into the fence that are released when you cut it. If there is any bow in wood it will be difficult to get a rabbet with uniform depth and your sled will be impossible to get square regardless.

2) Since you had a similar problem with both methods of cutting the the rabbet , make sure that your table saw is tuned. First, make sure that you table saw insert is perfectly flat and level with the table. if the piece doesn’t sit flat, you won’t get a uniform rabbet depth along the length of the cut. Check that the blade is parallel with the miter slots and that the riving knife is perfectly aligned with the blade (not used with dado stack). Once those are set correctly, make sure that fence is perfectly aligned with the miter slot. Any misalignment of the blade relative to the fence will cause a problem. After setting the fence before your cut. Measure the distance from both the back and front of the blade to the fence to make sure it set parallel. On some cheap saws (like mine), it sometimes takes a couple of trys before it locks down correctly. Finally, make sure that the blade is exactly at 90 degrees to the table suface. Make a test cut on a scrap and invert one piece while holding flat on the table surface and make sure you don’t see a gap on one side or other.
3) You might try using a feather board to help keep the piece firmly against the fence along the full lenth of the cut and make sure that the piece stays flat on the table during the entire cut You can clamp a board to the fence to act as a hold down, if necessary when cutting with the piece flat against the table.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View rwe2156's profile

rwe2156

2711 posts in 1316 days


#2 posted 10-26-2016 12:25 PM

The only way I can see this happening is your stock is not flat.

-- Everything is a prototype thats why its one of a kind!!

View cag0331's profile

cag0331

13 posts in 415 days


#3 posted 10-26-2016 02:54 PM

Thanks for the replies. I’ve attached a couple of photos of the rabbet with a straightedge on it and you can see that theres a bit of a gap in the middle. I did use baltic birch like Nick did; however, looking at it now, it looks like it may not be perfectly flat (you can sort of see a gap on the sides between the ply and my TS wing. I think I’ll try Lazyman’s suggestion and make another fence blank but clamp it to my TS or router table when doing the glue up. I’m thinking that will hopefully remedy the problem?

View HokieKen's profile

HokieKen

4510 posts in 974 days


#4 posted 10-26-2016 03:23 PM

If your stock isn’t flat, your rabbet won’t be either. I would try clamping the stock flat then cut the rabbet with a handheld router. Do the glue-up with the stock clamped flat as well. You’ll have to do the same for the rear fence which is tricky since you’ll need to adjust it. Honestly, I’d go get some flat stock and save what you have for smaller jigs where flatness isn’t as much of an issue.

-- Kenny, SW VA, Go Hokies!!!

View Lazyman's profile

Lazyman

1503 posts in 1223 days


#5 posted 10-26-2016 04:38 PM

If you use your table saw surface as your reference surface for clamping, make sure that you put some cling wrap under it to catch drips. PVA glue can leave some nasty stains in the iron top. If you have some flat and planed 8/4 hardwood laying around that is large enough, this makes a pretty good clamping surface as well (be careful not to glue your piece to it. .

Ken’s note about using a hand held router to cut the rabbet is a good idea too. As long as you use keep the base plate flat on the surface, its smaller area will be less affected by any slight imperfections across the entire length.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View cag0331's profile

cag0331

13 posts in 415 days


#6 posted 11-10-2016 03:42 PM

Thanks for the advice everyone. I ended up buying new baltic birch and trying again since the stock I had was not flat. It worked this time!

View HokieKen's profile

HokieKen

4510 posts in 974 days


#7 posted 11-10-2016 07:20 PM



Thanks for the advice everyone. I ended up buying new baltic birch and trying again since the stock I had was not flat. It worked this time!

- cag0331

Good choice ;-P

-- Kenny, SW VA, Go Hokies!!!

View MrRon's profile

MrRon

4492 posts in 3079 days


#8 posted 11-10-2016 08:03 PM

Just because it’s called “Birch” doesn’t mean it is “Baltic Birch”. There is a plywood that is a multi layer birch sold in the big box stores, but it comes from China. True Baltic Birch can be unflat if it has been stored wrong like vertically, leaning against a wall. I don’t think Baltic Birch will ever be perfectly flat like a sheet of glass or even a machine tool surface. Flatness tolerance could vary by a few thousands of an inch, but still be considered flat. Just try for the flattest material you can find, but don’t expert it to be perfectly flat like a machined surface.

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