Trying to cut perfect sliding dovetails.

[Joe Hillmann]

I am trying to cut a perfect sliding dovetail out of three pieces of wood. Two of the pieces will be screwed to a backer and the third is the wedge that slides between them. The problem I am having is getting the sides of the wedge perfectly parallel. I have tried using a joitner with the fence at an angle, that produced sides that were nowhere near parallel. I also tried twice on the table saw but I still can't get it perfect.

Can anyone suggest a way to get the center wedge to have perfectly parallel sides?

[Chris Friesen]

1) Rip on table saw with the blade tilted.
2) Router table using one dovetail bit for all pieces. Or use a straight bit and a fixture to support the board at an angle to give a bevelled edge.
3) Same as the first two but use a circular saw or router handheld with a straightedge to guide it.
4) Plane it to a line by hand.

[Kevin Groenke]

How big is the piece? I cut the tails for sliding dovetails on a router table. Prepare your stock, bury dovetail bit in sacrificial fence on router table, run stock through bit in a vertical orientation. rotate, repeat.

Here's a video (not mine) the dovetails are at the end.


http://www.youtube.com/watch?v=aVk_qHJaRL4

[pat warner]

The classical method has you trapping the stock on the router table.
Both sides of the cut being referenced from the same side of the stock.
But not for me.The essential criticality (to a safe cut) is flat uniformly thick stock (http://www.patwarner.com/images/dt.jpg).
If that is met then, whence fixtured, you can rout from both side of the stock, not trapping the work. Taking an equal amount from each side, centers the dovetail.
I regret that the fixturing, due to sawmill protocol, cannot be shown here.
****************************
Long grain dovetails can be made safely on the RT, routing from both sides of the sample.
Use a big stick, then harvest what you need from that.

[Joe Hillmann]

The center wedge piece is about 4 inches wide, 3/4" thick and eighteen inches long. The dovetail only has to slide about 1 inch but I can't have any slop in it any where in that one inch range.
260366
Hopefully this picture will explain what I am trying to do. The first diagram shows the end view and on the second one the black line represents what I want the center wedge to be like (parallel edges) and the redline is an exaggeration of I am getting, the sides are not perfectly parallel. As best I can measure the sides are maybe .015-.020 out of parallel over the 18 inch length. It is hard to measure because of the wedge shape of the piece there is no solid place to get accurate measurements from.

[pat warner]

I think I have it (understand).
Can't be done on the jointer. Width Parallelism almost impossible on the jointer.
Sawing is the best bet.
And, with trepidation, I would size and bevel in one trapped cut on the router table.
NOT recommended but it is very accurate.
Without push sticks, a plywood vest and eye protection I would not do it.
Notwithstanding, if all 3 parts are wood, they slide well today but jam tomorrow.
Moreover, if the way is not adjustable, the likelihood of a perfect fit over 18" is squat.

[Robert LaPlaca]

If I am understanding you correctly, you are attempting to cut an 18" wide sliding dovetail that has to fit dead on? Typically a sliding dovetail joint that wide would be tapered, where the fit at the back of joint is oversize and the fit at the front is where it's dead on. Even with the front 3 inches or so of the joint fitting dead on, it can be a challenge to get the joint to assemble..

[Chris Friesen]

If I am understanding you correctly, you are attempting to cut an 18" wide sliding dovetail that has to fit dead on? Typically a sliding dovetail joint that wide would be tapered, where the fit at the back of joint is oversize and the fit at the front is where it's dead on. Even with the front 3 inches or so of the joint fitting dead on, it can be a challenge to get the joint to assemble..

Due to this issue I would suggest "hollowing out" the non-visible parts of the interior section of the joint, leaving only a couple inches at each end that is actually fully tight.

[Joe Hillmann]

This is what I am trying to do. Only I building mine much larger and filling it with concrete for stiffness. Pages 2-4 show the dovetail I am trying to create.

http://www.exit45.com/shared/PinRouter.pdf

[Robert LaPlaca]

Wow, so we are attempting to create a sliding dovetailed 'way' like on a machinists milling machine (aka. a Bridgeport).

[Joe Hillmann]

Wow, so we are attempting to create a sliding dovetailed 'way' like on a machinists milling machine (aka. a Bridgeport).

Pretty much. I only need it to move up and down an inch. The rest of the length is just there to give it as much bearing surface as possible. At this point I am thinking of ripping the center wedge shape down the center then shimming it to be parallel.

[Chris Padilla]

Joe, you clearly need The Magic Square! :)

[Joe Hillmann]

Joe, you clearly need The Magic Square! :)

You're right. I wouldn't have to worry about a mere .010" off. The only problem is his site isn't taking orders at the moment. :)

[Pat Barry]

Joe, this is gong to be a very critical part of your finished tool, therefore I would suggest that wood is not the right choice for this dovetail. Perhaps you can find a steel dovetail (maybe someplace like Graingers??) That would give you a precise fit and therefore smooth operation. Also consider a cross roller bearing for this application. They come in many sizes.

[Kevin Groenke]

two words, linear bearings.

http://www.ebay.com/itm/261035049196 (http://www.ebay.com/itm/SBG-35-SLL-CNC-Linear-Rail-And-Ball-Bearing-Block-290mm-x-35mm-/261035049196?pt=LH_DefaultDomain_0&hash=item3cc6e6c0ec)

http://www.ebay.com/itm/220889260172

http://www.ebay.com/itm/110962515550


or similar.

[Bill Wyko]

LMAO that guy was hilarious. Clumsy as a baboon.


Joe, you clearly need The Magic Square! :)

[phil harold]

Kevin took the words right out of my mouth

two words, linear bearings.

http://www.ebay.com/itm/261035049196 (http://www.ebay.com/itm/SBG-35-SLL-CNC-Linear-Rail-And-Ball-Bearing-Block-290mm-x-35mm-/261035049196?pt=LH_DefaultDomain_0&hash=item3cc6e6c0ec)

http://www.ebay.com/itm/220889260172

http://www.ebay.com/itm/110962515550


or similar.

[Joe Hillmann]

How accurate are plunge routers when it comes to moving up and down? Are they accurate enough that I could replace the sliding dovetail with one?

[Alan Lightstone]

two words, linear bearings.

http://www.ebay.com/itm/261035049196 (http://www.ebay.com/itm/SBG-35-SLL-CNC-Linear-Rail-And-Ball-Bearing-Block-290mm-x-35mm-/261035049196?pt=LH_DefaultDomain_0&hash=item3cc6e6c0ec)

http://www.ebay.com/itm/220889260172

http://www.ebay.com/itm/110962515550


or similar.

That's how I would approach it. Perhaps not as good looking, but far more accurate and stable.

VXB makes a wide assortment, and I have found their quality to be stellar.

http://www.vxb.com/page/bearings/CTGY/LM