Making a router thicknessing jig

[Prostheta]

These are a common tool in many small luthier's workshops. I've seen a hell of a lot of variance in their quality both in the jig and the results. We're not re-inventing wheels here, but we can certainly make a wheel that does what it is meant to and better. The purpose of this article is to eliminate the flaws inherent in basic router thicknessing jigs in a simple manner. We can also enhance the function of a thicknessing jig to be more flexible from job to job, making the jig a reliable go-to from project to project.

Dimensioning body blanks is usually outside the capacity of your average thickness planer. Guitar and bass body blanks are simply too wide. A router thicknessing jig gives you a tool which can flatten and thickness stock accurately, and not take up valuable floor space like a large thickness planer would!

At their most basic, a router thicknessing jig comprises a flat bed with two raised rails running lengthwise down the long sides. A sled crosses over the bed, straddling both rails providing support for the router to mill stock fixed to the bed. The cross rail which the router motor is mounted to can be made in one of two ways:

1. The motor can be fixed onto a fully-mobile rail which is able to move across the bed. This allows greater control over the milling as the support is greater. It does however mean that the rail is wider and needs a larger work area to operate. The rail is controlled by a pair of handles depending on the user's personal preference.

 

The rail has two side stops preventing excessive side to side movement, protecting the jig itself from the cutter.

[Prostheta]

2. The router motor's base is "captured" within an area on the sled and capable of moving side-to-side. The cutter fits through a slot milled into the sled to reach the workpiece. The sled itself is restrained from movement other than back-and-forth through two stops against the bed rails:

[Prostheta]

Both jig configurations are valid, and have their respective advantages.

More advanced versions of these same jigs can be made using materials other than basic plywood/constructional wood stock. Jigs based on metal rails, bearings, etc. allow more freedom of movement by reducing friction from various bearing surfaces.

The most important aspect of a thicknessing jig is its own dimensional stability in use. How flat the bed is affects how true the bed rails will run along their length; bends or twists in the bed are transferred to the workpiece very quickly. The stiffness of the sled is paramount in being able to achieve a truly flat milled surface; sleds that sag under the weight of the router or heavy handling during milling will produce a rougher finish or worse, a concave surface.

[Prostheta]

Upgrading the bed

It almost goes without saying that using thicker material for the base will be more dimensionally stable and more likely to be flat. For the most part, this is true. A 1" to 1-1/4" (25mm to 32mm) thick piece of high-quality Baltic Birch ply should have very little variance across its surfaces, but should be checked in any instance since they are not made with tight tolerances in mind. This is a big chunk of ply that not everybody has to hand. An alternative is to make up a torsion box from lighter and more readily-available thin materials.

A torsion box comprises and upper and lower surface mounted onto a lattice grid which as a composite structure has far greater stiffness and dimensional stability than its mass would suggest. Paper honeycomb doors are a common day-to-day example, whilst some acoustic guitar makers might recognise the same concept in double-topped soundboards.

 

A basic gridded torsion box:

 

Internally, a torsion box is fashioned using a regular gridding of thin material. This can be made relatively quickly using a table saw to cut equidistant mating slots in thin wood stock:

 

Alternatively, different materials can be used for the bed. Lucky or deep-pocketed individuals may have access to polished stone slab, thick plastics, etc. A few viable off the shelf material is common melamine-faced kitchen worktop cutoffs; bothering your local kitchen fitters for their waste with a few beers should be your best bet. It keeps their waste bins empty and turns trash into cash. Or beer!

 

 

[Prostheta]

Workholding

The straightest, truest router beds and sleds won't be of any use if you can't keep your workpiece still! There are a number of options available, dependant on how far you want to go. The most ideal options are ones that are non-destructive to the jig and repeatable. Double-sided tape, hot melt glue and other temporary fixes are great in the short term but can be a pain to remove, impossible to adjust, slow to set up and wastes glue/tape, etc. Several better options are available.

 

Bench dogs

Thicker beds can be drilled for using home-made or bought bench dogs. A simple dog is a dowel of wood which drops into holes drilled around your bed. More advanced dogs incorporating surface vices, cam clamps, etc. are also available from companies like Veritas/Lee Valley.

A bed drilled for dog placement:

 

Veritas' "Wonder Dogs" have a small low-profile surface vice allowing the workpiece to be held from multiple locations:

 

If using a torsion box setup for the thicknessing jig bed, dog locations should be planned ahead of time so that thicker struts can be located specifically for their use. In general, dog-type retention works best with solid beds.

Generally it is wisest not to start adding metal objects anywhere the wrong side of high-speed cutting tools! Even though the low profile surface vises are well beyond the cutter's reach, there are plenty of simpler and cheaper options made of wood.

[Prostheta]

A step up from a basic dog dowel is one with a wider flat face. This face can have a pad of rubber or cork glued onto it to prevent marring and increase grip.

[Prostheta]

After butting the workpiece up against a pair of dogs in the bed, the other end can be secured with a second pair plus a pair of wedges.

[Prostheta]

In addition to lengthwise retention, cam clamps can be built into the side rails:

[Prostheta]

By adding scrap wood either side of the blank, four cam clamps located around the bed provide plenty of bearing pressure to securely hold almost any workpiece.

[Prostheta]

It is highly likely that any workpiece being thicknessed will also required a degree of truing. It's wise to check which face is most flat and use that as the initial reference surface before milling the least flat side. To allow the blank to seat adequately on the bed surface during the initial milling stage, it is useful to use thin slivers of wood as supporting shims underneath raised corners to prevent the workpiece rocking.

If using dogs on the bed, the first step in the sequence of clamping a workpiece up should be to butt the end directly up against two dogs. The workpiece can then be balanced with shims to prevent it rocking. After this is done, cinching the piece up between opposing dogs using wedges tapped together provides strong initial location. Using an assembly hammer to ensure that the workpiece is secure against the bed helps. After this is done, cam clamps make sure that the workpiece is going nowhere!

[Prostheta]

Your choice of sled is purely a personal one, and likely based on materials and the available room in your workspace. The outer stops for the sleds should be kept short enough that they do not interfere with the function of the cam clamps. Realistically, there is no reason why you can't try both sleds!

My own preference is for the wider sled on the basis of the wider control stance. Like a fixed base router, this setup has a much lower centre of gravity. It benefits from a degree of reinforcement in a similar manner to the captive sled, however using thick ply (1" - 25mm) and controlling the sled with handles makes this more of an afterthought than a requirement.

 

[Prostheta]

[Original]

I rarely use my surface planer, but my router sled is busy all of the time. I used 1.5" square steel tubing for the rails on mine. I think it's a good idea to make the jig long enough for fingerboards, and truing neck blanks. I thought about using bench dogs, but that's as far as it got. I use tape.

[Prostheta]

They're one of the simplest jigs in the workshop and definitely one of the most useful! If I weren't trying to keep the design relatively simple (short of the workholding) it could feasibly incorporate a way of dialling an angle into a workpiece, for example; the neck plane, pickup planes, etc. I think they more fall under the remit of a modified Myka jig.

[ScottR]

This was very nicely done, Carl. Impressive illustrations too.

SR

[Prostheta]

Thanks Scott! I'm hoping to get it built this or next week with either a photo series or even a video.

@Original - your avatar is broken because of a database SNAFU last night. Sorry about that. You might want to re-upload it :-)

[curtisa]

Good info here.  One other thing that may be useful to add is some way to raise the height of the work platform to reduce the depth required for the router bit under the sled when working thinner pieces.  Maybe something as simple as a second removable 3/4" ply bed plate that sits on top of the existing bed and can be secured to prevent it sliding around.

[Prostheta]

Agh, how annoying. I went into today hoping I could get a lot of work done on this, and maybe even get some videos of the process done. At best I have a few photos. It is what it is.

 

Cutting stock for the cam clamps:

 

Blanks marked up (I love that circle template, and my Incra T-rule still rocks) and prototype made:

 

Works beautifully.

[Prostheta]

Good info here.  One other thing that may be useful to add is some way to raise the height of the work platform to reduce the depth required for the router bit under the sled when working thinner pieces.  Maybe something as simple as a second removable 3/4" ply bed plate that sits on top of the existing bed and can be secured to prevent it sliding around.

An auxiliary bed that indexes off the dog holes is a great idea. It could be improved further by making it into a vacuum jig. That reminds me....I have another project idea, making a "reverse bike pump" to create reasonable quality vacuum. At least for workholding, if not good enough for a veneer press.

[psikoT]

It took me a while to realize that those are 3D renders...   very well done, love the textures!

[ScottR]

Is that a bamboo bed and rail you drew up? I missed that the first time.

SR

[Prostheta]

I didn't have a good surface texture for plywood or melamine. 

[Norris]

How about a couple of additional cam clamps that can slot into the dog holes in the bed? If you make those in more of a comma shape you could get quite a "throw" on the cam.

I think I might have to make myself one of these

[Prostheta]

Sure. Any of these methods are possible. If I recall, Veritas do some cam clamps on dogs which are ideal. The final article about router thicknessing jigs will be more about how to create one for your own personal needs. For my own part, I prefer to have zero metalwork below the cutter just out of good habit.

If I had a router motor which took a 1/2" collet, I'd love to do a review on the Infinity Cutting Tools 52-506 bit:
http://www.infinitytools.com/Dado-Planer-Router-Bits/products/1016/

Aside from their secondary purpose being as dado cutters, they'd make short work of wood in this type of jig if you're planing less than 3,0mm or 1/8" at a time. They're about the best for-purpose bit I've seen so far other than a cheapy Luna Tools (Metric) bit, which I don't seem to be able to source anywhere in Finland or Estonia right now.

[komodo]

I made a sled like this to make a sort of overhead pin router. The sled (which doesn't move) has locator pins that go into my table saw out feed table, and a centered pin mounted in that same table directly under the router bit. There are a couple pin sleeve bushings so I can change router bit sizes. The height is adjusted with the plunge router, and it has a dust collector chute attached. As with my other builds, I got to a point and mothballed it and have never even tried it! When I get my shop cleaned I'll get some pics of it.