Adjustable Router Sled For A Trim Router

If you want to flatten out a slab of wood and you don't have a planer or your slab is too wide, a router sled is pretty handy.  There are many options available to buy, and honestly they are probably better than this one, but they tend to be expensive.  This sled uses 16mm linear rails ( 2x1000mm for ~$40 on amazon ) and printed legs adjustable in ¼" increments.  The rails are extremely rigid and allow the router to glide smoothly from side to side.  The leg height adjustment is simple, secure and fine enough to work well with your router base and bit combination.  The cart that the router rides on conveniently detaches from the sled so you can bolt it to your router quickly.

I used 1000mm 16mm linear rails but, you can use longer or shorter rails.  Using 1000mm rails means the assembled router sled is a bit under 48" wide which means I can use any half or quarter sheet of 4x8 plywood as a flat surface.  The router cart on the sled adjusts from 0" to 3.5" in ¼" increments.

For the height adjustment knobs, I used 4 m5 knurled knobs designed by @FrankW.  You don't have to use these knobs if you have another type you like, but I would recommend printing something as you will be using them frequently to adjust the height of the sled.  Be sure that the diameter of whatever knob you choose is less than 28mm or they won't fit together in the body bracket.   If you want you can print knobs for the 2 m5 bolts that secure the cart to the cart bracket.  I opted not to as they don't need to be tightened very much.

Print settings:

• Supports: Yes
• Infill: 50%
• Filament: PETG (recommended) or PLA
  • Since this is a tool and will likely get some abuse, I'm betting PETG will hold up better over time.  I printed mine with PLA for the two legs, PETG for the 2 body brackets and matte PETG for the cart.  I only used PLA on the legs because I didn't have a color of PETG that I liked.  That may have been a mistake.
• Print the parts in the orientation that are in the model files.  You need 1 cart, 2 legs, 2 sled body brackets, 2 cart brackets and (optionally) 4 m5 knobs.

Additional hardware:

• 2 Linear rails with 16UU linear bearing blocks 
  • I used 1000mm rails, shorter or longer rails should be fine.  Just make sure to use the rails with the 16mm diameter.  Otherwise the body brackets won't fit right.
• 14 m5 nuts
• 4 m5x25 hex head bolts
• 8 m5x12 flat head hex socket bolts
• 2 m5x20 hex socket bolts or similar
• 4 m4x20 flat head hex socket bolts to attach a router plunge base to the cart or 4 m4x12 flat head hex socket bolts to attach a router fixed base to the cart.

Assembly:

1. Insert m5 nuts into each of the 6 hex holes in each body bracket.  Use an m5 bolt and washer to pull each m5 nut fully into its hole.
2. Insert an m5 nut into the hex nut hole in each cart bracket.  Use an m5 bolt and washer to pull each m5 nut fully into its hole.   Insert an m5x20 hex socket or similar bolt into the m5 nut on the cart bracket.  Hand tighten it just far enough to not stick out the other side of the cart bracket.
3. Loosely bolt two linear bearing blocks to each cart bracket, using 4 black hex socket bolts for each linear bearing block.  The hex socket bolts are included with the linear rails.  The flat side of the cart bracket should be towards the bearing blocks, leaving the chamfered face visible.
4. Slide a cart bracket and bearing block assembly onto each linear rail. 
5. Tighten the bolts holding the linear bearing blocks to the cart brackets.
6. Slide the 2 body brackets onto the ends of the linear rails.  Linear rails should be oriented such that the m5 nuts in each cart bracket are pointed away from each other (See photos).
7. Bolt the 2 body brackets to the ends of the linear rails using 2 m5x12 flat head hex socket bolts for each end of each rail.
8. If you printed knobs for your build, insert each m5x25 hex head bolt into its own knob, using a m5 nut and washer for fully pull the bolt into its hole.
9. Press a leg onto each body bracket.  Use the notches and the scale etched on the legs to make sure each body bracket is set to the same height.  For each leg, thread the bolts or bolts with knobs from the previous step through the center slot and into the corresponding holes in the body bracket.  Hand tighten these bolts to keep the leg and body bracket securely together.  

Use:

1. Place your workpiece on a flat surface.  The surface should be wide enough that legs of the sled assembly can rest on it as you slide the entire assembly across the workpiece.  It is important that the flat surface is rigid.  Using a piece of plywood spread between two sawhorses is not going to work well.  Using that same piece of plywood on flat ground or a workbench should be fine.
2. Level and secure your work piece to the flat surface.  I like using shims and hot glue for this purpose, but double sided tape, screws or clamps are also options.  Be careful if you use screws or clamps than the router bit never hits them.  Level your workpiece however it makes sense for your project and the effect you are trying to create.  Just make sure that the workpiece cannot tilt or rock once it is secured.
3. Measure the height of your workpiece from the flat surface.  Round the height up to the nearest ¼".  Set the height of each side of the router sled to this height.  Slide the router cart back and forth across the workpiece.  Verify that the cart does not contact the workpiece anywhere.  If it does, adjust the sled height up accordingly.
4. If the cart is attached to the sled, remove it.  Loosen the m5x20 hex socket or similar bolts in side of each cart bracket.  Lift the cart off the router sled assembly.  The cart and cart brackets are designed to fit snugly together.
5. You most likely want to remove the existing base plate from your router base as that will give you a few extra millimeters of vertical play when adjusting your router height.  Use mounting holes to bolt your router to the cart using the appropriate m4 bolts for your router base (m4x20 for the plunge base and m4x12 for the fixed base).  Slide the cart back into the cart brackets on the router sled.  Reinstall the m5x20 hex socket or similar bolts in the side of each cart bracket.   These bolts do not have to be more than hand tight.  They only need to extend through the cart bracket into the cart to make sure the cart cannot pop up out of the brackets while the router is running.  
6. With the router turned off, use the height adjustment function built into your router base and adjust it so that the router bit just barely comes into contact with the workpiece.  You'll have to use your judgement as to how much material you want to remove during each pass.  This will depend on the router bit, your router's power and the material the workpiece is made of.  I generally shoot for removing around 1mm of material per pass.
7. Turn your router on and slide the router and cart back and forth along the linear rails.  Turn your router off.  Slide the feet of the sled across your workpiece ( perpendicular to the linear rails ) no more than the width of your router bit.  Repeat until you have trimmed the entire top of your workpiece.  If you need to trim more material, repeat steps 6 and 7.  If your router base height adjustment has reached its maximum down position and you want to remove more material from your workpiece, adjust the height of each of the sled legs down by ¼" and continue with step 6.

Future improvements:

1. I used Makita trim router bases from @Qmodo_17187 (fixed) and @ruggibrante_789997 (plunge) so the bolt holes in the cart piece are unlikely to fit other tool brands.  If you are interested in remixing this for other similarly sized routers, I would recommend starting with one of the base plates from @GarageMaker_727088's excellent Ultimate Router Jig.  I would guess that remixing this design for larger, heavier duty routers would be much more involved as you would likely need to widen and strengthen the legs, cart and body brackets.  As it is these pieces just fit on a Prusa MK4S print bed.
2. I want to make tapered table legs in the future.  If I mount my workpiece at a shallow angle with respect to the linear rails of the sled, I can create tapers.  I also want to make cylindrical legs.  If I allow my workpiece to spin on its axis parallel to the linear rails I can create cylinders.  If I combine the taper and cylinder function I can make tapered cylindrical legs.  
3. The foot of each leg of the sled contains holes for m5 hex head bolts.  It might be useful to use those holes to bolt things on to the legs.  For example, if you wanted to flatten a workpiece thicker than 3.5", which is the sled's current maximum height, you could bolt on leg extensions.    Or maybe the table leg tool from the previous improvement idea should bolt on to the legs.
4. Dust collection.  After using this a few times I can report that you will generate a ton of dust.  It would be nice to have a vacuum hose attachment bolt on to the cart.  It would also probably be nice to have a broom foot around the bottom edge of the cart to contain sawdust.
5. Bolt storage.  I hate having to find the right bolts to attach my router base to the cart.  A magnetic storage box attached somewhere to this would give me an easy place to store them.  Or maybe just some m4 sized holes in the legs to screw the bolts into when I'm not using them.

Disclaimer:

Using a router is dangerous.  Using a router with this design is probably even more dangerous.  Use this design at your own peril. I assume no liability for any injury or damage that occurs while using this design.

I incorporated the general shape and hole locations from @ruggibrante_789997's plunge base into the router cart part, while removing the CMT300 guide ring slot.  I also added the bolt hole locations from @Qmodo_17187's fixed base to the cart.