|Project by LittleBlackDuck||posted 07-31-2016 02:23 PM||1403 views||4 times favorited||16 comments|
Boys and Girls,
I’m sure that most of us have been confronted by mitre cuts in long, narrow and thin stock. While most veterans have devised their own methodology and sacrificial fingers, I had to create a new ”skin” for a 22.5° set of cuts so I thought I might as well take some happy snaps and document my mitre sleds for any novices out there that are yet to design their own signature jig.
The reason why I made this jig is because I quickly discovered that I could never get two bevelled edges on opposing edges of narrow stock to finish up the exact size required. I always used to sneak up on the cut as the bevelled edges always seemed to slide under length stops and I continually struggled keeping the piece parallel and once moved you are really starting positioning again. Anyway I had issues with the traditional sleds.
Now what I do is cut the stock to exact size and use this jig to put the bevel onto it. As all it is doing is cutting the corner off the bevel, there is little work being performed and strong clamping pressure is not mandatory and medium pressure is sufficient.
The jig was initially cannibalised from a “traditional” mitre box jig for 45° cuts but got slightly mutilated when I had to make a few different degree cut. It finished up in 2 halves and morphed into a LHS sled. The dimensions are not critical as the width will be mutilated by the saw blade as you run it along the table… just ensure you have an adequate working space to the left of the blade for the workpiece and maybe a clamping system. The length is arbitrary, but as this is meant to be a small part jig, I can see no benefit in making it several meters long, so keep it short enough that you don’t need to revert to bifocals or binoculars to see the far end of it.
The secret to making it versatile is to make ”skins” for the cutting surface(s). I use a mitre track down the length of the base to permit further flexibility. I make up my skins of 3mm and 6mm DMF sheet goods to permit the use of standard 9mm T-tracks.
The base consists of 18mm MDF with a 3mm then a 6mm MDF sheet on top. These are “halved” by the track. One hint I will give now is to carefully measure the cutting side top 6mm MDF piece and place the screw holes so that the piece can be of any orientation and the screw holes will line up (more on this later). Now KEEP this as a template and create new ”skins” using the marked screw holes for subsequent drilling. Ensure the screw holes are placed in such a way that there is sufficient overhang to be able to cut a bevel edge. Note that when you flip the piece to use for another angle there should be a slight gap between the former cut edge and the T-track, but it should not be sufficient to be of concern (otherwise cut a sliver for a packer). This way you can replace the ”skin” as many times as you like without turning the base into a cheese grater with random placed screws.
As with all my projects I start off by designing something/anything in SketchUp and after looking at it for a few hours I decide what I can make out of it.
The screw hole placement on the left (non-cutting ”skin” is not paramount) for the equation. You may have noticed (and if you haven’t go back and have a look) the base has a 45° bevel, this is not really important but it does clear the timber to ensure it can accommodate a max of 45° ”skin” without additional scarring. It is the 6mm MDF top ”skin” that provides the cutting surface and should be more than thick enough for small stock.
To make the ”skin” reusable I highly recommend accurate angle measurement. I use a Wixey angle gauge and I feel that it’s mere .1° accuracy is sufficient not to be detectable by my failing eyes. The accuracy of the angle is required for both the build specifications and retainment of the top cutting sides edge (hey you can cut a new one each time if you want). I also pair up a dedicated skin for the backing board with the base which I use for visual alignment purposes only.
Now for accuracy I have devised this methodology for creating a fitting/alignment jig for the small piece along the cutting edge. The 10° in the diagram was for this specific ”skin”... You can choose your own personal 10° angle.
Take a piece of MDF the same thickness as your top ”skin” and ensure that one end is perfectly straight and preferably square, somewhere between 100-200mm wide. Place the width across your sled and cut a 24-30mm wide piece and put aside (remember it has a 90° edge and let us name this piece Aloysius). Now cut another piece about the same width which should now have two edges the same angle as your sled and call it Hephzibah. Place Aloysius on its end and glue Hephzibah with the long edge width against Aloysius. You should now have a bevelled edge leading into a 90° edge. Mark this edge as the action part. The other edge (should have two adjoining bevelled edges and is infertile).
You will need to make a packer/spacer (let’s call it Aristotle) the thickness of your base and any additional ”skin” if used (do NOT include the thickness of the top skin). In the diagram my spacer was 21.7mm thick (this needs to be as exact as you can make it for accuracy) which was made up of theoretical 19mm and 3mm MDF (should be 22mm but was probably made in China and they cut back on thickness to cut costs… also most likely lost in the katakana translation).
Now to set-up using Aristotle and Aloysiusand Hephzibah.
Place Aristotle next to the jig. Does not have to touch but provide a flat surface for A & H,
Now place Aloysius above Hephzibah on top of Aristotle and push the bevelled part Hephzibah into the cutting bed. The 90° piece Aloysius on top should protrude above the top surface parallel to the cut line.
Notice how Aloysius is against the cut line. Also note that the backing ”skin” kerf line is an handy and ideal alignment mark for this angled cut, that I find totally useless and have never used it.
Now push your piece to be cut (let’s call it Costlitimba) against Aloysius ensuring it does not move. Pieces should be small enough that no pressure need be applied to ensure snug fit without disorientation.
Clamp and cut, again ensuring the A & H is not pushed away by clamping pressure. I found that initially the way I set up my clamp it pushed the Costlitimba stunt double about 1mm sideways… I readjusted the clamp cut another victim and voilla…
This picture illustrates the “paired” end piece with it’s mating base ”skin” (both marked with the 22.5°).
If you position your screw holes properly, you can flip the ”skin” and use the other long end as a new cutting edge. This way you can use each ”skin” for 2 different angle configurations. You may notice the slight gap between the track and the flipped side (with the 6° cutting edge) small enough for even an elephant tom straddle.
-- There's two ways to do things... My way or the right way.. LBD