Thread: How do you cut an exact 45deg mitre?

Please ignore previous comments from this corner. That doesn't work. You would have to present one piece at 90 to the back fence for what I was suggesting, which might work to take a trimming cut of the last mm off but is fiddly.

I was not happy with yesterday's results. With the 90 degree cut line on the SCMS having a tiny 0.07 degree inaccuracy, I thought that the 45 degree pre-set position should be pretty accurate too.

Also, I was using the standard lower & push SCMS technique to slice across the workpiece at a 45 deg angle, which in itself could possibly try to push workpiece sideways off line or even deflect the blade slightly.

This morning I selected the 45 degree pre-set position again, clamped one end of the workpiece, held the other end with a push stick against the fence then lowered the blade on to the top of the workpiece like a drop saw action, cutting down on the line.

Result: Perfect 90 degree when I flipped one piece and joined the two cut edges. And no furry burrs on any of the top edges

I then proceeded to make four sides using this technique of doing two sides together to get the exact same length, clamped at least one side of the cut line, while holding the other side down into the fence with a push stick, and using a drop saw action to cut down on the line, rather than slicing across. The result was 3 out of four perfect right angle joints for a frame. Not sure where I went wrong on the fourth - possibly the sides were not exactly the same length, and the tiny errors at each joint around the frame cumulated at this last corner.

As I stated above, it was my intention of using a biscuit in each corner to align and possibly strengthen each mitre joint, especially i where the joint is not perfect and the mitred surfaces do not mate exactly for the full width of the join.

I am using 40mm wide timber frame for my test frame. After I cut the rabbett groove out, 10mm wide and 13mm deep, I find that the smallest biscuit that I have which measures 44 x 15mm is too large for the 40mm wide frame. I would really need a biscuit that is only 35mm wide, and I don't think anyone makes something that small.

Can anyone please suggest an alternative?

Jill,
If you know anyone with a Domino, the little 4mm domi's are perfect!

Rob

Cut spline on table saw
Insert into grove made by b jointer

Spline not so good, because wood is weakest across the grain.

With 40mm material, the miter is about 57mm long, leaving 6.5mm each end of a 44mm slot. If necessary, cut a bit off each end of the biscuit; most of its strength is at the middle.

Cheers,
Joe

Originally Posted by Wolffie

When I first started making frames, I bought a Nobex Pro. They are absolutely spot on.
Little bit of elbow grease (I called it exercise )

Did a Google search for Nobex Pro and soon realized your reference to elbow grease when I saw the photo, SOMEONE STOLE THE MOTOR

Speaking of Nobex - magic stuff.

To do setups and check internal/external angles I use the Nobex Multifix.

It also has the advantage of coming apart into 2 sections allowing the transfer of the in situ angle to a cutting jig or fence set up to get a perfect fit. Not cheap but it works for me.

eg

NOBEX MULTIFIX MITRE GUIDE AND ANGLE FINDER - Measuring tools. NOBEX MULTIFIX MITRE GUIDE AND ANGLE FINDER

PS I also learned a long time ago that often the mitres are fine but if the lengths and breadths are not 100% identical in their respective lengths then they will never fit properly.

I'm a bit late for your original question now, but perhaps if you decide to have another trial run...

I don't do enough mitres to warrant a "permanent solution" in the form of high-end kit, but I've come across the need often enough (usually when on-site and without access to my favourite tools anyway) that I've come up with a few workarounds.

One is to cut a square of MDF or similar, about the same dimensions as the thickness of the saw bed. Then scribe a diagonal line and cut along that to form two identical triangles.

I clamp those triangles to the bed on each side of the blade so they form a right-angled fence, tape the timbers to be cut with the "face-sides" together (so I cut both "pairs" at the same time, ensuring equal lengths and the face sides being clamped against each other minimises tear-out from the saw blade) then cut one end using the mdf fence on one side of the blade and the opposite end with the other fence.

I also mark which ends are cut with the left fence, to ensure that when I lay the pieces out a "left" cut will always be paired with a "right" cut and not two lefts or two rights in the same joint. (Been there, done that. Damned OOPS moments. )

This won't necessarily give a true 45° cut but will give a true 90° joint. If you take time and care when making the fences and adding pairs of screws for adjustments in the back faces - the ones that are pushed against the mitre saw fence - you can ensure a nice 45° cut... but spending that much time on what is essentially a throwaway jig.. well...

Nah! wrong.... the best way to cut a true 45 degree mitre is with a guillotine. A really good cheap one readily available is the Lion Mitre Trimmer in the Carbatec catalogue. It is around $300 and is worth every cent. Laminated blade is very sharp, and has a shearing cut. Better still is a full-size foot operated guillotine. I have both. Any commercial picture framer has a guillotine, and most would be happy to show you what they look like and how they cut. The blade angle is minutely adjustable, and a 100% true mitre is achievable, and a more smooth cut than you will ever achieve with a spinning saw....

Post 3 on this thread describes the best way I've found to get both length and angle correct.

Well, after much playing about with different techniques, tools and equipment, I found this jig here to be more accurate than my Festool SCSM for mitre cutting for frames for my family pics and my travel pics. It is not unlike what Skew ChiDAMN!! was suggesting above, but mine is not a throwaway item, I hope

The joint angle always turns out to be exactly 90deg if the fences are correctly aligned to the builders frame, and the opposing sides can be made exactly the same length using a stop block on the jig ( not shown) in the photos of my jig below.

You are supposed to make the first cut with the workpiece against the left fence, then the 2nd cut against the right fence. The right fence normally has a stop block so the lengths of the mitres are the same.

I made the mistake of not putting a bevel at the bottom of each fence for dust escape, so have to check and blow the dust away before putting each workpiece in position.

Also, with the Tassie Oak runners, I have found the sled moves quite freely in the morning, but by the afternoon it is quite tight and has to be pushed firmly across the table. I notice the edges of the runners nearest the sawblade are a tight fit against the edge of the mitre slot

Not sure if I should add a fence across the front edge to keep it all stable and flat