Thread: Mitre Gaps- 3 spot on, 4th always out

Your mitre cuts may be OK but the wood may have a slight bow.
Try assembling pairs i.e. top side and right end and, then left end and bottom side..
Once glued and set, assemble the two sections together. This way you only have two mitres to line up and pull into place. This may compensate and reduce the gap.
Use a band clamp or other methods to pull the corners in.

Hope this is of use
regards, Ned

How are you cutting the mitres? By hand in a jig? On a table saw with a mitre gauge? Sliding compound mitre saw SCMS? My SCMS has a half degree difference to the guidelines four cuts adds up to a hole degree out. Matching pairs is a good way to minimise error. The thickness of a piece of paper placed behind the wood at the fence is enough to fix up my difference.
cheers

I think you'll find that the 4th corner gap is an incremental result of the last 3 corners, not that 3 are ok and one is not. As mentioned previously you need to post a bit more info before anyone can diagnose your problem:

- How are you cutting the miters
- How are you ensuring that each piece is exactly the same length
- The final gap, is it gaping more at the inside of the mitre, the outside of the mitre, or is the gap uniform in width between the mating surfaces?

A photo would be extremely helpful, and doing a picture frame style mitre is an easier way to trouble shoot rather than a box miter. Wider elements magnify your errors and make them easier to faultfind.

I agree with what has been said earlier. The last mitre being out could be a combination of the other three all being out slightly. One thing you could try is to make a jig. Use a suitable size bit of flat 16 mm MDF which has an absolute square corner, then glue two guides with mitred corners down two adjoining sides leaving a gap at the corner just wide enough to accommodate a fine toothed saw, if your measurements and mitres are correct this should leave you with a perfect 45 degree angle at the corner of the jig.
To use it, clamp the two unglued sides of your project frame to the insides of your jig and then run your saw through the kerf in the jig and through the unglued mitre on your project cutting both at the same time and you should then be able to have a perfect mitre all around.
It would probably be wise to check the diagonal measurements to make sure that they are the same prior to gluing.
I use this system quite successfully when making picture frames.

All the best.

Router

thanks Guys,
I am using a table saw & incra jig which i believe is square to the blade ( spend hours setting it up ) ok I think I will have to take some photos over the weekend while i knock up another small box and get your feed back, this problem has me scratching my head as i believe i have got everything set up correctly.

On a different point a box making class is Melbourne would be great!!!!

The first step is to cut the corresponding sides to the EXACT same length while cross cutting at 45 degrees. Any errors in length and angle will accumulate and not be seen until the last joint - the other three joints will look good.

To test, put the corresponding sides back to back and stand them against a flat surface. The pointy ends of the timbers should be exactly the same length

Also, make sure the sides are not cupped along their length; the joint surfaces must be fully in contact prior to gluing

Good to see you practicing before you try on the good timber

I have found the most accurate cut for me is on a crosscut sled with two runners in the mitre guides on a table saw with the blade set at exactly 45 degrees with a digital angle meter. Others in the forum like to use a disk sander with the sides held at 45 degrees to the sanding wheel.

Keep at it, you will soon master it by diligent practice and good setting up

If the opposing sides are equal, the sides are all straight and the angles are all 45, the mitres must come in OK.

Check the sides as described by Dengue. To check that your saw is cutting the angles correctly, cut a piece of wood at 45, rotate one piece and put the mitre together. It should make a right angle.

When you clamp the box up, don't clamp it too tightly. Check the diagonals - they should be equal.

Originally Posted by AlexS

cut a piece of wood at 45, rotate one piece and put the mitre together. It should make a right angle.

When you clamp the box up, don't clamp it too tightly. Check the diagonals - they should be equal.

If you only cut 45 once and rotate it will always be 90 degrees I.e. If your saw cuts at 43 degrees when you flip the off cut it is 47 degrees always making 90. The problem is compounded error times four cuts, I have a simple test for your saw setup, it is from the kapex SCMS manual

image.jpgimage.jpg

Essentially you get a square piece of timber mark each edge 1234, cut with 1 against the fence rotate 90 degrees cut with 2 against the fence rotate etc. when you get back to 1 just cut a width using callipers measure the width of your off cut I.e. If it is 30mm at the bottom it may be 30.5mm at the top. Alternatively you can set a square up against it and measure the angle. My saw is 0.25mm out and if I place a piece of paper in between the fence and the timber being cut it reduces that 0.25mm down to a level I can't see.
Cheers

If you only cut 45 once and rotate it will always be 90 degrees I.e. If your saw cuts at 43 degrees when you flip the off cut it is 47 degrees always making 90.

Wrong.

If your saw cuts 43 degrees and you rotate, you will have 2 x 43 = 86 degrees.

Try it.

Thanks Guys, lots of great tips, it will be a busy weekend trying to master my mitre cuts!

To add another option.

I recently made myself a bevel shooting board, so I can cut a bevel Mitre on my scms to the approximate dimensions and then use the shooting board to finish off.

A sharp plane gives a beautiful crisp edge.

I made my first trial box recently and found that an angle of just a whisker more than 45 (ie slightly open on the inside of the box) helps to keep the outside edge closed and neat.

Originally Posted by AlexS

Wrong.

If your saw cuts 43 degrees and you rotate, you will have 2 x 43 = 86 degrees.

Try it.

My bad i thought you were using the one piece cut once and flipping. Two cuts on two pieces will give the above
cheers

Originally Posted by tonycrh

thanks Guys,
I am using a table saw & incra jig which i believe is square to the blade ( spend hours setting it up ) ok I think I will have to take some photos over the weekend while i knock up another small box and get your feed back, this problem has me scratching my head as i believe i have got everything set up correctly.

I think the most likely problem is the table saw blade is not tilted to a true 45° relative to the surface of your table saw.
If the blade tilt is actually 45.05°, the fourth corner of your box will be out by a noticeable 0.4° -- 8 x 0.05°

the other thing to check is that the top and bottom of your box sides are parallel

I dont have any fancy gadgets which tells me what angle the saw is tilted at (except for the digital angle readout on the saw...which isnt accurate enough for boxes). So what I have always done is grabbed a piece of scrap (3mm mdf is good cause its cheap, but anything longish and about 35mm+ wide is cool). it must be the same width along the entire length. Then I try to cut a perfect square box (4 pieces, each with a 45degree cut on each end). I dry assemble it and hopefully everything marrys up nicely. if i have cut at 44.92 degrees, then the last piece wont even be touching the first piece, there will be a gap between the two pieces, so i know i need to tilt the blade slightly more to a sharper angle. if all the outside corners meet, but the inside corners have gaps, it means I have cut more than 45degrees, say 45.3degrees for example, now i know I need to tilt the blade back closer.

a wide test piece allows you to see the discrepancies more clearly. start with a long test bit of scrap, cut 200mm+ pieces, that way when it doesnt marry up nicely the first time, you can cut those 4 pieces down slightly shorter. once you know your saw is actually at 45.00000 degrees, its just a matter of cutting a front/back the exact same length, and 2 sides the exact same length. cut one box, or 1million boxes, they will all be good