Fail - Equilateral triangle with 50/50 mitres

[CrazyArsedMonkey]

Good evening folks!

This is my 2nd attempt at writing this post, my cat managed to switch my computer off just as I was running the previous version through MS Word to spell check - I lost the lot...

I am looking for some advice regarding a little project I attempted at the weekend. I hoped to make a nice book holder with integrated drawer, instead I made some overworked firewood!

Setup
I machined a hefty block of CLS to give me three parallel sides to use as reference edges. Once finished I clamped it to my slide carriage, ensuring it was parallel to the blade, and extremely firm. I then set my blade to 30 degrees with respect to the table top and bolted a test piece to the assembly, leaving a gap for the riving knife. I cut the test piece and was pretty happy - the angle looked consistent, the cut was lovely and smooth, and there were no unwanted marks. The setup looked like this:

As I was happy with the test piece, I then ran my three prepared triangle sides through the setup, just moving the carriage forward and back to keep everything consistent.

All appeared to go well, the angles looked good, and the workpieces were still consistent lengths, giving me confidence that all was well.

Results
Unfortunately, when I assembled the sides in the correct order (to preserve grain pattern) I got this:

As you can see, the cuts are inconsistent and run out from one end to the other (pieces are 18cm long). I cannot work out why this happened. My only guess is that the setup wasn't as rigid as I thought, so it moved laterally slightly as it went through the saw.

The workpieces were well prepared and were exactly the same length, width and thickness. I remeasured the blade angle afterwards and it was rock solid and exactly 30 degrees.

Questions
- Can anyone shed any light on why this happened?!
- How would you attempt these cuts to get a better result?

Any and all advice will be gratefully received!

Thank you in advance

 

[Bingy man]

Probably one of my weaknesses is cutting mitres , if your saw was set to 30 deg and you confirmed it then what angle did you get in your work piece(s) I’m not the best at mitres as said but you seem to have achieved 2 out of 3 so you can’t be far away ..

 

[Sideways]

Something moved.
You can see it was good at the beginning and miles out by the end of the cut.
Blame it on your clamps and the way you clamped it up.

I've had something similar happen to me in a very different context when the batten I was using as a straightedge for cutting through floorboards moved during the cut.
The clamps are just not tight enough and if subject to any vibration (like my fein multitool) they move even more easily. All metal G cramps or those Bessey / Festool / Axminster lever action rail clamps apply much more force and don't slip if you crank them tightly.
Using a wooden batten between two clamps creates the opportunity for the batten to move under the plastic jaws of the clamps, for the clamps to flex at an angle, or the stock to move vs the batten. Too many vulnerabilities.

If it were me, I'd also have the stock horizontal not upright.
I'd have a square of mdf fastened hard down to the sliding carriage with T nuts and extending right to the blade, then have the stock fastened tight down to the MDF

There are lots of neat jigs for cutting smaller work with a slider. I'm not necessarily advocating it here as it may not give a 100% reliable clamping force but google "fritz and franz jig" for ideas.

 

[HOJ]

There is a lot going on there with all those moving parts, there is simpler way, which is the method I use for various needs (and it a can get you out of jail when cutting 45 mitres without obsessing about the blade angle being 100% bang on, that's another type of set up though)

Pictures first:



Its basically just a simple upright sled running along and over the fence, this set up was for me to get a 55 deg angle, on my saw the blade tilts to the left so in your case you would need to put your fence on the other side of the blade.

 

[CrazyArsedMonkey]

Probably one of my weaknesses is cutting mitres , if your saw was set to 30 deg and you confirmed it then what angle did you get in your work piece(s) I’m not the best at mitres as said but you seem to have achieved 2 out of 3 so you can’t be far away ..

I am not sure Bingy, it will be hard to tell when the margins are so tight. Next time i am out i will try and establish the margin of error!

As for managing two out of three pieces - i am not so sure, i suspect the picture shows an amalgamation of a smaller error, added together when the pieces are pushed together

 

[CrazyArsedMonkey]

Blame it on your clamps and the way you clamped it up.

If it were me, I'd also have the stock horizontal not upright.

I like that, i will blame the bloomin' clamps! I totally understand what you are saying, i almost used some parallel clamps, but the weight of them made the setup very heavy on one side and i was worried about it moving / toppling. I suppose that is where your MDF idea comes in - spread the weight and generate stability.

Regarding the stock being horizontal - i always strive to do this, however my table saw only goes to just beyond 45 degrees, and i needed 60, hence taking 30 degrees off the other side.

Thank you very much for the "fritz and franz jig", this is a new one on me - i will check it out

 

[CrazyArsedMonkey]

Its basically just a simple upright sled running along and over the fence, this set up was for me to get a 55 deg angle, on my saw the blade tilts to the left so in your case you would need to put your fence on the other side of the blade.

Love the jig HOJ - especially with your fence style! I could absolutely do this, will be a bit more complex, but definately achievable - i will look into it and compare with the Fritz n Franz that Sideways suggested.

Thanks for the suggestion!

 

[HOJ]

Norfolk is a big place but if your anywhere south give me a shout (Loddon)

 

[CrazyArsedMonkey]

Norfolk is a big place but if your anywhere south give me a shout (Loddon)

I am North West

Would have been great to have a local contact as well! I am still working, so can't participate in shed clubs

 

[Hornbeam]

I am not sure that your angles are out by that much. It looks like what you have done is cut a slight taper in the pieces so that they all meet at one end and so leave a gap at the other
Have you accurately measured the width of each piece top and bottom
Have you tried turning a couple of teh pieces round to see if they fit

 

[Adam W.]

...........And have you checked to see if the pieces are still flat?

 

[Jacob]

I wouldn't fancy having all that metal work passing over my TS!
I'd set out a simple rod and mark up each piece, and then see how close to or far from the marks the saw cut goes. Perhaps a few sample cuts to start with, on an over length piece.
n.b.they are probably all out by the same amount which is accumulative in one joint if the other two fit.

 

[Bradshaw Joinery]

Mitres.. saw blades have a habit of following grain so as you cut the blade will bend as uneven pressure on each side. even with brand new blades to a degree, and small pieces arent easy.
To get it accurate and repeatable id make some form of router sled to the angle then mitre it a few mm off and clamp/router the final mitre. easy and safe. could also drop the router by 0.1 in the middle to create a sneaky undercut so the edges pull tight.

 

[Jacob]

Also there are six 30º cuts. Hence the same error on each is multiplied by six when it comes to the last two not meeting.
Maybe a touch of careful sanding on each of the cut faces could reduce the cumulative effect?

 

[Terry - Somerset]

Looking at the picture in the initial post it seems the sides are joining at the bottom, but at the top there is what looks like a parallel sided gap.

If this is the case, it would suggest:

• the mitre angle is just about spot on
• that the sides are out of square - if you invert one of the sides does the join close up

 

[jetsetwilly]

Some thoughts
1 Is there any flex in your slider + fence? I’ve found that setups like that can flex a bit under load because the slide is relatively far from the blade. Could you locate your jig in the T slot?
2. Could you joint/shoot the ends with a sharp hand plane? Just fitting one of the three by hand could be enough unless you’re making it for a geometry class

 

[HOJ]

Back to my solution, I had finished with the setting I was using, so tilted my blade to 30 deg, grabbed a piece of ply and cut to 3 equal lengths.

Clamped on to the jig, quick cut on all the ends:



Results:



Probably took no more than 5 minutes and with a hard worked cheap blade, I know its not Oak, but it proves concept.

 

[Lefley]

Back to my solution, I had finished with the setting I was using, so tilted my blade to 30 deg, grabbed a piece of ply and cut to 3 equal lengths.

Clamped on to the jig, quick cut on all the ends:

View attachment 168338

Results:

View attachment 168339View attachment 168340

Probably took no more than 5 minutes and with a hard worked cheap blade, I know its not Oak, but it proves concept.

Some jigs are just better than others.

crazyarsedmonkey Also is it a thin kerf blade in saw. They are notorious for deflecting in cut.

 

[CrazyArsedMonkey]

Hello folks! Sorry, busy few days, just noticed the plethora of replies - thank you very much for the advice! I will respond properly tomorrow

 

[CrazyArsedMonkey]

Some jigs are just better than others.

crazyarsedmonkey Also is it a thin kerf blade in saw. They are notorious for deflecting in cut.

Hello Lefley!

Thanks for the reply. My blade is a Stehle, i think it is 48T with a 2.2mm Kerf (about 2 years old (has been sharpened), can't remember exactly) - don't think this classifies as 'thin' does it?

Please let me know if i should be considering a different blade for this work...