Prototype of Crosscut Sled for Angles

[smorgasbord]

This is an idea I had months ago (see this long thread: https://festoolownersgroup.com/threads/woodpeckers-protractor-ruler.71567/ . I still haven't gotten past the prototype stage, but I had occassion to use it recently and was very happy with the results. If it ends up being feasible I might turn it into a real product, but I've got at least a couple more iterations to go through first. Probably easiest to show a picture:



A little bit closer:


The idea is to use a CNC to accurately mill holes for pegs against which your place the fence and lock it down. I have holes for each whole angle, as well as angled for multi-sided polygons up to 20 sides. Here it is setup for a 25º (or 65º) cut.



You can see that the distance between whole angles is pretty large - something like 7mm. That enables use of a simple and cheap taper gauge to dial in any angle in-between. I've got 3 sets of radii, so the 7mm is only for the center arc - the other arcs are slightly more or less, but a known quantity and set up to a nice roundish number. It's easy to use a taper gauge to get 0.5 mm, which would be 0.07º, and even finer than that is certainly do-able.

I drilled holes to angle both ways, here's a setup for 25º the other way:


I've got a few things to work out:
• Will probably use tapered holes/pins so wear affects accuracy less over time
• Make from phenolic or aluminum instead of MDF
• Figure out a better fence lock-down mechanism than the Incra bracklets I'm currently using

And, I've been thinking for months about how to adapt this design to a MFT style top for precise angle cuts with a track saw.

Right now, the angular accuracy is better than I can measure in my home shop. I test by doing polygon angles and cutting wide pieces to see how they fit. And, I don't do the usual cut/flip/cut process which tries to compensate for angular inaccuracies, I cut both sides of each piece with the same setting, so on a 10-sides polygon, for instance, the error seen in the assembled polygon is 20 times the error in any cut.

 

[luvmytoolz]

Great idea!

 

[smorgasbord]

Just a photo showing how a taper gauge can be used to intermediate angles:


Here I've got the fence set to the pre-drilled 22.5º hole for octogons. I inserted another peg in the 22º hole and slid the taper gauge in. Again, in this radii, 7mm is 1º, so the 3.5mm taper reading indicates a 1/2º offset.

Let's say you're cutting some standard 52-38 crown molding and need a 31.62º miter angle for your 92º corner since the carpenter that built your house sucked. Your miter saw's built-in detents aren't right for this cut - you need 30.7º miter and 33.2º bevel (not the "standard" 31.62º miter and 33.86º bevel).

For the miter angle, put a pin in the 30º hole and slide your taper gauge to 4.9mm, then lock down the fence. Each 0.1mm is 0.014º so it's pretty easy to be accurate enough.

 

[Crazyraceguy]

That's a great idea/project. How are you drilling the holes now?

 

[smorgasbord]

That's a great idea/project. How are you drilling the holes now?

My CNC.

Next up is adapting the design into a MFT-type tabletop for tracksaw users. That will be somewhat easier as it won't have backside that needs to be aligned. On the sled, the miter bar needs to at the exact angle relative to the holes (which is why my prototype has it in two pieces today). But, given that one would want the track to ride in the same place and the stock positioned at different angles, where one locates the table becomes a challenge for small shops.

 

[Packard]

There are several commercial versions (all fairly expensive). I would read some product reviews on the existing versions and try to avoid the mistakes that the manufacturers have made.

A very challenging project, if done very well.

Look: https://www.google.com/search?clien...kGHRK8Dz0QtKgLegQIFRAB&biw=1320&bih=830&dpr=2

 

[Michael Kellough]

Very good! I like the use of the taper gauge.

Application to MFT will require more frequent use of the taper jig and more test cuts since the MFT’s cutting rail can’t be elevated for different thicknesses without its angular relationship with the fence changing slightly. Dashboard is much better but still not in the range of angular tolerance commensurate with the your jig.

 

[morts10n]

It would be a lot less work to do a curved slot with a bolt to tighten, and an off the shelf protractor for a machine. I did this 25 years ago

 

[smorgasbord]

It would be a lot less work to do a curved slot with a bolt to tighten, and an off the shelf protractor for a machine. I did this 25 years ago

A lot less accurate,too.

I suspect a lot of people have used Dubby's or clones thereof. This is at a different level, accuracy-wise.

 

[smorgasbord]

There are several commercial versions (all fairly expensive).

None of those have the kind of accuracy, or at least repeatable accuracy, that my design has. The best of them, like the Dubby or Woodpeckers or even the Rockler, use a large analog protractor with a cursor. I suspect you can get to 1/5º or even 1/10º accuracy pretty well with those - but the acid test would be doing something like a 7-sided polygon with wide sides and having the pieces fit with no discernible gaps. Obviously, not every angled cut needs a high level of accuracy, but repeatability often matters more.

Application to MFT will require more frequent use of the taper jig and more test cuts since the MFT’s cutting rail can’t be elevated for different thicknesses without its angular relationship with the fence changing slightly.

The legendary Peter Parfitt has a product on the market for homemade MFT top and track saw use, the UJK Parf Protractor:


Same basic idea (we're all triangle geometry math-based), except with his you have to drill holes for your specific angles, and those holes end up being the full 20mm dog holes, so you can't have too many on your benchtop before you need to replace the top. Also, I think his is restricted to half-degree increments.

But, I don't think he has issues with elevating the rail for different thicknesses, but I'd like to hear more about this, @Michael Kellough .

 

[smorgasbord]

Let me add that this was driven by our discussion here:

S

Thread 'Obtaining Unusual Angles with Precision'

Apr 25, 2024

An old review of a protractor devolved into a discussion of how to obtain non-standard angles (which are 22.5º, 30º, 45º, 60º nor 90º) with a high degree (pun intended) of accuracy. I'm moving that discussion to its own thread here.

There are various digital angle "finders" available, but their accuracy is at best +/- 0.1º. Most are worse at +/- 0.2º. For some purposes, that may be accurate enough, but for others, such as making a wide pentagonal frame, even just 1/10º off means there's a full degree of gap in the frame. And while trial and error techniques can enable us to sneak up...

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[Michael Kellough]

The vertical adjustment clamps for the rail on the MFT are fairly crude and the length of the interlocking fit reduces as the rail is raised for thicker workpieces.
However, once the height of the rail is fixed the fence can be adjusted to extremely fine tolerance, using a stop block as a reference and feeler gauges to move the fence for testing.

If your jig was used to cut 1/2” stock to a specific angle and you also needed to cut 1-1/2” stock to match the first piece exactly you would have to make some adjustments, unfortunately. Same thing happens with any fence when used with the stock MFT rail “elevators”. The DashBoard and TSO upgrades look like significant improvements.