pyRouterJig: Software to make Incra Templates - Page 3 - Router Forums
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post #21 of 76 (permalink) Old 04-02-2016, 02:40 PM Thread Starter
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Originally Posted by jd99 View Post
I thought about this a little bit. the program is fine for box joints as it is. I would change the input for dovetails.

Have the user enter the stock width, the bit size (3/8, etc.) and the angle (9, 14, 7 etc) but not the depth of cut. build a calculation sub routine that will calculate the even spacing, and also calculate the depth of cut using the bit parameters. then give the information to the user.

in my scenario, the depth of cut would have to have been a lot less than 1/4".
Unless you're doing a half-blind, I would think you'd want the depth of cut to match the adjacent board thickness. But I'll definitely think about this...thanks for idea.

At the least, I need to check the "fit" of the joint. For the case you have, there's no way it would have fit together, and I should have caught that issue.
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post #22 of 76 (permalink) Old 04-02-2016, 03:18 PM
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Unless you're doing a half-blind, I would think you'd want the depth of cut to match the adjacent board thickness. But I'll definitely think about this...thanks for idea.

At the least, I need to check the "fit" of the joint. For the case you have, there's no way it would have fit together, and I should have caught that issue.
I always do a half blind when making drawers. I make the depth of the cut 1/2 the material thickness. in this case it was 1/4".

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post #23 of 76 (permalink) Old 04-02-2016, 04:12 PM Thread Starter
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Danny, I need to think about this more. It should work for standard dovetails, too, although what you're proposing might be an option. I really appreciate your insights.

I do have a hunch where the error is, it's just this weekend I've dedicated to the IRS gods and Turbotax ... with a break here and there to read the forum
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post #24 of 76 (permalink) Old 04-02-2016, 06:16 PM
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Danny, I need to think about this more. It should work for standard dovetails, too, although what you're proposing might be an option. I really appreciate your insights.

I do have a hunch where the error is, it's just this weekend I've dedicated to the IRS gods and Turbotax ... with a break here and there to read the forum
No hurry, I have another way to do it. If I don't feel like doing the trig calculations. I draw it up on my cad, that gets me really close most of the time.

Good luck with IRS, they stuck it to me again....

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post #25 of 76 (permalink) Old 04-03-2016, 09:56 PM Thread Starter
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I finally had a chance to look at this. Danny, did you try to cut the joint with the 11/16" spacing with a 1/4" depth (what Incra suggests)? I would think you'd end up with kindling, too. Here's the long-winded explaination:

It's been a while since I've looked at dovetails. A while back, I had checked them against the "Master Reference Guide", and I knew pyRouterJig matched at least a few of the 7 deg. and 14 deg. cases. I own those bits, but not the 9 deg., which I admit I never had checked.

So I tried all 10 of the equally-spaced dovetails in the Guide. pyRouterJig matches all of them exactly, *except* Danny's case: 9 deg., 3/8" width, 1/4" depth case (the "bad case"). Of the 10 cases, it's interesting what is unique about the "bad case". The exact dovetail spacing is given by

exact_spacing = 2 width - 2 depth tan(angle)

I get 0.671" for the bad case. Because the Incra can only position in 1/32" increments, this number must be rounded to the nearest 1/32". I get 21/32", while the Guide somehow gets 11/16". Note that

0.671 - 21/32 = 0.015" (pyRouterJig)
0.671 - 11/16 = -0.017" (Guide)

That is, roughly 1/64" in either direction. Note that in either case, this difference is a lot larger than any of the other 9 cases, which typically need to adjust the spacing less than 0.004". I considered that the Guide "rounded up", so maybe it does that for all cases. But no, this case and the 7.5 deg. case are the only two that the Guide rounds up.

I'm not arguing that pyRouterJig is correct. It shouldn't even let you specify this depth for this bit, given how the Incra works. As Danny found out, you get kindling. But I would be surprised if the Guide's 11/16" spacing is any better; if anything, I'd bet it's even worse. But maybe I'm missing something. Maybe my formula is wrong above, and I'm just getting lucky for the other 9 cases. If so, please enlighten me.

For through dovetails, I need to figure out how to restrict the depths so that the error in shifting to the closest 1/32" is small enough. Danny, I like your idea for half-blinds. I'll get that in at some point, too.
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post #26 of 76 (permalink) Old 04-04-2016, 10:31 AM
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Rob, This is a very interesting read. I need to fuss with dovetails and PRJ as I have mostly focused on box joints. I'm about to do some drawers for a small project so it's good timing.

The Incra through dovetail approach seems overly complex - have looked at getting something like the Leigh Jig but then sticker shock sets in (more than the IncraLS!!). And for those that say "cut 'em by hand", yeah wish I had the patience for that.

Measure twice, cut once and CROSS OUT THE WRONG MARKS.
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post #27 of 76 (permalink) Old 04-04-2016, 09:55 PM
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Rob, This is a very interesting read. I need to fuss with dovetails and PRJ as I have mostly focused on box joints. I'm about to do some drawers for a small project so it's good timing.

The Incra through dovetail approach seems overly complex - have looked at getting something like the Leigh Jig but then sticker shock sets in (more than the IncraLS!!). And for those that say "cut 'em by hand", yeah wish I had the patience for that.
Ive done the through dovetail on incra, it's not that bad, but I do half blind mainly because I don't have to do the extra hand trim operation that is required on the through dovetail steps.

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post #28 of 76 (permalink) Old 04-04-2016, 10:20 PM Thread Starter
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I'm curious what a reasonable tolerance should be. 0.005"?
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post #29 of 76 (permalink) Old 04-04-2016, 10:41 PM Thread Starter
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I figured out that the pyRouterJig and Guide spacing for the "bad case" are equally bad. The Guide joint can be obtained in pyRouterJig by moving the Spacing slider to 1/32". The first attached image is the default (Spacing slider at 0), while the second image shows the Guide results (Spacing slider at 1/32"). Both of these joints will have the same fit. Changing "Spacing" does not change the fit.

For this bit, this is just a bad depth of cut for the Incra.
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post #30 of 76 (permalink) Old 04-05-2016, 10:08 AM
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Originally Posted by furboo View Post
I figured out that the pyRouterJig and Guide spacing for the "bad case" are equally bad. The Guide joint can be obtained in pyRouterJig by moving the Spacing slider to 1/32". The first attached image is the default (Spacing slider at 0), while the second image shows the Guide results (Spacing slider at 1/32"). Both of these joints will have the same fit. Changing "Spacing" does not change the fit.

For this bit, this is just a bad depth of cut for the Incra.
Its not really a bad depth, the 1/4 inch depth, is actually a suggested "Aprox. Depth of cut" for most dovetails using a 3/8" 9 degree cutter.

I just used the 1/4" depth, 3/8" 9 degree dovetail, on multiple stock widths, and I had no problem as long as the spacing was right.

I always try to set my depth of cut for 1/2 of my stock thickness. I know from past experience 1/4" depth, 3/8" x 9 degree the spacing is 11/16, and that works.

one thing you can provide is the spacing your program uses, in other words if you program creates a template with a spacing of .... say 9/16" then provide an output field for that information. That way the user knows that he or she need to use the 9/16" spacing to set the true depth of cut for the dovetail bit being used.

I always use 2 pieces of scrap material to set up my bit, I make two cuts first about 1/2 the width of the cutter, then another one at what ever the spacing is flip the pieces around and check the fit. Then I adjust the height (depth of cut) to either tighten or loosen the dovetail for correct fit.

Knowing the spacing your program generates would help in that setup process.
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Last edited by jd99; 04-05-2016 at 10:52 AM. Reason: add more
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