Thread: How to use Dovetail Jigs

Been a long time since i've used one, (reckon hand cutting is quicker) but from memory I started with a depth of about 3/4 the thickness of the timber I was using, which was usually 19mm thick, and fiddled about with the up and down game until it felt right.

Best of luck with it, hope someone-else can be a bit more precise for you.


Cheers


Steve

Hi Jill,

Did you buy the bit with the guide attached? If so, set the cutter so that the guide is tracking on the fingers, then adjust the bit up or down by small increments until you get a tight fit.
From personal experience, be careful with how close you get the bit to the fingers. The bit just loves aluminium for lunch

Hope this helps

Regards,

Rob

Jill, there is no exact setting for the depth. Save a couple of offcuts to use as test pieces. Get the joint nice and snug on your test pieces before you route the work pieces.
The manuals that are supplied with those jigs are pretty ordinary. Do a search for the Grizzly manual for those jigs. It is much better than the ones supplied.

Edit: just found it. Try this one

Thanks for this information, everyone. Looks like it is all trial and error, which annoys the hell out of my mathematical brain, and takes up valuable time making joints.

Surely there is some way of calculating the correct depth of cut for a specific dovetail bit ( with diameter and slope of cut) and timber sizes, and making a table that can be easily read.

But then, if it was all that easy, the manufacturer's would have done it long ago

Looking at the 2nd diagram below, I think it is a simple matter of determining the Pin Depth for which the width of the tail ( which varies according to the height of the bit) equals the Pin Width ( which is the diameter of the router bit)

I used the Carbatech jig a couple of years ago, and setting it up using scrap pieces of wood was quite simple even following their instructions. The bad thing was that the bearing on the supplied router bit died just after a couple of boards I had to get another bit, which had slightly different angle - and it wasn't making joints as good. Sorry, I forgot details, as I said it was some time ago.

Hi Jill,

I have an old jig built on a similar principle (I think its a chinese knock-off), and have grappled with the set up problems ever since I bought it.

The depth setting of the router bit is VERY critical to a snug joint.

To get the depth right, I pulled out a dial micrometer I had in the cupboard so I could accurately and repeatably set the depth. I found that a difference of about 0.010 - 0.015 of an inch would make the difference between a snug joint and one that was too tight/loose.

I also ended up having to tune up the base plate of my router to get it perfectly flat (a slight 'dishing' toward the centre was causing variations in the depth of cut.

The finger guide on mine is made out of phenolic type material and is prone to flex - another source of problems especially when making the cuts on the edges of the stock, if the router tilts at all from perfectly horizontal. I have resolved this by placing a spare piece of stock under the finger guide toward each end of the guide, so you can maintain firm pressure on guiding the router without flexing the finger guide.

The optimum depth setting for the router bit varies with the diameter of the guide bush on the router relative to the spacing between the fingers on the guide, the angle of the wings on the router bit and the width at the bottom of the router bit. The hardness of the timber you are using will also affect how tight you need to cut the dovetails.

It is also critical that your stock is cut exactly square and flat and that two upward facing surfaces of the stock when the stock is fixed into the jig, that the two upward facing surfaces (the end grain of the pin board and the inside surface of tail board) are exactly flush. I put a straight edge across the two pieces and check for light under the straight edge. If the pieces are not flush all the way along the length of the joint, there will be a gap between the pieces when they are assembled (either at the ends of the pins or between the two pieces of the stock).

Sorry if seems like a lot of detail, but in my experience these things are pretty fussy to use.

Hope this helps

Garry G

Thanks Garry G, this is really very valuable information. Thanks for passing on your experiences and knowledge. It is contributions like yours that make this forum so worthwhile

I found that a difference of about 0.010 - 0.015 of an inch would make the difference between a snug joint and one that was too tight/loose.

Crikey, that makes it just about impossible to get it exactly right and reproducible for boards of similar thickness

Hi Jill,

A few years back I spent several hours trying to sort this jig out. I thought I must be doing something wrong, so searched on here and was put on to the Grizzly instructions - which are a lot better than the garbage ones that come with it.

Full of confidence, I ventured out into the shed and spent almost a whole Saturday and Sunday trying to get the blasted thing right. Did I? It now just sits in the corner.

I bought a Gifkins jig for my dovetails (although that doesn't do half blind dovetails like your jig does). I figure if I want a half blind dovetail on a drawer, I will use the Gifkins jig and use a false drawer front.

I'd love to get the Carbatec jig working, but after reading on here it seems very difficult. Maybe this is why lots of people on here go on about the Incra jig?

Good luck, and let me know if you stumble across the massive secret!

Nathan.

Hi there, after 3 solid days of work on the problem, I have been able to calculate the depth of cut and the distance to the back stop for the Carbatec half blind dovetail jig, and confirmed my calculations by experiment on two different bits. Mind you, this is for a comb with a bearing guided dovetail bit that fits exactly between the fingers of the comb

But firstly, I would like to strongly endorse the comments by Garry G above on the need for accuracy. This is not a user friendly jig, or a well engineered jig, and the slightest inaccuracy will result in a poor joint. "Fussy to use" is a pretty fair description.

I ended up taking my jig apart, marked scribe lines on the outer fingers of the comb, then aligned the horizontal upper stops on the top of the jig with the scribe lines. It has to be exactly in line ( see photos below)

Aligning the vertical stops to a 1/2 inch offset to the horizontal upper stops is a much more difficult task, with lots of use of a set square to ensure that everything was aligned to the front upper edge of the body of the jig. Pity the manufacturer did not supply a small jig / bracket that could be used to set the vertical stops.

It is best too that you make the width of the boards being jointed a multiple of the jig finger spacing, resulting in half tails at the top and bottom of the joint, which looks really good.

Calculating Depth of Cut
There is only a single depth of cut for any dovetail bit, and that is the depth of cut at which the width of the tail ( measured at the top) equals the maximum diameter of the dovetail bit.

The depth of cut d = (2D-T) / 2tan (A)

where
d = depth of cut for that particular bit

D = maximum diameter of the dovetail bit

T = distance between the centres of adjacent fingers of the comb

A = slope of the the dovetail bit. Always check this first, by cutting a dovetail slot on a piece of scrap and measuring the angle. I have found 14 deg dovetail bits that were only 13 deg, and this makes a big difference to the required depth of cut.

NB. Don't forget to add the thickness of the jig comb to this depth of cut when setting the height of the router bit on the router, and before switching the router on, make sure the dovetail bit does not cut in to the comb

Calculating distance to the rear stop
Once the depth of cut is calculated, you need to determine the distance from the front of the finger to the rear stop, which determines the distance the router travels along each slot of the comb, between the fingers.

This is determined by measuring the distance F from the joint of the vertical and horizontal boards to the end of the finger on the vertical board. You also need to know the distance Y from the the outer edge of the router base to the corner of the dovetail bit, measured at the point of the largest diameter of the bit i.e., at the bottom of the bit

The distance Z from the front of the finger board to the rear stop is:

Z = 2 F +Y


PS Does anyone know where I can get a cheap replacement aluminium comb? I chewed mine up when I noticed one of the boards was moving when cutting, and in a moment of panic lifted the router, rather than sliding it out from under the comb. The bit seems OK, but the bearing is stuffed

Nice work Jill Now lets see some results.
The comb looks ok in the pics. Or is that pre panic?
Carbatec may have spares.

Here you go!

Yep, the comb was pre-panic, and Carbatec want $65 to replace it

WOW Jill! You've really nutted that out haven't you!

Thanks for the PM pointing me back here upon your fix!

My head is still sore from a Uni exam this afternoon, but I think I'm following what you have sorted out. I will give it a go when I am out in the shed - I've got four weekends off before next trimester - I haven't been in my shed for three months!

The results look great too - nothing to complain about there.

Again, excellent work figuring out what I had settled for 'never going to happen'. Its a shame that instructions like this don't ship with the jig - I'm sure a great percentage of the jigs sold are gathering dust.

Well done!
Nathan.