Gary's Dovetail Cutter - tweaking?

[gerald_martin]

We built a jig similar to Gary's for cutting half blind dovetails, and I am using his file with minor tweaking. Gary, thank you, you have been more than generous to share this excellent part file. We spent considerable time getting the jig right and I could hardly believe the excellent dovetails we got on our test cuts.

One issue I would like to resolve: it seems that when the dovetail part file is running, there is a distinct "bump" in the motion of the tool just as it leaves each arc of the pin cutting. I suspect this happens as it goes from the cut arc command to cutting a straight line segment. Move speed is at .5 Also my ramp is set at .4

Any ideas? Gary, my tool is a PRS Alpha similar to yours. Does yours make this bump sensation? Do I have some other ramping setting wrong?

The cut quality is still good, but I'm after the smooth, fluid motion we expect of the alpha Shopbot.

Gerald

[henrik_o]

Hey, thanks for making me see Gary's great article!

We do things a little bit differently in our shop. We have an old dovetailing machine we’ve used a lot and we figured that with cnc tech the whole idea of mating the joints in the jig/clamp was unnecessary – as far as I can tell this is done this way on low tech systems in order to secure perfect alignment, and I believe it is strictly speaking not necessary with cnc solutions where we can dynamically dimension the parts to be dovetailed during the actual dovetailing process.

Did that make sense? Ok, maybe this does: if we cut the mortises separately from the tenons, very very small differences in part thickness or tiny misalignment in the vertical jig can bust our joints – they won’t mate as well as they do if you align them to each other the oldschool way. But here’s the trick: we can create perfect alignment by machining the parts to our desired specification just before we cut the mortises/tenons, in the same program as the dovetail cutting, using the same bit. Then we won’t need to mate parts and can cut (say) fifty mortised parts straight out of sheetgods flat on the table, and as many tenoned parts as we can fit in an end-of-table jig without losing accuracy. The catch? Well, we need a very true table, it is a good idea to surface the spoilboard before making a hundred drawers. We also need a good vertical jig.

Here’s a couple of illustrations that should clarify how I think;

First, the problem (or one of the two, rather) that has haunted non-manual dovetailing until cnc arrived;


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And here’s how we solve that particular riddle;


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Simple, no? It is, it is a very simple solution, but so amazingly powerful in all its simplicity. We are no longer constrained by the rules of non-cnc; we can dynamically true our joints by machining the parts true on the table (or in a jig) in the same run as we actually cut the joint. The above is just an example. Yes, there’s a similar problem with the parts that have to be cut vertically – but what is important is that we can solve it in a similar fashion. And then we can cut several vertical parts in one program, without having to worry about perfect alignment.

Instead of having to mate parts and cut joint by joint, now we throw on a whole panel and cut twenty or thirty mortised drawer sides in one go, then we cut the tenon parts two-by-two in a jig, which could easily be made to hold at least four parts at the same time (==same program).

You know, when I first realized this it was like I was hit in the head with a hammer. Of course! Why had I not thought of this; so simple and yet so powerful. Maybe I’m like the 99,999th person to reinvent this particular thing and the fact it took me a while to figure out just means I’m daft, but for a moment it felt like I finally, FINALLY, understood something about this joint that I had never previously realized.

It has completely changed my way of thinking about joinery: before, I was concerned with ‘simulating’ on the Shopbot what I knew from more traditional experience. Now, I’m always looking to ‘hack’ any joint I want to make on the ‘bot. Instead of thinking about, for example, the dovetail joint “from the inside out” I’m thinking of it “from the outside in”. A complete volte-face, for me at least, and very challenging.

I think I’m starting to grasp what Brady is always telling us beginners: to “think in cnc”. It is a different way of thinking, but very rewarding when you make a discovery like the above was for me.

In parting, nothing of this takes anything away from the stunning jig Gary has built and shared. I am going to make one from his instructions, because there are times when I just want to make a few drawers and the whole approach above just involves too much setup time. But the great thing is we can have both. We can simulate the oldschool way when that fits our needs best, but we can also venture beyond it if we need that.

[gerald_martin]

Henrik:

WOW.

Yes, I can see your point. I appreciate your input.

I have made a jig similar to Gary's - and I used the CNC to both surface the Baltic Brich I made the jig from, and to cut the edge where the metal angle mates to the wood jig, for accuracy.

For my small production the Gary style jig is great.

Can any one help me on the speed/bump issue?

Gerald

[Gary Campbell]

Henrik...
Thank you for the kind words.

Gerald...
If you havnt read Brady's Ramping article, its great: http://www.shopbottools.com/BradysTricks.htm#Ramping It has given me enough information and confidence to experiment. Try seting the Move ramp to .1 (which is probably too low) and see if it improves the problem. IF it does, then find a spot in between that works the best for you.

I noticed a small amount of "noise" at that point, but I attributed it to the change from long grain on the slot to end grain of the arc. The end grain is harder to cut and seems to "grab" the bit more. Another thing to check: use a dial indicator on your bit sitting near cutting position to see if you have any machine flex. The YZ car and Z rollers need to be tightened every now and then to insure high quality cuts. Let us know how it works out.
Gary

[Gary Campbell]

Gerald...
I just went back and looked at the videos that I recorded a while back. The "bump" you refer to may be a combination of things mentioned above, plus some unknown to me. It seems more noticable, now that you mention it.

I will be tearing the Bot down for a move in the next week or so, so I cant test this for a while.

I did post a video on YouTube of my machine cutting the dovetails. My settings are the same as yours, but would have to confirm the ramp setting. http://www.youtube.com/watch?v=4Aw4lElTecg
Maybe someone can figure out what it is.
Gary

[gerald_martin]

Hi Gary: I considered that it might be the transition from one wood grain to another but I nixed that theory - it was the same in soft pine as it was in oak, and seemed like a machine "jerk". My dovetail bit is brand new and makes lovely shavings....

I wonder if it is trying to ramp when it shouldn't need to. I'm going to try increasing ramp speed and see what that does.

I'll post another update here later, anyone else feel free to chime in.

Gerald

[Gary Campbell]

Gerald...
It is probably a quick start/stop caused by the counter & width check the the resultant move into the slot. Here is that portion of the file:

CUT_LOOP_ODD:

[myxpykalix]

Gary,
From reading this post and then going back and reading your column, then downloading the file and watching your jig cut the files on youtube I think i want to try to emulate your jig and wanted to ask if there is anything different about your jig compared to an average dovetail cutting jig, that i would want to try to make a exact copy of yours?
My thought was to take an old dovetail jig and use it (minus the "comb") and wanted to know if you thought that was ok to do? I'm not sure of the measurements off the top of my head for the offset was why i thought of usng one already made.

[gerald_martin]

Hi Gary: Adjusting ramp values does nothing for this bump.

Uh, sorry, my binary decoder is not up to speed...

I suspect though that you are right. Is there any solotion, or is it the nature of the beast?

Gerald

[myxpykalix]

Gary,
I thought there were only 2 kinds of people...1's or 0's. I know a few 0's!

[mitch_prest]

can someone post a link to Gary's post about the dovetails...

thanks

mitch

[gerald_martin]

http://www.shopbottools.com/garysmusings.htm

Read this column Mitch, and download Gary's files from Shopbot Labs

http://www.shopbottools.com/LabFiles/GaryDovetail.htm

This is an invaluable resource, for about $100 in cost I have an incredibly accurate dovetail machine - my Shopbot and a smaller version of Gary's jig.

Gerald

[mitch_prest]

thanks Gerald

I was lucky to find this post now as I was going to buy a dovetail jig this week.. was looking at getting the Leigh... this may be a better way to go.. a lot cheaper for sure..

mitch

[Gary Campbell]

Gerald...
I dont know of a way to change this, or even exactly what is happening. Does it affect cut quality on your machine? Didnt on mine. When I get up and running in a couple weeks I will retry with the 5/8 file and a few species of wood. For now I am hoping that those wiser than me can diagnose and suggest a work around for you.

Jack...
My jig emulates the geometry of a Poter Cable Jig. The article has a link to a downloadable version of the PC instruction book for their jigs. There are a dozen or so joints that you can cut with it, if you follow the instructions in the manual and make 1 or 2 changes to the file.

Within reason... you can make the jig virtually any size that you want, but you must enter the dimensions into the file. Or you can mount that old dovetail jig on your bot too.

10 is "two" in binary!
Gary

[gerald_martin]

Hi Gary: No, the cut quality is gorgeous. I'm just after that smooth shopbot motion. I've talked to Frank at shopbot, I think he is looking at the part file for a solution.

I appreciate anyone's input as this is slightly beyond my capability. I get lost quickly with variables.

In the meantime, I'm happily cutting dovetails.

Gerald

[gerald_martin]

I thought I'd show you a shot of dovetails on red oak drawer boxes we built for a bedroom suite for my younger brother. We were very satisfied with the outcome, having never worked with dovetails before.

Again, many thanks to Gary - his contribution represents Shopbot community at it's finest.

Gerald


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[woodworx]

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8637
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Dovetails cut in 2 separate files. I couldn't get it right with the dovetail bit. It just wanted to blow out the wood. This if 5/4 cherry.

[Gary Campbell]

Guys..
I have just finished tweaking a dovetail routine that works for a 5/8" diameter bit. email add in profile for a copy, put "dovetail" in the subject line.

Gerald...
While working on this file, I looked, overlooked and tweaked on the ramp settings to try and get the "bump" out of the file. I couldn't eliminate it, but made it better, and I did determine it always happens on the switch from the circle (2 axis move) to the slot ( X axis only move) try these settings: (they seem to only help this file)

Move speed to .6 instead of .5

XY Move Ramp rate: .1
Move Ramp Rate: .5
Min Dist to Check : .04
Slow Corner Speed: 90

AIR CUT FIRST!
Like I said... these settings may have no value other than to operate this file
Gary