Using Pockets for positioning....

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Using Pockets for positioning....

Postby zaxis » Fri Sep 02, 2016 10:52 pm

I am trying to cut mortise pockets and, the tenons (rabbits) to match, on small components of a box. So I start out by pocketing a piece of MDF with the pockets the size of each part that has the mortise or tenon cut in it. Easy enough to calculate the tool path for this. Do you have to pocket the full thickness of your material? What if you only pocket down 1/8 inch or just enough to position the part? (with a small piece to DS tape) Do you keep the same tool paths but just set Z for the new thickness since the parts protrude above the pocket board? Some pieces need to be flipped for machining on both sides so the pockets would help positioning for multiple parts. I have always done this on a router table but with my new CNC I think this could be a lot faster and easier, if I knew how to do it. I am talking about small box parts that are in the 3"X 3"x 5/8" range of size. Maybe this is more trouble than it is worth. What do you think. :idea:
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Re: Using Pockets for positioning....

Postby dhellew2 » Sat Sep 03, 2016 4:47 am

You can cut pockets to any depth you want.
To cut on both sides you should use guide pins so when the part is flipped over it will be in the exact alignment position.
You can clamp a spoil board to your table so you don't cut through your table.
Everyone has their preferences... some like pins so the part can only be flipped one way along the desired axis.
I like 2 pins one on each end along the x-axis located at the part center along the y-axis.
Drill the pin holes first through the part into the spoil board, insert the pins, then carve your part.
I typically use 3/16" dowels but sometimes depending on the size of the part, as big as 1/2".

When carving a 3.5" thick x 25" wide x 42" tall headstone I used 1/2" pins.
rose headstone painted.jpg
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Re: Using Pockets for positioning....

Postby martin54 » Sat Sep 03, 2016 10:07 am

What control software are you using for your cnc machine?
I use mach3 which allows you to set up work co ordinates for doing repeat work. I have made most of my jigs from ply mainly because I was worried that over time mdf would warp/swell with moisture. My unit isn't the driest warmest place in the winter :lol: :lol:
The jigs were cut on the cnc & have dowel holes which fit into the spoilboard, means I can put the jig on the table in exactly the same place every time & then using the right work co ordinates my x & y zero is always set to the same place.

I have cut my pockets all the way through the ply but they probably don't need to be that deep, I wouldn't think 1/8" would be deep enough though, mine are deep enough that they aid holding the part as well as aligning it, with an 1/8" pocket you would be relying on just the tape for hold down unless you intend using clamps as well which means you cant machine the whole surface of your part :lol:

I use the same file all the time, I just open it, alter the material thickness & then recalculate the toolpaths, you will get a message about recalculating toolpaths if you alter anything on the material set up page which is handy as it means you can't forget to do it :lol: :lol:
If I need to change any of the artwork I will do that & then save the file as a different name & keep the original as it was. I set my z axis from the top of the material because that is just the way I have always worked but you could set it from the table/material bottom so your z 0 would always be in the same place regardless of material thickness.

As with most things it depends on what you are doing, it will take time to set up the files & cut your jig so for a one off it might not be worth spending the time to do it but if this is something you do fairly regularly then once you have done the initial work every other one is pretty quick to do :lol: :lol:
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Re: Using Pockets for positioning....

Postby zaxis » Sat Sep 03, 2016 5:11 pm

Thanks a lot for the info guys. I am using Vcarve for my machine software. My Axiom has a spoil board set up in strips on the aluminum table so maybe I could fashion some type of T-track positioning with some knobs to put the pocketing board on top of the strips that would always be in the same position. That way I could drill into the pocket board with dowels using jigs for various projects. Does that seem like a plan? BTW if you are cutting the pockets with end mills to put square parts in I am assuming you need to chisle out the corners, right..... :?
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Re: Using Pockets for positioning....

Postby FixitMike » Sat Sep 03, 2016 10:35 pm

For square parts you can use dogbone fillets so the corners will clear.
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Re: Using Pockets for positioning....

Postby zaxis » Sat Sep 03, 2016 11:27 pm

Ah Ha...... thank you. I am slowly catching on.. :roll:
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Re: Using Pockets for positioning....

Postby martin54 » Sun Sep 04, 2016 1:09 pm

I just square out the corners with a file, your not removing a lot of material so only takes a few seconds, could use fillets as Mike has suggested but when I made most of the jigs I am using I didn't think to do that :lol: :lol:

You will need to make your pockets fractionally larger than the parts that will fit into them but do this in very small stages or you will end up with a very sloppy fit which will allow the parts some movement in the jig.

This tutorial is about 2 sided machining but they use a jig for alignment.

http://support.vectric.com/tutorials/V8 ... rts_2.html

There is also a video by Andrew Pitts somewhere where he makes a jig that uses a wedge shaped piece to lock the parts in place but I can't seem to find it just now.
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Re: Using Pockets for positioning....

Postby SteveNelson46 » Sun Sep 04, 2016 4:34 pm

martin54 wrote:I just square out the corners with a file, your not removing a lot of material so only takes a few seconds, could use fillets as Mike has suggested but when I made most of the jigs I am using I didn't think to do that :lol: :lol:

You will need to make your pockets fractionally larger than the parts that will fit into them but do this in very small stages or you will end up with a very sloppy fit which will allow the parts some movement in the jig.

This tutorial is about 2 sided machining but they use a jig for alignment.

http://support.vectric.com/tutorials/V8 ... rts_2.html

There is also a video by Andrew Pitts somewhere where he makes a jig that uses a wedge shaped piece to lock the parts in place but I can't seem to find it just now.


On You Tube, its the one where he makes a dish. Here is the link.

https://www.youtube.com/watch?v=i5ojCcb-7GA
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Re: Using Pockets for positioning....

Postby zaxis » Mon Sep 05, 2016 2:25 am

Thanks for the great information....I really like the idea of the wedge jig. I have a lot to learn. Seems like the possibilities are endless. :D
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Re: Using Pockets for positioning....

Postby adze_cnc » Wed Sep 07, 2016 11:54 pm

zaxis wrote:I am trying to cut mortise pockets and, the tenons (rabbits) to match


Poor rabbits just hopping along minding their own business and along comes some rascally woodworker out to cut them.

That's why I call "rabbets" rebates, as the English do, so there's no confusion with bunnies---just confusion with getting money back on the purchase of consumer goods. :D
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Re: Using Pockets for positioning....

Postby tomgardiner » Fri Sep 09, 2016 3:00 pm

I use pockets often to position and hold parts with the aid of my vacuum pump. One added benefit of pockets can be controlling tearout. If you have cuts coming through end grain, having the workpiece supported by a full depth pocket helps to reduce tearout.
Another point, pockets for positioning need not be a fully surrounding piece. I will screw down blocks just inside the intended position of the workpiece prior to running a profile pass to establish the part outline.
If you make yourself some standard wedges from Russian birch ply you can drop the wedge design into your project drawings when needed.
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