Gary's CNC Musings ...

Dovetail and Mortise Jig:

One of the things that we always had in mind for our ShopBot was to use its automation capabilities to replace some of the operations that we had previously outsourced.  Due to increasing lead times and decreasing quality, our first chore was to start in house manufacturing of the solid Maple Dovetail Drawers we use in our cabinetry.  After dusting off our Porter Cable dovetail jig and making a few test cuts, it became perfectly clear that we needed another method. The chatter and inconsistent fits were as bad, if not worse, than the ones we had received from our supplier.  I used the general idea of the hold down and cutting methods from the tried and true PC jig as a guideline for my jig and toolpath geometry.  If you are looking for a way to cut half blind dovetail drawers on your ShopBot, this article should get you pointed in the right direction.  Other than a few “investment grade” wood scraps, the only purchase requirement is a ½” diameter 14 degree dovetail bit.  Since holding the drawer sides for cutting is probably more important than the cutting itself, we will start with construction of a jig.

 Jig Construction

 I made my jig from ½” PVC solid sheet and some aluminum extrusions I had at the shop.  I dimensioned it so that I could cut a pair of up to 11” wide drawer corners at one time.  I also made the fences and front lip from aluminum so that I could use the control software to zero the jig.  Others that I have sent the file to have mounted their jig and then cut the fences with the Bot the insure accuracy.  I used 8020 T slot extrusion, with T knobs to hold the material to the jig.  Your jig needs to be wide enough to cut a pair of corners on the maximum width drawer side that you anticipate.  My final width was determined by the T Slot mounted to my table, which allows me to use bolts and T nuts to fasten the jig to the ShopBot Table.   You should notice the lineup bracket behind the front clamp bar.  This bracket offsets the vertically mounted piece half the center to center distance of the lobes and slots.  (More on this later)

What you make your jig from is not important.  What IS important is that it is very square, the fences are exactly parallel, and the front and top are perpendicular.  You should also make sure that the fences and brackets are not as high as the thinnest material you plan to cut is thick.  5/8” is the thinnest material we use, so my fences are ½” high.  You will also need to be able to cut beyond the end of your table in the  –X direction.  On my setup, I am cutting between the limit switches and the hard stops, so I must turn the limit checking off to use the jig.  You will need to have a very secure mounting method as cutting end grain in dense hardwood puts a lot of stress on both the jig and the tool.  You may also want to consider building the jig and cutting the fences parallel with the ShopBot.  You will need to know the exact distance between the fences to be able to cut pairs of corners accurately.  Holding the material down to the jig is also very important.  In order to get my aluminum extrusion to work better, I sanded the contact side and then bent the center towards the jig 1/16” for better contact across the material.

SETTING UP MATERIAL AND USING THE JIG:

The following 2 illustrations show the orientation of the jig on the ShopBot table, the drawer parts and how they fit into the jig, and the orientation of the parts for cutting.

You should notice that the jig should be zeroed as shown by the arrows.  These zero lines correspond to the front edge of the jig and the edge of the “odd fence.”  The Z zero is the top of the material (shown in section “A” as “Must be Flush”  The cutting file also needs the exact distance between the fences. The dimension is shown as 24”, but can be any size.   Also note that the top edge of the vertically mounted drawer side must be flush with the top surface of its mating front or back. The 7/16” offset is required to make the top edges of the sides be flush with its mating front or back.  If you wish the sides to be lower than the fronts/backs, then increase the offset by that amount.  We do this occasionally to allow the use of radiused edge drawer sides.  You do not have to cut the mating corners together on the jig.  As long as you have proper orientation, you can make all the drawer cuts in 2 operations.

This illustration shows how I setup my drawer parts and mark them for cutting.  I pencil “X’s” and corner numbers on all the corners as shown.  The “X’s” go on the top edge of the parts to orient them to either the fence or the offset bracket.  The corner numbers go on the insides of all pieces, as they will be either up or out when properly oriented in the jig for cutting.  It took me a while to learn this marking method, but, once learned, it makes dovetails much easier.  The PC dovetail sheet linked to below shows a slightly different method, you may like theirs better.

The cutting lengths for the drawer parts are as follows:

                * The Fronts and Backs are cut the same width as the finished drawer width.

                * The Sides are cut to the finished length of the drawer less one material thickness.   Due to variations in cutting depths, you may have to adjust this length.

We use Blum Tandem drawer slides and our dado for the bottom is ½” from the bottom edge.  Other brands may have different requirements.  We use ¼” for a dado depth.  Care should be taken not to cut the dado so deep that the lobes of the dovetail are cut, as this will show in the finished product.

These pictures show test corners before and after cutting

Here is an assembled test corner:

The Cutting File

The ShopBot part file to cuts the dovetails is available from ShopBot Labs. It will give you the option to cut an odd or even numbered single corner, or a pair of corners.  When cutting a pair, the pieces must be an odd and even pair properly oriented.  As with all untested parts files, you should air cut with it before running with a cutting tool.  The file will give you options to cut trial pieces to make your adjustments, select cutting locations, adjust cutting depths and slot lengths, and repeat cutting for same width parts.  You will have to edit the parts file to match your jig fences and possibly to match your jig location on your table.  You may also have to adjust the cutting speeds to fit your machines cutting abilities.  In any event, if you don’t feel comfortable modifying a parts file, this may not be for you.

The file is designed to use Porter Cable #43704 (1/4" shank) or PC#43750 (1/2" shank) or equivalent bits.  I highly recommend the ½” shank bit.  There are many 14 degree ½” diameter dovetail bits that are made by other manufacturers that should work also.

If the fit is too loose adjust the Depth of Cut to be deeper. If too tight, make Depth of Cut shallower

If the side panel is inset, decrease the Slot Length value

If the side panel extend past the other panel, increase the Slot Length value

This jig and cutting file were made to replace the Porter Cable Dovetail Jig jig with a CNC router.  Any and all of the cutting operations shown in the manual should be able to be cut using this cutting file, with a few adjustments.  By changing the file variables, you will be able to change the cutting depth, the slot width and depth, the pin radius and the distance between pins and slots.  With a few different bits, and a little practice, you should be able to cut any joint shown in the manual. You can view or download the complete manual from the Rockler website

Moving forward:

If you study the Porter Cable manual, you will notice that changing the lobe and pin spacing, the slot length (distance the bit travels into the jig slot) and the router bit, are the only changes you will need to make in order to cut any of the dozen or so types of joints shown.  All of these adjustments use variables that you can either edit in the parts file, or fill in as inputs.

Whether you are looking to build a single drawer for a unique cabinet, or a couple dozen for the next kitchen coming out of your shop, with a little practice and a few adjustments, you should be able to use this jig and cutting file to make high quality joints.  As you use the jig you will find more uses for it.  We have also used it for end and edge drilling small parts, cutting slots in the edge of thin plastic and cutting mortise and tenon joints. 

Enjoy! 

Next Article: We kick the Vacuum up a notch

A PVC Vacuum Table Part 2: A little more on Plenum Design…

You may also want to make a couple passes at different depths to ease the transition into your vacuum port.

T-Track & Plenum Assembly…

I used 1 ½” wide t track so I cut a dozen blocks of plywood 1 3/8” wide for plenum assembly spacers. I used this width so that I could machine all the edges of the plenum zones true at the same time I cut in a shallow recess for the t track. The blank plenum sections are cut ¼” oversize in both directions from the zone design size. The 1/8” undersize spacers allow each section to overhang the track location by 1/8”.

• Sand sections and table thoroughly and glue to table using spacers



• Set a sheet of plywood over all 4 quadrants and add something for weight.



• When glue has cured, Install a surfacing bit and lightly surface the top of the plenum quadrants.



• I now used a cut file I created that matched the layout for the t track and machined it .060 deep into the table board. This levels the t track and cuts the edges of the plenum quadrants.

• If you want your t track flush with the top of the plenum, adjust cut depth accordingly.
  
  
• Check that the size of the quadrants matches your plenum design size, if not, adjust them at this time.
  
  
• Using your plenum file, cut the plenum and gasket grooves.
  
  
• Double check the locations of the vacuum entry holes and cut them. Make sure that no table cross supports will be hit.
  
  
• With all the machining done on the plenum, it is time to install the t track. I used stainless flat head bolts with locking nuts, 10-12” on center.
  
  
• Cut 4 spoil board pieces to fit in between the track and set is place.
  
  
• Hook up the vacuum and TRY IT OUT!

More on Removable Bleeder Boards:


Using a loose fitting bleeder/spoil board has both advantages and disadvantages and is not going to be the best solution for all users. The biggest advantage is the ability to have replaceable Spoilboards in any zone. Along with the economics of partial sheet replacement, you also have the ability to install custom made fixtures to hold parts you may cut repetitively. These fixtures, or pallets, can be used to “drop” a load onto the machine for cutting, and when that cut is complete, “drop” another pallet full of parts on to repeat. These pallets are usually sealed and allow vacuum to pass only under the part, between gaskets that are inside the cutting area. We have found that is convenient to do most of our cutting in the home or 0,0 position. With the price of Trupan what it is, it is nice to be able to replace our most used area, ¼ sheet at a time.

The biggest downside of having loose fitting Spoilboards is the tendency of the material to curl up at the edges, especially as we surface them thinner. This seems to happen with changes in humidity. Our recent switch to ¾” from ½” has helped a lot. We also turn them upside down when not in use. We are going to test spot gluing them down with silicone in the near future. This curling is more of an inconvenience than a problem as we have not noted any variation when the vacuum is on. So far, we have found that for us, the benefits outweigh the disadvantages.

Hopefully, if you had questions about building a vacuum table, this article has given you some information that helped your decision making process. If you were thinking about changing or upgrading an existing table, it is my wish to have given you a few things to think about. The cost of the PVC sheet and heavy duty T Track may exceed the budget for some. However, it is a very stable platform that should give better long term performance than any of the wood based products more commonly used in table construction.


Next time: Adding vacuum replacement motors directly to the table.

A PVC Vacuum Table - June 2008

As a relative newcomer to the ShopBot community, let me first say I am honored to be asked to join the likes of my fellow columnists. I am sure that all of you respect them, and their opinions, as much as I do.

Over the next few months I will try to use my woodworking and fabrication experience to hopefully expand your abilities and capabilities with your machine by adding a few “accessories” or by giving you some other construction options for things that many have taken for granted..

I was lucky enough to have hosted a Camp ShopBot in February of this year. There were a number of experienced, new, and future users that were a pleasure to meet. There was one common thread between the less experienced users that surprised me. While doing a short demo on vacuum hold down, I overheard, “Ooooooh, the vacuum goes THRU the Trupan!” This “revelation” by the newbie’s, coupled with a large number of emails that I have received since the camp, and a few suggestions from others, has prompted me to start out with a table construction article. Some of this will seem very basic to you “Old Hands”, but I hope it will help enlighten new users.

Glossary....

Bleeder Board: A material that is porous enough to allow vacuum to pass through it and hold down material being cut on a CNC table.

Plenum: A layer of material that has channels cut into it to allow even vacuum dispersal throughout a vacuum zone.

Spoil Board: A sacrificial (replaceable) layer that allows a CNC cutting tool to cut through material without damaging the table layers below. In many cases the spoil board and bleeder board will be the same.

Table Frame:
Table board:: The first layer in a CNC Table system. It forms the support for the subsequent layers.

Trupan: Specific brand of Ultra Light Weight MDF type material most preferred for bleeder/spoil boards due to its porosity. Also called ULDF or Ultralite MDF

Zone or Vacuum Zone: An area of a vacuum table that is used for vacuum hold down of parts being cut on a CNC router. These can be of any size or shape, usually based on the material to be held down.

Planning…

You should use the time between the day that you order your machine and when it is delivered to design your table and hold down system. I started searching the ShopBot Forum for every article concerning table design and vacuum hold down going back well into 2005. There is a wealth of information there from many that are more experienced and learned than myself.

If you have the chance to go to a Camp or visit a ShopBotter near you, especially one that uses his or her machine similarly to what you plan for yours, do it! You will gain a lot of information by just looking at a machine in full scale.

It seems that very few of us use our machines in the same way or cut the same materials, nor are we exactly sure before we even have the Bot what or where we need to hold down, I made the decision to make a table that incorporates as many options as possible. This decision, of course, made the table more expensive and heavier. It has also made it very versatile, so that in 10 months we haven’t had to put any screws into the table surface, which was one of my goals.


Material selection…

First let me preface my material selection information with me telling you that we are a commercial shop. We plan to have fairly constant use of the machine. We also are looking for every bit of accuracy that the machine is able to deliver. We will often do an operation on material that may cost hundreds to a couple thousand dollars. If you are a hobbyist, you may want to use methods or a material that is easier to obtain or more budget friendly.

Due to the high humidity here in south Florida I decided to not use any wood products in the table bed or plenum. We have many table surfaces in our shop on various pieces of equipment. They are both shop made and purchased from reputable vendors. I have been using them in various forms since the mid 70’s, here is what I know: Plywood will always warp. Fiber or chipboards will always swell.

I chose Gray PVC sheet, type 1, solid. This material has many advantages over any of the wood products: It is available in over size sheets. (I purchased 2 of the 5 by 10 sheets for the table and 1 of the 4 by 8’s for the plenum, all ½” thick), the weight helps resists vibration, there is little or no dimensional change with heat or moisture, and it is relatively easy to machine. It will glue to itself and other materials well with carefully selected adhesives. I have only found 1 disadvantage, the price, at $350 per 5x10 sheet, $250 per 4x8, it is much more expensive than ply or fiber wood products, but I feel that it is worth it. I purchased the pvc sheet from Piedmont Plastics, but there are many plastic distributors in the US and Canada that may have a similar material.

If you choose a wood based product for your table here are a few tips:

If you use plywood try to use one with a non wood “coating” on BOTH sides. This could be in the form of a factory applied finish (catalyzed) or a phenolic coating such as MDO (medium density overlay, sign plywood), or backer or liner board, made for applying laminates. If using a fibrous product like particle board or MDF use one that is as dense as you can get and laminate it from multiple layers using an adhesive that is not water based. Melamine coated sheets would give you a heavy stable surface, but I have not found an adhesive that bonds it to itself or other materials well. Others may have.

Seal any raw sides and all edges BEFORE assembly, reseal after assembly and/or surfacing. Use a good quality epoxy or oil base product. Try to get it all sealed at one time. Find an adhesive that is compatible with the coating that you use. Laminating epoxy will work for both. You can also add a fibrous thickener when using it as an adhesive. I have never had any good results using any water based finish on sheet goods.

REMEMBER: The material you cut will only lie as flat as your table. This applies to how flat your table STAYS, as well as how flat you can build or surface it.

TO “T” or not to “T”…

This is a short description of how I use the T-track for hold down on my ShopBot Table. Hopefully, it will give you some ideas of how or if you want to add t-track to yours. After reading David Buchsbaum’s article on his vacuum table, I made the decision to add t track into my design. David has many good ideas that you can follow, or build on, as I did.

There are many vendors for t track out there. Most of the woodworking supply chains carry lighter duty versions in one form or another for jig making. I chose a heavy duty version from 8020 inc. with a single track, 1.5” wide by ¾” high. It is available in lengths up to 12’. They have many other sizes and styles. They also have an eBay store where they sell scratch and shorts.

Here are some of the reasons I chose to use T track:

• Gives us an easy way to repetitively index parts at 0,0 or any of the other 3 zones.
  

• Affords us many options to hold material both down and laterally
  

• Can be used to help seal vacuum zones from each other
  

• Can be used to eliminate horizontal movement when using loose spoil boards.
  

• Removable spoil boards can be replaced with plastic or wood sealed special purpose pallets.
  

• If bolted down as I did, adds rigidity to table.
  

• Track can be machined in to be flush with plenum so that a spoil board can cover complete table
  

• Track can also sit above plenum to hold spoil/bleeder boards in place.
  

• Track can be used to bolt down vises and various jigs for holding odd size materials.
  

T Track location options...

If the T Track is flush with the plenum, it allows the use of a single large sheet for a bleeder board. The downside of this is that you will need a mechanical hold down method to keep the bleeder from both sliding around while cutting and to keep it from curling when not. Dabs of silicone every few inches would work for this, but makes removal of the bleeder harder when using the track for hold down. Flat head nylon bolts or screws could also be used. I do not recommend screws.

If the T Track is mounted above the plenum, and bleeder board sections are cut tightly in each zone, then lateral movement is eliminated. Vacuum then holds the bleeder very well while cutting, but there is still no way to control the “potato chip” effect that always seems to happen during humidity changes. We store ours upside down in place and may go to silicone in the future. This curling seems to increase as the bleeder material thickness decreases. The downside of this method is when the bleeder is removed to use the t-track it is not flush with the top of the track.

How many Zones?...

1 zone, 2 zones, 4, 8, 16? Read the many posts out there, and base your decision on the material that you plan on cutting and leave yourself options to go both larger and smaller. Remember, the more zones, the more plumbing. Fewer zones mean more masking to cut smaller parts. We have had good luck with 4 zones using thin Plexiglas for zone masks. They seem to work very well with very little vacuum loss. Trash bags, sheet laminates or visqueene also work well. We picked up 2 pieces of 3’ by 4’ by 1/8” Plexiglas panels from the big box store and cut them into 1 by 2, 2 by 2, 1 by 4, etc. sizes, and use them to mask off the unused part of a zone when cutting. This helps keep the vacuum loss to a minimum. We drilled a ¾” hole in each one and hang them on a peg on the wall when not in use.

Vacuum Plenum Design…

At this stage you should know what your materials are going to be for both the table bed and plenum. You may even have them on hand. You should have decided if you are using T track or not and how many zones that you are going to use. I had all of these, but no machine delivered yet. It was now time to design the plenum and table cuts. I sat down with Brady’s video and spent a few nights learning to be able to make the rectangles, circles and straight vectors do what I wanted. One of my biggest obstacles was learning how to reverse the individual straight vectors in the plenum grid so that the machine would cut them in alternating directions rather than all starting on one end. It’s not hard to do, just hard for me to find. Once I was able to get the results I wanted, I set these files aside to rescale based on my actual table dimensions once the machine was assembled. Both the control and design software have a preview mode that is a very good tool. It gets you used to the machine moves and you can zoom in to see what, where and how you are trying to cut.

Practice, Practice, Practice!!!

Next Month: Gluing and assembling the table