Log Clamping Jaws for Black & Decker Workmate - Cut With CNC Plasma

This particular Instructable isn't so much a step-by-step guide - it's really an overview of the steps I went through to design and manufacture this prototype.  I hope some of you find this useful. 

Step 1: The Brief

I try to scrounge logs for my woodburning stove whenever I can, and frequently end up with odds and ends and awkward sized branches to cut up.  

Cutting these logs up into suitable lengths for storage and burning can be problematic: a chainsaw is needed to do the work, and these are very dangerous tools - more so when the branch you're cutting is no longer attached to a tree!  It's awkward to position a log on the ground (particularly if it's small-ish diameter, and relatively light).  I've seen logs being held in position with one foot while being cut to length - a dangerous idea without chainmail socks!  On top of this, bending to ground level while holding the saw doesn't make a repetitive task like this any easier.

I was considering buying a 'Superjaws' type work clamp with purpose designed log gripping jaws, when I hit upon the idea of using my trusty Black & Decker workmate.  I've used this before to grip small logs (the max opening width is about 130mm / 5"), but maybe I could design something to widen the clamping capacity and offer better grip on the log?  A quick browse around the internet showed that it had been done before and I could buy a set for around £20 - but the only options out there look a bit flimsy, and log size is limited by the travel of the workmate  jaws.  So I decided to design my own. 

Step 2: Specifications

First, I had a close look at a selection of the logs from my current pile.  This exercise allowed me to establish the minimum and maximum diameter of log I would be likely to want to clamp.  

While having a little rest after lifting the largest log on to the Workmate, I decided that the safety benefits of clamping a log of this size would be outweighed by the manual handling risks of actually getting it into position.  Even with the workmate collapsed (it can be used for certain tasks in this position), it would take a considerable amount of effort to position a log of this size, and it had enough weight to be safely cut on the ground without movement.  So I decided that the next size down (around 200mm / 8") would probably be the maximum size I would want to cut, and any increase on this capacity would be a bonus.  Anything larger than this is a bit of a rarity for free firewood anyway - the tree surgeons usually take the big stuff away themselves to sell.  The minimum size worthwhile cutting up would be about 60mm / 2.5" - anything else is kindling for the chop saw.

Step 3: Ideas: the Design Takes Shape

I recently purchased some new plastic clamping pegs (used to hold wide boards etc.) and I liked the idea of using these - or at least the holes in the bench top - to hold the 'log jaws' in place.  Other designs I've seen wrap around the front of the bench jaws which limits the capacity but I hoped that using the pegs would be a way to maintain the clamping force whilst allowing versatility for positioning. 

Step 4: Sketches & Mock-ups

I sat down and sketched my ideas out.  While this allowed me to get my ideas on paper, I sometimes find that sketching isn't immediately helpful and it's easier to work through design flaws with a series of models. In this case going straight to cardboard mock-ups seemed to be the way forward.

After a little experimentation to gauge what seemed to be the right sort of size, I came up with a rough shape that I was happy with, adding bits of cardboard where necessary using masking tape.  I used a 20mm punch to cut a couple of holes corresponding to holes in the bench top, 75mm apart.  

Taking some measurements with the jaws at their minimum closing width, I realised that it would be tricky to position the jaws to allow for clamping smaller logs with two clamps directly opposite one another, and it wouldn't be convenient to position them offset as the log started to reduce in length.  For logs sized at the upper limit though, all seemed well.

Whilst mulling this over, I hit upon the idea of making the jaws reversible and adding a smaller 'v' to the other end.  A third hole was added to allow mounting in either orientation.  

When I was happy with this, I unfolded the cardboard and I had a development of the shape I would need to cut out in mild steel.

Further modelling in cardboard was possible at this stage, but I decided to go straight to the CAD drawing board and create the drawings to make a full prototype in steel.  Other materials wouldn't allow proper testing of the design and as I'm lucky enough to have access to the equipment, using CAD/CAM allows me to make a working prototype very quickly.

Step 5: 2D CAD

Working in TechSoft 2D Design V2 I drew up the design, taking measurements from the cardboard mock-up.  The screenshots below demonstrate the procedure used to create the drawing, but I'm not aiming to make this a CAD tutorial so you'll need to use your imagination a bit here.

Step 6: CAD > CAM - Preparing the Drawings for Output

Any CNC machine will follow the coordinates in the machine code.  If you translate a CAD drawing into machine code, the lines you have drawn will translate directly into movements by the machine.  For a knife cutter (for example) this doesn't always matter, but for a machine which removes a path of waste as the nature of it's process - such as a milling machine with a slot drill fitted, or a plasma torch where the small beam kerf counts - you need to compensate for this by adding the radius of the cutting path if you want to be accurate.  A quick glance at the plasma inverter manual told me that the beam kerf width for my planned material thickness (3mm) is 1.5mm. I therefore added a 'contour' around the shape in red at a distance of 0.75mm from the profile, on the waste side.  

An advantage of this program is that it is CAD and CAM all in one.  Speaking from experience, it is frustrating to create a design in a CAD package only to find that the machine post-processing software has different ideas about what you intended.  By creating the cutting path in red, I use this colour only to create the machine code - my original black lines are ignored.  Lines will be ordered in the machine code in the order that they are drawn, so I used this opportunity to make sure that the small inner circles were cut out first while the sheet is stable, by clicking inside these with the contour tool first, then the outer shape.  

At this stage the file is ready to output (File > CNC Ouput).  The file is saved on a memory stick for transfer to the machine console.

Step 7: Loading the Files and Preparing the Machine

So, here's the machine.  I loaded a sheet of 3mm mild steel.

The file is opened using the software on the controller pedestal.  A wizard based system guides the user through settings such as torch current and material thickness and allows a start point to be set relative to the origin of the CAD software.

About 60 seconds, and lots of sparks later and I had my first prototype.  Now to see if I could fold it up.

Step 8: Folding

This folder isn't designed for sheet of this thickness, but in the spirit of adventure I decided to give it a try anyway.  I've previously bent similar plate on this machine, but depending on the width I knew it may or may not work.  The most I was hoping for was to at least get the bend started along a straight path.  

No chance - back to the drawing board.  Note that I didn't cut four of these on my first attempt - just in case something like this happened!

Step 9:

The design might work with thinner material, but 3mm seemed about the right thickness for the strength I was looking for, so I chose to persevere with it.  I decided to make life easier for the folder by adding cuts along the fold lines, essentially just leaving a couple of short tabs in place.  I could have admitted defeat at this stage and just cut the part out in three pieces, but that would make setting up for welding a lot more fiddly and time consuming.  

This folded up nicely most of the way, and I was able to finish off with brute force by hand.

Now to make another three...

Step 10: Welding Up the Jaws

Time to roll out the welder.  A MIG welder is ideal for this task as it's quick and simple - not to say I wouldn't use a TIG set if I could afford one!  I won't go into detail here but will explain briefly the procedure I went through.

The first step was to set the welder up.  I used some scrap pieces of the same material clamped together in a similar way, and ran a few beads until I was happy with the result.

I then tacked all four jaws together at each end.

To prevent warping, rather than weld the full length of the join in one go, I first welded a few inches at one end followed by a few inches at the other end.  I then allowed this to cool while copying the routine on the other jaws, before filling in the gap in the middle.  (Actually, it would have been more than strong enough to leave alone at this stage, so the final step is more aesthetic than anything.)

Finally, I decided to weld up the ends of the fold lines underneath to tidy things up.  

Step 11: Dressing the Welds

I dressed up with welds with a flap wheel on the angle grinder.  I like flap wheels for this process as they give a nice finish and aren't too aggressive.  

While the welding was done indoors as it uses shielding gas which blows away in the wind, I moved outside to do the grinding as the filings make an awful mess.  

Step 12: The Finished Prototype

Now that it's been tried and tested, I'm quite pleased with the result!  It works well, and holds the log firmly with surprisingly little effort. Once you get the hang of it the jaws are very easy to re-position when necessary. 

One thing I was worried about was the jaws lifting away from the bench as they were tightened, pulling the pegs out - but I'm relieved that this doesn't happen.  (The new pegs I've got are a much better fit than the originals, so I think I'll get a second set just in case.)

If I have time to cut out a second prototype I think I would widen the angle of the jaws - it's currently 90°, but about 110° ish would make loading easier and increase capacity.  I'd also cut more holes to increase options when positioning.  

If anyone would like the CAD file or machine code to cut out their own jaws, please ask.