Yesterday the Red Zinc/Aluminium Alloy dog tags I’d ordered arrived through the post so I thought I’d put my CNC machine to work and have a go at engraving dog tags using the smallest ball nose end mill I currently have in my collection – 2mm Diameter.
Before I continue, this reminds me of something I want to point out to other new and unsuspecting CNC operators. When shopping around for bits for your CNC router, mill or engraving machine, make sure you know whether or not the bit size dimensions are given in Radius or Diameter measurements! Recently I ordered a 1mm ball nose end mill and when it arrived I found it was actually 2mm in diameter. I had not read the measurements properly and it turns out 1mm was the radius of the tool. Just something to be wary of when purchasing engraving or milling bits.
Making a Jig for Engraving Dog Tags
The first thing I needed to do before I could start engraving the dog tags was to create a jig / template to hold several tags firmly in place so that I could engrave more than one tag at a time.
I wont go into all the technical details of designing and milling out my jig (perhaps I’ll cover that in another article) but after about an hour of trial and error at various depths, I had my jig ready to go. The dog tags I’m engraving are 1mm thick and initially that’s the depth I milled out for my template. However I found that too shallow and ended up milling down to 2.3mm – which as you can see from the image above the machine did very accurately.
Despite milling the jig really accurately, there was still some play and movement in the tags (due to the tag measurements not being quite so accurate) so I settled on using bits of A4 paper tore off and laid across the recesses before pushing the tags into holes to wedge them in good and firm while protecting the rear side from scratches etc. This worked really well holding them firmly in place but there were still issues with how flat and level the tags sat.
Combating Uneven Surface Problems
As you can see in the image above, the tags sit nice and snug in their recesses ready for engraving but I still had issues with how level the surfaces were for engraving. Some tags sat lower in the recesses than others and the engraving bit would cut deep at one end of the tag and barely graze the other so I had to make further adjustments.
I had to resort to using wedges underneath the jig in several places made from bits of paper which I packed out until the CNC machine could engrave each tag cleanly without missing spots. It took a few attempts to get everything sat right so that the machine could engrave all the tags in one pass without cutting too shallow or too deep.
On a few occasions I had tags that still sat lower than the engraving bit so I had to resort to packing the recesses out with layers of tissue paper until the tag sat high enough in the recess for the engraving bit to get at.
Dialed it in, Time to get Engraving
Having gotten my dog tags to sit nicely in their jig recesses it was finally time to start engraving my text / designs.
I should point out before I forget that I am now using my new 400w motor as opposed to the stock 200W motor that came with my CNC machine.
At my current conservative speeds and feeds it takes my machine 10 minutes per tag to engrave the design on to both sides. Now I’ve got everything dialed in and a pretty consistent workflow going on without too many issues leveling and aligning the tags I’ll look at increasing my feeds and speeds and my workflow to maximise production. I’d like to cut the time it takes per tag down by 50% to 5 minutes per tag ideally.
I’d also like to get hold of a 1mm diameter ball nose end mill which I think would be better suited for such fine work. The 2mm has a tendency to cut a little wider than I’d like and where a tag sits a little high in the recess it cuts deeper and wider and as a result letters can end up cut quite close to each other, almost becoming joined.
Engraving Dog Tags: Some Final Thoughts, Observations, Problems and Notes
One problem I didn’t foresee until I actually got started with this project was the homing position for the jig and what happens if I remove it to do other projects.
If you look at the image below, you will notice that in the bottom left corner of the jig I had to drill and mark a homing position on the jig itself and the reason for this is because I don’t have a permanent home position for the machine to refer to. So if I remove the dog tag jig to work on any other project, I’ll lose my homing position with no frame of reference to get it back again. I could be mm out of alignment when I next try to use the jig which could mean I end up engraving the jig and not the tags!
The screw holes I’ve used to secure the dog tag engraving jig to the spoil bed will remain there so I can always re-attach the jig into virtually the same position as it is now but if I reset the home position to do a different project, the spindle will then have no idea where it is in relation to the jig when I come to use it again. DOH!
Fitting Limit Switches for Homing my CNC Machine
What I must do to get round this, now that I’m getting into projects like this, is to fit limit / homing switches on my CNC machine to tell the CNC machine when it is at it’s home position (Zero X,Y). By having this constant position to home to and refer to as Zero, I can then work out where the actual home position of the jig is in relation to that and know that I can get the spindle back there every time I want to use my dog tag engraving jig.
Up until now alignment and positioning of materials hasn’t been an issue for me because I’ve only been milling single projects out of larger sheets of material or engraving single PCBs with the corner of the board being used as the home position. But now accuracy and reproduction are coming in to play, I need to get these limit switches fitted and I can start making use of offsets to assist me.
So next item on the upgrade list for my CNC is getting and fitting some limit / homing switches.