Drag Engraving a Trophy with a 4th Axis and Mach3

[philf]

Last year our cycling club spent about £140 on trophy engraving and when you look at the engraving closely it’s not that good and I thought I could do as well if not better.

This year there are more trophies to engrave and so it would cost significantly more.

I've had a fair amount of success with engraving on flat or nearly flat surfaces with a diamond drag engraver but several of our trophies have the engraving on a silver band around the plinth which necessitate the use of a 4th axis.

When I modded my old pantograph engraver into a CNC mill I made a 4th axis which has remained unused until now. It uses a 200:1 harmonic drive which is virtually backlash free. The nose is 'a la Myford' so chucks and collets I already have can be utilised.

For all my engraving I've used Vectric Cut2d which has the ability to convert True Type fonts to vectors for CNC purposes. TT fonts are turned into outlines. It also has Single Line fonts which engrave very well. Single Line isn't a brilliant description because I used Helvetica 3L which draws the characters with 3 parallel lines.



Cut2D isn’t designed to run a 4th axis so, to get round this problem, it’s common to plug the 4th axis into the Y axis output to so that Y moves are made with the 4th axis and X moves with the X axis.

Rather than do that I thought it’s a very simple job to open the G code file in Notepad and replace every Y with an A. Then I could still use the Y axis to centralise the work and there’s then no risk of forgetting to plug the right plugs into the right sockets afterwards.

My 4th (A) axis is calibrated with steps per degree of rotation (in my case 888.88888 steps). 1 unit in A is one degree and so to get the correct distance it is necessary to apply a correction factor.  With a diameter of 114.591mm (=Pi/360) 1 degree of rotation will move a point on the surface of the object 1mm. Luckily with Mach3 you can apply a Scale to each axis. The number to enter in the A Scale DRO is 114.591/diameter. E.g. if the diameter was 229.182mm you would enter 0.5 into the DRO.



I’m using a home-made drag engraver with a tungsten carbide 120 degree cone. This was made after I managed to chip the diamond engraver I had. The carbide seems to work just as well as a diamond on everything I’ve tried - even 316 stainless. The cone was ground on to the shank of a broken 4mm carbide slot drill on the Quorn T&C grinder with a diamond wheel.

The trophy base with band were screwed to a backplate which was screwed firmly onto the 4th axis. The band isn't very tight on the plinth so I taped it to make sure it didn't slip. Professional rotary engravers usually use cones to hold and centralise round objects so that's another job on my to-do list.

Luckily, before I engraved the trophy, I chose to test it all out by taping some thin copper strip to the surface and found that the 4th axis was turning in the wrong direction and I got a mirror image of the text. This just needed a box ticking in the Mach3 settings to correct.

A video of it in action:

https://www.flickr.com/gp/16648905@N04/0ABSYk

A very satisfactory outcome.

Phil.

[m_kilde]

Hi Phil

Great video

Glad you did the test piece before running on the actual work, maybe we all would'nt have heard of your project then   

[philf]

Hi Phil

Great video

Glad you did the test piece before running on the actual work, maybe we all would'nt have heard of your project then   

Hi Mogens,

You're right - you wouldn't have seen that one being engraved. I still have 2 more to do but I have a month to do them so there's no hurry now I know I can do it. I wouldn't like to do it for a living - too much risk of getting something wrong.

When I look at some of those we've had done professionally you can see that on some they had two goes at engraving them and the 2 engravings are offset. My engraver is set to a depth of 1mm which means that the spring loaded cone can follow a certain amount of eccentricity.

I don't know about it being a great video. It was done with a £40 Fuji compact, hand held and edited on Microsoft Movie Maker - I'm sure no-one would want to sit through the whole job.
 
I just had a look at your website - some interesting looking plans there.

Cheers.

Phil.

[kstrauss]

If you tire of hand editing the Gcode you might want to investigate CNC Wrapper (http://www.cncwrapper.com/). It is only $25 and handles the output from Cut2D if you don't use arcs (choose a different output selection in Cut2D).

[philf]

If you tire of hand editing the Gcode you might want to investigate CNC Wrapper (http://www.cncwrapper.com/). It is only $25 and handles the output from Cut2D if you don't use arcs (choose a different output selection in Cut2D).

Hi kstrauss,

Since posting I looked at CNC wrapper but eventually found another (better?) solution called G-Code-Ripper which is free!

http://www.scorchworks.com/Gcoderipper/gcoderipper.html

Not only does it allow you to wrap the Gcode around an object but you can rotate, scale X&Y, scale Z and scale Feed Rates and if you like save that Gcode with or without wrapping. You can also type the wrap diameter in.

Another feature on large objects which are bigger than the CNC travel is to be able to split an object into two.

I think I mentioned before that Cut2D isn't brilliant at rotating text (if you rotate it and then come to edit the text Cut2D puts the text back to horizontal) so being able to do the rotation in G-Code-Ripper is very useful.

It does everything I want! (Except plaster my hall and stairs and mow the lawn to mention just two of the things it can't do!)

Cheers.

Phil.