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CNC Novice's (further) adventures in
cutting steel (May 2007)


Despite making a large dent in my tooling collection, I have made some hard won progress in my further attempts to work with steel plate on my homemade CNC machine.

Having drilled and milled a little steel, I thought I’d cracked it.  I then went on to try and surface my work plate which was an exercise in frustration.  I wrecked the end bearings on my die grinder, melted or wore out 3 x 6mm end mills and watched my machine churning for hours on end, day after day as I tried to shave off enough steel to make the rather bowed construction grade steel flat.

I learned a couple of valuable lessons.  Firstly that I was an idiot to attempt something like this in the first place.  I was a very naďve buying the couple of meters of 10mm x 200mm steel plate in the first place.  Presumably this is cold rolled (ie work hardened) steel, design for bolting to girders that hold buildings and sides of motor ways up etc.  I don’t know what grade steel it is, but it presumably has a pretty low machinability index.  I only just came across machinability indexes the other day and no doubt this topic will feature high on my Google searches in the course of my up coming metalworking career.

Despite some cunning CamBam scripts and coolant I was finding working with steel a very frustrating (and expensive) business.  I was extra frustrated by the fact that until the worktop was flat and grid drilled, I couldn’t use my machine for making more interesting stuff.

In the end I determined I had got the surface ‘flat enough’ and moved on.  End mills are so just NOT the right tool to face a steel plate.  I may take another crack at it if I get around to making a fly cutter or something like it.  That would be the way to go I think.

I’m having a week off work and my wife is magnanimously allowing me ‘play time’ in the garage, in between child minding and chores.  So I set out to finish the grid drilling off so I could get on with all the other jobs that have been mounting up.

I have added a couple of new drilling features to CamBam which I was keen to try out.  One is a spiral milling option.  This allows me to ‘mill’ a larger hole using an end mill and a spiral toolpath.   This is the theory anyway.

After melting 3 x 3mm end mills in that attempt I thought I would try out another new CamBam drilling feature, peck drilling.  If I could centre drill the holes first then mill them out that may help.   That was how I did the counter bored mounting holes for the work plate, using a CamBam visual basic scripts which worked fine.

Three snapped and melted drill bits later I was getting really cheesed off.  I had to make another collet adapter for my die grinder to fit a 4mm drill in my 6mm collet.  My last remaining 4mm drill snapped.  It was bank holiday Monday and I couldn’t be bothered making another adapter to fit another drill size from my ever dwindling supply.

I was resigned to calling it a day when a thought struck me.  I grabbed my 10 year old Stayer drill and measured the neck: 43mm! Before you could say, "Why didn’t you think of that before you big boob!" my aging power drill was mounted snugly in my 43mm tool mount and we were off again.

This drill is really not designed for CNC.  There is at least 3mm of slop in the bearings and I have my doubts as to the concentricity of the chuck.  But it has a nice beefy gear reduced motor that has grunted away for years and put up many a bookshelf.  It has a trigger lock and an adjustable speed screw.   It even has a forward and reverse switch and probably cost me about 30 quid (with a matching (not very good) jigsaw supplied.) Perfect! I just need to remember to switch the hammer bit off first. :-)

The problem with using my die grinders for steel is the speed.  They are just too fast (even on lowest speed settings, where they are also liable to stall).  I may try them again on if I get hold of some decent high speed tooling and better grade steel.

The drill was cranked back to a nice sedate RPM, the plunge feedrate only a couple of mm per minute, but it plodded on and filled my worktop with wonderful, glorious and perfectly formed coils of drill swarf.

Sadly the drill can’t be used for milling as there is far too much play in the bearings. But it has given me the idea to investigate further.  I may make a new bearing block to add to my existing drill or I may see what other drills are around that may be suitable.

Once the mounting grid was drilled I tapped them to M6 using my trusty Makita cordless drill.

I finished off the other six M8 mounting holes using a CamBam spiral mill drill.  This worked much better with the core of the hole already drilled out.  The M8 holes are for mounting my drill vice in a number of positions.

It’s been a long journey to get this far but I have learned loads in the process and both CamBam and my machine are much better prepared for the next wave of heavy metal madness I care to throw at them.