Drillcon 100 - The Mechanical Design


I'm no mechanical engineer and I was on a very tight budget.  I wanted to buy as little stuff as possible while still maximising my chances of making this project work.  Certain design decisions were therefore going to be based on what materials I already had in stock.  The drive mechanism I had decided on was simply a threaded rod held in bearings with a captive nut in the middle.
Several years ago I replaced the bearings in my original inline skates.  I didn't throw the old ones away in case they came in handy one day.  That day had come.  They were standard 608 bearings with an inner diameter of 8 mm.  That fixed the size of the threaded rod to 8 mm.  I did have a choice of material though and I thought stainless steel threaded rod would be stronger and stiffer than brass.  I would need some lock-nuts to hold the threaded rod to the bearings and some regular nuts to move along the rod.
I made some sketches on paper but I really needed a better way of drawing the mechanism.  The answer to this was the QCad computer aided drafting program.  Even better, the code is open source so all I needed to do was to download it and build it.  I spent a little while reading the manual so that I had sufficient working knowledge to use the program.
Here's my first design for the Y axis:

Y axis design

I had a chipboard with wood veneer sheet that would do for the baseboard.  I had well-seasoned marine ply that would make good mounting brackets and I had soft pine batten for miscellaneous mounting duties.
I could cut a square steel plate for the Y axis moveable bed from the case of the afore-mentioned Viglen Genie.  I wanted it to be 20.0 by 20.0 cm because that would give the drill a reach of 10.0 cm on either side of the midpoint.  Then the question of how to support this plate arose.  I looked at smooth sliding rods but I couldn't come up with something cheap enough or something I thought I could get to work.  Then I found the "DryLin N Low Profile Linear Guide" on the RS Components web site.  The bottom of the range version is a 30.0 cm long aluminium rail with mounting holes.  Into this rail are inserted 2.0 cm long carriages onto which can be mounted things like plates.  So I decided on two rails and four carriages.  This was the most expensive piece of the project at the time but I thought it would give me maximum chance of success.  The parallel rails would not only keep the plate in line but would also prevent any movement in the Z axis.  With two carriages inserted into each rail, the plate should essentially be self-aligning.  The other advantage of the DryLin system is in its name - they don't require any lubrication.  The problem with lubricating surfaces in a project like this is that drill shavings stick to the lubricant if it's something like oil.

Here's a picture of the plate with the mounting positions of the four carriages, the four holes to mount a wooden plate on top of the steel one and the mounting position of the central nuts which would attach the plate to the threaded rod:

Y plate schematic

Now the question arose of how I was going to accurately make all the parts to sufficent tolerances that would allow this machine to function.  If I had a drilling machine, I wouldn't need to build this in the first place.  Then I realised that I did have a high precision piece of equipment that would do the job - my new Canon PIXMA iP1000 printer that I bought to replace my Stylus Color.  The QCad program could print the designs with 1:1 scaling and all I had to do was cut them out and glue them to the pieces I was trying to measure and cut.  The external shape of most of the pieces wouldn't be that crucial - it was the relative positions of the internal mounting points that really mattered.
I had a plan.  If the Y axis didn't work then there would be no point in trying to build the Z or X axes.  I ordered the DryLin Linear Guides from RS Components and the remainder of the electronic and mechanical parts from Farnell.  Normally I would use Maplin but I could only get certain components from Farnell and they have a minimum order amount of £20 which I couldn't otherwise meet.

Time to start construction...

Y Axis Construction