A Hard Case to Crack

Good progress today. Linear rail for gantry drilled and tapped and gantry motor mount made!
I'll start with the motor mount as its very straight forward and need little explaining.
Here is a pic!

Its 6mm thick. I had to put it in the 4 jaw chuck on the lathe to cut the big hole.Drilled the 4 holes to 4.2mm and tapped M5. I've yet to drill the holes to mount on the gantry. I need to get the linear bearing first...

But the big thing today was the supported rail for the gantry.

 I'm going to go into a bit of detail here because

 it was a pain to get this done untill I got the knack!

 (and this might help someone trying the same thing)

In this project I'm using 16mm linear rails. They are case hardened, which means no drilling into the surface.
But...below the surface is normal mild steel! So I can work that easily if I can just get to it!
If your drills or taps touch the hardened steel the will snap.
So follow the steps below and your laughing!

1. Mark your drilling points with a permanent marker (don't bother trying to scribe a mark at this stage).

2. With an angle grinder, grind the spots you've marked. Be careful, try to stay inside your support width, 8mm in my case. The point here is to get the centre of the cut to be through the case hardening.

Here is the first cut

3. Remeasure, scribe and centre punch your drilling points.

 4. Get a tile drill at least 2mm larger that the thread diameter of your tie down bolts. M6 in my case so I'm drilling with an 8mm here. Naturally you want to use lubericant of some type here... an interesting side note, normally these drills are used for tiles and glass and water is the lubericant. But I have a suspition that the water is a coolant not a lubericant.

I noticed that it wasn't untill the oil I was using was pushed out of the drills path did it start cutting properly.

5. Drill with the tile drill untill you are at least 1mm larger than you tap diameter. So I drilled in to 7mm or so!
Now you can go ahead with a nornal tapping drill size as you are throught he case hardening...

 6. Once you've done one hole, go and tap it!
      You see the gap between the tap and the case hardened steel.

 Now scew in a short length of threaded bar. This will help ensure all the other holes are on the same axis!

7. Go ahead and drill the other holes. Tile drill then tapping drill always using the threaded bar as a guide to the alignment.

When you have all that done mark and drill the U channel. It will all line up... and you get:

Lovely.