welding steel base or just getting aluminium extrusion

[reefy86]

how much room play do you leave when drilling and tapping? for example i am using 20mm linear rails on the z axis and the screw clearance is 6mm so i would need to use smaller bolts then 6mm to make room for alignment.

[Nr1madman]

how much room play do you leave when drilling and tapping? for example i am using 20mm linear rails on the z axis and the screw clearance is 6mm so i would need to use smaller bolts then 6mm to make room for alignment.

I made a huge error when drilling/tapping my first plate. Went with m6 as the holes were 6mm and chinese drawing said m6 holes.
Turns out that m6 boltheads couldnt fit the countersink in the rails :D
So use m5 bolts, 4.2mm drill..
That gives you 1mm adjustment.
Done correctly thats plenty ;)

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[reefy86]

Thanks i had m5 in mind but wanted to double check :)

[Neale]

I used M5 as well - works fine and although you don't have a lot of adjustment range, there should be enough if you are careful. I made up a steel drill guide (a short length of steel bar with a 4.3mm hole through the middle) that had a reduced diameter one end that fitted the counterbores in the rail so it centred the drill accurately. You can then use a similar guide with a 5.1mm hole to guide the tap so that it goes in square.