Drilling And Tapping Cast

Getting ready to install Y-axis scale for DRO on my SB 9A.
Only way I can see to do it is to drill and tap hole on cross-slide end above acme screw to attach sclae slide.
Plan on removing cross-slide to do this.
Anything else I should be careful about or do to drill and tap here ?
Haven't done drill & tap on cast before, don't want to damage cross-slide.

Drilling and tapping cast iron is far easier than steel. [Assuming hand held drill] Center-punch, center drill, drill tiny hole through, drill tap hole, hand tap. The hardest part is getting the hole and the tap perpendicular to the surface.

I was planning on removing the cross-slide and drilling on drill press or mill in a vise.
Was thinking more about drill RPM fast or slow and if any issues with hand tapping threads.
Also don't want to risk marring up machined bottom surface.

JPigg55 said:

Getting ready to install Y-axis scale for DRO on my SB 9A.
Only way I can see to do it is to drill and tap hole on cross-slide end above acme screw to attach sclae slide.
Plan on removing cross-slide to do this.
Anything else I should be careful about or do to drill and tap here ?
Haven't done drill & tap on cast before, don't want to damage cross-slide.

Click to expand...

When I have drilled and tapped castings in the past, I have occasionally hit hard spots in the casting. Use care if and when you hit one, especially when tapping as it is very easy to snap the tap off. I prefer to back the tap out about every two turns to clear the chips. Cast chips are quite fine and can clog up the tap making it hard to turn in or out. If that occurs, have patience and use minimal force. Generally, rotating back and forth with increasing sweeps will unclog the tap.

If drilling in the drill press or mill, my preference is to complete both operations at the same setting. I hand tap and use a spring loaded tap guide to ensure the tap is running true. It greatly reduces breakage.

Good luck!

Bob

Don't over think it. Cast iron is way easier than most metals. Take your time, drill straight and tap in place. I clear chips every 1/2 turn when I'm nervous. I don't use any cutting fluid when drilling/tapping cast iron. As mentioned, hard spot can occur but a serviceable sharp drill should work just fine. Shop air will help clear the fine swarf during tapping.

tapping cast iron is not usually a traumatic event, at least not in my experience.
as others have said, tap it dry
most of the cast iron i've dealt with has had enough free carbon to help lubricate the tap as it's being used
just like any tapping operation, perpendicular bores and correct tap clearance are essential.
clear your tap often.
if you can perform the operation with the aid of a drill press for the drilling operations, and then, non-powered,for tap guidance, there should be little difficulty in achieving you aspirations.
good luck