Tapping small holes in cast iron

[Bob Korves]

I recently helped a friend install magnetic scales on his new to him lathe, and had to drill and tap holes in cast iron for the small mounting screws. The job came out fine, but I was less than happy with the quality and consistency of the work I did with drilling and tapping holes while holding the correct sizes and getting a good thread fits and accurate hole locations.

So, now I need to install DRO scales on my Millrite mill, 3 axes, all holes will be in cast iron, and all holes will be blind. The hardware all uses M4 x .0.7 screws (or I suppose I could change to another size, like 8-32.) All the holes will be drilled using hand held power drill motors. I have quite a few, and can hold whatever RPM range I want to. I have drill bits in all sizes imaginable, with backups, but only half millimeter increments in ISO sizes. I only have one M4 x 0.7 mm tap, a 40-50 year old unused Craftsman Kromedge carbon steel plug tap. Those are good taps, but I probably should have at least one more on hand for the job, and a bottoming tap may also turn out useful. The results do not need to be to high tolerance for this job, but I would really like to achieve good fits, consistently, and no breakage. I will definitely be hand tapping and taking my time.

Suggested taps for the job?
Tap drill size choice beyond the usual charts? 3.3mm (.1299") is the usual global recommendation, but other choices might be #30 (.1285") or #29 (.1360") or maybe others. I have lots of #29 and #30 on hand. One of my charts shows the preferred drill as #36, but that is definitely not correct, probably a typo for #30.
Specific advice for getting hole placement where I want it, considering hand held drill, awkward locations, hard spots in the iron, etc.?

I know for sure I will be making a drill guide to match the chosen tap drill size for keeping the drill normal to the surface while drilling the holes after starting them.

Other suggestions?
Thanks!

 

[Frank Ford]

I'd get rid of the carbon steel taps, and go with HSS, spiral point taps. Likewise, get some split point HSS drills if you don't have 'em already. The last thing you want is to break a tap, and the spiral point ones will make that unlikely! Drill the holes extra deep and you won't have to worry about where the chips go as you tap. Spiral point taps shoot the chips ahead, so on cast iron, I work dry and back the tap out to evacuate chips with a blast of compressed air if it's a blind hole.

 

[markba633csi]

I think of cast iron like wood, and in wood you use lag bolts, so I would use a coarse thread.
Do you really want to drill holes in your nice machine? I don't think I could bear to do it if I had one like yours.
Why not just epoxy some brackets? The forces are so minimal, I doubt it would ever come off..
Mark

 

[Tinkertoy1941]

Make a drill fixture to hold the Brown Tool cross hairs see link below.
The fixture could be no more than a flat plate about 3/4" thick that would accept a hole to hold a bushing for the drill and one for the tap
and the Brown Tool and held in place with c clamps.

https://www.browntool.com/Listview/tabid/344/ProductID/2322/Default.aspx

 

[markba633csi]

DON'T DO IT BOB! LOL
put that drill away son

 

[magu]

Why not just epoxy some brackets? The forces are so minimal, I doubt it would ever come off..
Mark

I would advocate this method. Whether one cares to preserve some percieved purity of a machine or not is a personal choice, but from a purely functional setup, if anything ever induced a force on my DRO scales beyond a light contact I would much rather they pop free than break. Epoxy won't allow them to shift about and reduce accuracy, but a correct joint will break like glass at a certain point.

 

[David S]

How about making some intermediate mounting plates with the correct hole patterns drilled and tapped in the drill press and epoxy those to the machine. Then you can screw the DRO's to the plate and they can be taken off for cleaning, service etc.

David

 

[Bob Korves]

DON'T DO IT BOB! LOL
put that drill away son

Excellent advice, Mark. I have been careful since I have had the mill to make no changes that could not be returned to original condition. Your post is a breath of fresh air and I will consider it strongly. I would have to try to get whatever oil has penetrated the cast iron out, and make sure to use enough surface area to make an adequate bond.

 

[Bob Korves]

Some of the mounting will pretty likely have to be over painted areas. How do I do that without drilling holes or ruining the paint? The X is easy, the Y looks perhaps possible, but on the Z there seems to be no alternative to mounting over pristine paint.

 

[whitmore]

I recently helped a friend install magnetic scales...
[tapping holes in cast iron]
Other suggestions?

Pin-drive anchors, screw nails, or aluminum pop rivets would be my preference; something
in plastic or aluminum that can be drilled out. I've been known to carve a wooden block
to mate to the cast iron, attach it with construction adhesive, and screw into the wood.