View Full Version : drill and tap thompson shafting, induction hardened shaft, hard chromed shaft

10-12-2014, 12:57 AM
I've built projects where I used thompson shafting for way rods, where I've drilled and tapped into the ends of it. Not easy, but it went ok. Now I have another project or two where I'll have to drill and tap into the side of some suitable shafting. Hard chromed is the suggested material as it's available and won't break the bank.

Whichever material I choose, my plan was to mark out the hole locations, then grind a flat at each point to both remove the surface and make center punching, drilling, then tapping easier. Wondering how much thickness of the surface I should be grinding away?

For the hard chromed, it would seem obvious that just getting rid of the chrome for a bit more than the area of the hole would suffice. Not sure about thompson shafting, or induction hardened. For the latter, it would seem that I'd need to grind away a deeper divot so the drill bit would only be into the softer material under the hard surface. Not sure how much depth to remove for this material- would say 60 thou be about right?

10-12-2014, 02:23 AM
Hi darryl
Can't help you with how far you have to grind, but if you are drilling through, you might want to grind a second flat on the opposite side or you'll hit more
hard material when you exit. Also, you'll want to grind a little deeper than the thickness of the hardened region to account for the fact that it's an arc.

10-12-2014, 02:32 AM
In the hydraulics field there are two common shaft types: CPO (chrome and polish only) or IHCP (induction hardened and chrome plated). Both can be found in better grades of material like 4140 but the commonly used stuff is usually just 1045--very nice to machine. CPO is pretty easy to work with; grind off the chrome and you're good to go. IHCP is usually (very) hard to a depth of about 1/8"...

10-12-2014, 08:43 AM
Don't know if it is still that way, or if all varieties are the same, but attempting to make foosball table rods out of Thompson rod in the 80's did not lead to any success.

The undrillable "case" extended 1/3 - 1/2 of the distance to the centerline of the shaft.


10-12-2014, 02:48 PM
Thanks- there's some good guidelines. The Thompson rod I end drilled was tough to drill and tap, but I did get through it. There were three holes in the end of each one, and I marked the center of those about 3/8 inch from the OD of the rod (these were 2 inch diameter rods). The holes were tapped 3/8, so it did come pretty close to the OD surface.

For these current projects I could probably get away with JB welding some threaded studs into drilled holes so I wouldn't necessarily have to tap the holes.

I shall have to ask again about the grade of material. CPO in 4140 sounds about ideal.

10-12-2014, 05:40 PM
Thompson shafting is case harden 1/8 inch deep per side . and you can bet your life on it. This id for 1 inch and 1 1/4 shaft . The smaller stuff may be less.We used to machine a lot of it at a shop I worked in.As far as drilling and taping holes thru the side we would drill down with a piece of a carbide end mill 1/8 deep then use a reg drill to depth then tap. Once you are thru the case ,no problem the material is soft on the inside.

10-12-2014, 07:35 PM
Can you cut/grind (what ever) off an end so you have a short slug ?
Then you can check or get a rough idea.

10-12-2014, 07:51 PM
That's an idea- I could cut off a piece and do some testing on it. But I think I can just go with some of the recommendations here and grind deeper if a shallow grind doesn't get me there.

These will not be through holes, so tapping will be blind. Most likely I'll be using the same diameter rods I did before, which is 2 inch. That will be lots for a pilot hole and a subsequent tapping to allow perhaps 1/2 inch of bolt to thread in.