Machinists: Drilling / tapping cast iron

[Brians356]

This was started on the reloading forum, was advised to move it here. I am not a machinist!

I want to enlarge (drill out) and tap the existing center hole in a cast iron drill press table. The current hole is 0.750" in diameter, I want to drill it out and tap it for a 7/8"-14 threads. (Yep, I plan to mount standard dies in the hole.) I need the larger hole on the same center (so that my drill press will align with it)

I have the plug tap for the job (carbide) but I don't have a drill bit, and frankly won't be able to drill it myself using my drill press - it's not nearly stout enough.

From tap / drill tables, I glean that 53/64" would be the new hole size, but 13/16" might work.

I took the table and tap to a local ironworks I have used before for such jobs, but he didn't have either 13/16" or 53/64" bit. He said if I bring him a bit, he will drill it out for me.

Than I started shopping for a bit, and found there are many configurations and qualities in this bit size.

Questions:

Will the carbide 7/8"-14 tap work ok in cast iron?

Which drill bit size do I need for the 7/8"-14 tap?

Which bit configuration will the large press at the shop likely require?

Do I need an HSS bit, or just a carbide?

Is there any likelihood the larger bit will wander and change the hole's center?

Any advise or tips appreciated.

 

[dalewoolumjr.]

Well 75% thread tap drill is 13/16" if your hole is already in the correct place then use a reamer to open up the 1/16 you need. If you want to buy a drill that will always fit buy a silver and deming drill. More than likely the shop will burn the corners of the twist drill. You will also be able to run this in your drill press. A better option would be to use a 3 or 4 flute core drill. These are made specifically to open up holes. Carbide is not needed for cast ductile iron.

Dale

 

[jonbearman]

If you turn the bit slower than normal it shouldn't dull or burn it up. A core drill will be a bit more money. How thick are you drilling through?

 

[Brians356]

The plate is 0.4" thick.

I have a Craftsman 1/2-hp, 6-amp, 1/2" chuck. I can only drop it down to 540 rpm.

 

[Gundoktr]

If I understand this, you are wanting the hole in the table to be in line (concentric) with the axis of the spindle? Fat chance! Seriously, I doubt this is do-able. Your may be able to drill and tap but the end result probably won't end up the way you want it. I think you would be better off making a separate piece and clamping it to the table where you could manually align it.

 

[fguffey]

I am sure I am missing something but If I wanted to drill and tap a hole that was aligned in the drill press table I would leave the existing hole as it is and rotate the table or drill head then level the table, then drill a hole and call it centered. If you took the table to a machine shop ask them to drill a perpendicular hole through the table then tap. A machine shop could use a boring bar to the exact diameter for threading.

Then for more money they could make an alignment tool when aligning the drill head with the hole. I have used a lot of portable equipment seems in the old days it was easier to carry a box of tools to the machine than it was to haul the machine back to the shop for repair.

F. Guffey

I made an adapter for an arbor press. I bolted the adapter plate to the arbor press and used C&H shell holders with 1/2 stud installed in the arbor. I also cut holes in plates that hold barrels for the purpose of forming cases to the chamber. Never forget to lube the case.

F. Guffey

 

[Brians356]

If I understand this, you are wanting the hole in the table to be in line (concentric) with the axis of the spindle? Fat chance! Seriously, I doubt this is do-able. Your may be able to drill and tap but the end result probably won't end up the way you want it. I think you would be better off making a separate piece and clamping it to the table where you could manually align it.

It'll be close enough for what I need.

 

[Brians356]

Thanks all. Several good tips and ideas, I will assimilate and proceed from here.

 

[Dusty Stevens]

how bout a boring head?

 

[fguffey]

The plate is 0.4" thick.

That would be 3 1/4 threads engaged, You could drill a straight hole through the table then use a nut on the bottom to hold the die if the bottom of the table is parallel with the top. Most tables have webs for strength.

F. Guffey

 

[TAJ45]

I'm curious about the tolerances you are wanting to hold. You say your drill press is pretty light duty, low speed of 540rpm.......this tends to suggest to me that the quill and spindle are on the low end of production tolerances. I can envision an easy .010" of movement.

It also follows that the further the quill has to be extended, the run out could increase. Will this be acceptable? Using a mandrel for neck turning? I'm thinking that sweeping with an indicator may be a good thing to see what is what.

This could get interesting, the results that is. Not having used a drill press of any stripe for neck turning, I'm just asking how well a neck turning op will go. If it works, it works.

I'm guessing you tube is next for me then sift the shade tree from the quality.
Interesting it is.

 

[Road_Clam]

Thanks all. Several good tips and ideas, I will assimilate and proceed from here.

The biggest challenge will not be enlarging the existing hole, but getting the tap started perpendicular(which in the case of you wanting to insert a reloading die in the hole sounds like it's critical). That's a big UNC tap for the home hobbist to use. I drill and tap the big stuff every day for a living I suggest grinding a small 3/16" piece of rod to have a point, and chuck it in the drill and use that as a "follower" inserted in the small countersink hole at the square drive end of the tap. Keep downward pressure on the tap and use a wrench to turn the tap. How big is your drill table ? You won't need a carbide drill as cast iron is a relatively soft material. You will however need to run the drill slowly if you are opening up a preexisting hole. Running a drill to fast in cast iron will "surface harden" the material, then you are screwed and you will need carbide at that point. As an option, ship me your table I will drill and tap your table as a member favor no charge (as long as I have that UNF 7/8-14 tpi tap at my work, I mainly work with 7/8-9 UNC threads)

 

[Brians356]

The bottom of the table is webbed, with a boss around the center hole.

I chucked up a 3/4" forstner bit and carefully lowered the bit through the 3/4" hole, allowing table to swing and self-center, and it goes right through with no binding, so the hole is under the chuck WRT distance from the support column's centerline.

Re: aligning the tap, I will rig up an alignment rod in the chuck, then it's just a matter of getting the required down pressure, nut sure what to expect.

So, if I do this myself, my main decision is, given the minimum 540 rpm speed, which 13/16" bit to use for enlarging the hole using the drill press itself. It needs to tend to self-center and not work harden the iron at that speed.

Road_Clam, where do you live? I would let you just keep the tap I send along with it.

 

[Road_Clam]

540 rpm is way too fast, you are going to get drill "chatter" especially attempting to open up a hole already pre drilled. A 13/16 drill should be run at about 150 rpm max with some cutting oil to help. Do you have a single speed drill ? Can you slow down the rpm ? Sounds like you have a drill set up more for woodworking than metal..

 

[Brians356]

540 rpm is way too fast, you are going to get drill "chatter" especially attempting to open up a hole already pre drilled. A 13/16 drill should be run at about 150 rpm max with some cutting oil to help. Do you have a single speed drill ? Can you slow down the rpm ? Sounds like you have a drill set up more for woodworking than metal..

It is a single speed motor, if that's what you mean. Speed adjusted by moving the belt to different pulleys, range is 540 - 3600 rpm. What's a high-speed twist drill for?

Someone mentioned a reamer. Could I ream this out 1/16" by hand?

 

[shortgrass]

SFM x 12 divided by (tool dia. x 3.1416) = RPM cutting speed for cast iron ranges from 140 to 200 (thats SFM) Still,, you're not dealing with a ridgid set-up. With the weak' set-up, I'd make a separate attachment that bolts to the table, like GunDoktr suggested (or buy a more ridgid drill press). The 'traditional' cutting oil for cast iron is lard oil, not cutting oil.

 

[Rustystud]

Remove the drill press table. Mount it on a lathe face plate and align it then bore it with a boring bar. Then thread it in the same set up. If your having problems with the material make an insert that is threaded and fit it into a horizontally mounted cat head.
Nat Lambeth

 

[akajun]

Are you doing this to modify cases using a die or to modify dies? If so you are going about this all wrong. When using a drill presss you want the work to " float". Very rarely do you want the work clamped down. Not only that, you are assuming that the hole you are using is indicated to your spindle, and I can 100% guarantee that it is not.
What you should do is to take a piece of heavy walled pipe that is tall enough to put your die in and keep it off the table, weld a piece of 3/8 or 1/2 on top of it, take it back to the machine shop and have them square it up on both ends, then center drill it and tap it for 7/8 x 14. Use this to hold the die while you perform your operation and it allows the workpiece to float and self center.
What you want to do rigidly requires a very heavy duty radial arm drill, or a mill, and the use of indicators and clamping fixtures. A standard shop drill press is too light for that precision work.

 

[Concha]

Hone the hole in your die. Use telescoping gauges, small bore mics or a dial bore gauge to ensure straightness. This is not a job for a drill press.

 

[Erik Cortina]

540 rpm is way too fast, you are going to get drill "chatter" especially attempting to open up a hole already pre drilled. A 13/16 drill should be run at about 150 rpm max with some cutting oil to help. Do you have a single speed drill ? Can you slow down the rpm ? Sounds like you have a drill set up more for woodworking than metal..

It is a single speed motor, if that's what you mean. Speed adjusted by moving the belt to different pulleys, range is 540 - 3600 rpm. What's a high-speed twist drill for?

Someone mentioned a reamer. Could I ream this out 1/16" by hand?

It appears your are confused by the "High Speed" name on drill bits. High speed drills are made of high speed steel, but the name "high speed" was from the old days as compared to hardened carbon steel. High speed steel tools are not made to be run at high speed (confusing, I know). To run at high speed you need carbide. However, to run carbide tools you need a ridgid setup.

Tell us your intended purpose and I bet we can help you with alternative options.