drilling/tapping in cast iron

[Jaroslav]

Hi All,
I'm installing my new Vega TS fence, as I have mentioned in the "Before I Buy" forum and I ran into a spot where I needed some help. Most of the information I got about the Vega claimed that it was already predrilled to fit all Sears saws made after 1955, and this appears to have been true for the front rail, but there are actually no holes on the rear of my TS table top. Thus, I will have to do some drilling in cast iron after all. I have two newbie questions about this:

1) Will this damage a drill bit that I might want to use later on wood in any way? Do I need to get a special bit for drilling metal?

2) The instructions say that if I have to drill holes in my table, I should "drill 17/64 and tap 5/16-18." What does this mean in English?

Thanks,
John

--------------------------

"The law, in its majestic equality, forbids the rich as well as the poor to sleep under bridges, to beg in the streets, and to steal bread." - Anatole France, 1894

"I like a woman with a head on her shoulders. I hate necks." - Steve Martin

[dcarter636]

For drilling iron a HSS (high speed steel) drill bit will work fine as long as you keep the tip wet with a few drops of just about any type of oil, motor oil works well. It is very important to keep the drill tip moist/wet.
The bolt thread is 5/16" diameter and has 18 threads per inch 5/16-18 UNC is the standard designation. A thread cutting tap and tap handle can be purchased at any hardware store. Same deal with the tap that will cut the thread into the 17/64" hole, keep it wet or at least moist with oil. When cutting threads in metal it is important to start the tap visually perpendicular/square and turn it clockwise two or three turns and then turn it CCW about a turn to break up the metal chips and prevent metal chips from clogging the tap. when the tap spins freely the work is done, and you clean chips from the hole and surrounds and assemble your fence.

[chasethedog]

Also, when drilling steel/cast iron keep the bit turning only as fast as necessary to keep it cutting smoothly without grabbing. Use the slower speed of a 2 speed drill. 650-850rpm is plenty.

Keep it realtively cool and lubricated and maintain enough pressure to keep it cutting. Spin it too fast without cutting and you'll just smoke the end of the bit.

Most of my HSS bits that have been used almost exclusively for steel are 20yrs old. I just keep them sharpened as needed.

Alternatively you could buy 4 or 5 bits and just 'sacrafice' each to a single use...

Chase

[bmorto1504]

One unanswered question is about the 17/32 drill bit. If you take a 5/16 diameter bolt and hold it so that you are looking at the side of the threaded part you will see the thread is “V” shaped. The deep part of all of the Vs will represent a cylinder that is the Minor Diameter of the thread form. Now take a 5/16 nut and look into the hole. You are looking at the minor diameter. So you see, if you drill a 17/64 hole you will have developed the cylindrical shape in the object that will allow the balance of the thread form to be cut by the tap. We have a standard the describes every size of thread in exact detail. Every thread size (1/4, 5/16, 1/2 and so on) has a recommended Tap Drill Size, in your case the standard hole required for a 5/16 - 18 is 17/32. The UNC note means Unified National Course. There is a Unified National Fine series thread as well . Having made a mess of that I will continue with getting the threaded hole in you machine. You may want to assemble everything you can to locate the new parts as carefully as you can. The bolt hole you are going to fasten can be a handy guide to find the exact location to drill the Tap Drill Size Hole for the new thread. With the new part in place find a drill bit that fits the new parts hole with no slop (side to side movement) It will be silghtly bigger than 5/16. Now firmly rap the end of the drill bit while it is in the hole. Rotate the drill bit 15 or 20 degrees and rap it again. Repeat as necessary. This activity will mark the hole location in the machine. Remove the new part, find the intersection of the little marks. Place a Center Punch point in the center of the marks an give it a good rap with the hammer. Now use your Tap Drill holding it as square to the surface a humanly possible drill the hole. Now you are ready to tap the hole carefully holding it square as well. The metal you remove will be somewhat like powder, that is normal. If the hole is blind (not drilled into the open air) you will need to back the tap our at some point and while protecting you eyes (you will have safety glasses on for all of the process) blow the chips out of the hole. Restart the tap to finish the job. OK I tried to help.
bmorto

[woodmannie]

Hi,
Just adding, As you tap the hole, turn the tap 1/2 turn in, 1/4 turn back.It keeps the tap from jamming and won't strip as quick.Once you learn how easy it is, you'll tap any bolt hole you find,lol. I just retapped the bolt holes in my ford 460 head. The new bolts sailed in.
Ok, I'll stop on Fords before I start.
wm

[Original Bart]

I find it interesting the recommendation to keep the drill wet or lubricated. When I went to high school & college machine shop they taught us for drilling cast iron on light duty type work like this cast iron is self lubricating. I've never ever had a problem drilling cast iron dry in the last 30 years.The drill bit just zips right through it. Did they change the composition of cast iron in some way that I'm not aware of? Its not very hard to thread either.

This is an excerpt from one of my machine shop books.

The machine ability of gray cast iron is generally quite good because of the presence of near continuous graphite flakes in the microstructure. The flakes’ presence promotes chip formation as well as lubrication during the machining operation.

[bmorto1504]

During high speed machining of cast iron, coolant is used. The coolant is applied in volume to keep the cutting tools cool and keep
the cast iron dust down. The coolant is water based synthetic or
water soluble oil. Many people machine cast iron dry, using carbide
cutters or during low production runs. Taps feel a little better if one uses something like Tap Magic (brand). I would not use lubricants
or motor oil they are after all designed to lubricate. Thread cutting
cutting oils are available if you wish, usually Sulfur types and are good for general threading jobs including steel. Dry would be better for home shops.

Generally it is not necessary to turn a tap in reverse while taping
cast because the process generates powder like shavings. Turning a
tap in reverse in steel will help break the chip in small bits if you
feel it is necessary. But, it should only be reversed until the edge of the flute hits the chip. If turned more the bump formed in the thread by the chip separation can prematurely dull the cutting edge tap.

Some taps are designated as chip drivers. They are designed to
produce a tightly curled shaving that will move out of the open end
of the Tap Drill size hold. They have a rather long lead on the end
and are not normally found outside a machine shop. They are easier to get lined up and started in the hole.
bmorto

[Jaroslav]

Wow, this is so much great information, thanks everybody! I'm somewhat embarassed to say that I ended up drilling slightly larger than the bolt and just using a nut like the set up with the front rail. Either way, this is all advice that will be useful to me in the future.
-John

--------------------------

"The law, in its majestic equality, forbids the rich as well as the poor to sleep under bridges, to beg in the streets, and to steal bread." - Anatole France, 1894

"I like a woman with a head on her shoulders. I hate necks." - Steve Martin

[DickLaxt 2]

testing
Dickie
A joker in the deck of life

[TDHofstetter]

Yup, ya passed.

Me, I'd do cast iron dry - just like brass (although whole milk is a good lubricant for copper). Steel really wants a lube, but cast iron's good about being worked dry.

-- Tim --

A novice has many options,
An expert, only a few.