Tapping and threading cast iron????

[shinnlinger]

Hi,

I would like to tap and thread some cast iron lathe tail stocks in my high school shop.  I read that you drill something like 13/64 for a 1/4 tap and then 2 turns in 1/2 turn out and use lots of oil.  These are completly new holes for a little experiment, not rethreading.

At the same time, I bet some one here has more precise advice that might help me out.  I think they will be blind holes so I think I need to get a special tap and put grease in the hole before tapping.

Thanks for any pointers, I can drill the holes on a drill press, what speed would you reccomend?  Slow?

Thanks.

Dave

[1938farmall]

13/64 is the standard tap drill for 1/4-20 thread.  drill speed not important as cast iron cuts easily & won't work harden.  it can be worked dry without lube but some tap-ease would make tapping easier.  use a regular taper tap and tap to the bottom of the blind hole - then switch to a bottoming tap to finish up if needed.  if you can stand a larger hole size, a 5/16 tap is much less likely to break. al

[MtnDon]

Dry holes. Production cutting will use a soapy lube, not oil.  The cuttings crumble and may make dust. It is best not to blow it out of the hole but to vacuum it. If you tap cast iron a lot there are special taps made with less hook. They cut nicer in cast and may be nitride treated to stay sharp longer. There are also solid carbide taps that are great in cast iron.

[CX3]

Sounds like some good tips there already.  All I could add is drill the hole a little deeper than you need as long as there is plenty of room to do so.  There is also nothing wrong with taking the tap all the way out and cleaning it off once in a while, makes for much easier tapping.  Good luck.  Also I read that you said 2 turns in and half turn out.  That sounds like a lot of turns in to me.  I like to do about half turn or 3/4 turn at most, especially with the smaller size taps like 1/4inch

[smalldog]

The correct tap drill would be a #7 drill. Most people do not have number drill sets so they go with the 13/64 drill because it is only slightly larger. If you are manufacturing new parts in a machine shop, most times the print calls out the #7. It is always nice to have a can of cutting oil in your shop. It's better than plain oil and usually contains more sulpher.

[Bandmill Bandit]

Also make sure you are using a high quality tap and not some Chinese knock off.

Just a suggestion from experience

I use heli coils in all cast and aluminum when I build or modify where new threaded holes are required. Haven't had to do a repair on any holes I have done that way in over 20 years of doing it. If you can get at least 5/16ths don't go smaller. if possible go up to 3/8ths. Where possible when i can't drill through, I will drill and 1/8th inch hole into the side at the bottom of the threaded hole to eliminate the problem of hydraulic fracture as a result of a screwing a bolt into a wet hole. ie anti seize compound and other liquids. I have on occasion, drilled 2 extra 1/8 inch holes beside the main hole so that the drilling of the main hole will cut away half of the small holes, leaving a notch for air/liquid to escape BEFORE I drill the main hole and thread it. But only on bolts  5/8ths and over and deep enough to take 2 Heli Coils    

[Slabs]

You didn't say if it was coarse(1/4-20) or fine threads(1/4-28).  The BOOK says #7 for 20 TPI and #3 for 28 TPI.  ALWAYS use tapping oil/lube for threading, unless you have only a few to do and the tapping fluids aren't available.  Then use oil.

Cast iron breaks it's own chips so reversals isn't necessary but take care that the chips don't accumulate in a blind hole and get compressed there by the tap or bolt..

[shinnlinger]

The holes are course of course...  would fine thread work in cast iron?  I actually just drilled my first hole and it seems to be a success.  Wether my experiment works remains to be seen but the thread part went fine.  Pretty easy actually. 

THANKS!!!!

Dave

[shinnlinger]

Here is what I did with your info.  I drilled the tailstock on my high school woodshop lathes and inserted a cutoff 1/4 -20 bolt so the handle that tightens them can't be loosened enough so that entire cylinder spins.  I suppose it also makes it less likely that the handles will "disappear" also.  What was happening was they would get over loosened and the cylinder would spin and then the wouldn't tighten the stock enough (unsafe) or when the handle was tightened after the cylinder had spun some, it was not in the groove and it would damage the cylinder.

Hopefully this will work, did it to 5 JET lathes.  Poor design really as there should be a pin that prevents the spinning in the first place.

Thanks again folks.

Dave

[beenthere]

Apparently a good fix, but would be great to have full size pics.

Then we could see better what you did.

[metalspinner]

I suppose it also makes it less likely that the handles will "disappear" also.  What was happening was they would get over loosened and the cylinder would spin and then the wouldn't tighten the stock enough (unsafe) or when the handle was tightened after the cylinder had spun some, it was not in the groove and it would damage the cylinder.

Hopefully this will work, did it to 5 JET lathes.  Poor design really as there should be a pin that prevents the spinning in the first place.

That is a good safety upgrade, shinnlinger.

While teaching a pen class once, a student was snugging the tailstock quill against the spinning work.  The quill lock was not engaged in the groove and the wheel turned in his hand. The palm of his hand was opened up pretty bad and he was taken to the ER for stitches.  This particular guy is one of those walking accidents and anything that could possibly go wrong always did with him. :-\

[Al_Smith]

A little tip here .On any 60 degree threads be it metric or SAE you deduct one thread spacing from the bolt size to get the tap drill size .

On 1/4" 20 the thread spacing is 50 thou [1/20 ] so the hole is .200 and a  number 7 drill is .207 . Say M6 -1 the hole is M5 .Metrics are given in thread spacing ,SAE in threads per inch .Trivia .

On steel the thread depth is about the thickness of the bolt -1/2" bolt ,1/2" deep .Cast iron is double and aluminum is 4 times in order to not get thread failure in the hole .A properly  torqued fastener is less prone to failure than an under torqued one BTW .

On cast probably lard oil or regular sulferated cutting oil works well .The old black smelly stuff .Use either a taper or spiral point tap and finish it off with a bottoming tap if it's a blind hole .Blow the chips out of the hole before you run that bottoming tap through it else you'll have the broken tap in the hole and that gets into another subject --How to remove a broken tap .