Hot rasping and tap & die work.

[divermike]

while reviewing the curriculums of a few organizations journeymans requirements, I noted 2 very specific areas that I had not been exposed to, one was hot rasping, and the other was tap and die. While in Mark Aspery's class, I specifically asked for and recieved a complete tutaledge in the craft of hot rasping. I had no idea why many smiths work was so nice to view, and it soon became apparent this was a way to really clean up your work. I believe I have a good handle on this operation, now I have to dabble in tap and die stuff, and don't really know where to start, any suggestions on purchasing and operation would be most welcome.

[Charlotte]

Tap and Die info is found in most machine shop intro text books. Or you can look at something like Audel's Machinest library, or Millwrights and mechanics guide,

Tap and die works is mostly knowing that there are number and letter drills for various sizes of threads. This information is available on tables. Blacksmiths have special problems with hand made work because the metal need to be clean and square to start a thread correctly. The hardest part, (for me), is getting the tap or die started squarely.
If it is not square it won't cut the thread to the length you need. Sizeing, measuring, and aligning are the keys to tap and die work.

Making dies as in a die maker is a whole other realm. Smiths make dies fairly often for their power hammers. But Precison dies are very special study.

[Sam Thompson]

Don't buy cheap taps/dies they will just snap.

[Bob JS]

When I do threads on things I support the tap, or the die in the jaws of either my lathe or drill press. Not tightened and not using the power, but simply to maintain good alignment. A trick my tech teacher taught me when I was a school.

If tapping into an uneven or rough surface I would drill a countersink first, so there is clean even material for the tap to start on.

Just a trick I discovered myself - maybe there is a better way but it has worked for me.

Uisng machinists tables is easy. Using a cutting lube really helps, and remeber two turns forward, one turn back to keep the cutter free from swarf.

Edited June 8, 2009 by Bob JS

[Sam Thompson]

If tapping into an uneven or rough surface I would drill a countersink first, so there is clean even material for the tap to start on.

Just a trick I discovered myself - maybe there is a better way but it has worked for me.

Spot facing is better.
It's also a good idea to anneal the work before any kind of machining operation, this avoids hard spots etc.
If you can avoid it never tap into a blind hole. Plug taps are just more expense. You can usually get away just using an inter.

[John B]

If you want to hold your tap square to the piece being tapped, drill a hole the same size as the tap you are using in a reasonably thick block of metal on your drill press, this will ensure it being 90 degrees to the face, the block can then be clamped over the hole, or merely held securely over the hole until the tap is biting squarely, the block can then be removed and the threading completed,

You can also drill a number of different size holes in the block to accommodate various taps you will be using.

Taps should come in sets of three, a lead tap (large taper on end to allow starting the thread)

a second cut tap (smaller lead on first few threads)

A plug or bottoming tap with very little chamfer on the first thread)

If you always use a tap wrench rather than try and use grips then it is easier to keep the threads square

Edited June 8, 2009 by John B
added data

[Iron Falcon 72]

Spot facing is better.
It's also a good idea to anneal the work before any kind of machining operation, this avoids hard spots etc.
If you can avoid it never tap into a blind hole. Plug taps are just more expense. You can usually get away just using an inter.

What is spot facing?

[Sam Thompson]

Using an end mill to cut a circular flat area:

Countersinking, Counterboring, and Spotfacing - 4: Spotfacing

[arftist]

One thing I would like to add to this discusion is theoretical percentage of thread. Most taps and tapping charts list the drill bit size for 70-75% thread. In many cases, this is more thread than needed, increasing the difficulty of the job as well as the likelyness of breaking the tap. Most jobs will be strong enough with 50% thread. A chart which lists all possible bit sizes and the percentage of thread which they yeild is very important to me. Additionaly, enough can't be said about using a high quality cutting fluid for tap and die work.

[jimbob]

One thing I would like to add to this discusion is theoretical percentage of thread. Most taps and tapping charts list the drill bit size for 70-75% thread. In many cases, this is more thread than needed, increasing the difficulty of the job as well as the likelyness of breaking the tap. Most jobs will be strong enough with 50% thread. A chart which lists all possible bit sizes and the percentage of thread which they yeild is very important to me. Additionaly, enough can't be said about using a high quality cutting fluid for tap and die work.

why not just use the drill size for the fine thread for the course thread instead of the larger bit

[jimbob]

One thing I would like to add to this discusion is theoretical percentage of thread. Most taps and tapping charts list the drill bit size for 70-75% thread. In many cases, this is more thread than needed, increasing the difficulty of the job as well as the likelyness of breaking the tap. Most jobs will be strong enough with 50% thread. A chart which lists all possible bit sizes and the percentage of thread which they yeild is very important to me. Additionaly, enough can't be said about using a high quality cutting fluid for tap and die work.

why not just use the drill size for the fine thread for the course thread

[arftist]

why not just use the drill size for the fine thread for the course thread instead of the larger bit

It's not that simple. The main factors which determine desired percentage of thread are:
1. Material strength. Soft Aluminum needs a higher percentage of thread than mild steel.
2. Length of thread engagement. 1/4'' thick plate needs a higher percentage of thread than 1'' plate.
3. Difficulty of tapping. No point using 75% when 50% would be more than adequate, or in more extreme terms, you may not even be able to tap the hole without the tap wearing out and or breaking. Small taps don't last that long anyway. Especialy in hard stainless or similar metals, would you rather get 6 holes per tap or ten? On the other hand, very large taps can be very difficult to turn, why waste all your strength?


So, if I were to tap 3/8"hard aluminum for a 3/8 bolt, I would use 80% thread.
For 1'' hard aluminum, I would use 60% thread.
For 1'' s.s., I would use 40% thread.
For 3/8 mild steel I would use 50% thread.
For 1/4 " brass I would use 90% thread. See what I am saying?

[ifitsmetal]

is to have a sharp tap. Being the thrifty type people that we blacksmiths are, we try to get the most life out of any tool. A dull tap can ruin your day and sometimes the whole job as usually it will break off at the very end of a long drawn out affair. So stay sharp.

[Ten Hammers]

Couple things I will add. Hotfiles are indeed very handy. Sometimes really the only smart way to remove some stock from a piece. Smoothing up a piece is handy in this fashion as well. A good stiff butchers brush is handy as well.

Tap and die work is indeed very handy as well. I waited 30+ years to finally invest a " C " note in a set (thats a hundred bucks for those out of the USA ). 1/4 to 1/2 taps and dies, coarse and fine and also a pipe tap or 2. The drill press is indeed a nice place to start square ( small upside down bit fits the end of the tap to help press down to get centered). A small counterbore is helpful sometimes too. There are percentages of thread listed for holding power. My drill index lists many drill sizes for given taps.

Dies ( the female threaded part for making bolts or studs or whatever male threaded part you need ) work best on cold rolled stock but can be used on hotrolled. Millscale or forge scale is tough on either ( tap or die ). Good quality cutting oil is the key and also as mentioned, cut(foreward) and back off to clean and remove chips or scale. A good blow gun and compressed air and more oil and half turn down, half back is the way it may go sometimes until you get down a bit. Patience is also the key. I always blow the tool off afterwards and oil with the cutting oil a bit before stowing the tool back in the box. I have many taps in a box that I formerly used over the years for given needs. Replace as necessary. I bought the set I guess a year ago and the other box (of loose taps and dies and handles) is unused for the large part. Having the set is handy for me. I was always waiting to get a set from #4 to 1" which was gonna be spendy. the set I bought has #4 to 1/2 and if I need others I have just bought them. Many things have been easier to build just by selecting to thread rather than weld or rivet. My 5

[Fe-Wood]

and did someone mention- Don't buy cheep taps... Sets are nice because they give you organized storage for the long term.
As far as cutting and clearing turns, on small taps like 10-24 etc. don't get to agresive on the cut turn 18 to 1/4 turn is about all I do... As the taps get bigger, so can the cut turns.

[Ten Hammers]

Small taps do indeed break easily. This said I have used impacts to chase threads (3/4 x 10) on farm tractor hubs prior to mounting dual wheels. 1/2 drive impact set to the weakest hammer stroke. These threads were factory but had been allowed to set without bolts or grease for couple years. I was handed a bottom tap and told to chase the threads. I was thinking someone needed their medication checked. 12 holes roughly 1 1/2" deep. I asked was there a taper tap available as well. Reply was "probobly". The owner then left and I was at the task at hand. Marvel Mystery oil in a spray bottle, compressed air (for the impact and also to blow holes out), a 1/2 drive socket (12 point ) that fit the end of the taps (size escapes me at the moment) and a stool to set on. There is a counterdrill to start the bolts so could start the tap ok. Soak the holes, allow to set 1/2 hour, blow the nasties out. Soak again and begin. Drive the taper tap, back off (repeat ad infinitum until the bottom found). This may or may not be achieved in one session and need to go on to the next hole to let the prior one soak a while. After the taper, the bottom tap used. Same game on the procedure. Duals mounted. Now the duals are off and the funny thing is, the holes are still open. Bolts in the holes with anti-sneeze would solve the issue. BTW I no longer work for this idiot but am greatful for the work he gave me. I seriously doubt that he knows I used an impact on the taps. The taps are still in pretty good shape (they were not new when I started). Brushed up stick welding skills making ice cream out of horse manure. Life goes on.