Lathe - Tapping



Tapping in the lathe (Mar 13, 2005)  
Tapping in the lathe
Would someone please explain the correct way to sink a large diameter tap in the lathe. Its a 7/8 inch x 20 tpi H4 tap, and the material to be tapped is carbon steel. Mark (26026)
Do you have the hole drilled yet? My charts call for a 20.5mm tap drill for 75% thread. The helical taps are really nice, and clear well. I usually go for 66% thread to make my life easier. I do rough work. Once the hole is drilled, replace the chuck in the tailstock with a center. Put the nose of the tap into the hole and the dimple in the end of the tap handle on the center. Lube it up well. You can turn by hand, or put the lathe in back gear and run it slow. An old timer I know swears that powered tapping results in less broken taps. IMHO the helical taps do not need to be cleared like the straight taps, so you can move right along. Keep the center following the tap handle to keep the tap straight. Keep a hand near the switch for powered tapping. If the hole is blind, use a bottoming tap. That way, if you bottom it out you'll have less problem. Mike (26028)
Mike, how would you hold the tap to rotate it for hand tapping or machine tapping. And why not use the recommended pilot hole size to get 100% thread depth? Is any light oil ok for a lubricant? Mark (26042)
When I tap on the lathe I will often chuck the tap in the tailstock. I will turn the spindle with one hand and push on the back of the tailstock with the other (tailstock nut not tight) This will put enough pressure to get the tap, tapping and not bind to break the tap. works for me not real fast but it gets the job done and the threads are very good. (26047)
That makes sense, but how do you "chuck" that large a tap (tap shaft is about 0.75 inch)? My Jacobs won't go that large? Mark (26050)
If the tap won't fit in your Jacobs, use the previous responder's suggestion: use a tap holder (or a wrench on the tap) and using a center in the tailstock, put the center to the pip in the end of the tap (or holder) and tap away. My Sterret tap holders have the center hole that I use and it works well. George (26051)
There should be a center drill point on a tap that big. Put a live or dead center in the tail stock. You can use a crescent wrench or a tap holding bar (not the correct name). You can power tap, with the lathe on very slow. I suggest you use manual tap though for a while. Yes, turn the wrench or tap wrench by hand. Also, from the previous post, you drill a hole big enough for about 60% thread engagement. You will have about 80% (or more) thread strength. Increasing the thread engagement doesn't add much strength and makes tapping harder. That will increase tap breakage and the force needed to tap. Usually, when I am hand tapping on the lathe, I will put the center in the tail stock, use a tap wrench and have one of its handles rest on the saddle, to keep it from turning. I turn the chuck with the chuck wrench. I turn the handle of the tailstock to keep the tap aligned. The tap will advance or move forward as it taps the hole. So you must take up this slack by advancing the center in the tailstock with the tailstock handle. Tom (26052)
I don't go for full thread since it makes the cutting harder and more likely to break the tap. Unless you are doing really high class work, full thread depth is not necessary. For instance, I drill my 1/4 20 holes with a #4. I usually just grab the handle of the tap with my hand to hold it still. Have the handle in a position that will clear the ways as the tap rotates. (Usually middle detent.) If it starts to cut hard, I can release it, turn the lathe off, back out the tap and go again. It gives a better "feel" for the cutting action to hold it by hand. (This may not be OSHA approved.) The back gear on mine gives plenty of time to react. I usually use cutting oil since the can is handy. High sulfur, but it's really made fast cutting. There is special thin tapping oil, but any light oil should do fine. Try some practice cuts. Mike (26055)
One of the first exercises I did was make a spring loaded tapping center, a 1/2" body about 2" long drilled and bored to take a sliding shouldered center, the sliding center extends from the front of the body by about 3/8" and is pushed forward by a strong spring in the body and and an end screwed to the back of the body. To use it mount it in the tailstock chuck, if the tap has a center hole use that, if not use a tap holder. You will find it a big help as I find any coarse thread over 3/8 is difficult to turn while holding pressure with just a tailstock center. For holes 1/2" and over it depends on what degree of concentricity you want and whether you have the full 3 taps. If its critical I screw cut to about half the required depth and then use a tap. If its big, and I consider yours is big, I again start by screw cutting. Without a 1st and 2nd taper I doubt if you could even cut the thread you want by hand, and without a proper tapping head I would be surprised if you could get better than about 5 degrees of alignment. Be extremely careful if you try power threading, its essential that the end of the tap is firmly supported at all times, without that support there are very large forces which are not in the axial plane and things, and body parts, get broken if you loose that support for even the shortest time. I've used tap magic for years, but a lot of people use normal soluble cutting oil. Bernard R (26058)
Class 1 fit= loose class 2 fit = standard hardware class 3 fit = in the neighborhood of a lock nut. class 4 fit = to the moon and back never come out interference fit. Close to 100% thd. 7/8-20 tap 60 % thd. .8662 hole 72 % thd. .8653 hole 80 % thd. .8642 hole 90 % thd .8628 hole If it were I: Drill then bore to the size I wanted. Single point rough thd. just shy of finish. Run the tap in to finish the thread via center in tailstock and a large wrench with plenty of black oil. Bruce (26064)
Machinists Ready Reference lists the tap drill size for approximately 75% as .8281 for 7/8X20 and this makes sense given the double thread depth of .065 (26106)
Thanks, I hadn't considered the tapping under power idea at all until I saw it done in a commercial shop last year. Even then I hadn't understood how to keep the tap from getting away from me if I was just using a center in the tailstock for alignment, via the small "pip" in the back of the tap. Bernard R's idea of a spring loaded center for this really makes sense to me now. I am still wondering what it was I saw in the commercial shop a while ago. The machinist mounted something into the tailstock that was used to align and keep the tap from turning. ? Seem to remember this device came out of a drawer with many similar items, perhaps it was not a commercially available tool? Mark (26131)
There are commercial tapping heads, fairly expensive and mainly used on milling machines. Also you can use a broached square holder, taps we use there are a limited number of standard size shanks. If you do a lot of lathe tapping a square holder makes sense as the standard tap wrench only grips a relatively small portion of the tap and the overall length usually means that there is a greater or lesser degree of misalignment, normally greater. Unfortunately a lot of taps don't have a back center so you are forced to use a holder, but a square holder that fits in the tailstock greatly reduces the errors. For smaller taps I normally use a 1/4" Jacobs with a 3/8 parallel shaft with the jaws of the large tailstock tight enough to maintain alignment but still allowing freedom to rotate, and then hand tap. Bernard R (26135)

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