| Tapping in the lathe (Mar
13, 2005) |
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| 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) |
