Help with better tapping

B

bluehawk

Registered
Registered
Joined
Nov 16, 2011
Messages
40
Hi Bill n here
my problem is how to internaly tape neat tight threads I only tape in the lathe or mill holding the tape in a drill chuck Iv tried 1/3 of turn
and then back turning to brake the chip and removing the tape every 3or 4 turns to clean the tape I also use cutting compound Iv allso tried
running the lathe on slow speed 80/125 and running the tape through in one go I allways use the correct size pilot hole but have played
around with slightly tighter size holes but no real improvement Has any one tried grinding there tapes slightly undersize ?
any ideas welcom (Oh Im tapping mild steel )
Bill N
 
To actually alter the fit of a tapped hole, you must specify a tap with the desired limits. If you'll look at the tap's full description, you should see an "GH" followed by a number. The number represents the amount above basic pitch diameter the tap will cut. H3 is probably the most common, if I remember correctly. This means the tap is 0.0015 over basic PD. Each H number represents 0.0005 difference in size. The G means it's a General purpose tap.

In some low carbon steels, it can be difficult to actually allow the tap to control it's own feed. If you push it too hard, it will cut wide threads because of the pressure from behind, and the same effect is possible, though less common, by holding back on the tap, forcing the back edge, if you will, to cut wide.

If you are trying to achieve interference threads, class 5 for example, you may even need to use a GL- number taps. This follows the same pattern. GL-1 is 0.0005 under the basic PD. The amount of interference is controlled by both elements, the male and the female threads. Class 2 is the standard allowance for a free running thread.


Here is a bit of light reading on the subject of tap sizes:

http://www.newmantools.com/taps/taptech.htm#tap
 
Tony has pretty well covered it there.
I will also add if you use spiral taps you can keep going all the way through, without backing out.
And if you want your bolt to tighten up in the thread, just stop short of going all the way through. The bolt will then tighten up as it reaches the bottom of the hole. If I am doing it, I usually just use an intermediate tap and when it is level with the bottom of the hole I stop.

Dave
 
Dave 7/16 unf and 3/8 unf size unimportant allways same result
Tony great imformation I never knew about different size/fit taps
the taps Iv been using last are 3/8 UNF GH4 HSSE streight flut ground thread
and a left hand tap 3/8 24 UNF with no other markings
Iv checked my other 65/70 taps mostly sutton brand and most unmarked
re tollerance any way Ill have to order the right tap in the future
many thanks Tony thats a great help
Bill N
 
Dave yes thats the best Iv been able to do in the past use an intermediate tap and only run it 3/4 to7/8 th of the way through
wich works not to bad I then wind the bolt in and out to get a tight fit but now I know I can get interferance taps I think thigs
will improve greatly
Bill N
 
Last edited:
Dave and Tony I did some further looking and found a referance to major diameter limit of taps in the machinerys handbook 25th edition
they list the diff limits as H1 through to H6 with H1 the most under size H3 prob around standard and H6 the most over size any way its
all on Pdg 888 under taps and threading dies for any one interested
Bill N
 
What type of material are you generally tapping,and what type of tap are you using? What I've found manually tapping cold roll and hot roll and 4140 and 4140ph and such like the paste type lubricants and the harder tool steels like the oils and fluids better..

As Tony has mentioned you can use a roughing tap and finishing tap. What i'll do is use a used tap to start and initially tap the hole, then use a nice lightly used tap and run back through the hole..


also, always counter sink the hole prior to tapping, you want the outer diameter of the chamfer larger than the major diameter of the tap.. for instance a 1/2-13" tap, put a caliper on the outer diameter of the chamfer and cut the chamfer to something like .560 diameter..
 
Thanks, I have book here, but they do get expensive when you go looking for them.

Dave
 
Thanks Brucer
mostly use 1018 or 1020 cold rolled mild steel (low carbon) but some times 1040 for higher strength parts wich may suit using
oil while cutting the tip of using a taper just larger than the MD is a great one Iv allready tried it and does seem to improve
the quality of the finished thread you just cant stop improving your technique if you want to make better parts
Thanks Bill N
 
Hi Dave
yes of course I didnt think of that but as soon as you get away from something standard things get expensive but I should
only need a couple of sizes the other expensive taps are Left handers !
Bill N
 
You must log in or register to reply here.
Back
Top