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Undersize Taps

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Paul Scholey24/08/2010 21:00:42
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I need to tap some 8mm holes in an aluminium engine case that have been welded up for re tapping to fit  the barrel studs. The problem is that the original 8mm holes are  always tapped under size so that the barrel studs are a tight fit. I have a set of standard Goliath taps but they measure 8.12 mm which is much to big for standard studs.  The engine case is for a Lambretta and as this is something I would like to help other people with I would like to be able to use standard studs rather than make thicker ones all the time.
Just as a matter of interest can anyone tell me  how factory holes are tapped under size and are undersized taps available? I cant find any.
Thanks in advance for any help.

Edited By Paul Scholey on 24/08/2010 21:02:41

macmarch24/08/2010 21:53:51
147 forum posts
1 photos
Hi Paul,
If you want the studs to 'lock' then use the taper tapt. The studs will then lock themselves in.  I wonder how you are measuring  the M8 taps. I have always repaired  Ali castings with 'helicoil' type inserts.
 
cheers
ray
KWIL24/08/2010 22:57:50
3681 forum posts
70 photos
Out of interest, have you done a trial hole in a spare piece of Ali, tapped with your taps, just to see how "loose" they are??
Gone Away25/08/2010 01:07:39
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Paul, I've seen oversized taps around - they're used on parts that will be plated after tapping for one thing so are not that uncommon - but I can't say I've ever seen undersized taps.
 
Commercially/industrially though I'm sure they could be obtained to special order in reasonable quantities. Or a manufacturer with a decent tool room may make them itself .... something you might consider. Should be feasible for tapping aluminum.
 
You could also dig out a tap manufacturer (or two) and call them. They would let you know the position and who knows, they might have the odd few kicking around.
JasonB25/08/2010 08:20:44
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You could try a thread forming tap, they push the metal into shape rather than cutting it and should be a little more accurate, just get the tapping drill correct.
 
Just tried my M8 taps in a calliper, cheap one comes in at 8.11, better quality spot on at 8.00.
 
Have you measured the studs as its likely these are what give the tight fit as its easier to control the size of an external thread than an internal one, are they a cut thread or rolled?
 
Jason
 
 

Edited By JasonB on 25/08/2010 08:23:54

Dinosaur Engineer25/08/2010 09:26:08
147 forum posts
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You could try a set of "serial" taps. These are different from normal taps in that the thread forms are progressively larger in diameter from the 1st to the 3rd tap . The 3rd tap being the normal full size thread . I often use the No. 2 tap to give a tighter thread fit on engine studs. The different sizes are denoted by machined or etched rings around the circumference of the shank . 1 ring for number 1 , 2 rings for number 2 and either no ring or 3 rings for number 3.
These taps are considerably more expensive than normal taps and are normally only available in sets of 3. the number 1 tap sometimes has a core diameter guide pilot in front of the normal thread cutting form.
A coarse thread such as American UNC is often recommended for aluminium as its deeper threads are stronger or as mentioned in previous post, "Helicoils" are much stronger than normal threads especially in Aluminium .
Most commercial studs are produced to quite a loose tolerance class . If you can select some larger studs from a batch or make from some long cap head screws this would help. Ideally if you could make your own studs to the maximum metal condition this would give a much tighter fit .
Dinosaur Engineer25/08/2010 11:01:53
147 forum posts
4 photos
I should have mentioned that "serial" taps were initially made for tapping tough materials. Each tap only removes approx one third of the total material to be removed to make the thread. Also the 3rd tap removes less than the other two and this gives a finishinig cut to give an improved surface finish.
WALLACE25/08/2010 16:13:55
304 forum posts
17 photos

 
I've found that the sets of metric taps sold by ARC will cut slightly undersize - the final 'plug' one needs to be used otherwise the (commercial) bolts are tight. Others in my motley collection (  Kennedy, Warrior etc) are ok even if using a 1st taper (obviously  the tap is run past the tapered section !).
 
They're cheap enough so might be worth a try.
 
W.  
KWIL25/08/2010 16:46:06
3681 forum posts
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It is not unusual to find that the normal, taper, second cut and then plug / bottoming tap each take an additional cut (beyond the taper obviously). As DE says above on the second Serial type is tighter than full thread.
 
It is noted that proper Whitworth threads are far better in aluminium for studs than metric, this is one (!) of the drawbacks of the metric system, being a relatively fine thread system, even the so called coarse range.

Edited By KWIL on 25/08/2010 16:46:50

Paul Scholey25/08/2010 17:29:02
87 forum posts
1 photos
Thanks for all your replies. I have tried a trial hole and it is just to slack, I am trying to get the original fit. There is a weak point on the casing on the inside of the stud holes where there is very little material, this is where the cracks develop, so I cant drill it larger to fit Helicoil because it would make that point even weaker. I will look into the Taps that have been suggested.
Paul Scholey25/08/2010 17:32:02
87 forum posts
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The average size of the studs seem to be  about 7.75mm if they are cut or formed.
Dinosaur Engineer25/08/2010 18:49:51
147 forum posts
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Assuming the studs are of a full thread profile then 7.75mm O.D. for an M8 thread means that the studs are at the bottom of a 6g tolerance which is only a medium fit class. If the threads are less truncated than normal threaded components then it is likely they are undersize to a 6g tolerance class. I would have thought that engine studs should be to a better tolerance class than this .On a good thread tolerance I would have expected a full profile O.D. in the order of 7.90 - 7.95 mm. But maybe I'm expecting too much !

Edited By Dinosaur Engineer on 25/08/2010 18:56:30

Edited By Dinosaur Engineer on 25/08/2010 19:00:12

Stub Mandrel25/08/2010 21:29:56
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My HSS metric set M3-12 (£20 at a boot sale!) opens up the thread with each tap. You can use the second tap for a tight fit (almost a force fit) and the plug for a free but close fit. If you use Tubal Cains 60% thread sizes for the drills the taper tap barely cuts a thread and you have to ve very careful about re-engaging so I often start with the second (says Pat*).
 
Neil
 
*I'm 1/4 Irish so I can get away with that
Paul Scholey25/08/2010 23:07:13
87 forum posts
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Do your Taps have a make on them?
Andrew Johnston26/08/2010 09:41:19
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Interesting, I had read somewhere that the bottom tap in a hand set was slightly larger than the taper and 2nd. Can't find anything about it in Machinerys Handbook though, but my older copies do mention the serial tap sets already discussed. However, for my metric hand tap sets it doesn't seem to be the case.Commercial screws fit fine in a thread using the taper tap only. I only use the bottom tap if I need a few more threads at the bottom of the hole, and I never use the 2nd tap. All the metric tap sets are by Dormer. It will be interesting to experiment with some of my older BSW/UNC sets to see what happens.
 
If the tap cuts a bit oversize you can always tighten up the thread a bit by putting a tapered rod in to the hole and hitting it with a hammer. This will distort the top few threads and tighten the fit. Similarly a ball bearing on the top thread and hit with a hammer achieves something similar.
 
Regards,
 
Andrew
 
 

Gordon W26/08/2010 11:41:24
2011 forum posts
My twopennoth:- the bottom tap usually cuts bigger, I'm assuming the holes are for head studs and are blind, so will need the bottom tap. Do not try to screw into a taper tapped hole, studs only held by two  (or one) thread, could crack hole. Are the studs genuine spares or maybe copies, and undersize ? I would use alloy epoxy for a simple job, weld up, machine etc. for a proper job.
Paul Scholey26/08/2010 17:27:10
87 forum posts
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Thanks for replies again,  all the studs are roughly the same size but they have to be to screw into the casing, full 8mm just wouldn't screw in. All the threaded holes in the casing for the studs see to be the same size if its a 40 year old engine case or a new one.
Does anyone know who stocks Serial Taps?
Dinosaur Engineer26/08/2010 18:04:57
147 forum posts
4 photos
Just checked Arc Euro Trade website details :-
P/No. 060 080 00600 M8 x 1.25   £4 for set ! Tel 0116 269 5693
They do seem very cheap !
Stub Mandrel26/08/2010 20:38:22
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No names on my metric tap sets.
 
My ancient Machinery's says that studs should ideally be oversize (not the thread) but in non-critical applications they lock on the thread runout. It also suggests for short runs choosing standard size studs by trial fit!
 
But why not use stud lock?
 
Neil W.
Stub Mandrel26/08/2010 20:38:35
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No names on my metric tap sets.
 
My ancient Machinery's says that studs should ideally be oversize (not the thread) but in non-critical applications they lock on the thread runout. It also suggests for short runs choosing standard size studs by trial fit!
 
But why not use stud lock?
 
Neil W.
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