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M4 carbide tap

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Glyn Davies30/08/2011 17:19:55
146 forum posts
56 photos
Hello there. I am trying to tap an M4 x .7 thread through about 10mm of a hard stainless steel. The material seems to have a hard spot at the end of the hole and my HSS M4 taper taps "squeak" and jam solid with about 3mm of the tap showing through the end of the hole. I have tried all the usual tapping lubricants.
 
Can anyone advise me where I can obtain a carbide tap at reasonable cost so that I can finish the job (ideally in the Leeds area).
 
Thanks
 
Ian P30/08/2011 17:53:27
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2747 forum posts
123 photos
Its possible that what you are suffering from is 'work hardening' of the stainless. As you say a Carbide tap might solve the problem but when I have had the same experience I have always managed to complete the tapped hole by using several taps.
 
I presume you are using a taper tap, if so change to a second or plug tap, you will find that you can probably advance another turn before that tap too gets stuck, when that happens change taps again. By repeating the sequence you will eventually get through.
 
Another thought,,, do you really need a 10mm length of full M4 thread? Either counterbore one or both sides or use a larger tapping drill. It will still be a very strong fixing.
 
I have been told that in some cases cutting a thread from scratch with a plug tap actually requires less torque than the conventional taper/second/plug method. It certainly works for me in aluminium.
 
Ian P
Tony Pratt 130/08/2011 18:19:46
2319 forum posts
13 photos
Hi, are your taps sharp and of good quality? I second what Ian says above, also I don't think carbide tap and reasonable cost go together!
Tony
JasonB30/08/2011 18:45:59
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25215 forum posts
3105 photos
1 articles
What are your existing taps made from? if they are carbon steel then you will have problems if the hole has hardened while drilling.
 
Try a material specific tap, the ones with green rings are for stainless steel, usually a good HSS with a few other metals thrown into the mix. J&L do them.
 
Not that many carbide tapos about I should think there is a real risk of snapping the smaller sizes which may explain their small numbers.
 
J

Edited By JasonB on 30/08/2011 18:46:37

chris stephens30/08/2011 19:05:41
1049 forum posts
1 photos
Hi Otley,
If you have some of the tap showing through the other side, there is a good chance that you can start the tap from the other side and still maintain alignment. Well, it is worth giving it a go.
Another thought, are you sure you drilled the hole to the correct size before tapping? The usual lists say for M4 use 3.30mm , Tubal Cain (the English one not the Yankee upstart) suggests using 3.50mm. That 0.20mm can make all the difference in tough materials.
chriStephens
Edit/  PS If you are going to buy a tap specifically for doing Stainless, buy either a spiral point or spiral flute, depending on whether you are doing through holes or not, rather than the usual set of three. With the work hardening that occurs with Stainless it is better to do the threading in one go, rather than repeatedly going in a bit then having to back out to clear swarf. 

Edited By chris stephens on 30/08/2011 19:14:34

Phil P30/08/2011 19:30:49
851 forum posts
206 photos
Hi Otley
 
I am curious about your user name, are your from "Otley"
 
Phil
Glyn Davies30/08/2011 20:48:27
146 forum posts
56 photos
Many thanks for all the most helpful replies. I have tried all ways to tap this hole. I started with my HSS taper and plug taps. Then I bought a new HSS spiral point tap from Cromwell Tools for £8 and succeeded in getting about another half turn before it squeaked and stopped. The geometry of the casting I am trying to thread means that I cannot tap from the end of the hole.
 
I suspected that I might be in trouble when I drilled the tapping hole. I got about 90% through when the HSS drill ground to a halt. I resharpened it and it blunted again. So I drilled through with a 1/8"carbide drill and then managed to open the hole with the HSS 3.3mm tapping drill.
 
Maybe it's time to admit defeat on this one.
 
I chose Otley at random because I often visit Dunn's cafe by the river there.
 
Thanks again.
Ian P30/08/2011 21:01:04
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2747 forum posts
123 photos
Mr Otley
 
Definately time now to open up the hole from 3.3mm, it should solve the problem.
 
Tapping sizes are really only a guide, they might be fine 99% of the time but with something as tough as your SS casting surely you would you be better off with 3.5 or even 3.6.
 
Ian P
ady30/08/2011 22:37:28
612 forum posts
50 photos
Open the hole up from the back with a carbide or equivalent drill.
John Stevenson30/08/2011 23:31:33
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5068 forum posts
3 photos
Try a set of serial taps from Arceurotrade.
 
These are tapered as per normal taps but they are also undersize so you have to use all three to get to the full thread size, work ace on stainless and aluminium bronze as the first and second tap hardly take anything out per pass.
 
John S.
Ian S C31/08/2011 09:44:58
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7468 forum posts
230 photos
I would not recomend using the serial taps, cut to size with the first attempt. I had problem putting a thread on a stainless rod, I opened up the die to ease the cut, then found that I had difficulty taking the thread down to correct size because of the work hardening. Ian S C
David Clark 103/09/2011 10:00:45
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3357 forum posts
112 photos
10 articles
Hi There
Are you suffering from the drill work hardening the hole?
Have you tried starting from the hard end and tapping towards the soft end?
regards David
 
John Stevenson03/09/2011 10:30:14
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5068 forum posts
3 photos
Posted by Otley on 30/08/2011 20:48:27:
 
 
The geometry of the casting I am trying to thread means that I cannot tap from the other end of the hole.
 
 
 
I think people are missing this part of the post.
 
John S.
mick H03/09/2011 20:15:04
795 forum posts
34 photos
 Otley.....could you enlarge the hole with, say, a carbide or diamond burr and then sleeve it, drill & tap.

Edited By mick H on 03/09/2011 20:25:05

Glyn Davies03/09/2011 22:49:47
146 forum posts
56 photos
Many thanks again to all those that have come up with such good ideas to solve my problem. I went to the autojumble at Rufforth today in search (in vain) of an M4 carbide tap - I still think that would do the job. The very nice lady at the carbide stall gave me a couple of contacts in Sheffield to try for a tap and if those fail, I'll try running a 3.5mm drill through the hole and then see if my HSS taps will get through.

Edited By Otley on 03/09/2011 22:50:43

Martin W04/09/2011 01:02:56
940 forum posts
30 photos
Hi
 
One for the experts here, is there any merit in trying to anneal the stainless in order to restore it to its pre work hardened condition then try to cut the last part of the thread. I found this link on annealing stainless here which made me wonder .
 
Cheers
 
Martin
ady04/09/2011 01:06:41
612 forum posts
50 photos
Posted by David Clark 1 on 03/09/2011 10:00:45:
Hi There
Are you suffering from the drill work hardening the hole?
Have you tried starting from the hard end and tapping towards the soft end?
regards David
 
Nice to see you back in one piece David.
 
ady04/09/2011 01:11:06
612 forum posts
50 photos
It's probbly too stiff for a carbide tap, it would fracture.
Stainless is a pretty amazing material once it hardens.
A carbide drill would give you more chance of success IMO

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