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Tapping a Blind hole

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Paul Scholey27/06/2011 19:04:38
87 forum posts
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I need to make a old Motorcycle part the same as the original but in stainless steel. My problem is I need to drill and tap a 6mm thread but its only just over 5mm deep, I have tried grinding the bottom of a plug tap but I still cant get it to cut enough thread, only enough for about one turn of the screw, where as the original will screw in about 3 full turns.
How would the original be threaded and how much can I grind of a plug tap and it still be able to cut? or any other ideas please?
Thanks in advance.
Gray6227/06/2011 19:25:19
1058 forum posts
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Hi,
If your taps are of good quality and are sharp, you so should be able to grind them to facilitate tapping to the bottom of a blind hole. I have several taps ground in this way. I always use HSS taps rather than the cheaper carbon taps for this purpose as they keep their edge better.
 
regards
 
Graeme
Andrew Johnston27/06/2011 20:16:21
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Use a spiral flute tap; you'll easily get at least 3mm of usable thread. This assumes that the thread is M6x1, or finer pitch.
 
Regards,
 
Andrew
David Clark 127/06/2011 21:03:38
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3357 forum posts
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Hi There
You can grind the end of a plug tap until you are virtually at the start of the thread cutting part.
regards David
 
Steve Withnell27/06/2011 22:00:33
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Pardon the expression, but does the hole have a square bottom? You will lose depth if the blind hole is just drilled with a jobber drill.   Cleaning up with a d-bit or boring bar might just give you an extra thread or so.
 
As everyone else said make sure the plug tap is ground to best advantage.   Some plug taps I've had were nearer "first" than plug!   
 
Best
 
Steve

Edited By Steve Withnell on 27/06/2011 22:06:05

Paul Scholey27/06/2011 22:15:22
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Thanks for all the replies, very helpful.
Paul.
Chris Trice27/06/2011 23:44:08
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1376 forum posts
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Is the part mountable on a face plate? If you can secure the tap in a drilled and split (or grub screwed) square block and mounted in the tool post, you can arrange for the saddle to move at the same rate as the thread pitch using the screw cutting gears thus removing the need for the cut thread to draw the tap in. Possibly worth turning the mandrel by hand on this one.
chris stephens28/06/2011 12:28:12
1049 forum posts
1 photos
Hi Paul,
As has been suggested a decent spiral flute tap is your only real option, but do not try to start it freehand. With spiral fluted taps you must use some form of tapping stand like a drill press or a milling machine or even the real thing.
chriStephens
The Merry Miller28/06/2011 14:30:35
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Or even the simple tapping guides as suggested in Harold Halls book "Model Engineers Workshop Projects"
 

Stub Mandrel28/06/2011 19:54:43
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I'd go as deep as I can with my decent HSS taps, and finish off with a carbon tap with the end ground off. But beware work hardening in stainless!
 
The cone on the end of a tap is for grinding it between centres. Unless you have some very expensive gear this is a useless bit of the tap, grind it off! (som of my larger taps are centre drilled, rather than pointed).
 
neil
 
Paul Scholey28/06/2011 21:06:41
87 forum posts
1 photos
I am tapping in the Lathe, Do spiral flute taps have a flat bottom then? I only have one and it has a small taper but it is the wrong size. Are spiral flutes better for tapping stainless?
Can I completely grind the bottom off a hand tap or does it have to have some taper?
Stub Mandrel28/06/2011 21:29:37
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I think that the best ones for stainless have no flutes at all, and mke the thread by deformation in one pass. I imagine hole size and lubrication are critical. If I'm wrong please will someone correct this!
 
The tap needs enough lead to enter the hole and guide the tap in without cross threading.
 
Neil
Andrew Johnston29/06/2011 11:14:21
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Posted by Paul Scholey on 28/06/2011 21:06:41:
I am tapping in the Lathe, Do spiral flute taps have a flat bottom then? I only have one and it has a small taper but it is the wrong size. Are spiral flutes better for tapping stainless?
Can I completely grind the bottom off a hand tap or does it have to have some taper?
 
Some of my spiral flute taps have a flat bottom, some have a small cone, which could be ground off.
 
Spiral flute taps are not designed especially for stainless steel, but are designed specifically for tapping blind holes. The helix angle and coating will determine which materials are best for a given tap, including but not limited to, stainless steels. Spiral flute taps generally cut a full depth thread within 1-2 pitches, in one go, as opposed to the 2 or more pitches on a bottoming hand tap.
 
As an aside the cones on the bottom of some of my spiral flute taps would not be problem if the blind hole was drilled, rather than being flat bottomed.
 
I would suspect that if you grind all the taper off a bottoming tap you'll just end up chipping the teeth. Either way a bottoming tap is intended to clean up a partially formed thread, not cut one in a single pass.
 
What grade of stainless steel are you using?
 
Regards,
 
Andrew

Edited By Andrew Johnston on 29/06/2011 11:14:54

David Clark 129/06/2011 11:35:40
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3357 forum posts
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Hi There
When grinding taps to length, try the following method.
Touch the point onto the grinder to remove a tiny bit of length.
Immediately touch the point on to a metal surface to draw the heat.
Continue to do this until the correct length is reached.
The tap should then last much longer.
 
Method was used with lots of 1/4in. unf taps that were £50 a go.
If this method was not used, taps tapped less than 10 holes.
With this method, taps worked for 100 or more holes.
It was hardened aircraft quality steel, S149 I think.
regards David
Ian S C29/06/2011 12:07:25
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7468 forum posts
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The only reason there is a piont on the end of a tap is for manufacturing, in the past taps had a female center, but it was found the cracks developed from the center hole during heat treatment, so the manufaturers changed to the male centers used on most modern taps. Ian S C
Paul Scholey29/06/2011 22:27:07
87 forum posts
1 photos
Thanks for that information,
I don't know what grade of stainless steel it is, just pieces I have collected over the years but it seems to file and turn ok.
Paul Scholey29/06/2011 22:29:25
87 forum posts
1 photos
Where's best place for spiral flute taps?
Andrew Johnston29/06/2011 22:53:29
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Posted by Paul Scholey on 29/06/2011 22:29:25:
Where's best place for spiral flute taps?
 
Any decent professional toolshop; J&L, Cromwell etc. I use CISDirect because they're a few miles down the road in St. Neots, but any professional supplier should have them. Somebody will prove me wrong, but I suspect that most ME suppliers will not stock them, as they're intended for machine tapping.
 
Regards,
 
Andrew

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