Tapping a Blind hole

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Tapping a Blind hole

This topic has 18 replies, 10 voices, and was last updated 29 June 2011 at 22:53 by Anonymous.

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27 June 2011 at 19:04 #21888
Paul Scholey
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@paulscholey17560
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27 June 2011 at 19:04 #70885
Paul Scholey
Participant
@paulscholey17560
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.
27 June 2011 at 19:25 #70889
Gray62
Participant
@gray62
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
27 June 2011 at 20:16 #70894
Anonymous
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
27 June 2011 at 21:03 #70897
David Clark 13
Participant
@davidclark13
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
27 June 2011 at 22:00 #70902
Steve Withnell
Participant
@stevewithnell34426
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
27 June 2011 at 22:15 #70903
Paul Scholey
Participant
@paulscholey17560
Thanks for all the replies, very helpful.

Paul.
27 June 2011 at 23:44 #70911
Chris Trice
Participant
@christrice43267
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.
28 June 2011 at 12:28 #70940
chris stephens
Participant
@chrisstephens63393
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
28 June 2011 at 14:30 #70943
The Merry Miller
Participant
@themerrymiller
Or even the simple tapping guides as suggested in Harold Halls book “Model Engineers Workshop Projects”
28 June 2011 at 19:54 #70954
Sub Mandrel
Participant
@submandrel
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
28 June 2011 at 21:06 #70963
Paul Scholey
Participant
@paulscholey17560
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?
28 June 2011 at 21:29 #70968
Sub Mandrel
Participant
@submandrel
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
29 June 2011 at 11:14 #71003
Anonymous
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
29 June 2011 at 11:35 #71007
David Clark 13
Participant
@davidclark13
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
29 June 2011 at 12:07 #71013
Ian S C
Participant
@iansc
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
29 June 2011 at 22:27 #71065
Paul Scholey
Participant
@paulscholey17560
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.
29 June 2011 at 22:29 #71066
Paul Scholey
Participant
@paulscholey17560
Where’s best place for spiral flute taps?
29 June 2011 at 22:53 #71068
Anonymous
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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