By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

what's the difference in taper, second and plug taps?

All Topics | Latest Posts

Search for:  in Thread Title in  
david liggins31/05/2014 21:16:42
7 forum posts
as above, i was wondering the difference between taper, 2nd and plug taps. ideally metric coarse. the reason for asking is i want an almost interference fit m6x1 bolt into the hole. its for a jig hence the need to try to eliminate play. thanks
JasonB01/06/2014 07:36:47
19098 forum posts
2100 photos
1 articles

You would be better off using a die to adjust the diameter of the male thread on a home made bolt than trying to make an off the self one a tight fit in a tapered hole left by going part way with a taper tap.


Michael Gilligan01/06/2014 07:39:11
16620 forum posts
723 photos

Welcome, David

Taper, Second and Plug [or Bottoming] describe the shape of the Taps, not their size.

The idea is that Taper starts easily in the drilled hole ... etc.

For through-holes you only need Taper and Second, but for blind holes, the Plug tap can take the thread to size right to the bottom of the hole.


If possible, you would do better to "fit" the plain shank of the bolt rather than relying on the thread.

Happy to discuss further.


Edited By Michael Gilligan on 01/06/2014 07:41:26

Clive Hartland01/06/2014 08:15:33
2612 forum posts
40 photos

With a good quality Metric set of taps the 1 Tap has a truncated thread and only cuts a shallow thread, the 2 Tap then opens up the dia slightly and the 3 Tap completes the thread to the correct crest form. This is a better method than the normal taps where the thread is tapered on the first 2 taps and the full form on the last one. I would think Jasons method is the bests, drill and tap the thread and then make a close tolerance male thread.


Michael Horner01/06/2014 08:21:15
207 forum posts
61 photos

Hi David

Your tap question has been answered..

To get the near interferance fit can you use a dowel?

That is what I have seen in the car world to align housings.

I have seen dowels with a hole in the centre for the bolt to pass through.


Cheers Michael.

JasonB01/06/2014 08:24:45
19098 forum posts
2100 photos
1 articles

Not so with all good quality taps Clive, my Dormer E500 are not like that. You may be thinking of "Serial Taps" which do increase in diameter. They are also a bit of a pain on through holes as you need to work up through teh sizes rather than just put teh taper through.


Mark C01/06/2014 09:15:43
707 forum posts
1 photos

You will find that taps are available in different classes of fit in the same way reamers are if you want to make the thread to a particular tolerance.


Dusty01/06/2014 09:18:44
480 forum posts
8 photos


If you are relying on a threaded hole, which it appears from your post, to locate a component forget it. You will never get a thread to pull up concentric and square, and I do mean never, even if the male and female threads are screwcut, the tolerances needed in order to screw the one into the other means that they will skew and throw concentricity out of the window. As an ex toolmaker I would reccomend that any location dia is press fitted to the base, of course all this is dependent on how accurate you need to locate your component.

Jo01/06/2014 10:22:08
198 forum posts

Pins or dowels are used for repeatable accurate location on a jig or between components, not fasteners.


Ian S C01/06/2014 12:00:05
7468 forum posts
230 photos

A one time I was making moulds for making rubber items, the only way to locate the two halves of the mould is with dowels, the bolts to hold the bits together should not be too tight in the holes, or you can use studs in one half.

Ian S C

Robert Dodds01/06/2014 12:25:05
275 forum posts
39 photos


You could try "socket shoulder screws" if you require to separate the parts and reassemble with accuracy They are a bit like a screw in dowel and you can get both M5x6dia or M6x8dia in various lengths. you will need to drill part way into the lower part and ream to size so that the larger shoulder diameter is a close fit and provides the register. The fit of the thread then becomes far less critical.

Bob D

Neil Wyatt01/06/2014 14:36:59
18316 forum posts
718 photos
77 articles

Standard metric sets have the first and second taps slightly undersize. I have both types and the undersize ones make tapping at large sizes or in tough materials much easier/less stressful, but as Jason says it does mean you have to make multiple passes. They are also handy for making small expanding mandrels.

For a jig, better to use dowels, hardened silver steel pins or even just plain bms pins for a temporary jig.


david liggins01/06/2014 14:54:40
7 forum posts
thanks guy's i didn't expect such a good response, been out since dawn il add detail of what im doing when i get home
WALLACE01/06/2014 14:55:07
304 forum posts
17 photos
The only taps I personally have ever come across to be tight on the first and second are the ones from ARC.

All others - mostly Sherwood ones from Cromwell Tools but a few others - appear to cut threads to the same diameter.

Michael Gilligan01/06/2014 14:55:19
16620 forum posts
723 photos
Posted by david liggins on 31/05/2014 21:16:42:
... the reason for asking is i want an almost interference fit m6x1 bolt into the hole. its for a jig hence the need to try to eliminate play.



You are getting plenty of answers, but none of us really knows the question

... it would help if you could describe or sketch your proposed jig




Edit:   Sorry, David, that just crossed with your follow-up post.

Edited By Michael Gilligan on 01/06/2014 14:56:47

Mark C01/06/2014 15:46:49
707 forum posts
1 photos

It will be interesting to see the sketch. There are times that you simply have to do things a particular way and if it needs to locate off the thread then that is what you have to do (although a dowel/pin is easier/more robust if possible).

You might find this page interesting (especially those who have the idea that 1st, 2nd and bottoming taps are different tolerance/size) **LINK** . Get a decent tap and this will be marked on it.


david liggins01/06/2014 16:35:08
7 forum posts
Ok this is the application: trying to cut a threaded hole in the bolts for the cranks on a mountain bike. I'm making my own frame from a few old bikes, the idea being that once i get the frame right il get it copied by a reputable frame builder and live happily ever after. I have already made the jig and just unsure of the best way of bolting the bottom bracket complete with cranks to the jig in a way that gives some left right adjustment. The bottom bracket bolts that hold the crank are m8x1 which even the nearest fastner shop can't get (in small quantities). I have made a proof of concept frame already, but was unsure of my welding so that was just carefully eyeballed and shimmed with blocks of wood and jubilee clips as i didn't want to potentially waste time making a jig. It came of so close to perfect i darent cut it until i make a better frame. Btw, so long as the thread has no play i can take a datum from the jig. Thanks
JasonB01/06/2014 16:53:04
19098 forum posts
2100 photos
1 articles

I'd be wary of holding the BB spindle to the jig just by a bolt in one end and relying on the small contact area at the end of the spindle to keep things true.

Better to have a substantial spigot 20-25mm dia with cones on that will centralise the BB shell and can be adjusted latteraly by spacers. Most of the jigs I have seen use this sort of setup and the one my Uncle who was a well know South London frame builder used also employed this method.

If you are going to bolt it to the jig then set the jig up so you can use the right hand threaded side not the lefthand

jason udall01/06/2014 16:55:37
2026 forum posts
41 photos
All the references I have seen define ground thread taps by the pitch and thread form with fit say h6...
The taper is applied after?thread form ground into blank...? truncating the thread at the start....
Unless the thread form is a taper form amyway......
Would be happy to see any work on alternative ways... ( ok you could make tapered blank but thats the same really)

david liggins01/06/2014 17:02:16
7 forum posts
id be inclined to think a series tap is what i need and the drilled and reamed to shank size on the tap? when i originally posted i didn't know of series taps

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Latest Forum Posts
Support Our Partners
Eccentric July 5 2018
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest