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Taper pins

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Peter Simpson 128/05/2019 09:41:18
118 forum posts
4 photos

I'm looking to fit some taper pins to the valve gear on my 5" gauge loco. Is there a rule of thumb regarding the drill size for a taper pin. For example if fitting a 1/16" taper pin. What would the drill size be ?

Andrew Johnston28/05/2019 10:18:23
4936 forum posts
560 photos

Probably, but I just measured the small end of the pin roughly where I thought it would exit the shaft and drilled a couple of thou smaller.


JasonB28/05/2019 10:25:39
16530 forum posts
1759 photos
1 articles

I do the same as Andrew, also if it is a long hole I will counterbore part way with a drill 0.1 or 0.2mm larger than the exit hole size which takes the strain off the fragile reamers.

HOWARDT28/05/2019 10:39:32
467 forum posts
14 photos

Taper pin holes are reamed through so that the small end, nominated diameter of the pin, goes through an extends slightly beyond. Then the excess length is removed to give a clean finish. I would think these days taper pins are used for shear pins mostly as Seloc type or roll pins are an easier fit.

mechman4828/05/2019 11:03:03
2518 forum posts
377 photos

You may still be able to get tapered drills to suit your application on eBay ( usual disclaimer ) I know I have a couple of tapered drills / reamers somewhere in my odd drill bits box.


Lambton28/05/2019 12:10:21
687 forum posts
2 photos


It is not good engineering practice to fit a taper pin in a parallel hole. Proper taper pin reamers (not drills) are made..

There are both imperial and metric standards for taper pins. It is worth getting the correct pin and reamer as your application is obviously important to you.


John Purdy28/05/2019 18:50:31
183 forum posts
59 photos


Check out the previous thread on this topic " Taper Pins 24/09/2017". There is a full table of drill sizes for taper pins from 7/0 up to 10 in "Machinery's Handbook" .


old mart29/05/2019 21:26:58
785 forum posts
77 photos

Imperial taper pins have a 1:48 taper, and I believe that metric ones have a 1:50 taper. Small TP reamers have a pentagonal section.

Andrew Johnston29/05/2019 21:55:45
4936 forum posts
560 photos
Posted by old mart on 29/05/2019 21:26:58:

Small TP reamers have a pentagonal section.

That's odd, my 1/16" taper pin reamer has four cutting edges, with machined flutes like a normal parallel reamer. If the reamer was pentagonal that would give a large negative rake to the cutting edge. Clockmakers broaches are pentagonal in section.


Michael Gilligan29/05/2019 22:17:47
14244 forum posts
627 photos
Posted by old mart on 29/05/2019 21:26:58:

Small TP reamers have a pentagonal section.


Those are horological broaches, and should only be used on brass [or, of course, anything else that is happy with a negative rake angle].



Oops : Sorry, Andrew ... I only read the first part of your post blush

Edited By Michael Gilligan on 29/05/2019 22:19:14

HOWARDT29/05/2019 22:41:18
467 forum posts
14 photos

Taper pin drills, drills with a taper, are available for those who wish to drill a tapered hole before taper reaming.

Paul Lousick30/05/2019 00:07:32
1212 forum posts
502 photos

Where do you get drills with a taper ? My understanding is that drills only cut at the end of the drill and not at the side and the hole made prior to reaming is made with straight drills in a series of steps of slightly different diemeter.


JasonB30/05/2019 06:59:27
16530 forum posts
1759 photos
1 articles

TP drills, and here think I may have one down in the workshop but too large for anything I'm ever likely to make.

Edited By JasonB on 30/05/2019 07:35:30

Andrew Johnston30/05/2019 11:37:38
4936 forum posts
560 photos

Drill Service stock them as well, but not cheap. I assume they must cut on the flutes pretty much the same as a reamer?


Boiler Bri30/08/2019 17:40:33
810 forum posts
285 photos

Right i am slightly confused, do we use a taper drill or not?


S.D.L.30/08/2019 18:40:17
176 forum posts
Posted by Boiler Bri on 30/08/2019 17:40:33:

Right i am slightly confused, do we use a taper drill or not?


If youre doing a few just use the reamer if youre doing a lot get the drills, its just the same with morse taper sockets.


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