Thread up to a shoulder.

[George TilleyParticipant @georgetilley96475]

Hi,

 

I am very new to practical engineering and would like some help please. I am currently trying to build a gauge 1 project 4F. Many of the drawings show components which are fully threaded right up to a shoulder. My dies leave an unthreaded section of about 1/16″ next to the shoulder and I don’t fancy cutting this type of thread using the lathe.

 

My question is can you cut a thread to a shoulder if so how or do you pack the uncut section with a washer or such like?

 

Thanks,

George

[George TilleyParticipant @georgetilley96475]

[CirclipParticipant @circlip]

When “Dieing” a thread George, the face of the die with the writing on is the one to present to the work, which leaves the unthreaded witness. If you want more thread, after the initial threading, turn the die over so that the “Back” face is presented to the work.

 

  The lead face usually has a taper relief whareas the back one hasn’t. If you want tighter threads, chat someone up with a suface grinder and have the back face skimmed.

 

 BEWARE trying to cut the whole length “Backwards”, the thread WILL run off.

 

  Regards  Ian.

[macmarchParticipant @macmarch]

Hi george and welcome to the club.

 

It is common practice to show the thread up to the shoulder. However this is not actually possible and you either thread as far as poss then use a washer or perhaps countebore the hole a little.  The best way is to undercut the shaft by the shoulder to the depth of the thread as you would if threadcutting. A groove about 2 threads wide will normally do it. The lead on the die will then pass on to the groove.

 

cheers

 

ray

[TerrydParticipant @terryd72465]

Hi George,

 

Welcome to Model Engineering, a normal die will not cut up to a shoulder.  One way to overcome the problem is to undercut the unthreaded portion next to the shoulder with a thin tool similar to a parting tool.  In fact I use a thin parting tool, about 2mm wide, myself for this job.  You just need to undercut to the depth of the thread.

 

If you don’t want to weaken the shaft, an alternative is to open out the start of the thread in the female part by sufficient to clear the unthreaded part.  Of course you can pack out the joint with washers etc but this may mean some adjustment in the components and I personally don’t use this method unless absolutely necessary as I consider it less than best practice.  Others may disagree of course but you will learn that there are often many ways to overcome problems like this.

 

Terry

 

 

Edited By Terryd on 11/03/2010 12:43:53

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

I must caution against grinding the back of a die, especially if you intend to hold the die in a standard die holder in the tale stock. The screws holding the die will no longer be in the right place to hold the die square to the job, which will cause all sorts of annoyances. 

chriStephens 

[Nigel McBurney 1Participant @nigelmcburney1]

Hi it was common practice when setting capstan lathes  making small parts for instruments to grind the back of dies,to enable threads to be cut up to a shoulder,special die holders were not used,but it is quite easy to make a special tailstock die holder to suit the modified die,Undercuts on small diameters tend to weaken the thread,if you have to undercut use a round nosed tool to avoid sharp corners,which will cause failure.

[KWILParticipant @kwil]

Since the thread strength is determined by the minor diameter, the undercut, if taken to that minimum, should not affect the strength. By all means round the undercut if you must, but the strength will not be significantly improved in tensile load.

[George TilleyParticipant @georgetilley96475]

Thanks for all your replies guys. Think I will counterbore the hole on this occasion and see how it goes.

 

Thanks again,

George 

[Gone AwayParticipant @goneaway]

Grinding the back of the die shouldn’t affect the fit in the die-holder as long as the die is always used with the ground face leading – only when the die is used the normal way around. That may mean having a separate die or dies ground exclusively for threading to a shoulder.

 

I’m by no means an expert and I’ve never actually tried but I see no one has mentioned the technique of lathe-cutting a thread by turning the machine by hand. It should be possible in that case to cut right up to a shoulder.

[Geoff TheasbyParticipant @geofftheasby]

I cut many threads in my Unimat 3 with a die in a tailstock dieholder I made, and turning the spindle by hand.

It causes cramp with a long thread, but I have survived so far.

 

Regards

Geoff

[JasonBModerator @jasonb]

Sid you will only get to within one turn of the shoulder doing it that way and even that would require a special tool ground to just the width of one thread.

 

As the left hand side of the tool hits the shoulder the right hand side would need to make one full revolution before the thread form totally runs out.

 

Jason

[Ian S CParticipant @iansc]

If you grind the die as has been suggested, and you want to use it the normal way later find/make a washer to fit in the holder to make up the space. One thing I often have to do is grind off the tapered center point on taps so I can get a good depth in blind holes (I,v read that normal center hoes in the end of taps cause/promotes cracks during heat treatment). Ian S C

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