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Making a M20 x 8tpi 'rope thread' tap

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Oily Rag21/02/2021 14:29:01
526 forum posts
172 photos

I have been asked to repair a piece of farm machinery that utilises a 'bastard' thread form. After managing to slice a part of the unworn threaded male component in half to allow a visual inspection of the 'form', we have come to the conclusion it is an M20 x 8tpi thread with a 'rope' form. Initially I thought it was a badly worn trapezoidal or Acme thread!

I am using a piece of D2 tool steel to make a tap to allow the female thread to be reproduced. I have ground a tool and checked it with a toolmakers microscope to ensure accuracy. What I propose doing is using the tool for making the tap and then for making the replacement 'bolt' (actually a complicated shaft which will be fabricated into the repair, using as much of the original as possible).

It has been many years since I made a tap and my conundrum is whether to thread the tap and then cut the flutes, or cut the flutes and then thread the tap. What do others do? - intuitively I reckon I should thread and then relieve, as the thread will be formed continuously rather than with intermittent cutting (which in D2 will probably deform the front edge of the thread form). Fortunately I have found a formed flute cutter in my collection of Horizontal milling cutters so that will get used, followed by a flute grind op on the Clarkson.

The female thread is in a casting which I will machine out and replace with a top hat bushing. I did consider a more standardised thread form but there is a seal which runs on the shaft and engages with the 'rope' form. These seals are available easily, all other components are not - unless you replace the whole 'unit' at a cost of upwards of £7k - the whole contraption is a £600k cost so it makes sense to keep it operational!

Your views on the operation sequences are welcomed.


noel shelley21/02/2021 14:40:41
995 forum posts
19 photos

What is the machine ? M20 X 8TPI ? Not M20 x 3mm ? A seal that runs in a thred ? All sounds like fun. I would cut thread and then flute. Noel

SillyOldDuffer21/02/2021 14:52:55
7883 forum posts
1724 photos

Thread and then cut for me too but I've never attempted a rounded form.

The extra work cutting a rounded thread might make it worth getting rid of the flute metal first, but then I'd worry about breaking the blank due to the heavy interrupted cut. More I think about it the worse it gets! Rather you than me, quite challenging!

Is it a knuckle thread? Not used for fasteners , rather something like a leadscrew or jacking thread with extra gaps for use in mucky places.


Howard Lewis21/02/2021 14:56:17
5733 forum posts
13 photos

+1 for Noel's method.

Not too long ago, I wanted a 4 mm pitch half round thread, so made up a holder to take a piece of 1/8" HSS toolbit.

First operation was using it to produce a run out groove at full depth of 0.062" (1.5 mm )

Ran at minimum speed in back gear, gradually, and gradually, increasing depth of cut, with plenty of neat cutting oil applied.

Something similar might be the starting point before hardening your tap?

Howard .

Oily Rag21/02/2021 17:37:10
526 forum posts
172 photos

It's a complex design and the seal element is to stop the pressure forcing the 'slurry' through the support plate from one side to the other. The shaft, which is stationary, supports the pump elements (they run on MDS impregnated plastic bearings) - I suspect that in the original design there was a 'rope seal' (graphited rope) but has now been replaced by a rubber moulded seal. Its definitely an 8tpi with semi circular roots and semi circular crests with a 'major' flank thickness of 0.060" with a 0.0625" 'major' nut flank thickness (as best as can be gauged). The measurement in the female thread is 20.2mm for the root dia. The thinking is to make the tap to this major diameter (20.2mm) and then cut the replacement thread to a nominal 20mm o/d.

I think the panel agrees then - cut the thread then flute!



Howard Lewis22/02/2021 14:20:50
5733 forum posts
13 photos

If the root and crest are both semi circular, it looks like a Knuckle thread.

Kempe's Engineers Year Book does not mention it, but Parkinsons " First Year Engineering Drawing" gives the form as thread depth being, P/2 with the thread form radius as P/4.

So with 8 tpi, thread depth should be 1/16".

Recently I made a "sort of knuckle" thread using a 1/8th diameter toolbit , eventually plunged into 0.0625". It was merely a semi circular spiral groove, with a 4 mm pitch, cut into a 1 1/18" diameter bar. With a 1/8 ball bearing in a close fitting ring, it is used as an adjuster. I used minimum speed and back gear with plenty of neat cutting oil.



Jeff Dayman22/02/2021 15:34:06
2200 forum posts
45 photos

There is a DIN standard for knuckle thread just FYI, in case it helps see link below.

("Runden" or Rd thread, in German, if I am not mistaken)

Edited By Jeff Dayman on 22/02/2021 15:35:06

vic francis22/02/2021 16:30:03
74 forum posts
7 photos

Hi its tough project! How are you going to harden and temper the tap (s) ? I have found they tend to warp on homemade smaller long tapered ones... and the actual taper of the tap? Load per tooth? Is it possible to rough out by sc the female thread first and then finish with the tap?? The roughed out cut helps avoid it horribly becoming stuck!!

good luck look forward to seeing the update!

Oily Rag22/02/2021 20:31:20
526 forum posts
172 photos

Howard, Jeff and Francis,

Thank you all for your additional comments and observations; especially the reference to the DIN 20400 thread data that will come in handy!

Certainly I'm looking to single point the majority of the 'nut' to reduce the tooth load. As for hardening and tempering the finished tap I'm looking to heat in a small induction furnace and plunge vertically into an oil bath. Temper by re-heating and cooling in the furnace.

Anything to look out for with D2 tool steel or do I treat it in a similar fashion to Silver steel?

Thanks again,


David George 123/02/2021 09:46:28
1711 forum posts
500 photos

Hi Martin I made a square thread tap for finnishing phosphor bronze nuts on lathe leadscrew. I made it from silver steel, firstly I turned the shape of the tap then I cut the thread and then cut the cutting edge grooves. I then hardened and tempered it and it works well to finnish the nuts. 1/2 inch x 0.1 pitch 






Edited By David George 1 on 23/02/2021 09:49:42

Bob Worsley06/03/2021 16:38:49
104 forum posts

My picture of this thread is not very deep but nice curved form to it.

Sounds just like what is needed to roll boiler tubes to make them not smooth, to increase the heat transfer from gas to water. These are made by Babcock & Wilcox, also over 100 years ago.

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