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Harrison 155 imperial thread cutting

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Ray Sub20/01/2018 10:31:40
5 forum posts


My Harrison 155 is a metric lathe with a 6mm pitch lead screw. The manual says that to cut imperial thread I need:

Top shaft. 30t

Intermediate shaft 120-127

Bottom shaft 60t

I have all the gears (30t, 120t & 60t) bar the 127t.

I can’t find/get a 127t change gear, if I used a 126t gear which I can get would the error be too great for thread cutting?

Any help/advice appreciated


norm norton20/01/2018 11:06:52
114 forum posts
6 photos

Hello Ray

Your metric Harrison 155 should be the same as my Harrison 140, which came as metric only.

The standard metric gear set is 25-100-80 (top-intermediate-bottom).

I think, that if you put a gear set of 50-63/40-120 on (which I have) then the metric gearbox will give the full set of imperial gear pitches, with the layout looking very similar to the imperial gearbox. I managed to buy the extra gears by asking around the dealers for a month. I don't think you will find a 127 for the Harrison, and I recall that it might not fit the quadrant anyway.

There is more on this and some tables on the Harrison Yahoo site, but it is not easy to find past threads. If you message me with your email I will send you a copy of the table that I made.


Jon20/01/2018 20:52:23
997 forum posts
49 photos

Error will be too great, 1 tooths a lot unless mak the threads real sloppy.

I did have a 126 and a 127T for my scrapped 140 along with a full set of change wheels about 12 of them from memory.

Change wheels near on impossible to get 8 years ago, might come across the odd one so make the most of it.

Pete Rimmer21/01/2018 01:09:46
685 forum posts
49 photos

If your lathe is supposed to use a 120/127 combination for the metric-imperial transposer then you can susbtitute this for 34/36 and the result is very close indeed - less than half a thou over an inch of thread.

So for a 6mm pitch screw:

6*36/34=6.35294 x4 /25.4= 1.00046"

You'd be hard-pressed to cut a thread that accurate.

Jon21/01/2018 17:05:09
997 forum posts
49 photos

Think you would be even harder pressed to get hold of a 34 and 36 tooth change wheel not to mention the size difference between a 120 to 34 and 127 to 36, may not mesh with the other gears needed about 16 to 18" away!

Clive Foster21/01/2018 17:29:41
2157 forum posts
73 photos

The extension gear sets for dividing heads often contain 34 and 36 tooth gears. Come up on that action site a time or five a year, usually sell cheap because folk don't recognise what they are. Once separated from their mating dividing head its nigh on impossible to figure out which one they actually fit.

HPC list 34 and 36 tooth gears in most sizes. Larger ones aren't exactly inexpensive tho'. Sometimes its best to bite the bullet and just buy something retail rather than wait ages for a bargain. Its what I did to fill out the set for my Smart & Brown 1024. Still be looking otherwise!


Pete Rimmer21/01/2018 18:08:04
685 forum posts
49 photos
Posted by Jon on 21/01/2018 17:05:09:

Think you would be even harder pressed to get hold of a 34 and 36 tooth change wheel not to mention the size difference between a 120 to 34 and 127 to 36, may not mesh with the other gears needed about 16 to 18" away!

The 127 and 120 are compounded so the actual diameter difference is 9.21-2.57=6.64 inches. You might need an idler in the cluster but you could choose any appropriate diameter for that. I haven't studied the Harrison gear cluster.

It's an option anyway.

Brian Wood25/01/2018 12:39:25
2151 forum posts
37 photos

Hello Ray,

I've just stumbled across your post and the problem you have.

Pete Rimmer is quite right with a working alternative of 34/36 for the translation gears of 120/127, it is based on a ratio module of 17/18.

I don't know what gears you are equipped with but by multiplying the basic ratio (17/18) both top and bottom by 5, you finish up with a combination of 85/90 which should nicely fill the gap to link your top shaft and bottom shaft. The threading error that gives you is still negligible.

If you still have a meshing problem with those sizes try 35 and 70 for top shaft and gearbox drive instead [if you have those] or an idler in the chain if not.

I hope that helps you

Regards Brian

Ray Sub26/01/2018 20:16:16
5 forum posts

Thank you all for the comprehensive and sound advice. I have now a way ahead with options and a better understanding, many thanks.

Kind regards


Carl Wilson 426/01/2018 20:40:32
670 forum posts
53 photos
Just to let you know that I can 3d print, highly accurately and in high impact materials any change gear you may require. Pm me for details.

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