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Left Hand Trapezoidal Warco Lathe

Tail stock spindle

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Trevorh02/11/2015 11:39:46
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303 forum posts
87 photos

Hi All,

Looking for some advice on how best to cut a left hand thread Trapezoidal

In a moment of madness I cut a short peice off the thread from the Tail stock spindle because it kept ejecting the M2 taper sleeves too soon, My lathe is the Warco GH1224

by this i mean as I retracted the post, at the 1" mark, it would eject the sleeve, so in my strange reasoning if I reduced the overall length it would let me retract further, so out came the hacksaw....

now having shot myself in the foot - because now I can only extend to 3" and not 4" I want to make a new shaft BUT how do I go about cutting this square form thread....

As best as I can measure, it appears to be a 2.5mm pitch 12mm dia

offers of advice please on what the tool bit should be as well as speeds and feeds

Material was going to be en1 but if you guys know better....

thanks in advance

pgk pgk02/11/2015 12:06:00
2361 forum posts
293 photos

A quick search suggests that metric trapezoidal type threads at 12mm diameter are usually 3mm pitch but 1/2in imperial and us threads are 1/10th inch pitch whichwould be closer to your 2.5mm...With the trapezoidal at 30deg and the acme at 29degs. My brain;s not in gear enough to hunt up the lead angle for modifying the tool side rakes and my machinery's handbook is down in the shed.

Ian Parkin02/11/2015 12:15:27
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987 forum posts
232 photos

Trevor

you still have the same travel surely

you now go from 0" on the scale rather than 1"

but you run out at 3" rather than 4"

I would keep the quill deeper in the tailstock

Trevorh02/11/2015 12:16:42
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303 forum posts
87 photos

Thanks pgk, it should be metric as the lathe is only 7 years old and chinese at that...

I just can't seem to understand what the profile of the tool bit needs to be to cut a square thread and yet traverse along to produce the angle

I was thinking it would be something like a parting off tool but with a side angle

I have found the correct gearing off the screw cutting chart - to cut either a 2mm or 2.5mm so that part is sorted

Cheers

Trevorh02/11/2015 12:19:02
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303 forum posts
87 photos

Hi ian,

This all came about firstly because as I mentioned earlier about the ejecting early but now I have a couple of jobs where I need the full 4" travel, I know I can just reset by moving the tail stock but it was originally capable of 4" travel so would like to get it back there

cheers

David Clark 102/11/2015 13:03:45
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3357 forum posts
112 photos
10 articles

Buy a new tailstock barrel or fit a tailstock lever feed attachment.

Thor 🇳🇴02/11/2015 13:46:02
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1435 forum posts
41 photos

Hi Trevor,

For small trapezoidal pitches you may be able to cut them with one tool. For larger you first use a tool resembling a parting-off tool - as you say, and finish with two tools that cut the angle part on each side.

Here is a link you may find useful, it is primarily for Acme but the technique is the same.

Thor

Neil Wyatt02/11/2015 13:53:21
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18807 forum posts
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Profile need to fit the thread from above, but the tool needs to be angled when viewed from the front to fit the helix angle of the screw.

Neil

pgk pgk02/11/2015 13:59:05
2361 forum posts
293 photos
Posted by Trevorh on 02/11/2015 12:16:42:

Thanks pgk, it should be metric as the lathe is only 7 years old and chinese at that...

I just can't seem to understand what the profile of the tool bit needs to be to cut a square thread and yet traverse along to produce the angle

I was thinking it would be something like a parting off tool but with a side angle

I have found the correct gearing off the screw cutting chart - to cut either a 2mm or 2.5mm so that part is sorted

Cheers

Chinese and modern but hey still make more for the US market so some parts may still be imperial.

As to the shape of the tool (never having cut one of these threads myself).. then from the top It's going to be a 29 r 30 degree point truncated at the tip either as a very shallow radius or as a flat with two tiny corner radii. From the front it's gong to look thinner at the bottom than the top but the front edge will 'lean' by the lead angle. You need to dig out a chart for that angle to add and subtract from the side rakes of your 29/30 deg tool for the material you cut and to find the width of the tip 'flat'. You'll be feedng it in at roughy half that 29/30 deg angle for most of the cut then going in square with the cross slide for the las pass or two as a straight form tool (so the half angle doesn't have to be perfect)

I'm fairly new at al this too. 'Tom's techniques' (youtube) describes shaping thread tools a lot better than I do and after following his tutorials I happily single point any i don't have a die for (lazy when possible)

...or ypu buy a length of acme/trapezoidal allthread <g>

KWIL02/11/2015 14:06:13
3445 forum posts
66 photos

Trapezoidal Threading inserts are not that expensive and you can make a tool holder, so why fiddle about with grinding a cutter?

Roderick Jenkins02/11/2015 14:07:24
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2129 forum posts
586 photos

I had my first go at cutting a square thread for my hardness tester. I ground the tool from a piece of 3/16" dia. hss. I think I used 2 degrees relief all round. The width is slightly less than the 1/16" that the 8tpi needed so that I could shave the last few thou off to measure to the correct size. You can rotate the tool in the holder to get the correct angle to give you relief on the cutting edge. I was pleasantly surprised how easy it was to cut the square thread in EN1A. If I was cutting an acme or trapezoidal thread then I would rough out using this tool to the correct depth and then grind a second tool to the correct form to finish off.

square thread cutting toolbit lr.jpg

HTH

Rod

Trevorh02/11/2015 14:17:21
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303 forum posts
87 photos

Thank you all,

I think I am going to try to attack this on 2 fronts

1st is a cop out but the easiest - just found out warco have an open day this coming Saturday so will go and see if they have a new one available or at least check out the price of it

2nd I like the idea of being able to machine my own - so will have a go following the instructions in the link above and see how I get on

Thanks again for the comments and advice - depending on how bad I am at cutting the thread I may or may not let you know how I get on

cheers

Peter G. Shaw02/11/2015 14:43:40
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1340 forum posts
44 photos

I have recently cut a trapezoidal thread for my Warco 220 lathe. This was a 12mm dia x 2mm pitch RH thread for the cross-slide leadscrew and was about 130mm long.

The tool was ground up from an old file as best I could (tip width was supposed to be 0.6784 mm wide so I made it almost 0.68mm wide) and has a 30 degree inclusive angle. I slowly cut the thread using my minimum speed (125rpm) increasing the depth of cut by 0.05mm or 0.1mm at a time until I reached the required depth of 2mm. This was actually the third attempt, the first I made a mess of and broke the tool, and the second being experimental was deemed a success as it almost fitted first time. This, third attempt, worked first time. I haven't actually finished what I'm doing but see no reason why it shouldn't be a success.

I used EN1A.

I see no reason why a LH thread should be any different - it just means that the clearances etc are the other way and the leadscrew turns "backwards" rather than "forward".

Regards,

Peter G. Shaw

Ajohnw02/11/2015 15:11:22
3631 forum posts
160 photos

If it's cut with one tool it's best to set the compound slide to the half angle as some one once said to me that way one of the sides can be close to the correct angle. It also reduces the cutting load a lot especially on acme types.

For truly accurate work a shadow graph or microscope is needed to check the size of tools made from hss however tips can be bought these days so it's a lot earier, maybe even making your own holder.

These types of tips can often be bought in 1 offs eg

**LINK**

winkI often mention them. No connection etc but I find them to be a good reliable supplier that also sell good tips at reasonable prices as well.

John

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Trevorh02/11/2015 15:42:19
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303 forum posts
87 photos

Hi John,

That looks like a 3rd approach and not too expensive

thank you for the information

I have just been looking on youtube and have found a couple of examples

Time to get organised and have a go

cheers

Ajohnw02/11/2015 16:28:05
3631 forum posts
160 photos

I'd hope the video's mention a travelling steady as you might need to use one.

John

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Trevorh02/11/2015 16:29:37
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303 forum posts
87 photos

Hi John

Actually they didn't but It is only 250mm long so I will just use a revolving centre to keep it true

cheers

ega02/11/2015 17:05:35
2324 forum posts
190 photos

Trevorh:

I don't think you mentioned the diameter but I think a travelling steady is likely to be essential for this kind of job. The steady fingers will need to be wide enough to span the crests and you will need to debur with a file after each pass.

Ajohnw02/11/2015 17:49:27
3631 forum posts
160 photos

I would if I were you. It's not difficult just fiddly when the od is being turned initially.

There is a decent video on use that shows setting up better than this one - but notice the comment about bulging in the middle on this without one - even on 12mm

**LINK**

This one shows screw cutting with one - where you really are likely to need it

**LINK**

Doubleboost is usually pretty good. On that one I would be inclined to work away from the chuck with a centre a bit loosely set. The reason being that the bar is being pulled throughout the cut - not pushed which might cause it to bend if there is any lathe misalignment. If the centre is well out of alignment it will have to be used to just stop the bar whipping about and not really be in contact with the work at all. Strange things can happen if it is. The centre will be trying to keep the work in one place and the steady in another. On that basis it could be argued that it's always best to work away from the chuck. You'd best have a play around as you might find best results are obtained when the slides are a little loose - depends on how precise the lathe is. When the same principle is used on a well worn capstan lathe I have seen both the chuck and the saddle move when the work went into the cutter box - good accurate parts came out as what the machine does hardly matters.

They give a rounder and more consistent size plus a better finish with sensible cuts that force the work up against the steady - this is also why the work comes out round and the same size all along if done correctly.

Boxford never updated their steady for modern tool posts. It's for a lantern type but I reckon it could be used with a qctp by using the side away from the chuck or angling it etc but the steady jaws ideally need to very close to the cutter.

John

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Trevorh03/11/2015 09:05:20
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303 forum posts
87 photos

Valid points thanks,

Had a but of an issue with my supports that came with the lathe - see Warco Supports thread

I hope to be in a position this weekend to attempt to cut the thread

Just experimenting at making a hss tool until the inserts arrive

cheers

Edited By Trevorh on 03/11/2015 09:06:05

Edited By Trevorh on 03/11/2015 09:08:19

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