Pratt Burnerd 6 inch 4 jaw independant lathe chuck

[sean logieParticipant @seanlogie69385]

I was thinking of getting a friend to do it . How would the part be held . Mark i think it's about 45 mm .

Sean

[mark smith 20Participant @marksmith20]

45mm should be ok, does the back plate have a boss on the back or is it full width?? I was thinking if it had a boss you could put it in the 3 jaw depending on the size of the 3 jaw using outside jaws?

The above hole that need bored is just over 1 /12" , more like 39mm  or 1 17/32"

Also are you certain the spindle nose is BSW thread ie. 55 degrees and not UNC 60 degrees?

Edited By mark smith 20 on 17/12/2016 19:56:01

Edited By mark smith 20 on 17/12/2016 20:14:41

[sean logieParticipant @seanlogie69385]

I'll take another reading in the morning of the thread . The back plate does have a boss of about 10-12mm .

Sean

[mark smith 20Participant @marksmith20]

But whats the diameter of the boss?

[sean logieParticipant @seanlogie69385]

I'm not sure,calipers are out in the shed and it's pissing down ,I've just downloaded a measuring app lol . Od of boss is 55mm or just over 2" ,thickness Id to od is about 10mm .

Sean

[sean logieParticipant @seanlogie69385]

Mark,I’ve added a photo of the gauge on the thread . Not sure how I tell what angle the thread pitch is .

Sean

[mark smith 20Participant @marksmith20]

Hi Sean i cant understand the thread gauge, is it a BSW gauge and is it definitely 8 tpi? The thread looks flattened slightly on the top. G often refers to whitworth pipe threads which can be 7 tpi.

Could you possibly take a photo like this,which shows my southbend nose which is 1 1/2 x 8 UNC (but 60 degrees).

 

Edited By mark smith 20 on 18/12/2016 15:22:56

[sean logieParticipant @seanlogie69385]

Mark I’ve just checked and it is a BSW gauge. Would the flatness not be ware and tear . The gauge seems a tight fit . And the backplate won’t fit in the chuck with the boss facing out ,so I’m not sure how I’m going to make the register .

Sean

[mark smith 20Participant @marksmith20]

If you plan on threading the back plate you can hold the back plate by the boss and bore out to the correct diameter ie. 39mm or 1 17/32".

Then you thread the hole for 1 3/4" x 8 whitworth right through the back plate.(making sure the thread tool can do this without hitting the chuck.

Once youve done this you mount the back plate directly on the spindle with the boss facing you. But using a temporary spacer (a piece of pipe or something just slightly bigger than register diameter and of the right depth minimum 0.17"  according to your measurements ) between the register and the back plate ,so that the boss side is clearing the spindle nose end..

This will allow you to machine /bore out the short area for the register, which you can keep taking off turnng around and checking until you get it right and its seating properly on the spindle register.

Once you have the back plate on the spindle you can then go about truing up the surfaces ,turning the chuck register and doing the mounting holes etc..

Edited By mark smith 20 on 18/12/2016 15:37:52

 

Edited By mark smith 20 on 18/12/2016 15:42:33

Edited By mark smith 20 on 18/12/2016 15:48:45

Edited By mark smith 20 on 18/12/2016 15:50:40

[sean logieParticipant @seanlogie69385]

Thanks guys ,I’ll pluck up the courage and do it in the 2 wks I’m off at Xmas . Been testing the run out at max distance betwwen tailstock and chuck . My gauge is telling me .05 mm over 45mm .

Sean

[Michael GilliganParticipant @michaelgilligan61133]

.

Sean,

I don't think I would be happy with this as an example of thread-pitch gauging … there is a lot of daylight showing.

… Have you checked the gauges ?

MichaelG.

.

P.S. … That first character looks remarkably like a 7 to me.

Edited By Michael Gilligan on 18/12/2016 20:39:59

[peak4Participant @peak4]

Sean & Mark, I hope I'm not muddying the waters here, but I've just checked both my sets of BSW thread gauges; first set only has the TPI marked on the blades and is stamped "7G", the other, like Sean's is stamped 7G 11/8" 11/4" but these diameters only refer to BSW threads, so don't be misled

N.B. The thread gauges are marked up as BSW, though they can be used for BSF, as they are the same thread form i.e.55 degree, rounded crests etc. just different TPI

This tallies with my thread tables as 7TPI for both these two BSW threads (11/8" & 1/14" ), on a normal standard nut and bolt, so I'd suggest that the lathe nose is actually 7 TPI not 8 TPI

In your case you seen to have a 13/4" lathe nose with 7TPI. This would equate to 13/4" BSF

The actual thread measurement for BSW/BSF will always be smaller than the actual thread nominal OD size as the thread form has rounded peaks.

Ergo, you seem to need a backing plate with a 13/4" 7TPI BSF thread form, with an appropriate register diameter to suit your lathe.

It does strike me a little odd that you've measured/shown 1.8" register and 1.7" thread diameter on your sketch. I'd have expected the thread to be slightly smaller to allow for the rounded crests (@ 1.6585" )and the register to be almost bang on 1.75" Are you sure your measuring equipment isn't about 50 thou over reading?

 

I think Michael & I  were typing posts at the same time, but I got delayed part way through. 

I wonder if the daylight is just due to half a century of wear on the thrust side of the thread on the nose? It won't make any difference to the accuracy of the chuck Sean, as it's the register which positions it; the thread is just to keep it there, but it might explain why you could partially engage what is effectively a 8TPI nut on an 7 TPI bolt.

Could it be the thread binding up, and not the register being the wrong diameter?

P.S. I'm not trying to teach anyone to suck eggs here.

Edited By peak4 on 18/12/2016 21:30:57

[Michael GilliganParticipant @michaelgilligan61133]

Posted by peak4 on 18/12/2016 21:12:18:

… I wonder if the daylight is just due to half a century of wear on the thrust side of the thread on the nose?

.

You may well be correct … I was simply commenting upon what I could see [maybe a fault of the thread, or maybe of the gauge]

MichaelG.

[MalcBParticipant @malcb52554]

Posted by Michael Gilligan on 18/12/2016 20:35:52:

Wha

.

Sean,

I don't think I would be happy with this as an example of thread-pitch gauging … there is a lot of daylight showing.

… Have you checked the gauges ?

MichaelG.

.

P.S. … That first character looks remarkably like a 7 to me.

Edited By Michael Gilligan on 18/12/2016 20:39:59

What you are looking at is a set of Unified Thread Gauges and not whit.

7 TPI covers 1 1/8 and 1 1/4" and thread angle as you know is different to Whit at 60 vs 55 deg

You need to re-gauge with Whit gauges

Edited By MalcB on 18/12/2016 21:43:49

Edited By MalcB on 18/12/2016 21:47:00

[peak4Participant @peak4]

What you are looking at is a set of Unified Thread Gauges and not whit.

7 TPI covers 1 1/8 and 1 1/4" and thread angle as you know is different to Whit at 60 vs 55 deg

You need to re-gauge with Whit gauges

Edited By MalcB on 18/12/2016 21:43:49

Edited By MalcB on 18/12/2016 21:47:00

Maybe, but my own thread gauges are certainly stamped up as BSW and have a blade engraved as 7G 11/8" 11/4"

Regardless, I'm still not convinced of the need to re-gauge anything as 7TPI is still 7TPI whether it be Unified or British Imperial.

I guess the lathe nose could be UNC thread form, but I'll still have a punt at 13/4" BSF, even though it did originate from Clausing in America.

Maybe look for a 1/2" thread somewhere else on the lathe and see if it's 12/16 TPI (BSW/BSF) or 13/20 TPI(UNC/UNF) , though of course it wouldn't definitively point to a 60 degree thread on a different bit of the machine.

Edited By peak4 on 18/12/2016 22:11:05

[sean logieParticipant @seanlogie69385]

Well I tried cutting a 8tpi and it doesn't marry in with the gauge . I bought the gauges off eBay , I check my purchase history and the description says BSW & metric . . I'll try a 7tpi tomorrow and see . I only have a cheap calliper set but they seem to be ok ,I've check them against a Brown & Sharp micrometer .

 

Sean

Edited By sean logie on 18/12/2016 22:14:42

[peak4Participant @peak4]

Posted by sean logie on 18/12/2016 22:09:37:

Well I tried cutting a 8tpi and it doesn't marry in with the gauge . I bought the gauges off eBay , I check my purchase history and the description says BSW & metric . .

 

Sean

I assume your newly cut 8TPI thread marries up with your 8TPI 1" engraved gauge, your lathe marries up with your 7TPI gauge.

Did you ever follow the links in my post at the top of the previous page?

Edited By peak4 on 18/12/2016 22:17:03

[MalcBParticipant @malcb52554]

Sean, Apolgies as I think i have them reversed. My M & W gauges show same as yours so when i put them together i get similar gapping error to your photo which looks as though you may have a unified thread at 7 tpi. Will try another post with pic of both if i can get it to download OK

[MalcBParticipant @malcb52554]

[sean logieParticipant @seanlogie69385]

Posted by peak4 on 18/12/2016 22:16:42:

Posted by sean logie on 18/12/2016 22:09:37:

Well I tried cutting a 8tpi and it doesn't marry in with the gauge . I bought the gauges off eBay , I check my purchase history and the description says BSW & metric . .

Sean

I assume your newly cut 8TPI thread marries up with your 8TPI 1" engraved gauge, your lathe marries up with your 7TPI gauge.

Did you ever follow the links in my post at the top of the previous page?

Edited By peak4 on 18/12/2016 22:17:03

Your right regarding the 7tpi I totally overlooked the obvious and yes I've been on that site a thousand time ,but I'm not paying £40 for a manual 😁 I'm a Scot for goodness sake lol . There are a lot of discrepancy between the causing 100 and the Fortis/Broadway 4800 lathes .

[Michael GilliganParticipant @michaelgilligan61133]

It doesn't get us along very far, but here is a nice clear table of BSW/BSF threads: **LINK**

https://mdmetric.com/tech/thddat8.htm

MichaelG.

.

and the Unified ones: https://mdmetric.com/tech/thddat9.htm

 

Edited By Michael Gilligan on 18/12/2016 22:51:46

[peak4Participant @peak4]

Posted by sean logie on 18/12/2016 22:29:39:

 

Your right regarding the 7tpi I totally overlooked the obvious and yes I've been on that site a thousand time ,but I'm not paying £40 for a manual 😁 I'm a Scot for goodness sake lol . There are a lot of discrepancy between the causing 100 and the Fortis/Broadway 4800 lathes .

For sure, you can get a very nice bottle of malt for 40 quid.

It's hard to tell from the photo of your gauge on the nose thread; you would be able to see better in the flest, so to speak. It looks to me a bit like the thread fits the gauge better on the tailstock side of each thread buttress than on the headstock side. I wonder if that's due to wear on the side which carries the force when the chuck is tightened against the spindle. If it fits one side better than the other, and is a good fit on that side with a BSW gauge, I'd suggest that the included angle of the thread, when it was originally cut, was 55 degrees. It's now worn, mainly on one side, so it appears to be even steeper on one side, hence the air gap.

To be honest, since the thread is worn anyway, I can't see it making that much difference if the internal thread on your backplate is 55 or 60 degrees, it's only there to hold the chuck in place. The important thing is the correct TPI and register diameter.

I've also got a couple of sets of cheap digital calipers, as well as some decent Mitutoyo ones, and I can certainly get them to over read sometimes. Whilst in the ideal world calibrated accurate reading are clearly to be desired, sometimes repeatable comparative measurements will suffice within the same workshop. Also my cheaper calipers measure different internal diameters to external. The external is pretty accurate as you can zero the display when they are closed, but to zero them for measuring internal holes I measure a set internal diameter on an external micrometer, and just add/subtract an inch from the caliper reading.

The issues arise when you get someone to make something to your measurements, and is doesn't fit as their micrometer is different to yours.

I can tell a tale of the Belgians having different sized inches to ours pre-war, but that's for another day.

Edited By peak4 on 18/12/2016 23:33:29

[mark smith 20Participant @marksmith20]

Im glad some others have piped up, i admit i was totally confused by Seans threadgauge which is why i asked if he was sure it was 8 tpi, and requested a photo with a ruler over the crests (i thought it looked like a BSW pipe thread gauge, especially with the G)

So Sean you need to make sure you have the TPI correct before you proceed, then try and figure the thread angle ,if it is BSW.

[sean logieParticipant @seanlogie69385]

My head’s minced with all the different measurements lol . Doubt if the 4 jaw will be here before the new year anyway, heard this morning that postal services are going to be severely disrupted due to strike action. So should have plenty time to practise this 7 tpi thread lol .

Sean

[sean logieParticipant @seanlogie69385]

I now have the 4 jaw so no excuses 😣. So we're agreed on a 7 tpi , but we haven't agreed on 60 or 55 degree ,a member on here whom I've been pming has the same lathe says his nose thread is 55 ,but he has a mt3 in his tailstock I have a mt2 ,see where I'm coming from (should've bought a ml7 &#128513 each machine have their differences even though they are the same model ,if that makes sense . I see different sizes for my bore out there . My thread major size is 1.75 + – a thou or two ,what's my bore size going to be . .

I need a holiday …. roll on Friday 😋

Sean

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