Pratt Burnerd 6 inch 4 jaw independant lathe chuck

[Michael GilliganParticipant @michaelgilligan61133]

Posted by sean logie on 20/12/2016 21:59:16:

… 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 …

.

Sean,

I think we are all now agreed that your thread gauges are Whitworth form [and therefore 55°]

… although I remain a little dubious about their quality.

We still don't know if the 'daylight' I circled is because of a worn thread or a badly made gauge

.

… But the first two threads on the nose appear to be a good fit to the gauge, and therefore [if the gauge is accurately made] will be 55°.

Appearances can be deceptive, especially on photos; so I suggest that you double check the 'fit'.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

This is pretty rough [finger-painting on the iPad] but

The slope of the thread flanks is steeper than the equilateral triangle; so 55° would be a good guess.

.

MichaelG.

[peak4Participant @peak4]

Posted by Michael Gilligan on 20/12/2016 23:08:04:

This is pretty rough [finger-painting on the iPad] but

The slope of the thread flanks is steeper than the equilateral triangle; so 55° would be a good guess.

.

MichaelG.

Which if memory serves me correctly is 1 3/4" BSF

Which I think is 1 19/32" or 40.5mm tapping size, as is your preference; Make sure you double check rather than taking my word for it and boring over size. That said, a very slightly loose fit is arguably better than a tight one in this case, but I'm sure others will advise if I'm talking hogwash.

You may well be aware of the advantage of setting the top slide at 27.5 degrees for a 55 degree thread, but if not, have a quick browse around the internet about screw cutting in the lathe.

Personally I've never made a backplate, but when I do get round to it, I'll likely rough it out to almost the correct thread dimensions and then use a tap to make it look pretty. 1 3/4" BSF taps are certainly available on ebay from about £17, but I wonder if you could either borrow one locally, or if someone on here would loan you one if you stumped up for the postage.

Enjoy the next phase of your project, and Happy Christmas if we don't communicate before.

Regards

Bill

Edited By peak4 on 21/12/2016 00:47:28

Edited By peak4 on 21/12/2016 00:48:08

[mark smith 20Participant @marksmith20]

Bill, are you sure that isn`t 39mm or 1 17/32" for 1 3/4x 7 bsw. Regardless id probably add 1/2 mm on to that if 39mm is correct , as you say better slightly slack on the thread than too tight (as long as the register is pretty good)

[mark smith 20Participant @marksmith20]

Sean, these type of home made threading tools work well,

Described here how to make ,ive used round o1 steel (silver steel ground rod)to make them in the past for boring and one or two threading jobs  as the design is very similar .

The cutting angle can be accurately made using your compound(topslide).The shank can be flattened to hold in your tool post if you dont have a boring bar holder. The cutting end can be heated to cherry red and quenched in water  (supposed to use oil but water works fine)and used directly without tempering(on soft steels.aluminium and cast iron,etc..)

**LINK**

I think the design is taken from a German/Swiss design. I have a few deckel ones (photo here) for boring ,that work superbly.

 

Edited By mark smith 20 on 21/12/2016 09:46:44

Edited By mark smith 20 on 21/12/2016 09:48:20

Edited By mark smith 20 on 21/12/2016 09:49:07

Edited By mark smith 20 on 21/12/2016 09:59:43

Edited By mark smith 20 on 21/12/2016 10:01:54

[sean logieParticipant @seanlogie69385]

Great minds and that ,was thinking about it last night . I don’t have 01 steel ,I have some 30mm rod that came off an elevator, this was what I used for the boring bar I tried to drill a hole for a m6 tap unsuccessfully I may add . It seemed very hard material . I might try it out on that . I can hardly mark it with a file .
I liked the idea of a 1 3/4 tap but that would kinda take the fun out of it ….. I think 😜

Sean

[peak4Participant @peak4]

Posted by mark smith 20 on 21/12/2016 09:20:43:

Bill, are you sure that isn`t 39mm or 1 17/32" for 1 3/4x 7 bsw. Regardless id probably add 1/2 mm on to that if 39mm is correct , as you say better slightly slack on the thread than too tight (as long as the register is pretty good)

Mark. it might be if it was BSW, but that would be for a for a coarser 5TPI thread. i.e. bigger ""teeth", thus smaller core diameter.

Sean's seems to be 7TPI, which is more akin to BSF.

I certainly wouldn't try cutting the whole thread with a tap though, just tidying up an almost finished one.

The other advantage of having a sharp tap on hand before starting, the ones I looked at were 4 flutes, is that you could measure the tap using 3 wires and a micrometer, and compare it to the lathe nose thread.

Speaking of threads, there's another one going on this forum about making backplates, which is probably worth a read.

[mark smith 20Participant @marksmith20]

Bill, you may be right i dont know that much about bsw/bsf lathe spindle noses. My southbend is 1 1/2 x 8 un which is said to be a special purpose tap,hence why so bloody expensive to buy.Plenty of  cheap 1 1/2" x 12 UN taps on ebay .

If 1 3/4"x 7 is a normal thread BSF then all the better. In that case yes i would drill to about 40.5mm

Edited By mark smith 20 on 21/12/2016 13:09:30

[sean logieParticipant @seanlogie69385]

Well…. bored the backplate out to the exact dimension I need ,then my son came out to see how I was getting on ,full of brovado in showing him and delighted that he was interested in what I was doing I stupidly stupidly bored it all the way through ,backplate is now useless. Lesson learned …. don’t get distracted 🤐

Marry Xmas

Sean

Edited By sean logie on 23/12/2016 19:29:14

[mark smith 20Participant @marksmith20]

Sean , what do you mean all the way through? At the thread root diameter?

Edited By mark smith 20 on 23/12/2016 20:21:03

[sean logieParticipant @seanlogie69385]

Bored through at 1.68 " major . Just been discussing it ,I should've left some meat for the nose The extreme front which is 1.5" .

merry Xmas

Sean

[peak4Participant @peak4]

Sean, are you sure that it's an issue?

My own backplates are all the same, as they all are for a Myford.

There's only two internal sizes, one for the register, and another for the thread. The end nearest the tailstock doesn't need to be any smaller with an additional step in it, just a through hole, threaded all the way..

As far as I'm aware, and others will correct me. You need a step for the register and then a thread all the way through for the spindle nose, machined at the same setting of the 4-jaw, so they are concentric with each other.

The plain bit, at the right hand end of the spindle nose, is just to help align the chuck before screwing it on to the spindle. All the location is done by the thread holding the chuck onto the register at the headstock end.

Edited By peak4 on 23/12/2016 21:08:49

[sean logieParticipant @seanlogie69385]

You are right Bill it won't be an issue , I'll have to cut the register after I thread the bore will I not ? .

[peak4Participant @peak4]

Sean, I'm honestly not sure the best order to do the two operations, as I've never made a backplate from scratch, just machined the tailstock side to fit a chuck.

Sorry must go evening meal on table

[Keith LongParticipant @keithlong89920]

Sean – I've made a few nose adaptors and backplates for my lathe and I have always threaded the item right through and then cut the register into the end of the thread, all at one set up, so you're working into what will be the "back" of the backplate / adaptor. That way you can see what is happening with the thread at the start without having to peer into a recess – although with the diameter of the one you're doing that shouldn't be too awkward. It also mean that you can clean up the start of the thread as you cut the register, and I tend to make the register recess deliberately slightly longer than needed to ensure that the register actually seats on the abutment face rather than get the backplate (or whatever) thread running into theleading edge of the register before the register is properly seated. So far for me that's worked out OK and I'm happy with the run-outs that I've got on the various items that I've machined.

[peak4Participant @peak4]

Lovely leek and butterbean bake, followed bt lemon cheesecake -yum yum.

I'd be very tempted to follow Keith's advice, depending on how rigid your boring bar is, and how much play you have in the various slided on what must be quite an old lathe. Hence my following caveat, as I know my lathe's well worn

If it's not very solid somewhere along the line, don't forget that, in effect, when opening out the threaded hole to register size, you will be taking an intermittent cut as you machine away the thread you've just cut.

N.B. I'm definitely no expert on this, but I'd be tempted to make an accurate plug gauge exactly the same size as the register diameter before starting the final machining of the backplate. That way you will have something to test fit into the plain hole to get the diameter absolutely bang on to size.

I'd say that's even more of a good idea if you are limited in your measuring equipment. One way to consider this is that you need accurate repeatable comparative measurements, rather than absolute ones.

It's going to be much easier to accurately compare a plug gauge to the register diameter, than it is to compare an outside measurement of the register, to the inside measurement of a hole, as in the former case you will be relying on a single instrument to measure both outside diameters. The register hole can then be machined to the fit the previously made plug gauge. It doesn't really matter, in this case, if your inches are a different size to mine, as all the measurements will be on the one instrument in the one workshop. Vernier or digital calipers, are effectively two separate instruments, one for inside and one for outside diameters, but on a common body; they use two different measuring faces.

Apologies if I'm teaching you to suck eggs; I would never intentionally talk down to anyone. Even for comparative sizes, don't just take the one measurement of each component; repeat it several times until you get consistent results every and every time.

[mark smith 20Participant @marksmith20]

Sean, as i mentioned several posts back thread it right through by holding it in your 3 jaw , you may have to remove the chuck with the piece still in to ensure concentricity and try the fit on the spindle nose ,hopefully you will have done it right and it will fit the thread, if not you may have to `pick up` the thread and take a little more off. If your unsure what i mean see these videos.The first is external and the second is internal on a back plate.

**LINK**

**LINK**

Once you have it threading onto the spindle ,you can remove the work from the chuck and use a `spacer`(as i described several posts back) with the backplate mounted backwards on the spindle to cut the register. You can keep taking it off and turning it around and trying the register .

This is how i would do it but others may have different ideas.

 

One question though where did you get the 1.68" value from?? Isnt that about 80-90 thousands of an inch more than what it should be?  A little extra is ok but 80 thou`s seems alot.

Edited By mark smith 20 on 24/12/2016 00:27:36

Edited By mark smith 20 on 24/12/2016 00:28:02

[sean logieParticipant @seanlogie69385]

Merry Xmas everyone , I've managed to thread the back plate ,threads weren't as tight as I'd like but the register was a nice snug fit,as was the register on the back plate so all in all very happy with my first proper job on the Fortis .Which leads me to think that my cheapo calipers can't be that bad . Put a dial gauge on the 4 jaw and it's not bad at all,I do intend to make another back plate with nicer fitting thread,not sure if it'll be made of cast iron or not (horrible dirty stuff ) .

I intend to make a mount for the cross carriage to do some light milling on ,so the first job for the 4 jaw was a biggy . I got myself a rough cut of 1 1/4" steel plate that was roughly cut to a 8" circle and i mean roughly cut . I have one side face and half the edge clean up ,which was a real test for the new 4 jaw and i suppose the Fortis and myself because of the interupted cut . Another job on the list is the adjustment of the motor ,at the moment it's the weight of the motor plus a long spring pulling against the drive belt and it's not ideal as the motor kind of bounces ,not a lot but i can see it moving .

Thanks again for every one's input very much appreciated . Very steep learning cure for me .

Merry Xmas

Sean

[mark smith 20Participant @marksmith20]

Well done Sean, at least youve learnt something I agree about the cast iron, last time it took 2-3 days to get it off my hands.

Edited By mark smith 20 on 26/12/2016 20:27:55

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