Thread dial indicator

[John CoatesParticipant @johncoates48577]

Had a snoop around my Barker  x 24 lathe this weekend, cross referencing to the pictures from http://www.lathes.co.uk. It’s not set up yet but I am sorting the garage out for its eventual home.

 

It would seem that the Barker originally had a thread dial indicator. One of the lathes in the pictures has it clearly visible but the other two lathes do not have one. Mine is one of the other two. There is the mounting on the lathe saddle/apron to accept a thread dial indicator.

 

Now first impulse was to go off to eBay to see if there was one there. Of course there is stuff all for my Barker but there were a couple for Myfords. Having been burnt before in buying stuff and thinking it will fit I held back from bidding or buying it now, afraid I would end up with something that would not fit.

 

Having spent some time reflecting, it would seem that these should be relatively easy to make (you can see why this is in the Beginners section can’t you!). They seem to be a tube fixed to the saddle/apron, with a rod inside which is threaded at one end to mesh with the leadscrew and with a dial numbered 1 – 4 at the top end.

 

As the chance of finding an original Barker thread dial indicator is less than Lord Lucan turning up astride Shergar, have I understood the construction of one of these devices correctly? Not having one to hand I am guesstimating from the photos on eBay and lathes.co.uk

 

[John CoatesParticipant @johncoates48577]

Are they easy to make?

[John Stevenson 1Participant @johnstevenson1]

Usual build up is 4 times the thread pitch for the gear so for an 8 tpi screw you need a 32 tooth gear.

Looking at the lathe in question this looks about correct

 

Bad news is that to mate with a 8 tpi screw you need a gear that is measured as Circular pitch, CP, as opposed to a normal gear of Diametrical Pitch DP

Also this gear has to be helical to follow the helix path of the screw.

 

So you are looking for a helical gear of 0.125 CP or  25.13 DP

 

Good news is because this is only driving a dial you can ignore all this and fit a normal gear of 24 DP and it will follow good enough for your application.

 

John S.

[Jeff DaymanParticipant @jeffdayman43397]

Your dial gear does not have to be helical if you angle the shaft at the appropriate helix angle for best mesh. Rather than try and measure/do the math/set it up it may be easier to just make the mount bearing adjustable with a pivot and clamp screw and set the angle for best mesh.

 

The gear does need to be a close mesh on the leadscrew, with least possible backlash, for good thread start accuracy. The adjustment above may actually enable this on its own, by permitting backlash to be taken up helically rather than axially as usual. (Easy to make as well.)

[Nigel McBurney 1Participant @nigelmcburney1]

Hi      I assume your leadscrew has a keyway for the feed,so any gear will require a face width of at least 3 times the keyway width to stop the keyway catching the gear teeth.I have a Colchester thread indicator which has a plastic gear,this may be the way to go as if there is any meshing problems it will not harm the lead screw. Thinking about this topic ,something occurred to me about involute gears meshing with a rack and after all the leadscrew is only a rotating rack,Most leadscrews are acme thread with a flank angle of 14.5 degrees and early involute gears used 14.5 degrees pressure angle,I wondered were the common angles deliberately selected so that gears could mesh with acme screws,or was it accidental and which came first. 

[Keith Wardill 1Participant @keithwardill1]

Hi, John,

 

Somewhere I have an old ME bound copy from around 1950 with a detailed article on construction of a thread dial attachment for a lathe (and I would not be surprised if there others around). V busy at the moment, but I will look it out in the next couple of days, and see if there is some way I can get you a copy, if you are interested. I think it was for a Myford (what lathe mod article isn’t?) .

 

Note to Moderator: David, this will not contravene copyright I hope?

[Frank DolmanParticipant @frankdolman72357]

     Nigel, HI.  A rotating rack would comprise a series of rings, it would not

    be a screw, so you are not quite right.  I can’t say what is right though,

   sorry.

                                             Regards

[mgjParticipant @mgj]

What it is is a worm and wheel.

 

A rack it is not, because it doesn’t have the right tooth form? Racks don’t have helix angles, and this most certainly does.

[Frank DolmanParticipant @frankdolman72357]

     Well done, mgj!  You are right, of course

                                       Regards

[John Stevenson 1Participant @johnstevenson1]

Posted by mgj on 23/03/2010 21:38:16:

 

Racks don’t have helix angles, and this most certainly does.

 

Helical racks do.

 

John S

 

[David Clark 13Participant @davidclark13]

Hi There

Not as a 1 off for personal use.

Don’t post it on here.

regards David

 

[mgjParticipant @mgj]

Yes, but . Oh sod it.

 

I don’t think anyone mentioned helical cut racks. We talked of racks did we not? Not the same thing at all

 

I just knew it was a mistake coming back.

Edited By mgj on 24/03/2010 10:58:46

Edited By mgj on 24/03/2010 11:24:23

[John CoatesParticipant @johncoates48577]

Once again a fantastic response for a newbie like me to digest. It’ll take a thorough reading of some of my Workshop Practice and Geo Thomas books to understand all of what you guys have said. Went a bit off topic on the racks issue though I thought !

 

Keith – thanks for the offer of the article which would be gratefully appreciated

 

DC – thanks for this site btw which has become my daily fix during work’s lunch hour and of an evening. The mag is excellent too and I am enjoying my subscription

 

I must admit I don’t know whether to invest in all the back issues of MEW that are advertised as I have bought both Geo Thomas books (and will be buying the UTP and VDH kits as my first foray into lathe work) and about half of the Workshop Practice books plus the Sparey book. Would they be a worthy addition as it looks like it will cost about £1 per issue so that’s £159 I would be in for?

 

As an aside, having bought the Barker as a total newbie and not quite knowing what I was looking at (which is why I will probably forever be posting in the Beginners section on here) I really enjoyed the article in this month’s MEW about installing the Chipmaster lathe. Once my over cluttered garage is sorted out to make space for the Barker I am really looking forward to getting it set up and starting to practice some of the tutorials in all the books I have bought and from MEW. I found the article a real inspiration and motivation. So thanks for that Dave !

[Michael Cox 1Participant @michaelcox1]

Hi John,

Most of the articles in George Thomas’s book are actually available on the home page of this website.

Mike

[mgjParticipant @mgj]

Mr Coates sir – you will find that it does make a difference as to whether its a rack or a worm. Or can do, depending on the accuracy you want.

 

A gear to fit a rack is simply that. A bog standard spur gear cut with a no1 cutter if you are not hobbing, which we amateurs will probably not be.

 

A wheel engaging in a worm is of a different form, and is normally relieved in the centre to fit “around” the worm.

 

Actually this is an unusual case of the device being both a form of rack, as well as a worm.

 

You can use the TDI as a distance travelled indicator, quite nicely for rough setting, when using the handwheel and carriage. When it acts as a rack driving a pinion. Its just a matter of counting whole turns and fractions thereof which are all conveniently marked up- excellent and quick for ruler length measurements.

 

And for picking up a helix as a conventional TDI, when it acts as a worm driving a wheel. 

.

 

 

Edited By mgj on 24/03/2010 18:15:53

[KWILParticipant @kwil]

Wormwheels are indeed a special form, they do wrap around the helical leadscrew, around 25% being the norm. This will also enable them to span the slot in the leadscrew used to drive carriage gearing. Wormwheels have to be hobbed with a cutter designed to match the leadscrew, both the hob and wormwheel blank have to rotated at the correct speed to achive the best result. Some slash cut the blank to try to achiev the drive but a sutable geartrain is the best way. Look at Ivan Law’s Book Bears and Gear Cutting (Workshop Series 17) if you want to pursue this.

[John CoatesParticipant @johncoates48577]

Posted by mgj on 24/03/2010 18:11:37:

Mr Coates sir – you will find that it does make a difference as to whether its a rack or a worm. Or can do, depending on the accuracy you want.

 

Forgive my newbieness mgj

 

I haven’t encountered racks thus far in my reading but I will go and look for them now

 

TBH most times I read detailed replies on here I go running off to the books to try and understand what you’re all on about. Problems of discovering lathes late on in life (45) and having to catch up with 20+ years learning you lot have on me

 

I’ll probably be a permanent resident of the Beginners section

[mgjParticipant @mgj]

A rack is simply a gear of infinite diameter – ie a straight line. It behaves like any other gear.

 

I wouldn’t reckon to be a permanent newbie. The experienced ones are only the ones who know which book to look it up in and machining as opposed to designing is pretty simple if tools are sharp and you take it one cut at a time.!

 

Kwil has it exactly right IMO. In many cases, while with amateur kit you can produce a reasonable result, nowadays specialist gear people will have catalogues of suitable gears, for not very much and which are so superior that its not worth trying to re-invent the wheel.

 

I did all the gears for my traction engine. That sort of thing is not complicated and well within the scope of the amateur. But I bought the 2 start worm and wheel, because I think my time has a value and HPC/Little Samson can do a far better job than I can on that sort of specialised item.(For some of those things you either have the specialist kit and can do the job, or you don’t have the kit and you cant, and its as simple as that.)

 

Springs are another case in point. .

Edited By mgj on 24/03/2010 23:02:42

[Ian S CParticipant @iansc]

I seem to remember that some of the old Harrisons that we had in the shool metalwork shop had no thread cutting dial, and we were taught how to cut threads without the dial by marking the lead screw and the lead screw bearing, and marking the bed(usually with the tail stock).You move the saddle to the tail stock. Rotate the lathe by hand until you can engage the lead screw, then mark the lead screw and the bearing(so you can put it in the same possition each time). Having previously set the tool square and to the required depth comence cutting, at the end of the cut disengage and return to the stop on the bed, rotate the lead screw until the marks are alined, engage and cut. I think this also was a question in our School Cert exam. Ian S C most of the machines(lathes, mills, shapers, drills etc) driven by line shafting and big flat belts-click click.

Edited By Ian S C on 25/03/2010 10:37:12

[MikeParticipant @mike89748]

My engineering instructor back in the 1960s had a method of screwcutting on a lathe without a tdi. All I can remember is that it involved chalk marking the points of engagement on the change wheels. Never did learn the finer points, because the Harrison on which I was instructed had a tdi. Anyone remember how the “chalk mark” method worked?

[Gordon WParticipant @gordonw]

Mike, I can only remember how the chalk marks on gears didn’t work, as an apprentice cutting a 1/4 in. square thread. Still have nightmares.

[Ian S CParticipant @iansc]

If you look at my last bit you’ll see that you use the chalk to mark the lead screw and its bearing, either end is OK, you need something more substanial to mark the saddle possition on the bed. Ian S C

[Ian S CParticipant @iansc]

Mike look at the previous entry, line 5. Chalk is used to mark the lead screw and its bearing, either end is OK. Something more substantial is required ie the tail stock to mark the bed. Ian S C

[Gordon WParticipant @gordonw]

The Amateurs Lathe by Mr. L H Sparey has a simple to follow description of chalking the gear wheels for screw cutting.

[MikeParticipant @mike89748]

Thanks, Gordon. I must be getting senile! I have a copy of Sparey’s book, so must find it and read it.

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