Thread: Nuttal 7 1/2 " Lathe

It is not exactly that simple..

Your leadscrew is a worm and the thread dial gear a worm wheel.. it gets complicated as gears and such are commercially made to either a metric module or diametral pitch, neither of which correspond to threads like 4tpi.. That is why if you cut worms on a lathe you use a different chart and usually change gears then you use for screw cutting..

But all is not lost as thread dial gears do not have to be perfect, they do not transmit power...

The gear will have 16 teeth (and eight graduations on the dial) as all 4tpi leadscrew chasing dials have, so with that knowledge should not be hard to make something...

bed time now..

Have seen indicators, possibly home made, with a straight cut gear with its axis mounted at an angle to match the angle of leadscrew teeth. The mounting pivot will need to be at a complementary angle.
Combustor.

If we go to the book Gears ans gear cutting by Ivan law starting at page 32 for those interested and have the book.. it says the circular pitch of a pinion gear of infinite radius, which is the definition of a rack, is the distance from one tooth to the same point on the next tooth.. Now a worm is a rack, or in this case it being a leadscrew acme thread of 14.5 degree pressure angle (PA) with helical teeth.. It is an involute as an involute rack has straight sided teeth..

So we know the circular pitch is 0.25", as that it the pitch of the leadscrew..

We move to page 29 that deals with circular pitch (CP). The circular pitch is the distance of one point on one tooth to the corresponding distance on the next tooth measured at the pitch circle diameter (PCD)..

To calculate the circumference of the PCD it equals the number of teeth in the gear multiplied by the CP

So 16 X 0.25 = 4 inches circumference of the PCD

To get the PCD we divide 4 (which is the PCD circumference) by pi, (to get us the diameter from the circumference)

so 4/3.1416 = 1.273237 is the PCD

To get the all important diametral pitch (dp) from the PCD we divide the number of teeth by the PCD

So 16/1.273237 = a dp of 12.56634.

outside diameter (OD) of the gear is number of teeth + 2 / DP so (16+2)/12.65534= 1.422" OD

There is no such thing as an off the shelf cutter with a dp of 12.65534. Whether you can buy cutters based on circular pitch I do not know..

One thing I cannot calculate is the required lead of the gear as we need the diameter of the leadscrew for that.


But no need to despair as today we have technology on our side..

We fire up the program gearotic and stick in the program... dp of 12.65534 and tooth count of 16 and PA of 14.5

The picture is not to scale

capture-20140815-073159.jpg

Originally Posted by .RC.

If we go to the book Gears ans gear cutting by Ivan law starting at page 32 for those interested and have the book.. it says the circular pitch of a pinion gear of infinite radius, which is the definition of a rack, is the distance from one tooth to the same point on the next tooth.. Now a worm is a rack, or in this case it being a leadscrew acme thread of 14.5 degree pressure angle (PA) with helical teeth.. It is an involute as an involute rack has straight sided teeth.. So we know the circular pitch is 0.25", as that it the pitch of the leadscrew.. We move to page 29 that deals with circular pitch (CP). The circular pitch is the distance of one point on one tooth to the corresponding distance on the next tooth measured at the pitch circle diameter (PCD).. To calculate the circumference of the PCD it equals the number of teeth in the gear multiplied by the CP So 16 X 0.25 = 4 inches circumference of the PCD To get the PCD we divide 4 (which is the PCD circumference) by pi, (to get us the diameter from the circumference) so 4/3.1416 = 1.273237 is the PCD To get the all important diametral pitch (dp) from the PCD we divide the number of teeth by the PCD So 16/1.273237 = a dp of 12.56634. outside diameter (OD) of the gear is number of teeth + 2 / DP so (16+2)/12.65534= 1.422" OD There is no such thing as an off the shelf cutter with a dp of 12.65534. Whether you can buy cutters based on circular pitch I do not know.. One thing I cannot calculate is the required lead of the gear as we need the diameter of the leadscrew for that. But no need to despair as today we have technology on our side.. We fire up the program gearotic and stick in the program... dp of 12.65534 and tooth count of 16 and PA of 14.5 The picture is not to scale capture-20140815-073159.jpg

Or, to save all that hassle, I might have one seeing as I got given a Nuttall leadscrew, half nuts and *I think* thread dial many, many years ago. I know I still have the stuff, question is, can I actually *find* it..... I'll take a look later today after the snow & ice melts a bit. PDW

I have seen thread chasing attachments with just a simple star wheel of the appropriate number of "teeth". There is no need for perfect tooth form , once the half-nuts are engaged, the chasing dial line stays stationary and only revolves when the half nuts are disengaged. There can be plenty of backlash so it does not have to be in full mesh. There is no load on the star/gear and normally runs very slowly. Perhaps if you had a Hardinge and thread at 1000rpm you would need something better.

Peter

Hi


I made a imperial thread chasing dial for my sheraton 9 ( same as hercus ) . The plans I found in an old magazine for a SB dial . The teacher at TAFE helped me make the little gear from brass stock.

Anyway I had problems , I discovered that the lead screw has a slot machined down its length, this slot is for the power feeds drive mechanism in the saddle . The mistake was, we made the gear with a concave profile on the teeth , not flat as It should have been. This happened- as the L/screw rotates, the slot in the L/screw pushed the gear out of mesh , the slot catches an edge on the gear and thus pushed it out.

Originally Posted by PDW

Or, to save all that hassle, I might have one seeing as I got given a Nuttall leadscrew, half nuts and *I think* thread dial many, many years ago.

yes that helps

Originally Posted by Peter Fou

I have seen thread chasing attachments with just a simple star wheel of the appropriate number of "teeth". There is no need for perfect tooth form , once the half-nuts are engaged, the chasing dial line stays stationary and only revolves when the half nuts are disengaged. There can be plenty of backlash so it does not have to be in full mesh. There is no load on the star/gear and normally runs very slowly. Perhaps if you had a Hardinge and thread at 1000rpm you would need something better.

Peter

That is sort of what I would do... I just typed out the above to show the workings behind the gear..

Now we have the OD, grind a single tooth fly cutter to the form, does not have to be super accurate.. put the cutter in the vertical head of a mill... Tilt the head to the lead angle of the gear and cut the gear... It will only be an approximation, but good enough, and pretty quick..

Originally Posted by .RC.

There is no such thing as an off the shelf cutter with a dp of 12.65534. Whether you can buy cutters based on circular pitch I do not know.

The next step is to make up a worm cutter with that pitch and then you hob the gear out - it's what I did when we were mucking around with Simon's bandsaw. The usual equations will tell you a blank size, so it's not too bad. As you say you need the lead screw diameter, but to get the cutter diameter right.

Michael

Originally Posted by PDW

Or, to save all that hassle, I might have one seeing as I got given a Nuttall leadscrew, half nuts and *I think* thread dial many, many years ago. I know I still have the stuff, question is, can I actually *find* it..... I'll take a look later today after the snow & ice melts a bit. PDW

Sorry guys been flat out sawing timber so have not caught up. Thanks for all the info and the time you all spent describing how to make one. Hopefully PDW has a spare one and if not with all the info RC supplied I will find a machine shop and get them to make me one.
Rgds,
Crocy.

Originally Posted by Old Croc

if not with all the info RC supplied I will find a machine shop and get them to make me one.
Rgds,
Crocy.

If PDW does not have one, I can cut you one, and you can pay me what you think it is worth after you get it and try it.. As it will be roughish in so far that cut with a fly cutter with the head tilted over so not a proper helical gear... I like a challenge..

Originally Posted by .RC.

If PDW does not have one, I can cut you one, and you can pay me what you think it is worth after you get it and try it.. As it will be roughish in so far that cut with a fly cutter with the head tilted over so not a proper helical gear... I like a challenge..

RC, thanks for the offer, if the real one does not become available, I will contact you. Also next time I come down to work in Rocky, I would love to have a look at a pink lathe
Rgds,
Crocy.

Originally Posted by Old Croc

Also next time I come down to work in Rocky, I would love to have a look at a pink lathe
Rgds,
Crocy.

It is a risk that... You have to be a real man to be able to handle such a manly colour.... Lesser men have been known not to be able to handle the colour...

Coming in a bit late on this thread...I recently made a new TDI gear for my Nuttal Herbert and from memory I used something like a 12DP gear cutter which was close to the original involute form, I think the theoretical size was 13DP. The leadscrew is something over 1" diameter and about 4TPI. I took the original gear and tried several of the 12DP cutters until I found the one that fit the profile closely. The original was missing two teeth which is a mystery as to how this came to be.

The TDI is built into the apron and on close inspection only half the gear tooth profile engages with the leadscrew which means I could've got away a straight tooth spur gear. Anyhow I set the dividing head to the helix angle of the leadscrew and cut an approximate helical gear. It worked out well and the TDI is once again doing its job. There's no load on these indicators so a true worm wheel isn't strictly necessary.

I've found the leadscrew but not the other bits. Thought all the parts were wired together so they *couldn't* get lost. It'll be there somewhere safe..... PDW