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Tooling for a 1/2"-5 Acme double start internal thread 1.25" long

Hi again SND:
You wrote:
"I'm looking at this as a 1/2"-10 double start with a 5 lead."

Agreed, it's certainly possible, which is why I asked way back in post # 4.
As you say it makes a huge difference. but even with a 0.2" lead it's not a good candidate for threadmilling IMO.
So if the choice was between threadmilling and single pointing with a nibble strategy I would pick the latter provided the material is not something gnarly.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
I'm looking at this as a 1/2"-10 double start with a 5 lead. Not a 1/2"-5 with a 2.5 Lead.
For clarity for those reading that don't know much about multi start threads, you have your terms arse about. The Pitch of a Tread is obtained by dividing the Lead by the Number of Starts and conversely, the Lead by multiplying the Pitch by the Number of Starts. Therefore, in your first example of a 5 Lead, the Pitch would be 2.5 with no reference to the 10 in 1/2"-10. What 1/2"-10 would probably refer to is a 10 Lead with a 5 Pitch.

In the second example of a 1/2"-5 Thread with a 2.5 Lead, the Pitch would be 1.25, with no reference to 5 in 1/2"-5. What 1/2"-5 would probably refer to is a 5 Lead with a 2.5 Pitch.

Regards,

Bill
 
Hi Bill:
I'm unclear on your terminology, so I'm going to re-state it and see if you concur.
Of the two threads this could be, based on the incomplete information the OP has, it could be a coarse thread of 5 TPI so a thread pitch of 0.200".
For a double start thread of that pitch, the lead for each start would be 0.400" would it not?

Or it could be a finer thread with a lead of 0.2" (5 TPI for each thread groove) and therefore a pitch of 0.1"(because there are two starts at equal spacing along the thread).
So that finer thread would be described as 10 TPI when both thread grooves are counted, would it not?

Of course this all mentally goes arse backward when we talk metric threads because we name them by their pitch, not by how many threads per unit length as we do imperial threads.
So does a double start thread described as the OP does in the title as a 1/2"-5 double start Acme have a pitch of 0.2" (5 TPI) or a pitch of 0.1" with a lead of 1/5 inch?

Nomenclature can be such a pain sometimes.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
Last edited:
For clarity for those reading that don't know much about multi start threads, you have your terms arse about. The Pitch of a Tread is obtained by dividing the Lead by the Number of Starts and conversely, the Lead by multiplying the Pitch by the Number of Starts. Therefore, in your first example of a 5 Lead, the Pitch would be 2.5 with no reference to the 10 in 1/2"-10. What 1/2"-10 would probably refer to is a 10 Lead with a 5 Pitch.

In the second example of a 1/2"-5 Thread with a 2.5 Lead, the Pitch would be 1.25, with no reference to 5 in 1/2"-5. What 1/2"-5 would probably refer to is a 5 Lead with a 2.5 Pitch.

Regards,

Bill

I don't have any helpful info for the OP, so I was just leaving that alone, but thanks for pointing it out.
 
Hello Marcus,
Yes, I've had a brain fog moment. For some inexplicable reason I took the 10 and 5 as mm, rather than TPI. Accordingly, 5 TPI
would be the Lead as SND stated and 10 the pitch. In the other example, 2.5 TPI the Lead and 5 the pitch.

So does a double start thread described as the OP does in the title as a 1/2"-5 double start Acme have a pitch of 0.2" (5 TPI) or a pitch of 0.1" with a lead of 1/5 inch?


It's convention that with Multi Start Imperial Treads, the TPI specifies the Pitch, therefore, in a double start 5TPI Thread, the Lead would be 2.5TPI (0.4"). Accordingly, it's more likely with the OP's example of 1/2" - 10, the Lead would be 0.2"

Regards,

Bill
 
Good morning Everyone:
So assuming the OP has actually been tasked with making a 1/2" 5TPI double start internal Acme here's what he's looking at for a tool if he wants to single point bore it:
The OD is 0.5" and the thread depth for a standard Acme is 1/2 pitch so 0.100".
That makes the minor diameter 0.300"
The helix angle at the thread root is 23 degrees for a lead of 0.400" (unless I got my calculation wrong)

So the tool cannot be bigger than 0.300" diameter at the head end, and has to be necked down to 0.200" diameter on the cutting edge side to provide for a thread depth of 0.100"
The leading edge of the cutter must be drafted more than 23 degrees so the tool has clearance.
So if the top surface of the cutting edge is horizontal, the leading flank will be a wedge that's skinnier than 67 degrees.

If the bar is necked asymmetrically the shank that remains will be roughly elliptical and the cross section dimensions will be roughly 0.200" x 0.300"
The necked length will need to be more than 1.25" long.

We're going to cut some kind of steel with this beast...does anyone here think it's doable?
I'm not hopeful...IMO if this is truly the thread spec, it needs to be burned by helixing a trode into it (You can't get away with orbiting a small trode...the thread flanks will come out wrong).

The OP is supposed to make HUNDREDS of these things, and I presume the customer is going to want them cheap.
Sinker EDM is not going to be cheap!
So does this project have the smell of success about it to any of you?

On a purely technical note:
The screw I assume is probably steel.
A double start Acme of this pitch is likely for motion transmission...I really can't think of any other purpose.
The nut is supposed to be steel too...WTF???
Is nobody worried about seizing the nut onto the spindle?

So I cannot see engineering success with this design either...I may be wrong but this is a pretty weird set of specs IMO.
What say ye?

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
Last edited:
Good morning Everyone:
So assuming the OP has actually been tasked with making a 1/2" 5TPI double start internal Acme here's what he's looking at for a tool if he wants to single point bore it:
The OD is 0.5" and the thread depth for a standard Acme is 1/2 pitch so 0.100".
That makes the minor diameter 0.300"
The helix angle at the thread root is 23 degrees for a lead of 0.400" (unless I got my calculation wrong)

So the tool cannot be bigger than 0.300" diameter at the head end, and has to be necked down to 0.200" diameter on the cutting edge side to provide for a thread depth of 0.100"
The leading edge of the cutter must be drafted more than 23 degrees so the tool has clearance.
So if the top surface of the cutting edge is horizontal, the leading flank will be a wedge that's skinnier than 67 degrees.

If the bar is necked asymmetrically the shank that remains will be roughly elliptical and the cross section dimensions will be roughly 0.200" x 0.300"
The necked length will need to be more than 1.25" long.

We're going to cut some kind of steel with this beast...does anyone here think it's doable?
I'm not hopeful...IMO if this is truly the thread spec, it needs to be burned by helixing a trode into it (You can't get away with orbiting a small trode...the thread flanks will come out wrong).

The OP is supposed to make HUNDREDS of these things, and I presume the customer is going to want them cheap.
Sinker EDM is not going to be cheap!
So does this project have the smell of success about it to any of you?

On a purely technical note:
The screw I assume is probably steel.
A double start Acme of this pitch is likely for motion transmission...I really can't think of any other purpose.
The nut is supposed to be steel too...WTF???
Is nobody worried about seizing the nut onto the spindle?

I cannot see engineering success with this design...I may be wrong but I don't think so.
What say ye?

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com


Just be glad it isn't a 1/2-10 5 start like mcmaster sells in cast iron. https://www.mcmaster.com/products/s...t-travel-precision-acme-lead-screws-and-nuts/
 
I wonder what this tool actually looks like?? https://www.ebay.com/itm/176265352683

Wonder if it would be accurate? Or accurate enough? Says 2 starts with a double helix illustration. Can't tell from their generic tap pictures whether what's pictured is actually a double start tap. The main pic shows 7/16 acme on the box.
 
Hello, gents. You've been busy here while I'm not getting notices about it.
First off, to clear up and suspicions, the "1/2"-5 Acme 2 start" comes right from the print. I'm not making any changes to that in typing it here.
Marcus, the only thing I noted about your dimensions is that the callout I saw for the 1/2"-5 Acme dimensions also include .02-.03" of clearance on that .1" thread height. That takes that root dia. down quite a bit further than the .3" you stated, correct?
I don't see how this can work, and the intended use in no way warrants such difficulty to make it happen. It's for a small, bench mounted hand vise made of plastic.
I agree that the job is too close to the impossible to be correct numbers, and I would definitely call for a bronze nut instead of steel.
PS. Oh, and I still haven't received a call back to clarify the info.
 
Hi again DMSentra:
You wrote:
"That takes that root dia. down quite a bit further than the .3" you stated, correct?"

Actually it brings the root diameter UP which works in your favour.
So the root diameter of the nut can be as much as 0.330" which does gain you quite a bit.
But it's still a ridiculously challenging part to machine...these will never be low dollar parts.

If this was my project, and I had to have this ridiculous thread to make it work, and if it was plastic I'd mold the nut in two halves that I can press and heatstake into the body.
Yeah, you gotta shell out for the mold, but it's a dirt simple mold so long as you make the nut in two halves and insert it into a pocket to hold the halves together.
If they're going to mold everything else out of plastic anyway, they may as well mold the nut too.
If they make it out of something really tough like Delrin or PEEK it'll probably outlast the rest of the contraption by miles...problem solved.

But from what you've been revealing about them during the course of this conversation, I don't see a whole lotta engineering experience with these guys.
You're probably better off if this never turns into a PO.

But it was an interesting conversation nonetheless, so I have no regrets about poking around with it.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
Last edited:
For clarity for those reading that don't know much about multi start threads, you have your terms arse about. The Pitch of a Tread is obtained by dividing the Lead by the Number of Starts and conversely, the Lead by multiplying the Pitch by the Number of Starts. Therefore, in your first example of a 5 Lead, the Pitch would be 2.5 with no reference to the 10 in 1/2"-10. What 1/2"-10 would probably refer to is a 10 Lead with a 5 Pitch.

In the second example of a 1/2"-5 Thread with a 2.5 Lead, the Pitch would be 1.25, with no reference to 5 in 1/2"-5. What 1/2"-5 would probably refer to is a 5 Lead with a 2.5 Pitch.

Regards,

Bill
The Lead is always how much a thread moves in 1 revolution.

Although it's been quite a few years, I've cut a few double start square and acme external and internal, triple start and other weird threads before.
 
Hello, gents. You've been busy here while I'm not getting notices about it.
First off, to clear up and suspicions, the "1/2"-5 Acme 2 start" comes right from the print. I'm not making any changes to that in typing it here.
Marcus, the only thing I noted about your dimensions is that the callout I saw for the 1/2"-5 Acme dimensions also include .02-.03" of clearance on that .1" thread height. That takes that root dia. down quite a bit further than the .3" you stated, correct?
I don't see how this can work, and the intended use in no way warrants such difficulty to make it happen. It's for a small, bench mounted hand vise made of plastic.
I agree that the job is too close to the impossible to be correct numbers, and I would definitely call for a bronze nut instead of steel.
PS. Oh, and I still haven't received a call back to clarify the info.
A thread that small with that fast a lead won't have hardly any locking power for a vice jaw. That project sounds like a WTF all around.
 
Could you use just a regular single start tap, clean up one thread, then insert again 180 degrees around and clean up the other thread?
 
Hi Elliotsghost:
You wrote:
"Could you use just a regular single start tap, clean up one thread, then insert again 180 degrees around and clean up the other thread?"

Sadly, no you can't.
The double start thread is a totally different shape from a single start thread of the same pitch.
The single start thread profile is roughly twice as big as the double start thread profile.

Also, the clearance angle of the leading flank must be twice as big for the double start thread because the tool drops into the hole at twice the pitch.

So although it seems like an obvious idea, unfortunately it won't work.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
My vote would be to make a tap or taps, and thread the nuts using some sort of lead screw tapping
machine where the tap advance is driven by the tapping machine and not the lead of the tap. Pretty sure that is how Kirk and the other lead screw companies do it. Sometimes, single pointing simply does not work or is possible because of the geometry. Great discussion though!
 
Hi again DMSentra:
I think you should be happy it's dead...it sounded like a ball breaker for sure.
But you started a great discussion, so thanks for that.
Seems to me, this is kinda what PM is supposed to be for.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
I am, unless the clarification simplified the nutty specs. I'm wanting to grow into the work a company like that could supply. But, oh well...
 








 
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