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Multi-Start Screw Threads

09/20/2014 7:00 PM

How do you make a nut that will mate with a 1/2-10 acme thread with 5 starts? Can the nut be made on a lathe? I know there are taps that can do it, but they are very expensive. Cutting the external thread is not a problem; just the internal thread.

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#1

Re: Multi-start screw threads

09/20/2014 7:54 PM

First, I've never done inside threads on a lathe.

But, I'd think that with a small boring bar and a properly ground tool one could do it. Very SLOWLY.

Or, Enco has them.

408-0220 - Enco - Guaranteed Lowest Prices on Machinery ...

Here's the MFG. catalog link:

Master Catalog Page 766

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#20
In reply to #1

Re: Multi-start screw threads

09/22/2014 11:16 AM

Lyn! You haven't lived not cutting an internal screw threads on a lathe, I have had to cut a vee form two start thread and a two start acme thread among several single start internal screw threads. Most of these I had to do whilst at sea, on lathes that were rather worse for wear.

One of the biggest problems is countering the "spring" in the boring bar and the secret is running the same depth of cut through several times, and it does take time; its far worse than straight out boring, because of coarse feed when screw cutting and working on multi start threads is a bigger problem because of an even coarser feed for each thread start..............and of course the longer the thread the greater the problem becomes...........but it can be done successfully, BUT I don't know about a five start thread on a standard lathe, "me thinks it would be rather difficult"........different matter on a CNC turning centre.

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#21
In reply to #20

Re: Multi-start screw threads

09/22/2014 11:29 AM

I guess a 5 start could be cut, but I'd not attempt it.

Yes, deflection is a big unknown here.

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#2

Re: Multi-start screw threads

09/21/2014 12:00 AM

You can cut internal thread on each lathe that can do external thread. Just need a proper knife with the right profile angle.

Because it is difficult to work in a hole, you'll need to give it some extra attention.

I clamp the nut close to the front, so that you can see the knife passing through. This eliminates some problems.

I also work upside- down to keep bearing play at minimum.

At the end it goes easier than a external thread, because there is no critical stop involved.

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#3

Re: Multi-start screw threads

09/21/2014 12:51 AM

Because of the thread "steepness", the boring bar "tooth" may need to be tilted, especially for side relief. Interesting challenge.

I didn't see a multi-lead (start) thread in the catalog, but maybe scanned too quickly.

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#5
In reply to #3

Re: Multi-start screw threads

09/21/2014 10:30 AM

I think the search engine didn't catch mulit-start.

Power's off here so I'm on my phone.

Looks like Ron will have to cut his own.

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#6
In reply to #5

Re: Multi-start screw threads

09/21/2014 10:46 AM

Think the "-start" was interpreted as "without the word 'start'". This should be better:

https://www.google.co.uk/search?q=how+to+turn+a+multistart+internal+thread

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#7
In reply to #6

Re: Multi-start screw threads

09/21/2014 10:54 AM

I'd think someone would sell them.

Maybe he just wants to use his lathe.

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#4

Re: Multi-start screw threads

09/21/2014 6:23 AM
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#8

Re: Multi-start screw threads

09/21/2014 2:26 PM

Is this for real or hypothetical? Here's a nut, but if I were doing it, I'd buy the screw and nut from the same source.

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#11
In reply to #8

Re: Multi-start screw threads

09/21/2014 11:22 PM

That picture does not look correct. No internal thread is visible, and the external thread is neither Acme nor 5-start.

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#17
In reply to #11

Re: Multi-start screw threads

09/22/2014 8:40 AM

I think that's just supposed to be a representative picture. Here's a prettier one.

Also available in plastic.

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#9

Re: Multi-Start Screw Threads

09/21/2014 11:03 PM

Mr Ron,

If the expense of a tap is daunting, and if multi-start lathe threading is too hard, you could consider CNC thread milling if you have access to a CNC 3-axis mill.

CNC Thread milling allows you to do things that are difficult to do with a tap:
Get 100% depth of thread;
Get a full, perfect thread within 1 pitch of the bottom of a hole;
Use the same tool for left-hand or right-hand threads;
Use the same tool for internal and external threads;
Use the same tool for different pitch diameters;
Cut multiple start threads;
Thread large diameters on machines that don't have the horsepower to drive a tap.

Most thread mill tooling suppliers make a program generator available. Here's an example:
http://www.guhring.com/tech/ThreadMillCNC/

Tapping still has its place, too. You can't thread holes by hand with a thread mill.
Mark Bingham
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#10
In reply to #9

Re: Multi-Start Screw Threads

09/21/2014 11:09 PM
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#12
In reply to #10

Re: Multi-Start Screw Threads

09/22/2014 1:30 AM

Are they stocked locally, or only at Lyn-Door Industries' huge central warehouse?

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#15
In reply to #12

Re: Multi-Start Screw Threads

09/22/2014 4:32 AM

If that proves fruitless, try KrisDelTM (usual disclaimer).

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#19
In reply to #12

Re: Multi-Start Screw Threads

09/22/2014 9:38 AM

They are stocked at one of our 200 satellite warehouses and can be delivered "same-day", by drop-shipment. Hard hats recommended.

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#13

Re: Multi-Start Screw Threads

09/22/2014 1:39 AM

When cutting a single start thread on the lathe, pitch (threads per inch) and lead (distance that the screw will advance in one revolution) are identical ie. a thread of 8 tpi (threads per inch) will advance the screw 1/8" per revolution. A dual start thread (in which one thread is cut within the helix of the other) of 8 tpi (although each individual screw thread is actually 4 tpi) will advance the screw 1/4" in one revolution.

If the lathe has an imperial lead screw then you will likely only be able to use the dial indicator for positioning for imperial thread pitches, if cutting a metric thread on an imperial lathe, the half nuts should not be disengaged as is also the case for multiple start threads, back off the cross slide and reverse the lathe or turn it by hand to withdraw the tool.

Set the blank up very accurately in the chuck to avoid later wobble errors and then turn the hole diameter to the root diameter of the mating screw.

To cut multiple threads - make sure that the gear wheel that is fitted to the lathe mandrel has a multiple number of teeth to the required thread starts ie multiples of five in your case, mark five teeth at equal spacings around the wheel and also on the point of engagement of this wheel's tooth with the next wheel in the train After cutting the first thread, rotate the mandrel by hand to line up two marks, disengage the gear train and rotate the mandrel to line up the next mark and re-engage the gear train (be careful not to rotate the gears in the train at this point), repeat the procedure for each thread. Don't disengage the half nuts during this operation as you will likely lose the relative thread positions.

Unfortunately, acme threads are possibly the most difficult to cut as they seem to not want to fit for no apparent reason, and with multi-start it's nigh on impossible to ascertain which of the threads is at fault.

First point is to make sure that your cutting tool angles are absolutely accurate, a purpose made acme thread carbide cutter may be a wise investment as you are unlikely to blunt it during the operation, removing a tool for sharpening can adversely affect accuracy of the finished thread.

It is important to not remove the nut from the chuck until successful testing of the thread on the screw has been done. small adjustments can then still be made if one thread is insufficient in depth or you need to move one thread slightly longitudinally. This will be impossible if you have moved the nut in the chuck. Often, widening the threads one at a time by one thou and testing between each cut will resolve the issue. Bearing blue is a good aid in this process.

I always set the top slide to half the internal angle of the thread angle, ie. 30° for SAE, 27.5° for BSW etc. (although this won't work for an acme thread) and then advance the top slide for depth. Care must be taken that this angle is taken from the axis of the cross slide and not that of the screw. This method means that only the leading edge of the tool is cutting and therefore produces a cleaner thread, you can also add about 5° top rake to the tool for easier cutting if desired with this method. You can still withdraw the tool for return to the start using the cross slide, but firstly set its dial to zero, advance the top slide the required increase in depth and wind the cross slide back to its original zero position.

Internal threads differ only in that the cross slide is moved in the opposite direction to advance the cut. If threading a blind hole, then it is wise to cut the thread from the back out so that you do not suffer an overrun disaster, you will need a left hand tool cutting at the back of the hole with reverse rotation for this operation, it's also much easier to see what's happening this way.

As an aside, IM Services in Michigan US make a range of Acme lead nuts in 1/2" including 5 start that may be an easier fix for you

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#16
In reply to #13

Re: Multi-Start Screw Threads

09/22/2014 5:35 AM

I am only an observer here, I haven't cut a thread on a lathe in many years.....

Firstly, yours was the first post that made any sense at all to me, thanks.

Secondly, does the OP require a 5 start external thread in a single start internal thread? Or did I understand that wrongly. I think it can be done, but severely reduced strength as far as I can see....forgetting the manufacturing problems for the moment.

Or did he mean that both are Acme and both have 5 starts? If yes, I think that you covered it in your post already.....

As the OP has not told us exactly why he wants this, its difficult to see a way to hlp hm further....

Again, many thanks for your excellent post!

GA

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#14

Re: Multi-Start Screw Threads

09/22/2014 2:16 AM

Here is a very good tool for multiple start internal threading. It is [ IFANGER Toolinf with the curved cutter tooling tip. They are not the chepest but the different models can accommodate many diameters in the differing thread forms IE:- ACME Metric 60 degree or other 55 degree etc. Hope this is of interest. The tooling and Holder shown costs AUD $120-00. The internal cutter is around AUD $60-00 an replaceable when I purchased them some time back.

This would be much cheaper than a single size tap which may cost upwards of $800-00 as it would be a special manufacture if you can get one machined and ground.

If you can cut outside threads, it is possible to make your own by making the thread form on a blank HT Grade 14.9 bolt shank and grinding 3 or 4 clearance flutes along the shank after you have softened the steel by heating to almost white heat & very slow cooling in a 'Fullers Earth' clay so it takes 24 hours or so to cool. This anneals the steel for ready machining. After the machining and grinding has been completed, it becomes necessary to reharden using a high carbon quenching oil or 'Hardite' in a boiling pot.

Just a bit of metalergy.

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#18

Re: Multi-Start Screw Threads

09/22/2014 8:45 AM

I have solved this in the past with a nut of epoxy metal cast around the thread, lightly oiled or greased to stop it sticking to the thread. Depends on the load and tolerance needed but to move a small load along an X-Y table it's OK with a good long nut

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#22

Re: Multi-Start Screw Threads

09/22/2014 11:43 AM

Draw it in CAD and have it 3D printed either in plastic or better in metal.

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#23
In reply to #22

Re: Multi-Start Screw Threads

09/22/2014 12:47 PM

See what technology can, or could do? I think it would still be entertaining to learn to cut inside threads like that, but I have not started to try.

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#24

Re: Multi-Start Screw Threads

09/22/2014 1:08 PM

They are for sale $16 each on the Roton site, from Kirkwood, Missouri.

Looks like a pretty common CNC arrangement, so not worth struggling with inside thread cutting.

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#25

Re: Multi-Start Screw Threads

09/24/2014 10:20 PM

A small cutter/boring bar like this can be ground from solid 1/2" sq HSS, this reduces spring.

Another way of changing the start position is to wind the tool post slide forwards ( or backwards BUT be consistant ) whilst leaving the half nuts engaged.

Soo. Keep your tool slide parallel with the bed ( I usually move the tool back and forward a small amount during the cutting process always returning to '0' ) and when you have finished the first thread move it forward the calculated amount to start the second thread etc.

Have fun,

Jim

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#26

Re: Multi-Start Screw Threads

09/25/2014 2:42 AM

Hi Ronseto, maybe I am totally off base with my thoughts, but just in case......

Is this nut part of a mechanism say for a CNC or similar? High accuracy movement under computer control?

If yes, you do realise that generally speaking, the more starts you have cause some possible problems with accuracy unless some mechanism, as in commercial units, that takes up wear as it happens:-

1) less metal is left in the nut, so wear is concentrated on a smaller area.....which may give accuracy problems in a short space of time.

2) with a many start nut, per 360° turn, the nut moves farther and faster.....which will give accuracy problems that will need to be corrected by the electronics. Which may eventually cost more to offset that.

I am only a rank amateur DIYer with regard to CNC, but if that is the case and long life and high accuracy are a requirement of yours, then you should look at commercial units. I do realise that they are far more expensive than home made stuff, but they will help you considerably....

The following is not a recommendation as such, as commercial units are far better, but if cost is important:-

For my woodworking DIY version, I used standard metric (8mm) threading rod with guides for the carriage (so that if the rod is not 100% straight, it does not affect accuracy in any way) and my own design of (very long, to spread wear and tear over a larger area) split nuts to allow me to take up any wear.....

I also completely re-cut each rod as they are generally sort of "pressed or rolled" and not cut by the manufacturer. It was a long drawn out process for me, but it took out the thickness variations along the length that I found.....

I also bought a MIG welder to allow me to make the parts needed, it cost about the same as having them (3 small parts!) made by a local company!!! And has been used for many other things since!!

It prompts me to say that life "after" buying a MIG welder, is far nicer than before for m as a DIYer!!!

I am marking this as Off Topic, just in case it really is......

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