Plastic Threaded Body Compatible With Metal Bolt?

[Guest Dastardly_Dave]

Hi, I'm currently working on a project that requires a rubber tipped bolt to travel a variable distance and compress against another part.

 

At the moment I'm thinking of using a nylon locknut in conjunction with a socket set screw.This will all be housed in a plastic body.

 

But the locknut adds an extra 8mm of dead-space to the part,

so I'm toying with the idea of completely removing the nut and threading the inside of the plastic part and possibly adding a nylon insert to simulate the locknut effect(this may or may not be necessary).

 

I've also been thinking through using a moulded insert but it adds to much complexity to the part.

 

What I'm concerned about is the metal bolt shaving the plastic threading. In theory this part is only tightened once but in reality it could be necessary to periodically tighten it.

 

So would a grade of ABS or similar material be appropriate?

 

If necessary I'll post sketches but the project is for a client & confidential so I'd obviously prefer not to

Thanks in advance for feedback,

Regards,

Dave

[Guest Buff]

If you look at the design of 'aerotight' locknuts, they use a distorted thread. There are nylon patch screws also.

 

You could easliy mould in the thread with a distorted section to give the locking capability.

 

It's unlikely if you're using a standard socket set screw for the screw to 'cut' the thread.

 

If you mould in the thread you will need a rotating core.

 

There are a number of ways of including rotating core including a simple rack an pinion that act upon the ejector plate.

 

In plastics you would normally (if not moulding the thread) form it with a fluteless tap, unless a very high shore hardness.

 

If the plastic is a low shore or something like HDPE or PP then the thread usually relaxes a little to form a snug fit.

 

You would want the thread depth to be 1.5 - 2 x the thread diameter.

 

If there is even a little amount of torque applied to a nut, it will normally distort the plastic and turn.

 

You would then be better with insert moulding

[Guest Dastardly_Dave]

If you look at the design of 'aerotight' locknuts, they use a distorted thread. There are nylon patch screws also.

 

You could easliy mould in the thread with a distorted section to give the locking capability.

 

It's unlikely if you're using a standard socket set screw for the screw to 'cut' the thread.

 

If you mould in the thread you will need a rotating core.

 

There are a number of ways of including rotating core including a simple rack an pinion that act upon the ejector plate.

 

In plastics you would normally (if not moulding the thread) form it with a fluteless tap, unless a very high shore hardness.

 

If the plastic is a low shore or something like HDPE or PP then the thread usually relaxes a little to form a snug fit.

 

You would want the thread depth to be 1.5 - 2 x the thread diameter.

 

If there is even a little amount of torque applied to a nut, it will normally distort the plastic and turn.

 

You would then be better with insert moulding

 

Hi Buff thanks for the reply, I've seen some of your other posts, found them comprehensive, and was hoping you'd reply

 

The product is basically acting as a clamp, which grips onto objects of varying diameters and shapes-triangular included.

At the moment- I'm running with a bolt of some sorts which makes up the distance applying compressive force.

 

The part is split in two, The split line runs along the side of the bolt housing (along the bolts centreline) with a negative space in each part to house the nut tightly(half the nut in each moulding)

 

As I've said previously I'm thinking of removing the nut altogether and moulding the thread, but the split line still has to remain where it is though because of the geometry of the part.

 

Instead of a rotating core would it be possible to split the threaded section in half using a simple split mould- I'm trying to keep things simple as I already have an unavoidable undercut.

 

From your experience though once you include one retracting core is it relatively cheap to add another?

 

Thanks again for taking the time

[Guest Buff]

depending on the axis of draw for the thread (I'll assume its in the same lane as the 'B' side) then you will be able to mould the thread without a rotating core.

 

You would need to spcify on the 2D drawing the acceptable tolerance for miss-match on the two halves of the thread, alos ovality, pallelness too.

 

Just follow good basic G&DT rules(geometric and dimensional tollerancing).

 

Of course you can use some ovality to help provide the self locking nature your looking for.

 

If your threads are co-axial, you can use one rotating core, but you would have to have the same thread pitch regardless of diameter. Otherwise the two (ore more) rotating cores would have to be geared and you have to be sure your toolmaker understands the efect of backlash on the gearing.

 

If you have two or more cores side by side you can control these with one rack, but it would be unusual as a lot would depend on positioning thread more with tool design in mind than the actual product..

 

Funnily enough I'm doing a toy (make that should be doing a toy ;-) ) which has two co-axial threads.

 

I've made them the same pitch though the diameters are quite differant. The larger diameter thread then in effect is a finer thread.

 

In this case the thread is Acme like in form as it suits the 'chunky' nature of toys.

 

With your part being split about the nut, I would definitely NOT use a hex nut. especially as your clamping and you can't always rely on the consumer not being ham fisted. In fact you can prety much rely on them BEING ham fisted.

 

The housing will just split!

 

I would also look into using Acme form thread. These have a 29 degree inclusinve thread angle, rather than 60 degree.

 

The 60 degree thread will produce a greater vectored force for any given clamping pressure and this force will try to cleave your assembly in two.

 

 

 

If you do decide to stick with a metal (steel?) nut, then go for square rather than hex. Try to keep the socket it fits into with minimal draft in that particular area. half to 1 degree max. put an ejector pin eitherside of the socket feature.

 

Once you have a mould that has a sliding core, your first core is going to add quite a bit of cost. perhaps 80% more. as a general rule of thumb each additional core obviously costs, but is less than the first. assuming of course that the slide operates on the same plane as the first.

[Guest Bowl of Soup]

It is difficult without seeing the part, but I understand secrecy, etc...

 

I understand that you are here trying to torque/tighten with this fastener, however my DFMA side is screaming in the back of my head that if you only expect to fasten once, don't use a bolt, use some more 'modern' fastening technique aka adhesive or a single solid part, even snap clips might do here.

 

Even, having explained the variable spacing of your design, I think there is a better design solution, but it may not be worth your time to re-design, so here are some other pointers:

 

Typically a nylon lock-nut is considered a one-time deal. If you use an insert or molded insert this will be the same reality. I know people use them more than once, but they are not designed that way. You could recess the housing of your part to allow a lock nut to slide into a slot at the end, that might solve your dilemma nicely, I have seen this technique a lot lately.

 

This picture is a bad example, but here is the general idea of recessing your nut:

 

http://www.rcuniverse.com/magazine/reviews...underhood15.jpg

 

Also, there are some older thread designs that carry loads and torques well, they typically have wide "square" profiled threads, metal on metal. But, they have gone out of favor in most design applications. The link here is a similar example to what I mean, it is for a specific application, but it conveys the idea, these threads are hard to find and are usually for precision machinery.

 

http://www.nookindustries.com/acme/AcmeGlossary.cfm

 

Hope this helps.

 

Cheers.

[Guest Dastardly_Dave]

It is difficult without seeing the part, but I understand secrecy, etc...

 

I understand that you are here trying to torque/tighten with this fastener, however my DFMA side is screaming in the back of my head that if you only expect to fasten once, don't use a bolt, use some more 'modern' fastening technique aka adhesive or a single solid part, even snap clips might do here.

 

Even, having explained the variable spacing of your design, I think there is a better design solution, but it may not be worth your time to re-design, so here are some other pointers:

 

Typically a nylon lock-nut is considered a one-time deal. If you use an insert or molded insert this will be the same reality. I know people use them more than once, but they are not designed that way. You could recess the housing of your part to allow a lock nut to slide into a slot at the end, that might solve your dilemma nicely, I have seen this technique a lot lately.

 

This picture is a bad example, but here is the general idea of recessing your nut:

 

http://www.rcuniverse.com/magazine/reviews...underhood15.jpg

 

Also, there are some older thread designs that carry loads and torques well, they typically have wide "square" profiled threads, metal on metal. But, they have gone out of favor in most design applications. The link here is a similar example to what I mean, it is for a specific application, but it conveys the idea, these threads are hard to find and are usually for precision machinery.

 

http://www.nookindustries.com/acme/AcmeGlossary.cfm

 

Hope this helps.

 

Cheers.

 

Hi guys, thanks for your replies both were helpful. There are a few more issues& restrictions on the product that I haven't mentioned, along with the variable spacing that make a bolt more appropriate solution. Believe me there are some really sexy ways of solving some of the issues, but when all aspects were taken into account the bolt solution seemed most appropriate.

 

I've mocked up some quick prototypes in the workshop, drilling a hole in cebatool a material I'd say you are familiar with? and let the bolt itself cut the threading for me, as the cebatool is soft enough to allow this.

 

I'll be tapping a hole soon when I can get my hands on a tap and seeing how it fairs up. The sketch model works fine with no adverse effects on the cebatool female thread when the bolt is inserted& removed a few times. But I'm not sure how a tapped thread will work compared to the thread formed by the bolt itself.

 

If anybody has any suggestions or has worked on something similar your experiences would be welcome.

Thanks again and I hope I can return the favour

Regards

Dave

[Guest Buff]

Dave,

 

In most cases with plastic, you use a flutless tap. much the same as your bolt.

 

http://en.wikipedia.org/wiki/Threading_(manufacturing)