Shelf screw vs. Custom (DFM)

[BiPolarMoment]

I work in the medical field frequently doing instrument design/redesign and often deal with custom or procedure specific instruments. Since i've been here I've tried to incorporate some DFM where there was often none (probably because we usually work with very small volumes and high unit costs regardless). One thing in specific is using one-off screws vs. off the shelf screws in our assemblies.

My question is sort of a "is it worth it" for this particular situation. Hypothetically I have an instance where I want to use a shoulder bolt (or screw if you prefer) as both the hinge on an instrument and carrier of a torsion spring, but have been unable to find one that I particularly like the dimensions of. Can someone estimate what sort of unit cost increase it makes to specify a custom screw on lots that will likely never exceed 30?

For example let's say I'd like a 1/8"D X 3/16" lg shoulder and would prefer a 3/8" diameter low-profile head--from all that I've found, it appears the standard head diameter for a screw with a 1/8"D shoulder is 1/4" and that particular shoulder length is unavailible.

Are there some online sources I can look at for more options perhaps? In stainless of course, but metric would be a bonus.

 

[ajack1]

Why not just ring up a local manufacturer and get them to quote? They will tell you the cost, rather than someone here guessing.

If you do want shoulder screws many companies now make two and three piece shoulder screws where the body and or the head are separate from the screw, theses may give options in size.

 

[BiPolarMoment]

I've considered it, but as it would be included in part of an assembly and through what I've experienced quoting from our manufacturers in the past, I don't think the price for 10 individual custom screws would accurately reflect the cost when included with the rest of the assembly.

Additionally, this is really mostly for my curiosity and I don't have anything particular to get quoted right now.

Though, at some point the cost of me looking for a particular component will exclipse the cost of just using a custom one .

 

[musashi99]

Can you revise the design? That is what DFM is all about steering the design to the lowest production costs while at the same time maintaining the functionality and quality of the finished good.

Another issue to consider. Say someday your product takes off and you need thousands of these shoulder screws. For your low quainity run or protoype run making them might be OK. Once you get into large volumes you can either make them yourself (there by consuming your capacity on a minor component)or pay the price to have a custom screw made by a screw house (read increased unit costs).

 

[BiPolarMoment]

For the item in question I was playing with the thickness of the material since I couldnt find an alternate shoulder height screw. That will necessarily increase the volume of material used--though perhaps reduce material removed depending on the size of billet they stock/order.

I think I have a screw/spring/material thickness combination I like but involves putting a standard screw into an otherwise completely metric assembly because of the increased number of options.

 

[griffengm]

BiPolarMoment,
Is there a reason that you cannot use a short sleeve or bushing on a more standard metric bolt to get the configuration needed?

Griffy

 

[BiPolarMoment]

I have thought about it but haven't investigated availible sizes yet.