Request for advice regarding manufacture of a spring anchor

[John2004]

Hi everyone,

I have uploaded a small 19KB .GIF file to rapidshare showing a drawing of a simple adjustable spring anchor I am making from a 10-32 screw. Here is the download link...

http://rapidshare.de/files/24739574/Spring_Adjustment_Screw.gif.html

Regarding the drawing, an extension spring hook goes through the .098" hole, and the other end of the screw gets a hex nut that can be no larger than 5/16" flat to flat, such as part # 90760A411 from

http://www.mcmaster.com.

Due to space limitations, the hex nut must be adjusted with a box end wrench.

The .359" to .390" OD spring ( similar to the #80583

http://www.centuryspring.com)

uses .08" diameter wire, but the problem I ran into is that the hole is not really large enough for the spring, due to the curvature of the hook.

Can a machinist cost effectively just make a .098" wide slot in the screw, about .145" to .155" long, instead of a .098" hole?

I know slots are generally easy to do, but since it is so small, I wanted to get some feedback. If the slot can be made cost effectively, It will work well with the spring.

If the slot cannot be made cost effectively, then I must use a larger screw that will allow for a hole of about .145" OD. A 1/4-20 or 1/4-28 screw would work great, but I could not find a nut that is 5/16" flat to flat max, and that has a 1/4-20 or 1/4-28 thread.

I did find a 1/4-32 nut (part # 91862A516) from

http://www.mcmaster.com,

which is only 5/16" flat to flat, but then I can't seem to find a screw for it. Mcmaster also sells a 1/4-40 nut that would work, but same problem, I cannot find a screw.

I would appreciate any feedback anyone can provide. Hopefully, just creating a slot in the 10-32 screw is viable and cost effective.

Thanks for your help.
John

 

[berkshire]

John2004 (Mechanical)
Any competent machinist should be able to give you a slot in your screw by using a milling machine.
B.E.

 

[John2004]

Hi Berkshire,

Thanks for your reply.

Ned Simmons from another forum just pointed out that it may be cheaper to machine a flat on the screw, like a standard spring hook shown on page 3114 of the Mcmaster catalog

http://www.mcmaster.com.

For example, see image of Mcmaster part # 9634k13. This should allow the screw anchor to clear the hook curvature of the spring.

If I do this, the flat will only be about .0625" thick in order to clear the spring hook. The spring will only have about 40 to 50 pounds on it so as long as a thin flat like that can be machined without problems, it may be a viable option.

Which would likely be cheaper, machining a slot, or milling a flat like a standard spring anchor and then drilling the hole ?

Thanks again guys,
John

 

[berkshire]

John2004 (Mechanical)
It depends on your machinist, but my guess would be it would be cheaper to just do the slot, this can be done in one operation. Putting a flat on, then drilling would require two operations.
B.E.

 

[John2004]

Hi Berkshire,

Thanks for your reply. I wish I could find some cheap off the shelf 10-32 spring anchors, but most seem to be kind of expensive at about $2.30 to $3.00 each, or not at the right lenght. I was hoping to have these made for less.

John

 

[berkshire]

John2004 (Mechanical)
Unless you are ordering large quantities most machine shops will charge more than $2.3 to $3.00 to put a slot in a screw. You will be very lucky to get away with less than a $20 setup charge.
B.E.

 

[John2004]

Hi Berkshire,

I was thinking down the line for larger quantities. It seemed to me the parts could be made in quantities of 100 to 500 PCS or so for less than $2.30 each, (I was hoping cents not dollars) but I could be wrong.

For prototypes, I would only need 20 to 40 PCS.

It seems a .15 cent screw could be cut to length, run over a grinder or milled to make the end flat, or just hammered or swedged into shape, then drill the hole, without too much trouble. I guess it depends on the shop and how they view things.

So far, the only stock spring anchors I found that seem long enough are here...

http://www.misumiamerica.com/PDFViewer.aspx?Metric=true&Page=721

However, if I use a 6mm OD, then the nut will probably be larger than 5/16" flat to flat. I need an OD that will allow a 5/16" hex nut to be used (or practically same size metric nut)and be in a length that allows me to cut them at 1.599 to 1.707" long like on the drawing.

Perhaps I will modify my print to look like a stock spring anchor, and give the shop the option of making them, or buying them and making simple modifications if that best (and if they can find something to modify, i.e., right OD and length).

If they have to take a stock part and drill the hole bigger and cut it to lenght, it may be best to just make them from a .15 cent screw.

Thanks again,
John

 

[GregLocock]

For small scale manufacture work on $1 per hole, plus a setup of say $30 per operation.

You may find a competent backyard place that will do it for half that. A retiree with their own workshop might be a good bet for small quantities.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[John2004]

Hi everyone,

After thinking about it, I am afraid if I drill a .098" OD hole through the threaded portion of a 10-32 screw, it won't be strong enough because it only leaves .02" on each side of the hole to the minor diameter of the screw. Plus, it was suggested to me that instead of increasing strength around the .098" OD hole, the threads may actually cause stress to rise at the root diameter.

I found a Part # 91251A353 from

http://www.mcmaster.com

that should work well for me. I can cut the head off and use the un-threaded .190" OD portion to drill the .098" OD hole through. However, this screw has a hardness of Rockwell C39 to C45. Can a flat be machined and a .098" OD hole drilled fairly easily with a carbide cutter and drill ? I just wonder if the screw hardness will be a problem to machine or drill, cost wise ?

I need an overall screw length of about 1.707" and about .69" of usable thread length. I did not see an un-hardened screw that met my dimensional requirements and still had an unthreaded portion I could put the .098" OD hole in.

Please let me know what you think.

Thanks again for your help.

Sincerely,
John

 

[John2004]

Hi everyone,

I found a part # 92196A279 from

http://www.mcmaster.com

made from 18-8stainless that should machine fine, but then on the 2D drawing it says the thread length can vary from 7/8" to fully threaded. I don't know if they mean the thread length can vary that much on that particular part #, or from part # to part #.

Thanks again,
John

 

[handleman]

Since that note is on the specific drawing for that part # I would assume that you will be guaranteed at least 7/8" of thread, but no more. This is probably due to McMaster-Carr getting the screw from whoever is selling it for the best price.

 

[zcp]

John2004,

1) What will machining tolerances do to your design?
2) Why only a 5/16 high nut? What happens to the threaded rod as you tighten the screw? Where does it go that a larger nut would not fit?
3) Can you post a sketch of your overall setup?



ZCP

http://www.phoenix-engineer.com

 

[John2004]

Hi everyone,

Mcmaster said the 92196A279 screw can have anywhere from 7/8" thread (which would be perfect) up to full thread (which would not work). I need to find something with a consistent thread length.

In regards to ZCP's questions...

1. The machining tolerances on the spring anchor can be very relaxed. It just tightens or loosens a spring.

2. It's not a 5/16" "high" nut, it's a 5/16" nut "from wrench flat to wrench flat". The reason I need the 5/16" nut is because a 5/16" (or smaller) box end wrench is the only thing that will fit in the limited space to adjust the springs by turning the nuts.

The springs go inside the stock tremolo system spring cavity of an electric guitar. I don't want to remove any more wood than is necessary for the design, as this is undesirable.

The screw just translates as the nut is turned.

3. I have uploaded a rough sketch to rapidshare.de at the following link...

http://rapidshare.de/files/25210820/Spring_Layout.zip.html

There is a top view looking down through the guitar and a side view. On the side view, the spring space is outlined in purple.

Drawing color code...

Guitar body = grey
10-32 Spring adjustment screw = blue
Spring adjsutment nut = red
Springs = black
Spring anchor = light blue

This is for a new type of tremolo system and I don't want to widen the stock spring rout in the guitar body if I can get away with it. This is why I have such a limited space.

You can see three springs on the left side that have a .454" center to center spacing. These three springs could be adjusted all at once if I had to, but the one spring on the right has to be adjusted independently of the other three.

The one spring on the right is .825" on center from the nearest of the other three springs. The location of the independent spring and the nearest of the other three springs, cannot change or at least they cannot get closer to each other.

Thanks again for your help.

Sincerely,
John

 

[John2004]

Hi everyone,

I found a Part # 92196A281 from

http://www.mcmaster.com

that is made from 18-8 stainless & should work for me. It has a length of 2.5" and the threaded portion can vary anywhere from 7/8" to 2". That will leave me enough room to cut the screw to length, and then use a 1/4" long portion of the non-threaded portion to drill the .098" hole through.

It kills me that the tell you the screw is partially threaded, then the drawing says the thread can vary from 7/8" to fully threaded or 7/8" to 2" thread. That's a big variance if you are working with or in need of the un-threaded portion. I wonder why they are so inconsistent with regards to thread length ?

Anyway, it looks like I have something that will work now.

Thanks again for the feedback guys.

John

 

[handleman]

Have you prototyped this yet? It looks to me like you're going to have problems with your nuts vibrating off. The guitar is going to be a high-vibration environment, and a tremolo unit is going to experience quite a lot of cyclical loading. The higher quality screw and nut you have the easier the nut will spin and the more likely the nut will back all the way off and your spring will go bouncing around the inside of the guitar body.

 

[John2004]

Hi Handleman,

Thanks for your reply.

I've tested some prototypes and I'm finishing the drawings for the final prototypes now.

The nuts will always be pulled into the spring anchor (steel bar stock) by the springs, so the nuts will always have an axial load on them. I've designed this so the springs always have some tension on them.

There does not seem to be much vibration or problems with cyclic loading. I did not really think about the nut backing off because it's just a slow moving hand operated device, not a industrial machine with heavy vibrations that requires a lock nut. Plus, the nut always has an axial load on it.

If there were problems I suppose a second nut could be used, i.e., after adjustment, the second nut can be tightened down into the first nut for a locking effect.

Thanks again for your feedback.
John