Smithy lead screws/nuts

[markba633csi]

It's more economical to make a composite piece with a threaded brass/bronze section and steel for the body portion- your choice
of methods to attach the two together: screws, brazing, epoxy
Make it so the two pieces fit/lock together for strength in the long direction- I'll do a sketch to show what I mean
something like this:

 

[markba633csi]

You could also consider making the whole piece from cast iron- it would wear well on a steel screw

 

[Extra Fox]

Hi Everyone,
I have the smithy Midas 1220 LTD, and have been going through those brass half nuts "on the regular". From conversations with Smithy over the years of owning this tool, they've always made this part from brass, and consider it a wear part, as they don't want the screw to experience wear.

Now Smithy just discontinued the line completely and has no back stock in these.

So the lead screw is NOT anything standard. It's a 20mm with a 10TPI pitch. It's not an ACME screw and won't fit that profile. The trapezoid angle of an ACME screw is 30deg, and this is more like 20deg.

If anyone's interested, I have a CAD model and am 3D printing one in bronze/steel right now. Once it's tested working I'll be happy to make the file available to all takers.

 

[mac1911]

Hi Everyone,
I have the smithy Midas 1220 LTD, and have been going through those brass half nuts "on the regular". From conversations with Smithy over the years of owning this tool, they've always made this part from brass, and consider it a wear part, as they don't want the screw to experience wear.

Now Smithy just discontinued the line completely and has no back stock in these.

So the lead screw is NOT anything standard. It's a 20mm with a 10TPI pitch. It's not an ACME screw and won't fit that profile. The trapezoid angle of an ACME screw is 30deg, and this is more like 20deg.

If anyone's interested, I have a CAD model and am 3D printing one in bronze/steel right now. Once it's tested working I'll be happy to make the file available to all takers.

that would be awesome.

Do you have pictures?

 

[Extra Fox]

Sure, why not?I went ahead and spent the $$ to have it printed in bronze/steel with shapeways...which is how I found out it's NOT an ACME profile. That's pictured below with the worn out oem brass piece. What I have on order now is a MUCH cheaper high accuracy plastic model of just the threads to check fit before I reorder in the steel.

 

[GeneT45]

Yet another reason to learn to grind HSS...

GsT

 

[Extra Fox]

I agree that’s a good thing to have in your capabilities if you have the time, shop space, and funds for the tooling. I don’t. I also don’t think grinding or tool steel would be either needed, correct, or even cost effective for this project. 3D printing a part is perfect when you have something with complex geometry and a small production run.

 

[GeneT45]

I agree that’s a good thing to have in your capabilities if you have the time, shop space, and funds for the tooling. I don’t. I also don’t think grinding or tool steel would be either needed, correct, or even cost effective for this project. 3D printing a part is perfect when you have something with complex geometry and a small production run.

The only tool you need is a bench grinder and a tool blank. You don't need a tool and cutter grinder for these things, you need a gauge (your lead screw would do) and a bench grinder, preferably with a rest.

I've cut internal and external acme threads - I see no reason this should be different, other than small changes in geometry.

I'm guessing that I could buy a reasonable bench grinder and tool bits and come out ahead in both money and time. I don't say that to criticize your approach, just stressing that if you have that knowledge in your arsenal you can readily deal with issues like this. (i.e. I am more or less advocating learning the skill of hand grinding cutters, which I've found invaluable yet which seems moribund even among those who own equipment.)

GsT

 

[Extra Fox]

That’s reasonable. I’m not sure how one would grind an internal thread on a bench grinder though. If there’s a straightforward way to do that, it would be a good solution for the part I already have. The ACME thread it was made with is too fat and needs material removed for fitment anyway.

 

[mac1911]

Sure, why not?I went ahead and spent the $$ to have it printed in bronze/steel with shapeways...which is how I found out it's NOT an ACME profile. That's pictured below with the worn out oem brass piece. What I have on order now is a MUCH cheaper high accuracy plastic model of just the threads to check fit before I reorder in the steel.

That’s cool. When I was talking to my friends, dad who is a lifelong machinist when I was asking about the lead cruise on these he laughed and said the only thing inch on on that is the 10 threads per inch.

If you don’t mind me asking, how much did Shaw charge you to print it in metal

The fit and finish are good?