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Now I'm looking for a .250-20 ACME Tap!

Yep, change your design, you are going to have a hard time finding ANY acme that is 20tpi. I would redesign to use 1/2 x 10tpi, a very common size.
 
Tell me why it has to be an Acme screw?
Is the load that much more than what a 60 deg thread will take?
or is the accuracy required that much more?
or is it just cuz ya want one?
 
Tell me why it has to be an Acme screw?
Apparently PM has been invaded by Hobby CNC guys


Based on his other posts, I'm guessing he's trying to replace the Z-axis of a Harbor Freight Micro Mill.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=47158

Laminar: if you're trying to do this on a really tight budget, your best best will be to make an Acme tap out of the same Acme screw stock.

If you do this, and you're not set up to harden the tap, just tap a Delrin nut with it. There are a bunch of posts on CNCZone describing how to do this.

Let us know if you want to try to harden it yourself (which you'll need to do if you want to tap bronze Acme nuts) -- there are several folks here who can walk you through the process.

Robert
 
You guys are something. Not a lead screw for a micro mill. It's part of an electromechanical actuator I'm designing for a %&*@$%#^??Ó#, which is used on a $?Á„‰Á‡°Øˆ, which your taxes are paying for, so money is not a problem. Space is very limited and it weight is also important. Efficiency is important and the final parts will have to be electropolished and will run in most likely a teflon filled delrin nut. This is why a V thread will not be as good. We probably could use a V thread for testing and just factor in the efficiency loss. And if we have enough transfer through the #$*%^*@# coupling, a V thread might work in the end.

Thanks for the help!
 
Laminar, my appologies -- my response wasn't meant to be critical, just trying to determine the scope and budget of what you're trying to do. The 1/4" x 20 is a strange Acme size that happens to be the size screws used in the Micro Mills, and there's been several new members that have arrived from CNCZone asking for help on CNC'ing Micro- and Mini-Mills...

Back to your original question -- if you're building an actuator, and mechanical efficiency is a major concern, why not a ballscrew?

The Delrin AF will help a lot on efficiency, as will a multi-start thread. You can also find Teflon-coated acme stock at Nook, Green Bay Manufacturing, etc.

As far as the Acme tap, the smallest I've ever seen is a 3/8", and the smallest "common" tap and thread stock is really 1/2".
 
Ditto to what Lazlo said...3/8" is the smallest ACME tap I could find as well. I ran into the same problem in a design I was doing (I need something with a faster pitch). Greenfield has a dept. that makes specialty taps, but they won't make an ACME tap smaller than a 3/8".

One other possiblity is thread milling. But you'd have to see if you can get one that small, and can do the length you need.
 
Nook shows a .250-20 so they have a tap. Ball screw is probably out. No room and other reasons. Checked MSC.

If it is this hard to get, I'll probably use a V thread. It will work OK at least for testing.

Thanks guys!
 
You may need to call someone like vermont tooling (tap and die, I forget the actual name but it is Vermont) or a vendor like greenfield that cound make this for you or may have this tap. This is after all, their specialty. The tap may be expensive, the calls may take a bit of time, but would probably be worth it.

Did you check the normal suspects like MSC?

I also have to work designs that are quite specialized like this (though I am not familiar with ^%$#%&&* as a company).
 
It's about time someone asked that. Good question. The "nut" on my design is large and has features for other things on it. I might be able to cut the Nook nut and press it into our part, and I will look into that for a test. But if there is any distortion, it blows the torque test so we have to be careful. Good thinking John!
 
Nook shows a .250-20 so they have a tap.
I'm not sure the former implies the latter. Nook probably doesn't use taps in their manufacturing process, for example.

An Acme tap that small is going to have a lot of trouble shearing a trapezoidal Acme thread. It would probably need to be a tandem tap, but even then you're applying an immense amount of torque to a very thin cross-section of metal (the root diameter of a 1/4" - 20 Acme thread is 0.215").

You might be better off single-pointing or thread milling a nut this size...
 
Couldn't it just be done on an EDM?
If you're eventualy gonna make a production run out of it, it seems to be the way to go. Mostly since money isn't a problem.
 
You will need a set of three taps to make a 1/4 -20. The first two open up the threads and the last is a finish tap. Here's a link to Western Tap. http://www.westerntap.com/ I have no connection except that I have a set of their 1/4-20s. Mine are double lead and are excelent.

Rick
 
McMaster-Carr http://www.mcmaster.com has a plastic nut on page 1006 for $30, catalog number 95072A450. It's made of internally lubricated Acetal rather than Delrin. I don't know if that would work or not.

It's rated load capacity is 150 pounds, which is the __lowest__ rating of any plastic Acme nut in the catalog.

Leigh
 








 
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