Half Nut

If you have read Robs post on the Deckal Jam you will Know we found the problem and freed every thing of and now all seams well, this is apart from the half nut which is worn out so we need to make a new one and guess who got the job, screw cutting is not a problem as I've done quit a few on the lathe and seam to do ok, but I've never cut an acme lead screw thread so I will need some help along the way, my first question is the ID as the thread inside the half nut has almost disappeared am I right in thinking if I measure the OD of the thread between the thread on the lead screw is that my ID, help with this would be much appreciated and I hope to post pictures as I go, don't want to cock this up as the price of the material is expensive phosphor bronze.

Regards Alan.

Yes, but; there's usually an allowance on both ID and OD so that the thread engages on the flanks, not on the crests. All the details are in Machinery's Handbook. It is traditional to make a practice piece in a less expensive material, such aluminium, to check form and fit.

Andrew

Thanks Andrew I don't have a copy of the machinist hand book you talk about is there a guid for the allowence you talk about.

Al.

Enter the OD* of the leadscrew and tpi here and it will give you the rest

The OD will be a nominal size eg 0.625", 0.750" etc

And the formulas here

Edited By JasonB on 23/08/2013 16:35:40

Hang on.

If it's a Deckel chances are it's metric and a trapezoidal thread form at 30 degrees.

You might want to look into that choice of material. Phosphor bronze can be quite hard and can wear non-hardened steel shafts and leadscrews. For a soft steel, a softer leaded bronze/gunmetal may be a better choice. Depends I suppose on how hard the leadscrew is and how concerned  you are about it wearing.

Edited By Hopper on 24/08/2013 10:05:41

Will hopefully try to get the new half nut made this weekend, I've started with grinding up the Acme thread cutter if you look at it and see anything I've done wrong please let me know, Andrew thanks for the advice it makes sense to make a trial one out of ally first so that's what I shall do, Jason thanks for the link that will be very handy, John you are right most of them appear to be metric but this one isn't it's imperial, Hopper I see what you mean but as we only have the phosphor bronze it will have to do, spoken to Rob whose mill it is and his thoughts are if the miller was in use for 8-10 hours a day we would look for some thing else like you suggest but the use the miller will get we will all be dead and buried before the lead screw is worn out.

Al.

Al, will that make a v type of cut? I thought the thread was straight sided! More like U with a flat bottom,

Rob.

Start with cutting the v

Then square it up as required wit a second tool

Cutting it out in a single chunk is a PITA

Edited By Ady1 on 30/08/2013 14:46:53

mrpete222 (tubalcain) has a set of videos on YouTube showing how to do Acme.

Series starts with his Machine Shop Tips #24

Geoff - Run out of things to say. Bummer!

That's the form it should be so it will be ok I think, Andy I can't start with a v shaped cutter because it's an internal left hand thread, one I start to cut the thread I don't want to risk anything moving so I thought best to go in straight with the cutter I ground but very slowly and 5 thou at a time maximum, unless any one can give me a better way to do it.

Al.

Finally got the lead screw picked up today and it measures 8TPI and the OD = 0.950 so I have gone for 0.828 for the ID don't know if this is correct but I will find out tomorrow when I thread a piece of ally which I've bored out ready in the lathe, if I have the ID wrong just tell me before I cock it up.

Regards Alan.

0.122 divided by 2 = 0.061 sounds a little tight, but easy to take off not so easy to put back on.

Rob.

Sorry al completely misunderstood, I have sent the guys on the forum into a spin,

rob

So today started to make a trial nut just to check I've got everything right, cut and faced of a piece of ally round bored out and cut the thread, lead screw would not fit after abit of head scrathing thinking I've got a measurmet wrong I grabbad the old half nut and screwed it onto the lead screw and to my amazment its a right hand thread the one I cut was left hand, can't tell you the names i was calling myself I couldn't belive I made such a stupid mistake all I needed to do was check it befor I started, (don't know why I did it) anyway made another one and it fits well so next will be the phospher bronze.

This is the left hand Acme thread I don't need.

This is the right one.

Got a bit more done yesterday it's ready to bore out to size will do some more this afternoon after Moto GP if the wife lets me.

Al.

Finished boring out the half nut ready for threading, drilled out to 14mm

Made a boring bar for the job and took it out to size.

Cut the Acme thread.

Checked the fit just what I was hoping for, thank's to every one for all the advice and help with this one, only ever cut one internal thread before so I'm pleased how it turned out.

Cheers Al.