Cross Slide Screw Material

[YotaBota]

I heard back from MSM, they can get the C1144 rod but would need to bring in a 12' length, he'll get me a price hopefully tomorrow.
If this ends up being to much of a pita I'll use 4140 as Brent H did, if they can get that.

 

[DavidR8]

I heard back from MSM, they can get the C1144 rod but would need to bring in a 12' length, he'll get me a price hopefully tomorrow.
If this ends up being to much of a pita I'll use 4140 as Brent H did, if they can get that.

Let me know if you want something from McMaster.

 

[Brent H]

@YotaBota : if your crossfeed nut is bronze or brass I would suggest a nice cold rolled dowel would suffice. If the nut is cast iron, a better steel quality would be required

 

[YotaBota]

Let me know if you want something from McMaster

I should know tomorrow if MSM is viable, thanks

Brent H - it's a brass nut so is 4140 a good way to go? All suggestions welcomed.

 

[Brent H]

@YotaBota - yes, quality steel and machines well. It cut nicely fit me and works well. Probably better than original

 

[YYCHM]

What tool are you going to use to turn the acme thread?

 

[YotaBota]

Thanks Brent, I'll ask MSM about 4140 tomorrow.

YYC - I've ground an HSS bit for 10tpi acme.

 

[Brent H]

@YotaBota : when you ask if you need say “9/16” finished dimension (0.5625”) or 5/8” (0.625”) be sure you get either precision 4140 (0.001+\-) over a given length - if you get the annealed, not hardened 4140 it might be under size and you just can’t machine out a 9/16” precision cross feed screw it it’s 0.005 under

 

[Chipper5783]

I have made three "long" screws: one for the compound on my 15" lathe (acme), one for the cross slide on the 11" lathe (acme) and a 15" long 3/4-10 tpi (regular V thread) for a meat slicer. It is challenging - to get the fit all the way along. I suggest you first figure out how you are going to measure the thread size (my recommendation is to take a deep breath, throw caution to the wind and get a good thread micrometer - there are other options, but a thread mike is very nice to have). Second figure out your follower rest - it is impossible to eliminate all "tool push". I found I could do most of the screw without the follower rest - basically zoom along, lots of shallow passes, figure out the sizing at the head stock and tail stock ends (still leaving material for the finish passes) and only used the follower rest on the last few.

 

[PeterT]

I actually don't know - can you get screw micrometer anvils for big acme threads like that?
The sets vary but for example I see #5 anvils (5-8) TPI exists, but I think that's 60-deg not... what is Acme... 29-deg?
https://www.shars.com/0-1-screw-thread-micrometer

 

[Chipper5783]

You are correct, I was mixing two ideas. The point to take away is that you need to be able to compare the thread. You want the thread to be consistent over the entire length. There are probably a few approaches to compare one area to the next, or to an example thread (the actual number doesn't really matter). Three wire would likely work, perhaps a blade mike, or even the jaws of some calipers, sort of whatever lands on the sloping side of the threads and is repeatable. I don't recall what I did for measuring when I cut the acme threads (I know I tried to find a length of acme rod, but was not able to find that size & pitch in the correct rotation). The project for the 3/4-10 thread is why I bought the thread mike.

 

[RobinHood]

I would make the nut first (if you don’t have one already) and use it as a “thread gage”. The produced screw will be unique (and a one off) to your machine. That does not matter as long as the two parts fit together with as little (and consistent) clearance as possible.

If you were making screws for production and sale, that would be a whole other story... You would have to use a go/no go thread gage that is produced accurately to reflect the acceptable tolerance standards. You can buy those gages.

Here is a link to a discussion of how to measure acme thread using a single wire...

https://www.practicalmachinist.com/vb/general/measuring-acme-threads-187704/

The Machinery’s Handbook also discusses measuring ACME threads using the three wire method (29th Edition, pages 1995 through 2004).

Happy threading!

 

[YotaBota]

IIRC that is a 1/2” - 10 LH acme, correct?

Correct, the rod from Mcmaster is -.003 +.0 in tolerance

you just can’t machine out a 9/16” precision cross feed screw it it’s 0.005 under

What about going up in size and machine it down to what is required?

The screw portion is under 8" so I'm not sure the follow will be needed. As for DOC, I have a new nut nut made so, as RobinHood suggests, I was going to use it for the thread gage. I have some unknown rod so I'll make a couple of test pieces to get the process right. I'll also try one at .497 to see how much difference .003 under will be.
I've seen the thread mics but I'm not into a production run so I think I can better use the $ for tooling, and I might need the $ just to buy the rod.
MSM just quoted me $55 for .625 x 36" of 4140 so that's not cheap, it's not even inexpensive.

 

[Brent H]

You may be Ok @YotaBota - be sure to check your fit as you get close so you don’t go over.

 

[PeterT]

This link might be useful for your sizing. Its probably in the The Book but then I'd have to get up & find my other glasses :/
https://www.engineersedge.com/hardware/acme-threads-calculator.htm

I've never really paid attention to this before but the min/max tolerance on the major diameter (0.6250-0.6200) represents a tighter range than the Class II example pitch diameter (0.5687-0.5544). I'm pretty sure V-threads around this size are the other way round, no?

 

[Brent H]

@YotaBota - looks like the chart Peter posted allows for up to 0.005” under the major diameter with no ill affects on the fit. You should be good with only 0.003” and like I mentioned earlier check your fit to be sure you keep things snug.

 

[Chipper5783]

The chart from Peter helps in pointing out that the OD is a non-critical dimension- it does not touch anything. I believe the sloping sides of the thread are what determines the fit. when cutting an acme thread, there is quite a bit of tool in contact with the material - there is going to be deflection. Perhaps with a piece only being 8" long X 1/2 minor dia, the deflection of the finish pass will not be objectionable? My experience on cut to fit with a "long" thread is that it is pretty easy to get the starting end correct, but then the mid-region is too tight.

Lots of great ideas shared here, looking forward to hear how it turns out.

 

[YotaBota]

Ya gotta luv interactive charts. thanks for the link. I use the LMS speed and feed chart a lot.
We'll see how the under size test rod works.

looking forward to hear how it turns out.

Me too! lol

 

[PeterT]

I wouldn't say OD is non-critical. Its a min/max range for that class of fit. If you are outside this range, there will likely be a downside. But like I was saying strikes me as odd is the OD min/max tolerance range is tighter than the pitch diameter tolerance, which is opposite to vee theads. I don't have experience turning acme, there must be a reason.

Also be aware of typical catalog material specs when it comes to mill tolerance. I've encountered this on O1 tool steel. They aren't being sneaky, its more of understanding. Say its +-0.005. Now picture an ellipse. That could that a 1.000 nominal shaft cross section is 0.995 on the skinny axis and 1.005 on the fat axis. But that is different than a CIRCULAR section that varies between those two dimensions down the length of shaft. There is probably an official definition out there but I don't see it referenced. This might not be enough for you to worry about but just mentioning FWIW

 

[YotaBota]

PeterT - I've read about the "ellipse" factor, if it's that tight a tolerance then they only way I see around that is to buy over size rod and bring it down to the size you need.
Definitely worth the mention, I don't know if it is critical with a screw but I'll measure the rod before cutting it.