Single Pointing Metric Acme Thread

[Rogue_Machinist]

So for some off reason my boss decided to take a waste of a job and now I have to get it done. So I need some help. I need to single point an M40 x 7 Metric Acme Thread. And I cant seem to find any guide as to what the pitch will be in Imperial. Im not changing the units of my machine for 1 part. So I used an online guide and it appears the thread pitch is going to be F.276 but need to verify. any input would be great.

 

[angelw]

So for some off reason my boss decided to take a waste of a job and now I have to get it done. So I need some help. I need to single point an M40 x 7 Metric Acme Thread. And I cant seem to find any guide as to what the pitch will be in Imperial. Im not changing the units of my machine for 1 part. So I used an online guide and it appears the thread pitch is going to be F.276 but need to verify. any input would be great.

You should use as accurate a lead as you can, rather than rounding to 0.276. With your machine set to Imperial Mode, you can go to 5 decimal places and therefore, the lead will be 0.27559, provided the thread is a single start.

Getting the lead conversion is the easy part, you're going to have to deal with a fairly extreme helix angle of a 7mm lead thread on a 40mm diameter as clearance on the Threading Insert will probably be an issue. You would also want to either grind., or crank the tool over somehow, so that the top surface of the insert is close to perpendicular to the helical path and it's centre (along the Z axis) is on centre height. Cranking the tool over will fix both the cutting action of the insert and the clearance issue you will have with a 7mm lead thread.

Regards,

Bill

 

[Mark Rand]

The other point is that it is probably 30° trapezoidal, not 29° ACME. Make sure you've got the ridht tool or insert.

 

[RC Mech]

A “metric” ACME thread does not exist. It’s a trapezoidal thread.

My sympathies. Wouldn’t be the first time a moron business dweeb couldn’t begin to comprehend the difference.

 

[Rogue_Machinist]

You should use as accurate a lead as you can, rather than rounding to 0.276. With your machine set to Imperial Mode, you can go to 5 decimal places and therefore, the lead will be 0.27559, provided the thread is a single start.

Getting the lead conversion is the easy part, you're going to have to deal with a fairly extreme helix angle of a 7mm lead thread on a 40mm diameter as clearance on the Threading Insert will probably be an issue. You would also want to either grind., or crank the tool over somehow, so that the top surface of the insert is close to perpendicular to the helical path and it's centre (along the Z axis) is on centre height. Cranking the tool over will fix both the cutting action of the insert and the clearance issue you will have with a 7mm lead thread.

Regards,

Bill

Yeah im going to grind some clearance below the cutting edge as this is going an internal thread. For a lead screw of some sort of machine that makes wire rope. And its going to be cut from a 2 piece assembled in cast iron. So this job has some challenges. But thank you for the input.

 

[guythatbrews]

If you don't have the tool yet try to find a lay down insert with the appropriate angled shim. Sandvik probably makes one. Sandvik tech support can tell you what shim. Or you can calculate the angle.

Thread angle = ARCTAN(lead/pitch diameter circumference).

For high helix angles you can mill the bottom of the tool shank, then the top parallel and use a shim to bring the tip back to center.

I don't think any on edge holders can compensate for the thread angle but I may be wrong about that.

 

[Rogue_Machinist]

A “metric” ACME thread does not exist. It’s a trapezoidal thread.

My sympathies. Wouldn’t be the first time a moron business dweeb couldn’t begin to comprehend the difference.

Whats worse is hes takes these CRAP jobs all the time. And I spoke with our industrial rep and he knew exactly what we needed. Hes been doing this for 30 years and he got me the correct insert.

 

[guythatbrews]

After my initial post you added this was internal, so no milling the tool!

Sandvik makes laydown threading bars.

You also added the material was cast iron so it's tons easier. If it's only low quantities it's not too hard to just grind a high speed tool. I hate cast iron because of the mess but makes stuff like this tons easier.

 

[Bobw]

And I cant seem to find any guide as to what the pitch will be in Imperial. Im not changing the units of my machine for 1 part. So I used an online guide and it appears the thread pitch is going to be F.276 but need to verify. any input would be great.

25.4 25.4 25.4

Tattoo that number to the inside of your eyelids, that's the only number you need.
Just simply 7/25.4 and there is your answer.

On the helix angle, its not that steep, only 3.5 degrees, for comparison a 1/4-20 is 4.185 degrees.

I hope this is a "it just has to work" job.. Do you have the rod so you check the nut you're making?

 

[sinha]

So for some off reason my boss decided to take a waste of a job and now I have to get it done. So I need some help. I need to single point an M40 x 7 Metric Acme Thread. And I cant seem to find any guide as to what the pitch will be in Imperial. Im not changing the units of my machine for 1 part. So I used an online guide and it appears the thread pitch is going to be F.276 but need to verify. any input would be great.

In mm mode, up to 4 decimal digits can be used for lead.
In inch mode, up to 6 decimal digits can be used.
Therefore, it is better to use F0.275591 in inch mode, though it would not matter much in single-point threading.

 

[Mark Rand]

In mm mode, up to 4 decimal digits can be used for lead.
In inch mode, up to 6 decimal digits can be used.
Therefore, it is better to use F0.275591 in inch mode, though it would not matter much in single-point threading.

Except that 7.0 is pretty exact in this instance.

 

[Mtndew]

Except that 7.0 is pretty exact in this instance.

Except that the OP is in Inch mode as he states in the first post. So as usual, Sinha is correct.

 

[Mtndew]

Whats worse is hes takes these CRAP jobs all the time

Ahh, but that's where the money is, if nobody wants to tackle a job, and 1 shop will, they can basically charge a premium. Your boss isn't as dumb as he seems.

 

[Rogue_Machinist]

25.4 25.4 25.4

Tattoo that number to the inside of your eyelids, that's the only number you need.
Just simply 7/25.4 and there is your answer.

On the helix angle, its not that steep, only 3.5 degrees, for comparison a 1/4-20 is 4.185 degrees.

I hope this is a "it just has to work" job.. Do you have the rod so you check the nut you're making?

Yes, we were sent a gage.

 

[Rogue_Machinist]

The other point is that it is probably 30° trapezoidal, not 29° ACME. Make sure you've got the ridht tool or insert.

And you were 100% on point. I believe our tool rep got us the incorrect insert. As we ran the part the gage would NOT fit. So we put in a call to Sandvik and were told the inserts we were sent were incorrect. So we ended up getting a tap to "fix" the part. It will be fine for the application. But was told today that the company who we quoted the job too, needs 15 more SETS of these. So, We have ordered a new bar and inserts directly from Sandvik.

 

[winger]

A DoAll (Harrison) lathe I was fixing at work had the cross-slide screw and nut worn badly. The lead screw was inch and Acme but with metric diameter. Ended up purchasing precision ground acme from McMaster and making a new nut to fit.
The same with the tailstock screw on a different DoAll. It was a double start thread (inch and metric diameter) At least only the nut was worn, but still took me quite awhile to make that nut.

These were both over 40 years of hard use.
Thinking back I wonder if they were originally trapezoidal. But that wouldn't have been TPI. I wasn't aware of the difference back then and it looked like acme. Everything else on them was metric.

Dave

 

[Rogue_Machinist]

A DoAll (Harrison) lathe I was fixing at work had the cross-slide screw and nut worn badly. The lead screw was inch and Acme but with metric diameter. Ended up purchasing precision ground acme from McMaster and making a new nut to fit.
The same with the tailstock screw on a different DoAll. It was a double start thread (inch and metric diameter) At least only the nut was worn, but still took me quite awhile to make that nut.

These were both over 40 years of hard use.
Thinking back I wonder if they were originally trapezoidal. But that wouldn't have been TPI. I wasn't aware of the difference back then and it looked like acme. Everything else on them was metric.

Dave

Very well could have been a trapezoidal thread. But we knew something was off when we measured everything and it came out to be metric dims. and none of the acme thread pitch gages worked. And thats when we realized it wasnt metric but something else. And I had some old kennametal trapezoidal inserts too big for the bore, but they fit the gage threads perfectly.

 

[johnmontrose]

... The lead screw was inch and Acme but with metric diameter.

This comes up regularly on the UK Model Engineer board.

You have to think as if you are the machine tool manufacturer making machines for a world market. Whichever units system you work in, it is more efficient in terms of stock inventory and tool inventory to use the same diameter stock for your leadscrews whether they are imperial or metric and just to vary the pitch of the thread cut on them (keeping the form/thread angle the same).

It also gives you an agile capability - to switch from making an imperial leadscrew to a metric one, just flip a lever to change the gearing of the machine.

No need to change the stock, the workholding for the stock or the cutting tool. No chance of mixing up imperial stock with metric stock. No chance of picking up the wrong toolbit. No need for the tool grinding department to fixture for 29 and 30 degrees.

You can do the same mental exercise for the nut with which the leadscrew works. Same stock, same tapping drill, only the tap itself changes. It might not conform to any conventional standard, but it makes very good economic sense.

 

[winger]

Not sure why but I wasn't aware of trapezoidal thread forms at the time. So I probably cut a 29 deg form into the tailstock nut. (only thread gage I had) That was at least 5 years before I retired. I'm sure it is still working better than before I fixed it.
I've done a couple trapezoidals since at my part time job and have the proper gage.
Looking forward to using a cnc for metric threads. Real pain with inch lead screw.

Dave

 

[Rogue_Machinist]

Not sure why but I wasn't aware of trapezoidal thread forms at the time. So I probably cut a 29 deg form into the tailstock nut. (only thread gage I had) That was at least 5 years before I retired. I'm sure it is still working better than before I fixed it.
I've done a couple trapezoidals since at my part time job and have the proper gage.
Looking forward to using a cnc for metric threads. Real pain with inch lead screw.

Dave

Well I tried to use the 29° ACME like I was told to use. And there are clearance issues all over the place. Gage wont go even with making it wider using the workshifts. So I told my company we need either a Tap for these 2 parts or spend the $1000 for 10 inserts and a bar to hold them. Im walking away from this abortion of what should have been a no quote.