Screwcutting an 8TPI ACME internal thread?!

[NickG]

Hello,

I need some advice on cutting an internal 8tpi ACME thread on the lathe. This is to repair my milling vice which I recently found out had a part broken as some of you may have seen in my Poppin flame licker thread.

My lathe has a screw cuttting gear box with full range of feeds and speeds so that isn't an issue, it's also easily man enough for the job. The real problem is, I don't really know how to grind the tool. I guess I could work it out from the thread geometry, probably on CAD but it would take a while. I need to know what clearances I need on the tool and what size cuts I should be taking etc, also whether it is critical to feed in and an angle etc.

Please don't suggest any other options for repairing the vice, I know there are other ways to skin a cat and I have a few options but I want to have a go at repairing it the right way first.

Thanks,

Nick

 

[rake60]

It may not be much fun to cut that. Acme threads are notorious for chatter.

For a basic Acme thread, the depth is approximately ½ the pitch.
An 8TPI would have a pitch of .125" so the final thread depth will be
somewhere close to .0625", a little deeper wouldn't hurt.
You can calculate the other dimensions by using this drawing.

If you cut it deeper that bottom width will end up being smaller than .046"
You need to hold the top width to that .046" If you grind the tool to a 29°
included angle with a .030" flat at the nose, you will have .016" to move back and
forth in the cut. I do that by indicating the compound perfectly square to the chuck.
Then I can use the compound to adjust in and out to clean up the sides of the thread.
It's real tough to measure. You basically need to get it close then check it for fit after
each finish pass. There may be a lot of them!

Rick

 

[Maryak]

If you cut it deeper that bottom width will end up being smaller than .046"
You need to hold the top width to that .046" If you grind the tool to a 29°
included angle with a .030" flat at the nose, you will have .016" to move back and
forth in the cut. I do that by indicating the compound perfectly square to the chuck.
Then I can use the compound to adjust in and out to clean up the sides of the thread.
It's real tough to measure. You basically need to get it close then check it for fit after
each finish pass. There may be a lot of them!

Rick

I agree with Rick's comment about the difficulty of getting accurate measurements of tool tip and the top land of the thread.

Without fancy gizmos the only way I know of to get pretty close, is to set a micrometer to the tool tip measurement, (0.030") and then use it like an outside caliper. When the jaws will just, and I really mean just, allow the tool to enter you are within about 0.001" of the true measurement. The only other thing I would add is try and determine accurately the backlash of the compound leadscrew before you start in earnest.

Hope this helps.

Best Regards
Bob

 

[rake60]

Good point on the compound leadscrew backlash Bob.

I use an indicator to make those adjustments.
Go past the wanted shift then move back to that point to
place the pressure of the leadscrew in the direction of the
pressure of the cut.

Some things become so automatic I just forget about them.

Rick

 

[Stan]

If this is for a 5/8" or thereabouts screw it is going to be real tough to cut a thread of any significant length. The boring bar will have to be so small that it will chatter if if you have more than about 1/2" of unsupported bar.

One solution may be to cut the majority of the material out with a conventional thread bit and then make the final shape with the acme bit.

 

[Deanofid]

Nick, is there an obstruction of some sort that would prevent you from cutting these threads with a tap?
Just thinking you might have better success cutting internal Acme threads that way.
Like some of the other guys have let on, Acme threads pose a few difficulties that will be magnified in a small bore due to tool rigidity troubles, not to mention, it's a pretty quick pitch.

Not trying to suggest a different way of fixing the vise, but a different way of getting those threads cut. You would probably have to make two or three taps, but at least you could see what you were doing, and no tool flexibility issues!

Dean

 

[Cliff]

Another way to do it is to grind the tool in the shape of the thread groove and set your compound on 29 degrees to the left and take light cuts. If you are lucky and you use 1144 ( Stressproof steel ) it may not chatter very much. Cliff

 

[steamer]

Cutting small ID ACME is doable

I cut a .354-10 for my Waltham and posted it...I'll search for it

There's a good trick for sizing the thread. I used what has been described as the english method. I set the compound perpendicular to the chuck as Rick Described and fed out until I got the full depth of cut plus my OD clearance. My tool was very narrow.

This cut is hard as your cutting with the full width of the cutter to full depth. ITS SLOW GOING! I did this cut in 10 or 12 passes.

Once to depth I then put on feed during the pass with the compound. The cutter is now cutting on the side and is progressively making the thread wider.....in the direction that you want to reduce your backlash. Worked a treat...but was tedious.

If you can beg, borrow or buy a tap that size, you'll be better off...but you can thread it.

My bar was 3/16 diameter and the tool was held in place with 4-40 screw......

I don't remember how long it took me.... a while.

I'll be back with the post or pictures

Dave

 

[radfordc]

Here is a tutorial on cutting Acme threads
http://www.homegunsmith.com/cgi-bin...0b2db1e8cbb295fac76d093b8d;act=ST;f=8;t=22418

 

[Blogwitch]

Nick,

This contains some good info on grinding and using

http://www.neme-s.org/Shaper Books/Square Threads/TMT Square Thread Info.pdf

Here is a little calculator that might help you in your quest.

http://www.americanmachinist.com/Calculators/ThreadCutting.aspx

Make a note of the helix angle.

Plus a bit more info

http://www.diracdelta.co.uk/science/source/a/c/acme thread/source.html

Although you said you didn't want to go that route. But just in case you change your mind.

Beg, borrow or buy the correct sized tap to do the job. You don't say what size your acme thread is, but you can buy reasonably priced ones from RDG.

http://rdgtools.co.uk/acatalog/ACME_TAPS__RIGHT_HAND_.html

I have a couple of taps, but don't know what size you need.


If it is a metric trapezoid thread, maybe you could buy a ready made nut, and somehow fit it into your casting.

http://www.worldofcnc.com/main.asp?category=Trapezoidal

Best of luck on your quest.

Bogs

 

[steamer]

The boring bar and bit

The thread complete

The old nut and new nut.

Like I said, IF you have a tap , its going to be a lot easier................

Dave

 

[steamer]

Here is the run down on how I did it.

I cut the tool narrow.... .040 wide instead of .050 and attained the width of the thread by applying feed with the compound.

Dave

 

[don-tucker]

Yep,thats the way to do it,you can work on both flanks of the thread using that method to obtain a good finish,it works for any large thread.
Don

 

[NickG]

Wow,

I am somewhat overwhelmed by the response! Thanks to all :bow:

I need to digest this information now - there's a lot there. I'm kind of thinking I'm going to end up tapping it esp since bogs suggested RDG - I never thought of them, it's 7 squid cheaper from there so not bad.

But I think I aam going to try a couple of times to screw cut it first - if nothing else, it will be a good exercise for me.

Steamer, I was thinking last night when ni bed that I would have to make a tool the same as yours if I'm to stand any chance. I had tried to grind one out of a square bit of HSS but I need a round bar as close to the hole diameter as allowable to keep it as rigid as poss. Steamer so you didn't bother setting the compound over to 14.5 deg to feed in at flank angle then? Would that be worth doing in this case - couldn't widen it by your phase 2 then thoguh as settings would all be lost. The calculator bogs sent link for agrees with your 12 passes for that size.

Many thanks again all for taking time to respond to this, I never expected all of this! :-[

I now think I'm armed with enoguh theory to give it a proper go.

Fingers crossed!

Nick

 

[Stan]

I try every way possible to avoid cutting a small inside acme thread. When I made a new tail stock ram, I silver soldered a pre-threaded nut into a round hole. For an old horizontal I had years ago I had to replace the X axis leadscrew and nut. Even at 3/4", cutting the thread in the 3" long nut put me off that exercise for life. This is a case of being able to do it, doesn't mean you should do it.

 

[Mainer]

I think I'd try to get it reasonably close on the lathe, then finish with a tap. Taking out most of the material with the lathe will make the tapping a lot easier.

 

[steamer]

I try every way possible to avoid cutting a small inside acme thread. When I made a new tail stock ram, I silver soldered a pre-threaded nut into a round hole. For an old horizontal I had years ago I had to replace the X axis leadscrew and nut. Even at 3/4", cutting the thread in the 3" long nut put me off that exercise for life. This is a case of being able to do it, doesn't mean you should do it.

YUP....agree...as stated above in my post, if your goal is an id acme thread of a common size....RDG is the next stop. .I didn't have the tap....didn't feel like making one for one hole either......as it was oddball .354 diameter x 10 left hand thread.

But....there is some fun in trying......if you can afford the time and some stock.....and some noodling......what the hell.....

Dave

 

[Maryak]

But....there is some fun in trying......if you can afford the time and some stock.....and some noodling......what the hell.....

Dave

Yep, my take on it too. A new hobby skill is a hobby in itself. ;D

Best Regards
Bob

 

[steamer]

Wow,

I am somewhat overwhelmed by the response! Thanks to all :bow:

I need to digest this information now - there's a lot there. I'm kind of thinking I'm going to end up tapping it esp since bogs suggested RDG - I never thought of them, it's 7 squid cheaper from there so not bad.

But I think I aam going to try a couple of times to screw cut it first - if nothing else, it will be a good exercise for me.

Steamer, I was thinking last night when ni bed that I would have to make a tool the same as yours if I'm to stand any chance. I had tried to grind one out of a square bit of HSS but I need a round bar as close to the hole diameter as allowable to keep it as rigid as poss. Steamer so you didn't bother setting the compound over to 14.5 deg to feed in at flank angle then? Would that be worth doing in this case - couldn't widen it by your phase 2 then thoguh as settings would all be lost. The calculator bogs sent link for agrees with your 12 passes for that size.

Many thanks again all for taking time to respond to this, I never expected all of this! :-[

I now think I'm armed with enoguh theory to give it a proper go.

Fingers crossed!

Nick

Nick

How ya making out?

Did we answer your questions?



Dave

 

[NickG]

Hi,

thanks again. I am going to try making a proper boring bar for the job and I think the d bit thread gave me a good idea of how to accurately get a tool bit with the right angles. Am going to try this, might not work on the rock hard steel stuff but should on the gunmetal.

will keep you posted.

Nick