Debugging Acme threads

[Metmachmad]

It is usually OK to cut fine pitched ACME thread with one tool and in one operation. With coarser ACME threads it is best to complete the thread cutting in three operations. You are cutting coarser ACME threads here.

With right hand ACME threads
1) Cut out a square thread to the size of the bottom of the thread first.
2) Then finish the left hand side with a right hand tool.
3) Then cut out the right hand side with a left hand tool

As you are cutting left hand threads swap the order of operations 2 and 3 above.
Also you should use an ACME thread gauge to accurately grind your tool bits.

[Bryan]

With right hand ACME threads
1) Cut out a square thread to the size of the bottom of the thread first.
2) Then finish the left hand side with a right hand tool.
3) Then cut out the right hand side with a left hand tool

That's how I always imagined it working best, but without trying it I didn't like to say. Anyone else used this method?

[Graziano]

It is usually OK to cut fine pitched ACME thread with one tool and in one operation. With coarser ACME threads it is best to complete the thread cutting in three operations. You are cutting coarser ACME threads here.

With right hand ACME threads
1) Cut out a square thread to the size of the bottom of the thread first.
2) Then finish the left hand side with a right hand tool.
3) Then cut out the right hand side with a left hand tool

As you are cutting left hand threads swap the order of operations 2 and 3 above.
Also you should use an ACME thread gauge to accurately grind your tool bits.

Thanks, I was aware of that method but thought it applied to the larger size acme screws, but not to coarser pitch threads. I've been treating it like a screw thread in that I could single point the thread due to it's fairly small size. For now I just have a standard thread gauge with the 104.5 (90+14.5) degree angle for aligning the tool steel to the job in the lathe.

I have been using a small engineer's protractor to check the angle of each side of the tool individually and then grinding the tip and comparing to a micrometer set to the correct size according to a chart I have off the 'net and using the 10 tpi dimensions.

[Graziano]

Have run some more experiments:
1. Used about 20mm diameter 1214 steel bar to reduce deflection and honed the tool on an oilstone and plunged in at 90 degrees using the cross slide: no change, i.e. tears on the left hand flank and right hand flank has a smooth improved finish.
2. With the same setup as 1., came in at 75.5 degrees (90 minus 14.5): no improvement over 1.
3. Sharpened the left hand side of the tool (1/2" wke45 tool steel) to that it has no side clearance (90 degrees) other than that provided by the 5 tpi helix, honed and plunge cut straight in: The left hand flank now cuts smooth with about one small shallow tear per revolution, no burring either and the bottom of the thread is mirror smooth.
4. Same as 3., but sharpened the left hand side of the tool past 90 degrees so there is minimal side clearance to the LHS flank, honed and plunge cut at 90 degrees: results the same as 3. and no improvement in slight tearing of 3.

Will try some other steels and some aluminium bar next to see if that improves the finish, and try the travelling steady.


Thread as cut in No.4 but LHS of tool is not honed and depth of cut was 0.001" so the bottom is a bit rougher than ex 4.




Photo showing the WKE45 freehand ground and honed.




Right hand flank close up




Left hand flank close up



Acme pt2-2.jpgAcme pt2-rh flank 1.jpgAcme pt2-lh flank 1.jpgAcme pt2-1.jpg

[morrisman]

Hi

Those threads don't look too bad to my eyes. Are you being a bit too fussy ? The hard part will be achieving a constant or even depth of thread over the whole length of your new screw as the piece you are threading tends to flex in the middle as you traverse the cutting tool along. The travelling steady is supposed to hold it all in alignment but you may have to adjust it as you go Mike

[Graziano]

Hi

Those threads don't look too bad to my eyes. Are you being a bit too fussy ?

Mike: I reckon they'll do for my purposes, they are a big improvement over the earlier experiments.
For the length of acme I need the travelling steady is a must, this short piece will be made as accurately as I can manage and used as a test piece for making the bronze nut.

[Bryan]

Have run some more experiments:
1. Used about 20mm diameter 1214 steel bar to reduce deflection and honed the tool on an oilstone and plunged in at 90 degrees using the cross slide: no change, i.e. tears on the left hand flank and right hand flank has a smooth improved finish.
2. With the same setup as 1., came in at 75.5 degrees (90 minus 14.5): no improvement over 1.
3. Sharpened the left hand side of the tool (1/2" wke45 tool steel) to that it has no side clearance (90 degrees) other than that provided by the 5 tpi helix, honed and plunge cut straight in: The left hand flank now cuts smooth with about one small shallow tear per revolution, no burring either and the bottom of the thread is mirror smooth.
4. Same as 3., but sharpened the left hand side of the tool past 90 degrees so there is minimal side clearance to the LHS flank, honed and plunge cut at 90 degrees: results the same as 3. and no improvement in slight tearing of 3.

Will try some other steels and some aluminium bar next to see if that improves the finish, and try the travelling steady.

Mark it's not clear what the starting point was for trailing clearance, but it sounds like you had too much because less worked better. Is that right?
How did you get your pics to display? No-one else can.

[Graziano]

Bryan: Yep it was way way too much side clearance, once it approached 90 degrees or slightly over it was fine. I'm just hand editing the [img] [/img] around the image links.


This is a photo of the left side of the tool showing the excess side clearance at bottom, then a honed 90 facet in the middle, then up top an unhoned 90+ degree facet used for the latest screw pics only the top two are any good, the bottom would suit a RH tool.
Edit: I should add this is how my machinist mate told me to cut the acme thread, with the condition that the final cuts are taken with a freshly honed tool. The main problem was getting the tool geometry right.




Blurry end shot showing the tool tip with only a small part in focus and maybe three facets on the LH side.



Acme pt2-tool left.jpgAcme pt2-tool tip.jpg