Debugging Acme threads

[Graziano]

This is my fourth attempt to cut a double start left hand 5 tpi 9/16" acme for my Bantam lathe. It's only a stub sample in 1214 to get the single point tool sharpened correctly and practise thread cutting. I asked a fitter and turner how he checks an acme thread and he suggested cutting a short section in half and see how they fit together. At first glance it looks like it meshes together pretty good but once you shine light from behind you can see slight pitch variations.

The tool is tearing a lot on the flank away from the tool's travel (not in the pics but it's bad) and I don't think I can put much more side relief on that side so the next thing to try is going to be honing the tool as sharp as I can get it (thanks for the post Morrisman) as that's what the machinist suggested I try next. Also I had to clean all the crud out of the leadscrew and halfnut to ensure a good mesh and snug up the compound and cross slide gibs a bit.

[BRADFORD]

It could be wear in your leadscrew.

[.RC.]

What about tool deflection or work deflection...

[Graziano]

What about tool deflection or work deflection...

That's going to be a big issue on the full length piece, right now it's only 90mm long and I've got the tearing mostly on the left hand flank of the thread away from the direction of threading as the tool cuts towards the tailstock. The cross slide and compound gibs are snugged up very tight and the tool steel piece is a 1/2" thick piece with as little overhang as I can get on it. The right hand flank of the thread looks much smoother at first glance(except when magnified).

The cuts appear pretty smooth with 0.002" cuts up until the last few passes when I do 0.001" depth of cut....come to think of it there's got to be some deflection as I can do the same depth of cut for three passes and still get some material coming off the tool. I was told to keep the tool sharp and honed especially for the last pass so it may be time to follow that advice.


Bradford: there was a lot of engrained crud in the leadscrew and halfnut that I've cleaned out now just in case, at this stage I think it's the tearing of the metal causing the pitch variations. The leadscrew and halfnut appear to be in pretty good nick but had engrained crud, I don't think the original owner did much threading, it was even missing the clip for the dog clutch to hold the leadscrew engaged so I had to make one. I'll see if the cleaning has an effect.

[Michael G]

Just a thought - if you think about a tool in a thread (assume RH for the sake of illustration), the left side of the tool will be positive rake, the right side will be negative rake. (assuming the tool is set flat across the top face). If you are cutting a LH thread, it will be reversed.
Do you need to rotate or grind your tool a little to reduce the negrative rake from "quite a bit" to only "a little"?

Michael

[Graziano]

Just a thought - if you think about a tool in a thread (assume RH for the sake of illustration), the left side of the tool will be positive rake, the right side will be negative rake. (assuming the tool is set flat across the top face). If you are cutting a LH thread, it will be reversed.
Do you need to rotate or grind your tool a little to reduce the negrative rake from "quite a bit" to only "a little"?

Michael

Hi Michael, I am sharpening it that way with the top face horizontal, not rotated to the right into the thread. I did increase the left side relief at first in case it was rubbing on the rough side. I have been plunge cutting too so it this stage I'll try honing first, as per the fitter and turner's advice then try angling the compound feed to 14 & 1/2 degrees instead of plunging in at zero and then try angling the top face to match the helix angle better.

[morrisman]

Just a thought GRAZ

You can buy pre formed ACME cutters in 5 tpi .. Ive seen them on EBAY .. a seller in the USA ... I will try to find them ...Mike

[Graziano]

Just a thought GRAZ

You can buy pre formed ACME cutters in 5 tpi .. Ive seen them on EBAY .. a seller in the USA ... I will try to find them ...Mike

True, but this thread is a double start so it's effectively got the 10 tpi thread form dimensions but the 5 tpi pitch....and left handed to boot, it might be a custom grind job. I've had hassles trying to find a pre made threaded rod, 9/16" ACME seems to be a bit uncommon .

[Steamwhisperer]

Hi Graz,
Have you made sure the width across the face of the screwcutting tool has remained the same. Also did you cut a square thread first then clean up the flanks to finish full form.

Phil

[Graziano]

Hi Graz,
Have you made sure the width across the face of the screwcutting tool has remained the same. Also did you cut a square thread first then clean up the flanks to finish full form.

Phil

Hi Phil.....erm, um.... no to both questions. When you say the width across the face do you mean the end of the screw tool?.

[Steamwhisperer]

Hi Phil.....erm, um.... no to both questions. When you say the width across the face do you mean the end of the screw tool?.

Yep. The part of the tool which cuts the bottom of the thread. It is the most critical dimension. I made a tool to measure the width. I am in mildura on a long weekend break so am using my phone. I will try and find it and then try to send a link.

Phil

[Steamwhisperer]

Found it. Hope it works.
It is about post # 494.

Phil
https://www.woodworkforums.com/f65/project-121056

[Graziano]

Yep. The part of the tool which cuts the bottom of the thread. It is the most critical dimension. I made a tool to measure the width. I am in mildura on a long weekend break so am using my phone. I will try and find it and then try to send a link.

Phil

Hi Phil, I wasn't sure if you meant the tool is ground so the end is parallel: I grind the 29 degree flanks with relief so that the end face tapers towards the bottom (not much). To check the width, I was setting the micrometer to the required width and bringing the tool up to the gap and eyeballing it with a 20x magnifying loupe. I'm reading through the posts now to find your tool, cheers!.....ok, found it:

https://www.woodworkforums.com/f65/yo...me#post1496681

[Bryan]

Mark from your last photo it looks as though the thickness of the 'tooth' varies. If true that seems like a backlash problem - the carriage floating slightly. Though I can't really see why it wouldn't take up as normal. This is on the SAG right? Wouldn't be any end float in the spindle...?

[Graziano]

Mark from your last photo it looks as though the thickness of the 'tooth' varies. If true that seems like a backlash problem - the carriage floating slightly. Though I can't really see why it wouldn't take up as normal. This is on the SAG right? Wouldn't be any end float in the spindle...?

Hi Bryan, it's on the Bantam lathe I'm making the actual screw for. In the rear illuminated photo, there's actually a bit of dirt in the valleys of some threads mixed with oil which makes the bottom seem a bit uneven in that shot. The main offending tooth is the top meshed tooth third from the left which is a bit fatter than all the others. There is significant tear on all of the left flank which is random and at this stage seems to be the main cause of the unevenness. If I can get a smooth cut I suspect a lot of problems will disappear. The worn cross slide screw still advances the cut exactly according to the dial in thou's if the gibs are snugged up for tight movement over a narrow range in and out of an inch or so.