Author Topic: Screwcutting  (Read 15546 times)

Offline Firebird

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Re: Screwcutting
« Reply #15 on: September 01, 2015, 10:24:41 AM »
Hi

I have managed to do a bit of screw cutting on my Myford and have used the set over method as described in the Myford users book.

What is the "plunge in square" method  :noidea:

Cheers

Rich

Offline jadge

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Re: Screwcutting
« Reply #16 on: September 01, 2015, 11:10:24 AM »
What is the "plunge in square" method  :noidea:

Cheers

Rich

Presumably the toolbit is plunged straight in for a cut, rather than at an angle as set on the top slide? All my screwcutting has been done using this method, although being idle I have moved on to using carbide threading  inserts, which are happy cutting on both flanks, rather than grinding my own toolbits. It has the advantage that the depth of the thread is directly related to the numbers on the cross slide dial rather than having to work out the feed on the top slide via trigonometry.  :ThumbsUp:

Andrew

Offline Allen Smithee

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Re: Screwcutting
« Reply #17 on: September 01, 2015, 11:24:41 AM »
What is the "plunge in square" method  :noidea:

Set the compound at zero degrees and just advance the cutting tool straight into the job. It means that the tool is cutting on both flanks, so you need to make sure their both sharp, but if your setup will do it without chattering it's simpler and easier to control the thread depth.

When I first started machining stuff I always cut threads this way because no one told me to do otherwise, and I've generally found that the set-over method is only really necessary for "difficult" materials.

YMMV,

AS
Quidquid latine dictum sit altum sonatur

Offline sshire

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Re: Screwcutting
« Reply #18 on: September 01, 2015, 12:22:36 PM »
The toolbit is, in fact, a form tool, so the plunge-in straight method works just fine as with any form tool cut. The insert on the Aloris threading tool that I use is properly ground on both sides so it works quite well.
Some have suggested using the 29 (29.5, 30°) set over for most of the cut and the plunge-in straight for the last few passes. With Aluminum, brass and 12L14, I've had nice results plunging in. Haven't tried it with "difficult" materials.
I also paint the part with Dykem. When the Dykem is completely gone, I begin measuring.
Best,
Stan

Offline Stuart

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Re: Screwcutting
« Reply #19 on: September 01, 2015, 12:28:44 PM »
Allen

You may or may not need to slot the banjo a wee bit to get the new gear on the output shaft of the tumbler , I had to

As to mesh I am not saying this but I set it somewhere near and adjust but sound whilst it's running , note I did not say that at all

They need to be a bit sloppy printer paper is about right  slacker than tighter is the way to go

For lube I use the wd40 brand of chain lube you don't need much as it's very sticky , but you do need to clean off all the oil that's on them first
Just remember to keep the half nuts closed ,best to use a spindle handle for short threads, or make the Grayham Meek dog clutch drawings can be found on the web my set is for the big bore lathe do will not fit yours then you leave the half but closed and set the stops and the ad like a hardinge

Stuart

Stuart
My aim is for a accurate part with a good finish

Offline jadge

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Re: Screwcutting
« Reply #20 on: September 01, 2015, 01:47:33 PM »
I've cut threads in 303 stainless steel using the straight in technique with no problem. Although the thread was only 32tpi. Oddly these particular bits of 303 were a total pig to get a decent finish on, unlike most other 303 I have used. Despite that, the screwcutting was fine. To get a decent turned finish I was down at 260rpm with coolant, but I did the screwcutting dry at 370rpm.

Andrew

Offline Ian S C

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Re: Screwcutting
« Reply #21 on: September 01, 2015, 03:18:55 PM »
I use straight in thread cutting for finer threads, but after they get to around 10/12 tpi, I prefer to go in at the angle, reducing the load on the tool as it's cutting on a wide area by the time it's getting near full depth.  My first threads were cut using a tool ground up from an old 3/8" square file, working in back gear over short distances, with fine cuts the tool does not get hot, so it holds it's edge.
Ian S C

Offline Dan Rowe

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Re: Screwcutting
« Reply #22 on: September 01, 2015, 03:35:37 PM »
Finally - am I right in believing that I can't use the thread position indicator when running the 33 or 34 tooth gears unless U do the trick about setting a saddle stop position and re-engaging the half nuts with the saddle held against the stop or similar?

The usual advice for strange gear combinations not covered in the lathe thread chart like cutting metric with an inch lead screw is do not disengage the lead screw. This is always how I did it when I needed to cut  the wrong way on the ships I worked. I usually had a lathe with a foot brake so with a slow speed the brake can be stomped the tool backed out and reverse the machine. I could reverse with an apron control so it was a simple operation.

You should read this post to review another very workable method:
http://www.modelenginemaker.com/index.php/topic,5237.0.html

Cheers Dan
« Last Edit: September 01, 2015, 04:10:06 PM by Dan Rowe »
ShaylocoDan

Offline Allen Smithee

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Re: Screwcutting
« Reply #23 on: September 01, 2015, 05:30:53 PM »
You should read this post to review another very workable method:
http://www.modelenginemaker.com/index.php/topic,5237.0.html

That seems to be similar to the saddle-stop method (unless I'm missing something). The method as I understand it is:

1. Put a saddle-stop on the RIGHT of the saddle in a position that has the tool clear of the front of the work.

2. Start the lathe, with the saddle held against the stop wait for the thread position indicator to get to the desired number (say "1") and then close the halfnuts

3. Make your cutting pass down the work and then disengage the half-nuts at the run-out groove or shoulder

4. Back out the tool and wind the saddle back to the saddle-stop, then set the tool (croos-slipe or compound) for the depth of the next cut.

5. Start the lathe, with the saddle held against the stop wait for the thread position indicator to get to the right number (say "1") and then close the halfnuts again.

..and so on. So essentially you engage with the saddle held against the stop, then disengage and wind the saddle back to the stop, wait for the number to come around and re-engage. Simpler and no need for continually stopping/starting the motor.

If I have it right this method ensures that the halfnuts will always close "in synch" on the same place on the leadscrew. I've evn heard of very confident people who dispense with the saddle stop and just zero the saddle DRO for the synch point.

Do I have this right? Would it work?

AS
Quidquid latine dictum sit altum sonatur

Offline Firebird

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Re: Screwcutting
« Reply #24 on: September 01, 2015, 06:19:00 PM »
Hi

Thanks  :ThumbsUp: I'll try that method next time I do some screw cutting

Cheers

Rich

Offline Dan Rowe

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Re: Screwcutting
« Reply #25 on: September 01, 2015, 07:47:05 PM »
Not to be argumentive but your method makes the unstated asumption that for a  fixed position of the carrage that there is a SINGLE position that the "1" will come up for any rotation of the spindle and gear train

This might be the case but I certanly would check it to make sure that the "1" does not come up twice in a single spindle revolution. To check simply use a marker on the stock on the top and a mark on the lead screw gear on the top and make sure that for every instance of "1" both marks are still on the top.

The method in the linked thread is a positive way to make sure all the gears and the lead screw are in the exact same relationship similar to the advice to never disengage the lead screw.

Just my 2 cents worth....

Cheers Dan
« Last Edit: September 01, 2015, 07:50:14 PM by Dan Rowe »
ShaylocoDan

Offline Allen Smithee

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Re: Screwcutting
« Reply #26 on: September 01, 2015, 08:47:49 PM »
Not to be argumentive but your method makes the unstated asumption that for a  fixed position of the carrage that there is a SINGLE position that the "1" will come up for any rotation of the spindle and gear train

Good point, and I will check. But as far as I can see the method described by Marv makes the same assumption!

AS
Quidquid latine dictum sit altum sonatur

Offline Dan Rowe

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Re: Screwcutting
« Reply #27 on: September 01, 2015, 09:11:15 PM »
No the method posted by Marv is a method to engage the lead screw and the gear train EXACTLY the same way.

You turn off the lathe at the same time or slightly before you disengage the half nuts. Then you move the tool bit out and reverse the lathe and engage the lead screw when the "1" comes back. The lathe will not have turned much in slow speed as this is done so you can be sure that the lead screw and the gear train is in the same orentation as before which is similar to not disengaging the lead screw.

If done correctly the "1" will only turn a few degrees and reversing will line it back up to the very same spot.

Cheers Dan
ShaylocoDan

Offline mklotz

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Re: Screwcutting
« Reply #28 on: September 01, 2015, 09:37:44 PM »
Thanks, Dan.  I couldn't have explained it more clearly myself.
Regards, Marv
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Offline jadge

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Re: Screwcutting
« Reply #29 on: September 01, 2015, 09:49:12 PM »
Well I was going to say I didn't quite get the method, but following the explanation from Dan it now makes sense. Not that it helps me as I don't have a thread indicator dial on my lathe.  :o

I took it off to fit an Ainjest high speed threading unit in its place. That allows me to screwcut any integer, half or quarter tpi thread without the need to bother with a thread indicator dial. The trip mechanism is pretty consistent; I set clearance to a shoulder, internal or external, at 3 thou. The only thread listed on my gearbox that it cannot cut is 2-7/8 tpi.  But that wasn't listed on the original thread indicator dial either so I'm not sure how you were supposed to cut it?

Of course the Ainjest unit fails when needing to cut a metric thread on my imperial lathe. There are add-ons (the Metradial) that enable this, but they are an approximation, and have a cumulative error. So the more passes taken to cut a thread the worse the alignment gets. As and when I next need to cut a metric thread I'll use a bed stop on the right hand side of the saddle to return same to the original starting position. And then stick an encoder disc on the main spindle so I can count the number of turns the spindle made during the cut and then reverse it, and by definition the leadscrew, to their original starting positions.

Andrew

 

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