By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

Thread cutting problem

All Topics | Latest Posts

Search for:  in Thread Title in  
SillyOldDuffer29/11/2020 14:57:20
Moderator
6653 forum posts
1495 photos

I'm confused! What thread is this? I assumed ³⁄₁₆" x 40tpi Whitworth, but my books don't list it. There's ⅛" Whitworth x 40 tpi, but ³⁄₁₆" Whitworth is 24tpi. Likewise UNC 40 tpi dies for ⅛" and 0.112" diameters exist, but the UNC equivalent ³⁄₁₆" size is 24tpi.

Could the problem be that a ⅛" x 40tpi die is being forced on to a ³⁄₁₆" rod?

Dave

JasonB29/11/2020 14:58:46
avatar
Moderator
19506 forum posts
2131 photos
1 articles

SOD its 3/16" x 40 ME, 55deg form but constant pitch over the diameters.

Your maths from the first page was also wrong, generally for 40tpi the tapping size can be 1/32" less than nominal not 1/16"

I've had a few dodgy ME dies and that's what I would put SOD's money on.

 

Edited By JasonB on 29/11/2020 15:12:02

Buffer29/11/2020 17:04:44
207 forum posts
84 photos

thanks again to everyone who has taken the time to reply with advice.

I did what Andrew suggested and cut a thread perfectly on a piece of 3/16th brass. So there is nothing too wrong with the die it would seem.

I managed to cut a thread in steel but only by taking the bar down to about 0.18 but it didn't look very nice. It seems that despite me pushing the die towards the chuck it just wouldn't start pulling itself forward along the bar and it appears to just grind the steel away until it can get going and then it pulls itself along. So I did what I didn't really want to do and set the lathe up to screw cut. In the end I managed to get a very nice test piece cut on the other end of the brass rod so later I will try it on the steel.

I tried to find out at what point I could engage the leadscrew by looking at the workshop practice series book but soon realised it was going to take ages to work it out. So I reversed the lathe after each cut and engaged on the same number each time.

To speed this up does anyone know any easy way to work out when you can engage the leadscrew for a 40tpi thread with a 4tpi leadscrew?

Thanks

SillyOldDuffer29/11/2020 17:12:01
Moderator
6653 forum posts
1495 photos
Posted by JasonB on 29/11/2020 14:58:46:

SOD its 3/16" x 40 ME, 55deg form but constant pitch over the diameters.

Your maths from the first page was also wrong, generally for 40tpi the tapping size can be 1/32" less than nominal not 1/16"

I've had a few dodgy ME dies and that's what I would put SOD's money on.

Ah, all is explained, ta

Unfortunately you're not the first to notice I got the maths wrong again! Sack cloth and ashes for me, and a Dunce's cap for xmas...

Dave

Andrew Johnston29/11/2020 17:20:22
avatar
5824 forum posts
662 photos
Posted by Buffer on 29/11/2020 17:04:44:

..............when you can engage the leadscrew for a 40tpi thread with a 4tpi leadscrew?

According to the chart for my lathe (Harrison M300, imperial with a 4 tpi leadscrew) any number from 1 to 8. But I've never tried it as I replaced my thread dial indicator with a high speed threading unit which automatically looks after all that.

Andrew

Buffer29/11/2020 17:54:41
207 forum posts
84 photos

I never thought to do that but there it is in the manual a chart with1-6 what a numpty!

duncan webster29/11/2020 19:26:56
avatar
2929 forum posts
34 photos

4 tpi is 0.25 pitch, 40 tpi is 0.025 pitch, so you can drop it in anywhere. As long as tpi of thread to be cut divided by tpi of leadscrew you don't need to worry

Rod Renshaw29/11/2020 20:58:40
230 forum posts
2 photos

Andrew J, 14.17 pm. I agree.

My point about dies being made for hand held use is that the manufacturer does not always bother much about concentricity, which hardly matters when using a "manual" die-stock.

Even a non- concentric die will work under power as long as there is enough slack in the system to allow the die to line itself up, the power produces the turning motion - and the die wiggles about, to use a technical term, to stay concentric on the part formed thread. With high quality dies this may not be an issue.

Rod

Andrew Johnston29/11/2020 21:19:24
avatar
5824 forum posts
662 photos

There's virtually no "float" in the Britan system. So they must rely on the thread in the die being concentric?

Andrew

JasonB30/11/2020 07:00:18
avatar
Moderator
19506 forum posts
2131 photos
1 articles

Which brings us to another possibility - How slack is the die in the holder and what is it's OD. I've seen similar issues where a metric OD die was being used in an imperial holder which means the die sits off ctr as the adjusting screws push it to one side of the holder.

Not usually a problem with imperial threads as the dies all tend to be imperial but you never know, metric threads can be had in both metric and imperial ODs.

Hopper30/11/2020 08:09:06
avatar
5031 forum posts
111 photos
Posted by Buffer on 29/11/2020 17:04:44:

thanks again to everyone who has taken the time to reply with advice.

I did what Andrew suggested and cut a thread perfectly on a piece of 3/16th brass. So there is nothing too wrong with the die it would seem.

I managed to cut a thread in steel but only by taking the bar down to about 0.18 but it didn't look very nice. It seems that despite me pushing the die towards the chuck it just wouldn't start pulling itself forward along the bar and it appears to just grind the steel away until it can get going and then it pulls itself along.

Testing your die on a piece of brass does not tell you anything much about its ability to cut much tougher steel. Test it on a piece of 3/16" steel in the vice by hand. It sounds to me like your die is not sharp or not correctly ground from the manufacturer. It happens.

And you should not have to take the bar down to .180" to get the die to work. A 3/16" die should thread a piece of 3/16" (.187) bar with no trouble. Something is wrong there for sure.

Like Jason, I would put SOD's money on the problem being a duff die.

Hopper30/11/2020 08:34:45
avatar
5031 forum posts
111 photos
Posted by Buffer on 29/11/2020 17:04:44:

To speed this up does anyone know any easy way to work out when you can engage the leadscrew for a 40tpi thread with a 4tpi leadscrew?

On the other hand: Holy cow, dude. If your lathe is big enough to have a 4TPI leadscrew, it's probable that the tailstock is too heavy to get pulled along by that tiny, tiny 3/16" x 40TPI thread with a thread depth of 16 tiny thou, even with you pushing it. Sledgehammers and mosquitoes come to mind.

A sliding-type die holder might be in order. Or a regular manual die handle pushed up against the job by the tailstock barrel under handwheel pressure might work better, so you dont have to try to move that massive tailstock mass.

Martin Connelly30/11/2020 09:18:08
avatar
1602 forum posts
175 photos

There is also something I wrote about the threading dial here Album 46824

Martin C

Rod Renshaw30/11/2020 10:50:12
230 forum posts
2 photos

Andrew, I guess so.

I use an Arrand tailstock dieholder which also has very little float but it has three hex headed screws to hold the 2 halves of the holder together and these can be left slightly loose to allow the die to line itself up. One half (end?) slides on a mandrel that goes in the tailstock taper and the other holds the die. By experiment I find some dies cut well enough with the screws tight and some do not. Perhaps I should buy some better dies and pension some of my older ones off!

Jason. I read somewhere about someone who had approached this aspect by fitting an additional pinching screw diametrically opposite the 3 normal screws and then used the 4 screws together to centralise any dies which were very loose in the holder. Sounds like an idea which might help in some cases. Or I suppose one might try a slip of shim opposite the pinching screws to take up some of the slack.

It's odd how tailstock dieholders work well, straight out of the box, for some workers while others struggle.

Rod

Andrew Johnston30/11/2020 11:05:10
avatar
5824 forum posts
662 photos
Posted by Rod Renshaw on 30/11/2020 10:50:12:

Jason. I read somewhere about someone who had approached this aspect by fitting an additional pinching screw diametrically opposite the 3 normal screws and then used the 4 screws together to centralise any dies which were very loose in the holder.

It's interesting to note that the die holders for both the Britan and Arc systems use four screws as described. But in both cases the dies are a snug fit, no more than a barely detectable shake. So I can't see the fourth screw being much use for aligning an off centre die.

I'm not a fan of split dies. Where possible I prefer to screwcut or use Coventry dieheads.

Andrew

JasonB30/11/2020 11:32:34
avatar
Moderator
19506 forum posts
2131 photos
1 articles

Up until recently I use a bit of shim on a couple of 25mm OD dies as my holder is imperial but did break the rule of a lifetime and actually made a metric holder after a bought a few more 20 and 25OD dies from Uncle Ketan.

I would imagine the 4th screw would also allow for the odd occasion when a smaller 25 or 20mm die was being used in a 1" or 13/16" holder where they would not be such a snug fit.

Buffer30/11/2020 12:18:18
207 forum posts
84 photos

I just did what Hopper and a few suggested and tried cutting a thread on the bench by hand on a 3/16th steel rod and I got the same problem. So I have filed the die in the round filing cabinet in the corner of the room. Just to be sure I turned the same bar down a bit and cut a perfect 4mm thread on the same material. So that's that as some said duff die.

Hopper30/11/2020 21:35:36
avatar
5031 forum posts
111 photos

Frustrating! It pays to buy quality tools -- but even that seems to be no guarantee these days.

Martin Dowing30/11/2020 22:13:27
avatar
292 forum posts
4 photos
Posted by Buffer on 29/11/2020 17:04:44:

thanks again to everyone who has taken the time to reply with advice.

I did what Andrew suggested and cut a thread perfectly on a piece of 3/16th brass. So there is nothing too wrong with the die it would seem.

I managed to cut a thread in steel but only by taking the bar down to about 0.18 but it didn't look very nice. It seems that despite me pushing the die towards the chuck it just wouldn't start pulling itself forward along the bar and it appears to just grind the steel away until it can get going and then it pulls itself along. So I did what I didn't really want to do and set the lathe up to screw cut. In the end I managed to get a very nice test piece cut on the other end of the brass rod so later I will try it on the steel.

It is common for die to cut well on brass of nominal diameter but on steel only on slightly undersized bar.

From my experience 2-3 thou undersized steel bar is getting threaded very well.

I have M10 die where manufacturer have stated that steel bar to be threaded must be 9.85 mm of diameter!

Of course die must be pressed square. Proper holders are best but some shabby settings employing pressure of tailstock barrel for squaring while starting your thread also can work in absence of holder.

Phil P30/11/2020 23:10:08
705 forum posts
175 photos

Buffer

Just as an experiment, go retrieve the die from the bin and see if you can make any impression on it with a file.

I had a similar experience with a new (cheap) die a while back, it was made from something other than the HSS that was written on it and was quite easy to make a nick into with a needle file, so it was probably as soft as the material I was trying to thread.

I bought another new (expensive) die and had no further problems.

Phil

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Latest Forum Posts
Support Our Partners
EngineDIY
Eccentric July 5 2018
cowells
emcomachinetools
Warco
ChesterUK
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest