Cutting threads

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CMV
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Cutting threads

Post by CMV » Sun Jun 10, 2012 9:57 pm

Having a hard time cutting threads. Some came out perfect while others got all boogered up. I referred to the grizzly manual for their lathe that's similar to mine, but the gearing is different since I use a different setting for the TPIs than their lathe.

My threading dial wasn't working. I had to take it apart & polish it. Was really poorly machined & would spin about 3/4 of a revolution & then bind up. After getting it back together it works great, but I'm wondering if the dial needs indexed. I didn't pay any attention to its orientation before taking apart & there's no keyway or anything so after the dial is disassembled it could be set on any number & reinstalled. I wouldn't think that matters but maybe that's screwing me up.

Anyway, I'm making some machinist jacks - easy project - & was doing everything in 1/2x20 tpi since I'll be doing lots of that eventually. First one came out perfectly. After that, on my 3rd or 4th pass on the threads I'd wipe out the existing threads & cut new ones, ruining the piece.

According to the Grizzly manual 20 tpi can be engaged on any number. I engaged on the same number every time. First time was 7 so I though maybe the odd # was the problem so next time used 8 & same thing happened.

6061 al (yeah not the right material for the job), cutting at 60 rpm, using cutting oil, freshly ground HSS tool that's nearly perfect 60° on the leading edge and a sharper angle on the trailing side, .010 DOC per pass.

I might be doing something wrong but I start by turning the work down to .500. Then I add a heavy chamfer on the end at about 45°. Then I switch to threading tool which is on center and oriented 90° to the lathe axis. I adjust so I'm about 1/4" behind the work & dialed in for a .010 cut. I engage the half nut on #X noting what that was. When the cut is complete I disengage the half nut, back the cross slide off, run the carriage back behind the work, run the cross slide back to where it was +.010, clean chips, add another squirt of cutting oil, reengage half nut on #X. Repeat until done. I *think* that's how you thread on a lathe - what am i missing? It confuses the heck out of me how engaging the half nut on the same # on the threading dial will sometimes have the tool follow the path perfectly and other times it will be off so much that it just cuts off the threads.

It would take a lot longer but i guess I could try never disengaging the half nut? Just back off the work, stop the lathe, reverse direction & let the carriage creep back to the right. That sounds like a workaround for what is probably a simple fix or some step I'm missing.
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Re: Cutting threads

Post by CMV » Sun Jun 10, 2012 10:23 pm

OK - compound should have been at 29.5° not 90° & .010 is way too deep. Will try again tomorrow if I get off work at a reasonable time.

How do you know how deep to cut the threads? Is there a rule of thumb or just keep taking light cuts until the female threaded part passes? I don't have thread gages unless you count the assorted nuts & bolts in my garage as thread gages.
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Re: Cutting threads

Post by Shift1 » Mon Jun 11, 2012 6:39 am

CMV wrote:OK - compound should have been at 29.5° not 90° & .010 is way too deep. Will try again tomorrow if I get off work at a reasonable time.

How do you know how deep to cut the threads? Is there a rule of thumb or just keep taking light cuts until the female threaded part passes? I don't have thread gages unless you count the assorted nuts & bolts in my garage as thread gages.
There is a way to do that....the Machinist handbook has the thread dimensions for virtually all the threads. a quicker way is to aquire the "fishtails". These have the thread dimensions for the more common threads engraved in them. You want to use the double depth of cut. For instance a 28tpi is about .023 deep or .046 double depth. When dimensioning the plug you need to add this dimension for the threads. You can profile your part to start into the tube to the min dimension, and turn the threads until you hit this minimum dimension with the threading tool. You will be at full thread depth.
You will need to get the indexing wheel working reliably before trying to thread or it will never work. You need to pick up the threads the same place every time. If the index wheel is not consistant, you will have a difficult road ahead.
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Re: Cutting threads

Post by Shift1 » Mon Jun 11, 2012 6:39 am

double post.......oops
Last edited by Shift1 on Mon Jun 11, 2012 6:26 pm, edited 2 times in total.
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Re: Cutting threads

Post by epicdoom » Mon Jun 11, 2012 2:37 pm

CMV start rite here http://www.youtube.com/watch?v=q7UvwyLSdsM This guy has all the information you need he has a ton of videos HQ I will send you some information through the mail as well. I will also post links to your PM box we'll make a machinest out you. There are many folks here on the forum Who can answer questions along the way. Look through this guys stuff and I'll go gather more info for you is there any name or info on your lathe at all can you take a picture of it for me some can be told by color or atleast narrowed down to a couple brands.

Joe
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Re: Cutting threads

Post by Bendersquint » Mon Jun 11, 2012 8:42 pm

epicdoom wrote:CMV start rite here http://www.youtube.com/watch?v=q7UvwyLSdsM This guy has all the information you need he has a ton of videos HQ I will send you some information through the mail as well. I will also post links to your PM box we'll make a machinest out you. There are many folks here on the forum Who can answer questions along the way. Look through this guys stuff and I'll go gather more info for you is there any name or info on your lathe at all can you take a picture of it for me some can be told by color or atleast narrowed down to a couple brands.

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Re: Cutting threads

Post by CMV » Mon Jun 11, 2012 10:27 pm

Just spent a couple hrs watching him :) Thanks for the link. Lots of interesting stuff.
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Re: Cutting threads

Post by Historian » Tue Jun 12, 2012 7:01 am

CMV,

Here is information for making a hand crank for the spindle that for short
threading small lengths allows complete control. The movie shows its use.
[Disconnect power, of course.]

<< http://www.deansphotographica.com/machi ... crank.html >>

For my Atlas 618 I used the collet draw bar with a 1/2" collet holding a small 1/2" rod and added a
handle. When I tightened up the draw bar I had control forwards and backwards with no slipping.


Cutting a thread in the 1/2" length range, say 14 threads worth, gave absolute control.
Then I did not have to disconnect anything and depend on the rotating thread dial to reengage.
I simply pulled the cross slide back, cranked right, reset the cross slide to 'zero', move in the
tool, with the compound set at 29.5 degrees, a few thousands, and repeat. I learned
a great deal about metal properties with this intimate feel.

For me it took a few botched up practices to have a slap-the-forehead insight that
I should not also crank backwards without first withdrawing the tool as reversing would have
the tools right side dig in reverse and ruin the thread.

When a CNC is given at some future Birthday Party I shall let the program do all of this. :) :)

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Re: Cutting threads

Post by ghostdog662 » Tue Jun 12, 2012 9:37 am

Usually for me the best gauge is to put layout dye on the part and cut until all of the dye is gone. Then start fine tuning fitment.
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Re: Cutting threads

Post by elginrunner » Tue Jun 12, 2012 10:18 am

CMV,

If your using a grizzly 4003g or clone, you have to set the compound at 60.5 degrees.... (is really 29.5) I couldn't understand why mine wouldn't end up right after following all the directions!! I hope this helped....

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Re: Cutting threads

Post by Bendersquint » Tue Jun 12, 2012 12:12 pm

CMV - I got your letter today........however the envelope the bar stock was sent in didn't hold keep the materials from escaping, who knows where they were lost but they didn't make it to me, just an empty envelope.

Sorry can't help you on this endeavor.

You really should buy the Machinist Handbook it will answer most of the questions you are asking.

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Re: Cutting threads

Post by CMV » Tue Jun 12, 2012 6:14 pm

That's a shame that 1 lb of round bar stock can't make it 100 miles without failing :|

I'll try again this week
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Re: Cutting threads

Post by Bendersquint » Tue Jun 12, 2012 6:21 pm

CMV wrote:That's a shame that 1 lb of round bar stock can't make it 100 miles without failing :|

I'll try again this week
You shouldn't ship bar stock in an envelope. At least use a padded envelope it will give it a fighting chance, or packing tape the daylights out of it.

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Re: Cutting threads

Post by epicdoom » Tue Jun 12, 2012 7:33 pm

My sample was still in place but there was a large hole in the package could have fallen out with a shake or two. CMV I posted results in the bad finish thread you started.

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Re: Cutting threads

Post by mg81 » Wed Jun 13, 2012 8:43 am

My guess is if you are completely "wiping" the old threads out then you are not engaging the half nut at the correct time. Not sure if it is a technique error or a faulty machine part or one with to much lash.

You can thread without a threading dial. My very old SB never had one, I just withdraw the tool and run the lathe in reverse. Normally the thread I am doing is not long and it takes no time at all to reverse to the start.

But you have a threading dial, so you should enjoy it and get it figured out.

If I were you, I would keep using the threading dial but at the beginning of each pass turn the lathe by hand to bring the tool up to your work. Then look very carefully at how the tool is starting into the cut. Is it going into the previous thread groove or is it "off" and is starting to cut a huge part of your previous thread off.

By bringing up by hand you will know within the first tiny bit of thread if things are lined up right. If not, withdraw the tool back the carriage up and try engaging the half nut again.

Once you get it figured out, you won't have to spend nearly as much time on a job. But learning takes a while and by doing this method you won't be ruining projects as you learn. I never minded projects taking me a long time when I started out, but ruining something was frustrating and non-motivating.

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Re: Cutting threads

Post by wolf » Wed Jun 13, 2012 10:30 am

As you already found out 0,01 is way to much

have look

Image

by the way ,, are you using the the left or right method ??

the tool has to ground different for each method

as you can see IF you avance the same each time , then the first cut only has to take the two sides of the first tiangle in the first cut



but next time the sides are double
and on the third they are triple



the depth of the cuts has to get SMALLER each time , THEN the amount that IS cut will stay the same

like

Image

here is a chart , its for a emparial thread , but in metric values

Image

as you can see , for a 20 tpi . the first cut is made at 0,2 MILIMETER

that is 0,0079 inches :wink:

the next is then even smaller at 0,19 MM
the last is 0,08MM = 0,0031inches

maybe some can give a imperial version for the inch only people :wink:

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Re: Cutting threads

Post by wolf » Wed Jun 13, 2012 10:39 am

HMM , i just got a bad gut feeling :?

is your lead screw a metric one perhaps

check out that if metric , you can not use the threading dial for cutting inch threads (or vica versa)

engage the half nut ,,zerro you main carried , disengage the halfnut . move the carried one , two or more and engage again

see if the numbers ad up to a hole number in metric OR inch

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Re: Cutting threads

Post by Historian » Wed Jun 13, 2012 1:41 pm

wolf wrote:HMM , i just got a bad gut feeling :?

is your lead screw a metric one perhaps

check out that if metric , you can not use the threading dial for cutting inch threads (or vica versa)

engage the half nut ,,zerro you main carried , disengage the halfnut . move the carried one , two or more and engage again

see if the numbers ad up to a hole number in metric OR inch
Wolf, you correctly warn against using the threading dial. Using my hand crank on
the headstock spindle allows me to accurately disconnect, wind back, position forward,
and repeat.

I have practiced cutting metric threads on my Atlas 618 lathe with the
lead screw 16 TPI, using just Imperial change gears. Mathematics is a wonderful tool to overcome limitations.

I made an adapter for the 10 TPI headstock spindle to make an ER25 collet closure chuck ( M32 x 1.5 )
Please look at the great articles: << http://www.deansphotographica.com/machi ... /ER25.html >>
and
<< http://conradhoffman.com/metricthreading.htm >>.

Metric threading with the change gears:

" This change gear setup cuts a 1.5mm pitch thread that is calculated to give an error
of only 0.06%.
That's very little error, in case you were wondering.
... I used a short ground test bar to check the runout to see if I had done any good. It showed
less than .0005" on my old Atlas. Not bad, since the collets are only guaranteed to .0006!"

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Re: Cutting threads

Post by CMV » Wed Jun 13, 2012 6:29 pm

I *assume* I have an imperial lead screw because the data plates for threading gear changes are only in TPI - it would list metric sizes if that's what it was made to cut. But assumption is the mother of all f---ups, so that very well could be the case.

Once I get the new toolpost on I'll work on it some more. I think after watching tubalcain I should have the process down. I really like how he explains things.
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Re: Cutting threads

Post by ghostdog662 » Wed Jun 13, 2012 7:44 pm

CMV wrote:I *assume* I have an imperial lead screw because the data plates for threading gear changes are only in TPI - it would list metric sizes if that's what it was made to cut. But assumption is the mother of all f---ups, so that very well could be the case.

Once I get the new toolpost on I'll work on it some more. I think after watching tubalcain I should have the process down. I really like how he explains things.
Practicing on PVC is an inexpensive material to begin on.
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Re: Cutting threads

Post by epicdoom » Wed Jun 13, 2012 8:13 pm

ghostdog662 wrote:
CMV wrote:I *assume* I have an imperial lead screw because the data plates for threading gear changes are only in TPI - it would list metric sizes if that's what it was made to cut. But assumption is the mother of all f---ups, so that very well could be the case.

Once I get the new toolpost on I'll work on it some more. I think after watching tubalcain I should have the process down. I really like how he explains things.
Practicing on PVC is an inexpensive material to begin on.
+1 When I worked in the Machine shop I was cutting metal from the start it was scrap basicly when I bought my own machines I bought PVC to play around with Used it for threading practice mostly I now keep several sizes on hand and use it to check my set up before I commit to metal.

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Re: Cutting threads

Post by calinb » Thu Jun 14, 2012 1:07 am

Historian wrote: Wolf, you correctly warn against using the threading dial. Using my hand crank on
the headstock spindle allows me to accurately disconnect, wind back, position forward,
and repeat.
I once turned down a couple of Carlson Beretta/Benelli Mobil turkey shotgun choke tubes to make thin wall tubes for my Turkish S&W shotgun. The threads are square and a weird pitch (not metric or inch). I found a set of gears that got me close enough but had to stop, reverse the spindle and carriage drive direction, and back it up for each pass. I don't have a hand crank so I just turned the carriage back with the motor.

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Re: Cutting threads

Post by mg81 » Thu Jun 14, 2012 12:14 pm

Wolf, great images they really show what is hard to describe in words.

I guess I am spoiled by having an old overhead motor belt drive machine. No need to make up a hand crank to turn things. I just disengage to gearbox and grab onto the vertical belt that is right in front of me and pull it to rotate the lathe. If I am in backgear it does not take a hard pull to make a cut, I often finish up a threading cut if I don't have a threading groove to go into by doing this.

I am not sure why you want to reverse the lathe by hand if you are not going to disengage the half nut, just seems time consuming. Just use the motor in reverse to get you back to the beginning of the cut, especially if you are in backgear. That would be lot of hand cranking.

Historian did mention a great trick that I am not sure many people know. You can take a combination of gears that were never made for metric threads (on a change gear lathe) and combine them to cut metric treads even if you have a non-metric lead screw (most are 8 TPI)

I made an adapter so I could combine two of my change gears and put them on my banjo instead of the standard compound gear that goes onto it. If you have a full set of change gears you can cut all of the common metric treads close enough for them to work. If you do a web search you can find people who have posted combinations of gears for different threads. You must not disengage you half nut if you do this method, you will be reversing your lathe for each cut. Your thread cutting dial will not work.

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Re: Cutting threads

Post by CMV » Thu Jun 14, 2012 8:56 pm

wolf wrote:HMM , i just got a bad gut feeling :?

is your lead screw a metric one perhaps

check out that if metric , you can not use the threading dial for cutting inch threads (or vica versa)

engage the half nut ,,zerro you main carried , disengage the halfnut . move the carried one , two or more and engage again

see if the numbers ad up to a hole number in metric OR inch

Can you explain that again for the machining challenged [me]?

1. Engage half nut
2. Set handwheel indicator to 0
3. Disengage halfnut
4. Do something with the carriage - I'm not sure what you mean to do
5. Add something - I'm not sure what

Probably 99% of the people reading what you wrote get it... But it makes sense that it's something I should check out.

In your pictures I'm set up like the one on the left with the compound at 30°. I'm feeding in with the compound and not the cross slide.
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Re: Cutting threads

Post by Bendersquint » Thu Jun 14, 2012 9:33 pm

CMV wrote:
wolf wrote:HMM , i just got a bad gut feeling :?

is your lead screw a metric one perhaps

check out that if metric , you can not use the threading dial for cutting inch threads (or vica versa)

engage the half nut ,,zerro you main carried , disengage the halfnut . move the carried one , two or more and engage again

see if the numbers ad up to a hole number in metric OR inch

Can you explain that again for the machining challenged [me]?

1. Engage half nut
2. Set handwheel indicator to 0
3. Disengage halfnut
4. Do something with the carriage - I'm not sure what you mean to do
5. Add something - I'm not sure what

Probably 99% of the people reading what you wrote get it... But it makes sense that it's something I should check out.

In your pictures I'm set up like the one on the left with the compound at 30°. I'm feeding in with the compound and not the cross slide.
Did you buy the Machinist Handbook yet?

How about taking a course at your local community college?

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