Do you need a screwcutting gauge if you have a DRO?

[brewdexta]

I have never done any screwcutting but can see a few requirements coming up so I thought I would ask the stupid questions up front. I haven't thought this through so the answer may be really obvious but I thought I would ask it anyhow.

My new lathe hasn't got a screwcutting gauge as per the following picture that sits on the carriage and is turned by the leadscrew (this is not the model for my lathe, just indicative of the thing I'm talking about)

However it does have a DRO so do I need a gauge, is it a nice to have and complements the DRO or completely superfluous?

Cheers
Andy

 

[ford34/2]

Hmnnn not too sure, but getting the half nuts engaged at the right time is one of the secrets and I`m not sure the DRO will not give you that information, the carriage will be in the right place but the leadscrew might not be in the right spot to engage half nuts. you can of coarse just reverse leaving the half nuts engaged all the time.--if the lathe will reverse.

Stan

 

[brewdexta]

Thanks Stan, I believe it does reverse, it does according to the manual but its not under power yet so haven't tested it but I have no reason to believe it won't.

Andy

 

[711jrp]

You cant screw cut with just a dro as it won't tell you the spindle orientation only the carriage position. Fords advice is the way to go...

 

[piman]

Hello Andy,

the old school way was to mark the leadscrew with some chalk and use that as a reference to a fixed point on the lathe, usually at the leadscrew bearing pedestal.
Wind your carriage back to the same fixed point, using the tail stock as a stop. If you are unsure about the procedure have a look on You tube for screw cutting tutorials. You will need what I call a screwcutting gauge which is a bit of flat shaped metal so you can accurately align your tool. Something like this:-

http://www.ebay.co.uk/sch/i.html?_n...o_id=32251&keyword=screw+cutting+gauge&crdt=0

Alec

 

[brewdexta]

I'm beginning to get it now guys

To summarise

• Chalk or a screw-cutting dial to get the spindle orientation
• DRO or bed stop (such as the tailstock) to wind back to the starting point
• If you have reverse and don't disengage from the lead-screw then winding back to the starting point could do both

however Colchester triumph lathes came with a screwcutting dial by default and can do reverse so there must be a reason for them.

 

[Wallace]

The screw cutting dial is a useful addition, accurately gives you the start point each time irrespective of where the carriage is and allows you to turn multi start threads.

 

[Luke]

I've cut metric threads on a myford, which means you can't use the gauge. I just wound the spindle back by hand after each cut.

Tedious, but got there in the end.....

 

[brewdexta]

Thanks guys, it sounds like the dial is a useful addition.

Cheers
Andy

 

[Spark plug]

The dial makes it quicker and easier, as you can move the carriage out of the way quickly to test your thread. But sometimes you have no choice but to not diss engage the half nuts and reverse the lathe instead.

The most important thing is to get in to a routine when screw cutting as catastrophes can be just one move away.

 

[hareng]

Andy if its a Triumph or most other lathes its needed.
With the 600 Groups gearbox the lathe might be metric or imperial, work well. Used an earlier Triumph years ago cutting metric, wound carriage back
and just dropped the dogs in though this may have been specific for that pitch and not worked on others. Imperial had to line up on the guage before starting next pass.
Harrisons 11"/140 onwards are metric, no problem winding carriage back and dropping dogs in any where on a guage mark 1-8. If cutting imperial the start mark on guage would have to line up on each pass and takes an eternity for the guage to revolve 1 rev before dropping the half nuts in or a long way to travel to enter job.

My M300 is metric and no way will a metric thread line up winding carriage back and dropping the dog in any where. Guage has to index round and line up every pass, same with Myford imperial and metric pitches. Metric drop guage in to any mark 1-8 but imperial is specific positions dependant upon pitch. For this purpose and it has copped me out, I reverse without disengaging dogs rather than scrap an expensive product that can have two metric pitches and diameters and end up with an imperial thread on same shaft working on all in line true.
Its possible to drop the half nut in between any mark so will not line up.

 

[brewdexta]

Thanks H, I worked out its got imperial dials so must have an imperial leadscrew because of the variants they sold (Imp dial - imp LS or metric - imp or metric - metric) however that is about as far as I have got.

 

[graffian]

Your refering to a chasing dial.
You have to get the tool and thread timed correctly on subsequent cutting passes, after the first one.
The chasing dial is geared to the leadscrew and lets you see when you can drop the halfnuts in so the screw
and carriage are in the right place.
So is it necessary, well you can work without one but I would rather have one.
But I have a leadscrew clutch and I can just leave the half nut's engaged;-)

 

[ford34/2]

• DRO or bed stop (such as the tailstock) to wind back to the starting point

• If you have reverse and don't disengage from the lead-screw then winding back to the starting point could do both

you dont need to wind back to the exact same spot with the carriage, engaging half nuts on the correct indexing line on the screwcutting gauge will be correct, as you think it is imperial theres a remote possibility if you have the options to cut Metric threads you might have to keep the half nuts engaged and then reverse the carriage, stop change to forward increase cut and off you go for another pass.

Stan

John Mills---Doubleboost on here has several videos explaining screwcutting on his you tube videos there are countless contributions from other you tube members
this one is a similar situation to your lathe, imperial lathe need it to metric screwcut

 

[stormin]

If you dont disengage the half nuts then it doesnt matter where you wind back to the carraige will always follow the same path relative to the spindle.
Trying to reverse the lathe without backing the tool out is asking for trouble, youll have too much slack in the gear train and that has to be taken up each change in direction.
Winding the cutting tool in and out for every back and forward pass soon gets tiresome, and youll forget the amount of rotation of crossfeed dial at some point (possibly easier with dro).
Get or make a threading dial. Far far easier. Got one on my DSG and dont thread on my Student or Myford which havent anymore.

 

[Spark plug]

The method I always stick to to avoid confusion is to back the cross slide off 1 turn so you just go one turn back to zero for the next cut. Then using the compound slide witch should be set to the appropriate thread angle you apply the desired cut for each pass.

 

[Bill Edwards]

I was taught to reverse, and it's a foolproof method as it can't go wrong.
Let it cut, when it reaches the end you turn the cross slide back slightly - very easy with a DRO - and at the same time flick the lathe into reverse. Then stop at the end, push the tool in again and you're good to go again.

The other way is to use the threading dial. I can certainly see times when this might be best - working at very slow speeds on long work pieces with fine threads might leave you very bored while reversing. But for me, most lathe threading jobs are pretty small, one off jobs where it's not going to slow things down to reverse and accuracy and removing any chance of error is more important.

 

[ronan]

I use the reverse or clenched buttocks method of screwcutting too, as my colchester is missing its threading dial. I reckon there is an evil genius somewhere hoarding thousands of the things. Anyway its the method i was taught both in work and tech when serving my time and its always worked for me.

 

[MrFluffy]

If you dont disengage the half nuts then it doesnt matter where you wind back to the carraige will always follow the same path relative to the spindle.
Trying to reverse the lathe without backing the tool out is asking for trouble, youll have too much slack in the gear train and that has to be taken up each change in direction.
Winding the cutting tool in and out for every back and forward pass soon gets tiresome, and youll forget the amount of rotation of crossfeed dial at some point (possibly easier with dro).
Get or make a threading dial. Far far easier. Got one on my DSG and dont thread on my Student or Myford which havent anymore.

THis is where swingup threading tool holders excel. They swing up out the way for the return pass. I have a threading dial but its only for imperial threads so I use spindle reverse and one of these to keep everything engaged. As I rarely cut imperial threads I cant remember the last time I used the dial.

Mike weeblys run through of them :-
http://mikesworkshop.weebly.com/swing-up-tool-holder.html

 

[MrFluffy]

Ignore the fact it has a milling cutter mounted in it (being used for boring ops instead), this is the multifix version of the swing out toolholder but for internal threading, the hex head on the bottom right advances and retracts the tool during passes for rapid advance and retract and the micrometer stop adjusts accordingly. They also do a external threading version but I dont own one of them to photograph.