Colchester 1600 Bantma mk2 threading issue.

Hi All,
I have been scratching my head with this.
I have been trying to cut a 16tpi thread,the problem is that it will not cut in the same place.I am engaging the half nut in the same place.
I have tried leaving the half nut engaged and stopping the machine and reversing back that seems to work.
The other problem I have is that the finish is really rough, tried 85 rpm and 180rpm the faster speed was little better, sorry for got to say,the material is stainless and using a threading insert..

Regards
Neil

Are you using the lever on the left of the apron (half nut engage) or the lever on the bottom-middle of the apron (power feed) ?.

griff25-06 said:

Hi All,
I have been scratching my head with this.
I have been trying to cut a 16tpi thread,the problem is that it will not cut in the same place.I am engaging the half nut in the same place.
I have tried leaving the half nut engaged and stopping the machine and reversing back that seems to work.
The other problem I have is that the finish is really rough, tried 85 rpm and 180rpm the faster speed was little better, sorry for got to say,the material is stainless and using a threading insert..

Regards
Neil

Click to expand...

If your dialing in with the compound then I'd just set a zero on the cross slide,dial in on the cross slide, engage half nut and take a pass stop the lathe with the brake in your thread relief, back off the cross slide, reverse the spindle till your at the end of the thread again, dial back to zero on the cross slide, dial in on the compound then rinse and repeat, you never have to disengage the halfnut using this method.

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Erstwhile said:

If your dialing in with the compound then I'd just set a zero on the cross slide,dial in on the cross slide, engage half nut and take a pass stop the lathe with the brake in your thread relief, back off the cross slide, reverse the spindle till your at the end of the thread again, dial back to zero on the cross slide, dial in on the compound then rinse and repeat, you never have to disengage the halfnut using this method.

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Click to expand...

Meant to say dial in on the compound...

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SAG 180 said:

Are you using the lever on the left of the apron (half nut engage) or the lever on the bottom-middle of the apron (power feed) ?.

Click to expand...

Been using the lver on the left of the apron..

regards
N

Erstwhile said:

Meant to say dial in on the compound...

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Click to expand...

Hi,
tried this method second, and it seemed to work ok, but the finish was abysmal..Need to get a solution for that..

regards
N

Are you going far enough back past your starting point so when you engage all the slop in the drive train is pulled out in that direction before you engage the half nuts. . Any change in direction includes all the gears in the drive trane. If leaving the half nut engaged works that may be your best bet.
I'm not a lathe expert but am experienced in drive trains.

Usual issue with stainless is work hardening and tendency to gall. Generally with threading once its work hardened you are pretty much stuffed because you can't apply enough depth of cut to shift the hard surface. Generally takes the edge off the tool too when it goes hard too which doesn't help.

With inserts you need to select an appropriate type and run them at book feed and speed values. Folk who know can go off-piste but I've rarely got satisfactory results when venturing far from what the insert maker recommends. Pretty much never use inserts for threading. If a Coventry die head can't handle the job HSS is more user friendly as I can sharpen them easily if something goes wrong.

I do stainless very carefully with an uber sharp HSS tool with the top slide set close to the thread angle using the zero-2-zero method to avoid tedious in-feed calculations. Synthetic coolant ensures that things don't get too hot. Takes far too long to make money by threading alone but gets the job done.

Clive

You don't say what the diameter of the work you're screw cutting is. 16 tpi at 180 revs screwing something 5" in dia is different from screwing something 1/2" dia.

Regards Tyrone.

griff25-06 said:

Hi All,
I have been scratching my head with this.
I have been trying to cut a 16tpi thread,the problem is that it will not cut in the same place.I am engaging the half nut in the same place.
I have tried leaving the half nut engaged and stopping the machine and reversing back that seems to work.
The other problem I have is that the finish is really rough, tried 85 rpm and 180rpm the faster speed was little better, sorry for got to say,the material is stainless and using a threading insert..

Regards
Neil

Click to expand...

griff25-06: Does your lathe have a metric lead screw? If so, that is probably the problem. Here in the ststes, if a lathe has an inch lead screw, we have to leave the half-nut engaged when cutting a metric thread. I noticed that same process worked for you, only the situation was reversed. If I understood you correctly, you are using a metric leadscrew and wanting to cut an inch thread. I believe you will need to run with the halfnut engaged to stay on-thread. Good luck.

JH

griff25-06 said:

Hi,
tried this method second, and it seemed to work ok, but the finish was abysmal..Need to get a solution for that..

regards
N

Click to expand...

Yeah man If that method worked it shouldn't have anything to do with the finish, probably just a feeds and rpm problem there

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With the Bantam,it pays to check that the half nut doesnt have swarf embedded.......In fact ,if you have never had the apron apart,it will be full of swarf and a right mess inside.( there is only one half nut......it works,about all you can say.)............One thing I do know.....take a threading tool off a little lathe thats doing a poor job,and put it on the Cri Dan at 2000 revs......mirror finish...Seems speed (and rigidity) is everything.

Tyrone Shoelaces said:

You don't say what the diameter of the work you're screw cutting is. 16 tpi at 180 revs screwing something 5" in dia is different from screwing something 1/2" dia.



Regards Tyrone.

Click to expand...

Hi Tyrone,
1" diameter stainless 416.

regards
N

Hi John,
what kind of a job is taking the apron apart?

Regards
N

Hi Clive,
I presume that when you say zero2zero then we are talking about a straight plunge in and not the 29.5 degreee compound slide method..

Regards
N

The apron isnt complicated as machines go,but there are a couple of tricky bits......including a shaft in a blind hole,where you will need to drill and tap the end for a screw to get it out...IMHO,pulling the spring loaded "shoe" out is enough to clean out the apron pretty good.Im pretty sure there are detailed instructions on some forum or other.............I havent had one apart for a lot of years,and the memory fades.....But one of those little USB cameras might be OK for having a look inside.

griff25-06 said:

Hi Clive,
I presume that when you say zero2zero then we are talking about a straight plunge in and not the 29.5 degreee compound slide method..

Regards
N

Click to expand...

Nope.

Zero-2-Zero AKA Zero-to-Zero (and probably known under other names as well) is an angular in-feed method where the lathe itself effectively "calculates" the exact in-feed needed to get the desired depth of thread for any arbitrary top-slide angle close to, but less than, the thread half angle. My topside lives at 25° angle offset. I get good results for both Whitworth form 55° threads and US/Metric 60° ones without faffing around. Being an angular indeed method you can use side rake on the tool which can be a great help with difficult materials when using HSS. Threading inserts frequently have top geometry functionally equivalent to rake on both sides but this usually only works correctly when run at the book speeds and feeds. Usually far too fast for manual lathe work.

Basic idea with Zero-2-Zero is that you touch the tool to the work piece to be threaded and set both cross and top-slide dials to zero. Then bring the tool clear of the end of the work and feed the cross-slide forward past zero by the desired depth of thread. Reset the cross slide dial to zero.

Move the top-slide back to clear the work and cut the thread in the usual way applying suitable cuts for each pass via the top-slide. The cross-slide is set to zero on the dial for each cutting pass and pulled back a suitable amount to clear the work on the return pass.

For the final pass both slides will be set to zero on their respective dials. Once you have worked all the spring-cuts out the finished thread will cut to the depth set by the initial cross slide in-feed. Which is about where you discover that book thread depths are only correct if the tool is of the right full form shape for the TPI or pitch in question. Insufficient depth is easily corrected by feeding the top-slide past zero until you get the right depth. Adjusting the top-slide dial zero to reflect the correct depth of cut means that all subsequent threads will be right the money. If the thread is too deep you have to try again starting with a known too shallow setting. Organised types (not me!) will make a note of the correct depths of cut with their tools rather than working to book and correcting.

For me the big advantages of this method are:-

1) you get exactly what you set so any errors can easily be tracked down

2) technique is exactly the same for all threads promoting fast, efficient work.

Main disadvantage is that working up to an exact stop is tricky.

Clive

400 series stainless is as hard as hell and you need speed to get a finish.

Might be worth spinning the tool upside down or working on the back side , so you can thread away from the chuck

(or sell the bantam and buy a hardinge;-))