First attempt at threading on a bantam - all didn't go well

Not a good evening playing with my still new to me Colchester Bantam Mk1, and as I sit down to write this i realise almost all of it was my own fault (long time since I used a lathe).

I set it up to cut my first thread, a 9/16 UNC external thread in a bit of EN16 steel using a cheap Chinese indexable carbide tool from Banggood (which actually seems not too bad).

First pass went fine, until i overshot slightly when the inertia kept the chuck turning a bit longer than i anticipated, even at a lowly 35rpm. It didn't occur to me to use the brake!

Reading the instructions, the Bantam should be reversed as it doesn't have a thread dial (and I was cutting imperial on a metric machine anyway). So, i engaged reverse direction and set out to cut on the reverse pass (spot the obvious mistake). To my horror it didn't line up properly with the forward pass, and then to add insult to injury chipped the top off the carbide tip.

Oh well, turn the tip round and use another point. Look at lateral movement in feed screw, of which there seemed to be quite a lot, adjust out end float and have another go, forward. It seemed to line up with the first forward pass OK. I didn't even overrun this time.

Now for cutting on the reverse pass again. It didn't line up, again. Although this time was slightly better than first go. Then i brole the tip off again. Doh! I was now getting frustrated (and had still not realised the obvious mistake).

Lots of looking and measuring the lateral play in the carriage with the feed nut engaged, about 0.1mm. Starting to conclude the feednut must be worn and in need of replacement, which sounded expensive.

At this point things went from bad to worse. Traces of smoke started to emanate from the back of the lathe, the motor to be precise. Ever since I bought it it would occasionally buzz loudly when the forward/reverse lever was in the off position. On this occasion it had been doing that for a minute or so whilst I was distracted looking at the play in the feednut (i'd usually turn it off at the wall when it did it). I suspect something is wrong with the original electrics and it was managing to somehow power one coil and not the other, or one in forward and one in reverse. Anyway, I quickly turned off the power and the faint trace of smoke quickly stopped. Not what you want when you're already frustrated.

Time to give up for the evening and consult the forum as to the state of the feednut. Then, as I started to type it dawns on me. The reason I was breaking the tips and the forward and reverse passes weren't lining up properly was because I'm not meant to be cutting on the reverse pass, only on the forward. Under normal circumstances I'm pretty switched on, but this evening I'd clearly left my common sense in the house. The work was turning backward and the compression force on the underside of the tip was causing to to break. The feednut was loaded against the opposite face of the screw on the return and the backlash mean it was never going to line up. Even it it was a new screw and nut it probably wouldn't have worked.

As usual, the problem was the operator not the machine (except for the now poorly motor).

Hopefully the smoke was just burning oil and not insulation/enamel or the 3 phase and VFD conversion might be upon me earlier than I was planning. I'll have a second attempt tomorrow after checking out th eNVR and reversing electrics. I'll use a HSS tool this time, as carbide at 35 rpm probably wasn't the best choice anyway, even when cutting in the right direction.

Edited By Mark Davison 1 on 11/01/2019 23:07:22

Edited By Mark Davison 1 on 11/01/2019 23:08:05

The people who make it look easy have skills

That'll be you one day

Hi Mark,

If you can source or make a thread dial, you can use the half nut to stop the carriage even if you are cutting an imperial thread on a metric lathe. The method is as follows:

1) Engage the half nut when the thread gauge reads 0.

2) Disengage the half nut at the end of the thread and turn the lathe off. Back the tool off.

3) Start the lathe in reverse and engage the half nut when the thread gauge reads 0.

4) When the tool is clear of the work, stop the lathe, leaving the half nut engaged.

5) Reset the tool and start the lathe in the forward direction.

I made a thread gauge out of PVC for my old imperial Boxford lathe and turn metric threads quite happily using this procedure.

Regards,

Alan C.

You may realise you cannot surface in reverse unless you turn the cutter upside down.

If your motor is energised while apparently switched off, you should not be touching anything that might injure you!

Your motor is close to toast and likely has burned wiring/windings already, so needs checking out before any further damage occurs.

Learn about backlash - all normal installations have it, unless expensive lengths are gone to, to fit anti-backlash parts.

Take note of AC’s order of operations, apart from disengaging the lead screw in your particular instance (no thread dial). It is important and standard practice. Not followed for no good reason!

Hi check the contacts on the reversing switch as they don't like reversing whilst in motion and the power can burn out the contacts, been there and done it, and then motor has only starter windings running and getting hot.

David

Sound so you have a problem with the direction switch contacts. Also I don't think it is good idea to change from For to Rev with and inverter instantly, as where does the energy stored go at the zero position. I suggest you switch every thing OFF and check the switch. Sorry to hear about the screw cutting problem, John

You can use carbide tips at any speed. They may work best at high rpm but as long as everything is rigid enough any speed above zero will work. As you have found out though carbide does not last long when under tension due to upward forces without support above it.

Regarding reversing the lathe when doing metric on an imperial machine or vice versa lots of posts saying do it but not many say back the tool away from the workpiece first, see Alan's item 2 above. It seems a lot of people who offer this advice assume it is an obvious step.

Martin C

When cutting a thread and using reverse i always set the cross slide dial to Zero at the start of each cut. When you get to the relief at the thread shoulder. Stop the lathe, move the cross slide back to clear the already cut thread then reverse the lathe to bring the cutter clear of the front end of the work before moving the cross slide back to zero then apply another cut with the cross slide and re-zero. I rarely use the compound slide set over at an angle but if you use this method then the same will apply.

Backlash will get you every time if you try just reversing without moving the cutter back.

I have located a photo of a thread I cut with a carbide insert. This shows one way of ensuring the thread cutting is rigid enough to cut with low rpm and carbide. This was probably done at the lowest speed on my lathe, 66rpm.

Martin C

Not sure EN16 is the easiest steel to learn screw cutting on. Try a bit of EN1B which I believe is free cutting mild steel - lovely stuff to turn.

There was a thread (we Brits love a pun) recently on screwcutting where the finish improved no end when the OP ditched the cheap insert and bought a professional one. I'd also agree with Speedy; if EN16 is anything like EN24 you'll need a high surface speed to get any sort of decent finish with insert tooling. I'd start with EN1A.

Andrew

En1A leaded is even better, single phase motors do not like too many stop /starts or forward to reverse,most larger motor s have a rating of so many starts per hour,usually specified in the manufacturers literature but not on the motor plate.

Bob

Resetting the dial to zero after each cut is tedious and slightly error prone as you have no reading for the total depth of cut.

Far better to refine things by using the "Zero-to-Zero" / "Zero-2-Zero" (and probably other names too) system where the successive cuts are put on with the top slide. Threading passes are still made with the cross slide on zero.

The purest version I've seen is that due to Geo. H. Thomas which has the top slide set at an angle close to, but not greater than, the thread half angle and uses the lathe itself to calculate the top slide in-feed required for that specific angle.

Basic procedure is:-

1) bring the tool up to touch the workpiece then set both cross and top slide dials to zero.

2) withdraw the tool a little to using the topside to clear the work and move the cutting tip into clear air past the end of the workpiece.

3) wind the cross slide in past zero to the required depth of cut and reset dial to zero

4) bring the topside back sufficiently to clear the workpiece and move to start of threading pass

5) apply the first cut with the top slide and make threading pass with cross slide is on zero

6) at the end of the threading pass pull back tool for clearance using the topside and move back to the start point

7) move cross slide back to zero position

8) move top slide forward to apply next cut

9) Make threading pass

10) repeat steps 5 to 9 making last threading pass with top slide reading zero, probably need one or two more spring passes at that setting to finish the job properly

The great thing about this method is "what you set is what you get", within lathe accuracy limits of course. So if the thread doesn't fit you have good chance of figuring out what went wrong. Usual issue with home ground tools is the point being sharper than the book value or the thread angles being a touch off. Usually you need to cut the thread a little deeper to accommodate the errors. Make the extra cuts using the cross slide leaving the top slide at zero. Once you have it right make a note of how much extra cut you need for your tool so next time will probably work out fine. If you are doing several threads reset the cross slide to zero at what you have now found to be the right thread depth so subsequent threads will automatically come right.

Clive.

PS Far faster to do than to read!

Even if all the backlash is removed - including that between the gear teeth, and otherwise in the drive train bushes etc, you cannot rely on a backwards move matching a forwards one. This is because of the elasticity in the parts, each bit of stretch allowing a small discrepancy, but all adding to it in the same direction.

And I recall a mod to the standard tool holder which held the tool in position against a spring. As the tool reached the end of the thread, a catch was released automatically and the tool sprang back a short distance. When it had been wound back to the start, it could be pushed forwards to the stop again, where the catch held it, and then it was adjusted for the next cut. One day I will get round to making one ...

Cheers, Tim

Machine still works. I dismantled and cleaned the NVR/contactor. There was a loose wire which I suspect was at least part of the problem. Also dismantled the barrel reverse switch. The buzzing seems to be down to slop in the forward/stop/reverse lever on the carriage not always returning the barrel switch back to the neutral position. I'll investigate further later and in the mean time will keep checking it.

I finished the thread. Given the mess I made at the start it has come out "usable" but not pretty. It will suffice for what I want it for (it is a stud that will be semi-permanently screwed into a large T nut/plate to secure a new tool post). I'm going to get some EN1A to practice on, the EN16 seems to tear.

Same as EN24 and, to some extent, EN3B. That's why you need to run fast with carbide inserts on these materials.

Andrew