Jet or Wivamac

[paulm]

Done a bit more reading - my son is interested as he is doing an engineering course. It seams that torque drop off starts at 8% of motor speed for simple v/hz inverters and improves down to 2% for open loop vector inverters and 0% for closed loop vector inverters. No idea what type of inverter is in my 1628VS but I expect an open loop vector (I think these tend to be used more where braking and reverse operations are needed). Taking worse case of a v/hz style it will deliver full torque at about 100 RPM and up on the lower belt setting. If it is the vector type it will deliver full torque at about 27 RPM. In either case I can't see additional belt ratios helping much? With the 6000/diameter rule you would need to be turning greater than a 60" bowl at this speed to start getting torque drop off with a v/hz inverter. Sure, many people start at slower speeds than that so lets say a 3000/diameter giving a 30" bowl.

EDIT: Sorry, I get question in my head and need to dig away at it. The drive is a simple V/hz (V/f) drive but with a torque compensation at start up delivering up to 150% additional torque for 60 seconds. So, the short answer is on the 1628VS under normal running conditions full torque is delivered at about 100 RPM. I suspect that is why the lathe is stated as starting at 50 RPM. Maybe lathes whose variable speed starts at near zero use the vector style inverters?

BM

Interesting, makes me wonder why we don't all drive around in cars with just two gears, or bicycles even ?

Perhaps there are fundamental differences in efficiency or power delivery characteristics that render the analogy invalid, I'm not technical enough to know !

Seems to me that the two pulleys is a compromise solution, it seems to work okay most of the time for some users, but it seems unlikely to me to be able to deliver the same efficiency across a range of applications as more pulley wheels will provide. That may not matter to some users some of the time but others may want and need the extra efficiency of a less compromised solution.

Not to say that the two pulley compromise isn't an acceptable one that is fine for a lot of situations, just that technically I don't see that it can ever be as efficient a solution across a range of scenarios.

I know that I could ride my mountain bike around the local countryside using only two gears, but it's a lot more efficient and enjoyable to use a wider gear range and I'd certainly struggle to get up the steep hill to my house which I can barely manage in any gear :lol:

If I'm misunderstanding or overlooking something though I'm happy to learn more !

Cheers, Paul

 

[paulm]

I've remembered one annoyance with the Wivamac - the box with the inverter takes up a lot of space under the lathe where I previously had a waste bin. I guess it could be mounted somewhere else but I've just never got round to it

Same here Duncan, I mounted the box on a free standing timber frame at a good working height and can move it around to be close to hand as needed. Also wired up a magnetic mounted emergency stop button that can be positioned anywhere useful such as hip height for no hands use.

Cheers, Paul

 

[duncanh]

Unfortunately I'm limited on space so there aren't many places the box can go. The magnetic control box it came with is good enough for me as it has a long enough cable to place pretty much anywhere on the lathe. How did you wire up your emergency stop?

 

[paulm]

Unfortunately I'm limited on space so there aren't many places the box can go. The magnetic control box it came with is good enough for me as it has a long enough cable to place pretty much anywhere on the lathe. How did you wire up your emergency stop?

I think they do ship now with a separate magnetic control box Duncan, so that's probably fine then and obviates the need to move the main box. On my earlier model they didn't have the separate control box on a lead, hence my mod, gives me some magnetic rack storage for tools too on the same stand which is quite handy.

Can't recall how I wired up the stop switch, it was a few years ago now. If you're interested I'll open the cabinet and take a pic later, it's just simple two strand cable I recall so just a case of figuring the correct two terminals to attach it to inside.

Cheers, Paul

 

[themackay]

Well Ive gone and ordered the Jet 3520B
Alan

 

[gregmcateer]

Well Ive gone and ordered the Jet 3520B
Alan

Good on yer - exciting times ahead.

Do take some photos of the beast arriving and set up, (and first turnings, of course :wink: )

Best of luck with it, you lucky @"£|$%"

Greg

 

[bogmonster]

Done a bit more reading - my son is interested as he is doing an engineering course. It seams that torque drop off starts at 8% of motor speed for simple v/hz inverters and improves down to 2% for open loop vector inverters and 0% for closed loop vector inverters. No idea what type of inverter is in my 1628VS but I expect an open loop vector (I think these tend to be used more where braking and reverse operations are needed). Taking worse case of a v/hz style it will deliver full torque at about 100 RPM and up on the lower belt setting. If it is the vector type it will deliver full torque at about 27 RPM. In either case I can't see additional belt ratios helping much? With the 6000/diameter rule you would need to be turning greater than a 60" bowl at this speed to start getting torque drop off with a v/hz inverter. Sure, many people start at slower speeds than that so lets say a 3000/diameter giving a 30" bowl.

EDIT: Sorry, I get question in my head and need to dig away at it. The drive is a simple V/hz (V/f) drive but with a torque compensation at start up delivering up to 150% additional torque for 60 seconds. So, the short answer is on the 1628VS under normal running conditions full torque is delivered at about 100 RPM. I suspect that is why the lathe is stated as starting at 50 RPM. Maybe lathes whose variable speed starts at near zero use the vector style inverters?

BM

Interesting, makes me wonder why we don't all drive around in cars with just two gears, or bicycles even ?

Perhaps there are fundamental differences in efficiency or power delivery characteristics that render the analogy invalid, I'm not technical enough to know !

Seems to me that the two pulleys is a compromise solution, it seems to work okay most of the time for some users, but it seems unlikely to me to be able to deliver the same efficiency across a range of applications as more pulley wheels will provide. That may not matter to some users some of the time but others may want and need the extra efficiency of a less compromised solution.

Not to say that the two pulley compromise isn't an acceptable one that is fine for a lot of situations, just that technically I don't see that it can ever be as efficient a solution across a range of scenarios.

I know that I could ride my mountain bike around the local countryside using only two gears, but it's a lot more efficient and enjoyable to use a wider gear range and I'd certainly struggle to get up the steep hill to my house which I can barely manage in any gear :lol:

If I'm misunderstanding or overlooking something though I'm happy to learn more !

Cheers, Paul

Hi Paul, I don't fully understand this and may be completely wrong. I am interested so trying to work it out in my own head. Assuming your interest is genuine this is my thinking on the matter - which may be completely wrong

Bicycle and cars have similar issues to overcome. High torque is initially required to overcome inertia. When the bike / car is moving the forces working against the cyclist / driver are very non-linear when taken as a whole, rolling resistance, wind resistance (definitely not linear) and gradient (this one is linear taken on its own). Also the power output and torque of a car engine is also very non-linear and dependent on revs (a cyclist also has cadence ranges that are much easier on the rider ). So there are a lot of non-linear variables to match up get the optimum performance. In particular the narrower the power and torque band in an engine, the shorter the shifts need to be to get optimal performance. Also, energy efficiency is much more important, petrol is expensive and people run out of puff.

Compare that with a lathe. An invertor controlled motor has very linear torque output throughout its RPM range except at the extremes. Also the important work variables are more linear. The energy to take a 1 cm deep cut at the edge of a 30" bowl turning at 250 RPM is practically the same as taking a 1 cm deep cut on a 10" bowl at 750 RPM as a 1 cm cut on a 5" bowl at 1500 RPM (which is probably a bit extreme as you will end up with an egg cup very quickly). With the above examples the same amount of wood is being removed because the RPM is adjusted proportionally with the diameter to achieve a safe and sensible turning speed. Absolute energy efficiency is also probably not so important (I can see me getting a bashing by the greens).

Obviously it is not quite as simple as that. We tend to take bigger cuts on larger diameter pieces. As we get into small diameter stuff the cuts tend to be much smaller so turning small spindles requires little torque in comparison

With a decent size motor with a linear output and relatively linear max torque requirement regardless of bowl size 1 pulley that cover bowl turning speeds (and the relatively constant max torque requirement largely irrespective of bowl size) will likely cover most of the bases with another that covers spindle turning speeds with the high RPM and low torque requirements associated with taking small cuts.

I am not saying that more pulley ratios are not better - of course they are as you have more choice, I am just not sure they are necessary for the majority of wood turning applications and this would be low down my criteria list. Motor size, mass, rigidity, ergonomics etc appear likely to have a much bigger impact of the suitability of a lathe.

EDIT: It is also worth considering that many electric vehicles have a single gear with few having more that 2 gears.

As I say, I may be completely off base here.

More important is how the actual machine performs in reality, not theoretical exercise.

BM

 

[duncanh]

Unfortunately I'm limited on space so there aren't many places the box can go. The magnetic control box it came with is good enough for me as it has a long enough cable to place pretty much anywhere on the lathe. How did you wire up your emergency stop?

Can't recall how I wired up the stop switch, it was a few years ago now. If you're interested I'll open the cabinet and take a pic later, it's just simple two strand cable I recall so just a case of figuring the correct two terminals to attach it to inside.

Cheers, Paul

Not a huge problem for now, but thanks.

 

[paulm]

You've lost me there BM, way too technical for me, I'll stick to my instinct that more is ultimately better :lol:

Enjoy your new lathe when it arrives Alan, as I said up front, they are both good machines and I'm sure you won't be dissapointed

Cheers, Paul