Another lathe question-Speeds

[Randy_]

I've always wondered about this; but never knew quite who to ask, so I'll try some of the experts here. My JET mini will run from 500 RPMs to 3975 RPMs in 5 steps. If the speeds were evenly divided, each step would be 695 RPMs; but JET didn't evenly space the steps through the full speed range. As the machine is set up now, the steps are...from LS to HS...340, 400, 560, 830, and 1345 RPMs.

Does anyone "KNOW" why the arrangement is the way it is or care offer an opinion/speculation?? Also, any thoughts on why they didn't have a lower "low" speed? 500 RPMs seems to be a little fast to me for the low end?? Thanks.

 

[rtjw]

I have often wondered the same thing but have never really got up off my butt and asked. It could be a conspiracy![]

 

[wdcav1952]

Originally posted by rtjw
<br />I have often wondered the same thing but have never really got up off my butt and asked. It could be a conspiracy![]

Conspiracy? Frank, they are playing your song!

 

[Daniel]

Randy, it's sort of complicated and you need an understand of electricity and electronics. but as for the lower low end. it is harder and more expensive the slower you make a motor able to run. not nearly impossible though.
also the speeds on a switch type speed change, (meaning belts don't get changed) are a factor of the cyle of the electricity coming into the lathe. Most people are aware that AC Current moves forward and backwards through the wire in what is called a cycle. it is this cycle that allows electricity to cause most electric motors to turn. if the motor turned once for every cycle it would be spinning at thousands of RPS thats seconds not minutes. so the cycle has to be reduced by some factor that fits into the original cycle speed. each speed on your lathe has it's own factor to go by so they are not evenly devided spacings. Hope that makes since. in short it is a cost over performance issue. My small metal lathe has a dial for the speed control. also called a mosfet control. the speeds are infinit from 0 to it's top speed. the draw back is you never know what speed it is really running, the low end is realy hard on the lathe unless you switch it into low gear. and I blew the mosfet when I first got it(trying to cut metal with it in high bear but slow speed). tha one had to do with it coming with low quality components and I was aware of that. I had spare parts in anticipation so it was only a minor problem. They could start making the Jet with a mosfet type speed control. but it would triple the cost of the lathe.

 

[Paul in OKC]

My guess would be that there is not enough room for the size of pulley needed. You would need a larger pulley on the spindle to a smallest pulley on the motor for slower speed. I haven't looked at mine to check it out, but... My thought is that they figured how many steps and size they could get in that space and just 'winged it' from there[]

 

[Rifleman1776]

Originally posted by wdcav1952
<br />

Originally posted by rtjw
<br />I have often wondered the same thing but have never really got up off my butt and asked. It could be a conspiracy![]

Conspiracy? Frank, they are playing your song!

Yes, it's obvious. These are designed by the same people who force strange drill sizes on us. It's all a big conspiracy, I tell ye.

 

[Mac In Oak Ridge]

To layer on to Daniel's post. A motor must dissipate the heat that it creates out of the electrical energy it does not convert to motion. Your lathe motor is a totally enclosed motor, no air passes through the motor when running it only passes over the motor. That term for a motor is TEFC, Totally Enclosed Fan Cooled. There is a fan inside the motor that circulates some small amount of air, it may only be small fins on the rotor. There is a fan on the outside of the motor that is covered by the fan guard on the end opposite the output shaft. It blows air over cooling fins on the outside of the motor housing.

The slower the motor turns the hotter it's operating temperature, mostly because of less air moving across the motor surfaces. The maximum operating temperature is determined by how many copper coils are inside and the surface area of the heat dissipation fins.

A motor can be designed to turn very very low RPM's by putting more coils of winding inside and making the outside case bigger. In extreme cases a fan with it's own motor is added to cool the main motor. That is called "Externally Fan Cooled", this is usually done in industrial applications for big motors. All this costs money and you have to be able to justify the cost against the return you get from running the motor and the fan motor.

So what you have in a small inexpensive lathe motor is a compromise. The motor is running as slowly as the construction of it will allow without the motor burning up. This is put in the smallest, least expensive package that can be sold to the average purchaser of a lathe.

It's just the nature of the beast.

 

[Mac In Oak Ridge]

The above is assuming that you change the speed of the lathe by changing the speed of the motor, not by shifting the belts on a step pulley or on a variable speed drive arrangement.

 

[wdcav1952]

CarbeTec 4SE. It goes faster when I turn the dial up, slower when I turn the dial down. I have had it for more than two years and have never found the need to experiment with belt positions. Is it a true VS, or is it part of the great drill conspiracy? In the words of the great Jimmy Buffett:

"Hell I don’t know and I don’t care
If you’re looking for a quote from me
I’ll be under the mango tree
I just can’t say how I’ll get there
Hey I don’t know and I don’t care
I don’t know and I don’t care."

 

[Randy_]

Originally posted by Daniel
<br />Randy, it's sort of complicated and you need an understand of electricity and electronics. but as for the lower low end. it is harder and more expensive the slower you make a motor able to run. not nearly impossible though.....

Thanks, Daniel et al; but I guess I wasn't as clear as I should have been. I have a 6 speed, not a VS so the question really revolves around how JET decided to size its pulleys.

 

[rtparso]

Randy I probably was what the engineer found in the catalog. Pulleys are rarely custom.

 

[Randy_]

Originally posted by rtparso
<br />Randy I probably was what the engineer found in the catalog. Pulleys are rarely custom.

Ron: These are 6 step pulleys. The one on the the motor shaft probably has a standard size hole in it; but the one on the spindle almost certainly is proprietary. I doubt the pulleys are stock items; but it's certainly possible??

 

[Mac In Oak Ridge]

The pulley selection on a piece of machinery is a compromise between what you can do and the space you can do it in.

A given V-belt has a minimum radius that it can be bent around and still function as a driving element in the arrangement.

You bend a given size belt around a sheave that is too small for it's function and it will slip. There has to be a certain surface area of the driving portion of the belt in contact with the sheave or it cannot get sufficient traction to pull the load that is designed for the driven arrangement to accommodate. If the driving sheave doesn't have sufficient traction on the belt to turn the load then the load won't move and the driving sheave will move, that results in the belt burning or a lot of noise like someone stomping on a squirrel as the sheave slips against the belt. When that happens the manufacturer of the machine gets a lot of phone calls from unhappy customers.

On my Delta Midi the belt has a grooved surface where it contacts the sheave and the sheave has a grooved surface to match the belt. The reason for that is to make the contact surface of the belt as large as possible for the belt and sheave that can be fit in the space that the machine design can accommodate. If you had a way to measure the grooved surface area you would find that it has about 50% more contact with the sheave than a standard V-belt. The larger the contact surface the smaller the driving sheave can be made.

Soooo..... What you see is what is feasible to provide to make a functioning machine within the limits of design of the technology?

Get it?

 

[Mac In Oak Ridge]

As a little aside. There is a compromise calculation in choosing a sheave/belt combination. If the driven sheave is too large then that reduces the amount of the belt that contacts the driving sheave. So if you have a marginal small driving sheave and it slips you can reduce the diameter of the driven sheave. That effectively allows more of the belt to contact the driving sheave, hence more traction surface in contact with the driving sheave.

But when you do that then the driven load will not turn with as many RPM's because the driven sheave is smaller. If that is acceptable in your machine design then you have solved the problem. If the RPM change is unacceptable then you start tampering with the drive/driven sheave ratio and in worst case the RPM of the motor.

I guess the point is that the lathe manufacturer has to take the available technology and make a machine that will function well enough for a customer to buy it and keep it.

To slow that driven shaft down using V-belts could be done by building that part of the machine larger to accommodate a different sheave design. Or putting a larger HP motor on the machine, that would entail a larger diameter sheave because the motor shaft would be larger. Or putting a slave shaft in the system, that is have the motor sheave drive a shaft that has two sheaves on it, a belt goes from motor to slave shaft and another belt goes from slave shaft to the driven shaft. All three shafts have step sheaves on it. This would give you a whole bunch of choices of speed, depending on how many steps on each pair of sheaves.

All of this costs money. The manufacturer knows how much Joe Schmuck will spend for a given machine and all the design has to fit inside that price. Say you could make that Midi lathe have 15 speeds instead of 6. It may have to sell for $350 instead of $200. Would Joe Scmuck buy that lathe or kick in a few more bucks and buy a bigger wiz bang lathe from a competitor?

Most machine manufacturers are looking for a vacuum in the machine market and then coming up with something to fill it, first. Then they look at the competitor and see if they think they can match a successful machine that is selling already. The market sets the price and the engineers make the machine to fit the price. They get it right the company makes a profit, they get it wrong and the company gets a new engineer.

 

[Daniel]

Randy,
I was thinking in the back of my head while I wrote my earlier post that I have never even looked at a jet. So I could not be shure what sort of variable speed you have. to slow down the pully type speed change you have to change the size of the pulleys. the dangers (as the posts have pointed out) are many. My guess would be the biggest problem would be not overheating the motor though. If you have six pseeds though you shoudl then have six sets of pullys. each of these have to be the same distance around or close to it for the same belt to work on each position. that is one I don't know the math for.

 

[Randy_]

Originally posted by Mac In Oak Ridge
<br />.....On my Delta Midi the belt has a grooved surface where it contacts the sheave and the sheave has a grooved surface to match the belt.....Soooo..... What you see is what is feasible to provide to make a functioning machine within the limits of design of the technology? Get it?

Thanks, Mac. I understand what you are saying about the operation of a pulley and belt system. What are the speeds of your Delta midi??

The JET has the same sheave(pronounced shiv, BTW, for those that don't know) arrangement with the flat grooved belt. Always wondered why they didn't use something similar in car disk brakes?? Seems like you would get more bearing surface for friction and more surface area for better cooling with grooved disks??

As to your "limits of design" comment, the smallest drive sheave is about as small as it can physically be. It is only slightly larger in diameter than the drive shaft, itself. But there is clearly some room in the headstock for a larger driven sheave and, if the engineers has wanted to, they could have made the headstock casting slightly larger to accommodate a larger sheave. Case in point, the Delta midi has the access door to the upper pulley on the front of the headstock housing where it is easy to access the upper pulley to change belt position.....good engineering. On the other hand, the Jet designers put the door on the back of the headstock where it is a minor pain to get at.....poor engineering. I got a good deal on my JET; but if the JET and the DELTA had been similarly priced, I may have very well wgone with the DELTA for that "one" reason, alone!!

 

[Randy_]

Daniel: I thought I posted a response to your comments; but they are not here?? Maybe they went as an email to you direct. I do want to thank you for your ideas.