Mini Lathe belt slipping

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SmithDoor

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Just purchased my first mini lathe in November.
I did know about slipping problems.
I have all poly V belts but the belt slips.
https://www.walmart.com/ip/8x14-225...Motor-for-DIY-Wood-Metal-Work-More/5116832219
What best way to stop the slippage and how tigh can I tighten the belt?

Dave

20231121_124001.jpg


off internet_PT0_SX300_V1___.jpg
 
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Your manual should tell you how to adjust the motor belt tension. I have a Cummings . Under the switch box there may be 3 screws. These are supposed to be adjusted to tension the belt. Your lathe may be different, check the manual.

I could never get the belt tight enough with the procedure described in my manual. Instead I cut a wedge that forced the motor down slightly , this worked. Make sure the motor is in a plane with the pulley on the drive shaft spindle. If the motor has a twist it will throw the belt off the pulley.
 
Every problem is different... I had a V belt slipping and after much head scratching found that the sheave was not deep enough forcing the friction to be on the bottom surface rather that the wedging action of the V flanks. A quick deepening of the sheave fixed it.
 
Your manual should tell you how to adjust the motor belt tension. I have a Cummings . Under the switch box there may be 3 screws. These are supposed to be adjusted to tension the belt. Your lathe may be different, check the manual.

I could never get the belt tight enough with the procedure described in my manual. Instead I cut a wedge that forced the motor down slightly , this worked. Make sure the motor is in a plane with the pulley on the drive shaft spindle. If the motor has a twist it will throw the belt off the pulley.
The manual has all most nothing on belt tightening.
But I am worry about over tightening and braeking the belt.
This maybe just problem with this design on low speed setting. Maybe needing more grooves/width for low speed?

Dave
 
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Every problem is different... I had a V belt slipping and after much head scratching found that the sheave was not deep enough forcing the friction to be on the bottom surface rather that the wedging action of the V flanks. A quick deepening of the sheave fixed it.
I did check the death of grooves and the angle.
It maybe not enough grooves/width of belt on low speed.
I did l check on other manufacturers they use about same belt design either 5 or 6 groove Poly-v belt. So does everyone have this problem on low speed setting of belts?

Dave
 
From vee-belt design experience I would suggest a worn belt. Factory made pulleys are always made to pretty close standard dimensions and tolerances, so "any " good belt will use the wedge action to grip onto the sides of the belt vee, with clearance at the bottom. Even a slacks belt can transmit full power as the wedge forms the grip, pulled into the pulley vee by belt load. Pre-tension only needs to be adequate to prevent initial slip, otherwise you endanger overloading the bearings.
So is the a home-made pulley, of factory produced? DID you try a new belt? On many vee-belts, wear is hard to measure unless to make a gauge. But bottoming on a proprietary pull is almost always just a worn belt. So it fails (slips) at high load - as with your low-speed loading....
QED,
K2
 
From vee-belt design experience I would suggest a worn belt. Factory made pulleys are always made to pretty close standard dimensions and tolerances, so "any " good belt will use the wedge action to grip onto the sides of the belt vee, with clearance at the bottom. Even a slacks belt can transmit full power as the wedge forms the grip, pulled into the pulley vee by belt load. Pre-tension only needs to be adequate to prevent initial slip, otherwise you endanger overloading the bearings.
So is the a home-made pulley, of factory produced? DID you try a new belt? On many vee-belts, wear is hard to measure unless to make a gauge. But bottoming on a proprietary pull is almost always just a worn belt. So it fails (slips) at high load - as with your low-speed loading....
QED,
K2
I just got lathe and trying to find out what best way to go.
So hoping some group can help me improve this lathe.

Dave
 
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I have what is essentially the same lathe, but built with simple Vee pulleys and belts and tensioner. It hasn't slipped (Yet!). I regularly stall the motor with low-speed thread cutting above 5/16 ths in or 8MM steel, or 3/8in Brass. So cut larger threads by hand power, not motor.
I am amazed the poly vee belts slip!
When cutting metal normally, I do not allow much drop of revs, as the speed control simply turns up the power against torque, until it reaches the max to maintain the selected speed. - Then speed drops, and it will cook electronics or motor after more than 5 to 10 mins at full power. A 600W motor is good for 0.010" cuts at most speeds and light feeds, but really small for proper metal removal as industrial machines work.
I strongly recommend you ensure the whole lathe is mounted on a STRONG (STIFF) base that is accurately set, as if not correctly set and shimmed level the base will distort the lathe when bolted down. e.g. a 50mm kitchen worktop will twist the lathe bed if not absolutely flat and true....
What are the dimensions of the poly vee belt? - If you bought the lathe NEW you will have a guarantee so can chase South Bend for advice?
K2
 
I have what is essentially the same lathe, but built with simple Vee pulleys and belts and tensioner. It hasn't slipped (Yet!). I regularly stall the motor with low-speed thread cutting above 5/16 ths in or 8MM steel, or 3/8in Brass. So cut larger threads by hand power, not motor.
I am amazed the poly vee belts slip!
When cutting metal normally, I do not allow much drop of revs, as the speed control simply turns up the power against torque, until it reaches the max to maintain the selected speed. - Then speed drops, and it will cook electronics or motor after more than 5 to 10 mins at full power. A 600W motor is good for 0.010" cuts at most speeds and light feeds, but really small for proper metal removal as industrial machines work.
I strongly recommend you ensure the whole lathe is mounted on a STRONG (STIFF) base that is accurately set, as if not correctly set and shimmed level the base will distort the lathe when bolted down. e.g. a 50mm kitchen worktop will twist the lathe bed if not absolutely flat and true....
What are the dimensions of the poly vee belt? - If you bought the lathe NEW you will have a guarantee so can chase South Bend for advice?
K2
Belt is 145J4

Dave
 
Found a belt tension guide...
https://simplybearings.co.uk/shop/files/chal_belt_tensioner_p168-169.pdfBut I don't think this covers Polly-Vee belts.
For tensioning Poly-Vee belts, it is relatively easy to measure tension by a calculation from the frequency of the note generated from the belt in tension. Load this App to your phone.
https://www.hutchinsontransmission.com/resource-center/apps/easy-tensionMeasure the centre to centre distance of the span, pulley diameters "mid-depth" of the grooves.
Or measure the belt-span from pulley to pulley at the tangential contact points.
With the data I think the app will give you a frequency of the note you should have when you Twang the belt. - Tap it with a spanner. It is "what we did" on the car production line I.E. tap the belt while using the frequency meter, then tension the belt until the correct frequency. Rotate for a full belt rotation, re-check/adjust. Doing 200 cars a shift, the lads knew exactly what tension to apply so when rotated it was correct.... But doing 1 belt you need the rotation and re-adjust, until it is correct. That was back in 1986~92...
But maybe the App is easier?
K2
 
Does the belt slip during normal running or only during heavy cuts?

I keep the belt on my mini lathe on slack side.
I can take a 1mm DOC in aluminium without the belt slipping. During parting off or grooving operation the belt slips if I feed the tool aggressively.
For me a slipping belt is better than a broken lathe.

Regards
Nikhil
 
Hi Nikhil,
The slipping belt is a torque limiter that saves some parts of the machine, but is sacrificial on belts! - But I guess you consider a belt to be a low cost consumable. Just remember that while it is not slipping, it will generate a small amount of heat (1% or 2%) that has been considered by the engineers during the design stage. But if slipping, it could generate maybe 50% of the motor's power into heat in the pulley - which is then carried along the main-shaft into the bearings. This extra heat may affect the bearing lubricant and shorten the bearing life?
So for a bit of care in using the tool, you can set the belt properly, and have no belt slippage and use your skill as an operator to listen to the speed of the lathe and if it starts to labour (slow down more than normal) you can reduce the feed-rate or size of cut, or adjust the speed of the lathe so it cuts properly. That is a part of the skill, and pleasure of using machine tools.
e.g. As you face the end of a piece of rod, more noticeable with larger diameters, you set the rotational speed "to the book value". Then this is too slow at half diameter, and quarter diameter, etc... With a variable speed you can increase speed at the half-way point, and again at the 3/4 point. But with a fixed speed lathe, you can adjust the feed-rate of the tool to "that sweet spot" where the curls of swarf keep on curling nicely. And without feeding too fast and tearing the metal. But it is all a compromise that we learn with practice - except the blind machinists who set the CAM system and let the computer to do its stuff. (A bit like buying a ready meal and "cooking" it in the microwave oven, IMHO).
Enjoy!
K2
 
Hi Guys,

If the lathe in question is the one in the picture, then the belt tensioner is the bolt sticking out on the extreme middle left of the small picture.
 
Hi Nikhil,
The slipping belt is a torque limiter that saves some parts of the machine, but is sacrificial on belts! - But I guess you consider a belt to be a low cost consumable. Just remember that while it is not slipping, it will generate a small amount of heat (1% or 2%) that has been considered by the engineers during the design stage. But if slipping, it could generate maybe 50% of the motor's power into heat in the pulley - which is then carried along the main-shaft into the bearings. This extra heat may affect the bearing lubricant and shorten the bearing life?
So for a bit of care in using the tool, you can set the belt properly, and have no belt slippage and use your skill as an operator to listen to the speed of the lathe and if it starts to labour (slow down more than normal) you can reduce the feed-rate or size of cut, or adjust the speed of the lathe so it cuts properly. That is a part of the skill, and pleasure of using machine tools.
e.g. As you face the end of a piece of rod, more noticeable with larger diameters, you set the rotational speed "to the book value". Then this is too slow at half diameter, and quarter diameter, etc... With a variable speed you can increase speed at the half-way point, and again at the 3/4 point. But with a fixed speed lathe, you can adjust the feed-rate of the tool to "that sweet spot" where the curls of swarf keep on curling nicely. And without feeding too fast and tearing the metal. But it is all a compromise that we learn with practice - except the blind machinists who set the CAM system and let the computer to do its stuff. (A bit like buying a ready meal and "cooking" it in the microwave oven, IMHO).
Enjoy!
K2
I agree
The belt is like fuse saving the lathe.

A South Bend 9A has ½hp but do turning at 50 RPM.

The mini lathe uses a DC motor and you can take a cut at 50 rpm using 1 hp the lathe would have destroyed the lathe.
Do have the same torque at 50 as you do at 1,000 rpm. But hp will very a lot.

Dave
 
There is no single max cut "for steel" or anything else.
Think of this. You are shearing metal with the cut. Say b0.010" cut at a feed of 0.010" prerevolution. On a 2 in diameter, you will shear the meatal 2 Pi long per revolution, and on a 1/4 in diameter you will shear the metal 1/4 x Pi long per revolution. So 1/8th of the metal is sheared on 1/4in dia as on 2 in dia per revolution.
Torque and power.
The force required to shear the metal per cut is the same for 2in dia and 1/4in dia. : But multiplied by the radius, 1in, or 1/8 in means a big difference of torque = 8 times the torque at the higher diameter of 2in. compared to the 1/4in dia metal.
But the speed of cut at 2 in will be much slower than at 1/4 in. - Maybe 50rpm compared to 1000rpm...? - depends on what the lathe and your super sharp tooling can do, the state of the steel alloy, temper, etc.
Now at 2 in dia, and 50rpm. you will use the shearing force for 2 x Pi per rev x 50 revs per min = 100 x Pi of shearing force/min. .
Whereas the same shearing force (cut and feed) on the 1/4in dia will be 1/4 x Pi per rev., x 1000rpm = 250 x Pi x shearing force per min = 2 1/2 x the POWER required of the low speed cut...
BUT your lathe is not linear. The variable speed unit is current limited. I.E. It cannot deliver more current than a safe limit set inside the unit to prevent the semi-conductors from exploding (been there, dunnit). Inside the motor, (Assuming a permanent magnet motor like mine?) the current in the windings creates the torque from the motor. If at max torque, the torque at max current at 50 rpm is the same as the torque at 1000rpm... but the power at 1000rpm is 200 x as much. SO you may expect the motor to make 600W at 1000rpm (Lathe spindle) but only 3W at 50 rpm.... Except is isn't like that except in some imaginary unreal world.... because of linear and non-linear friction losses, electromagnetic losses, controller limitations, etc.. I can easily stall my lathe at 50rpm (torque) but cannot at 1000rpm... I think it is because of a thing called "back EMF" so the controller has to vary the voltage at various speeds and loads to keep the speed constant, so the non-linear voltage screws-up the simplicity that I have just explained.
I am sure some expert will correct me, on motors, controllers, etc, and also on cutting metal, size of cut, feed rate, shearing forces, lathe power, etc.
Suffice to say a lot of practice of both "feel" and "listening to the lathe" is worth far more than my explanation, so get in the workshop and make unimportant things (like at school) - Screwdriver handles, round door knobs, flywheels, scribers, punches, and anything else you can think of (that doesn't matter if it goes in the scrap bin).
Get in the workshop and do it! Better use of your time than reading my prattling.
K2
 
Found a belt tension guide...
https://simplybearings.co.uk/shop/files/chal_belt_tensioner_p168-169.pdfBut I don't think this covers Polly-Vee belts.
For tensioning Poly-Vee belts, it is relatively easy to measure tension by a calculation from the frequency of the note generated from the belt in tension. Load this App to your phone.
https://www.hutchinsontransmission.com/resource-center/apps/easy-tensionMeasure the centre to centre distance of the span, pulley diameters "mid-depth" of the grooves.
Or measure the belt-span from pulley to pulley at the tangential contact points.
With the data I think the app will give you a frequency of the note you should have when you Twang the belt. - Tap it with a spanner. It is "what we did" on the car production line I.E. tap the belt while using the frequency meter, then tension the belt until the correct frequency. Rotate for a full belt rotation, re-check/adjust. Doing 200 cars a shift, the lads knew exactly what tension to apply so when rotated it was correct.... But doing 1 belt you need the rotation and re-adjust, until it is correct. That was back in 1986~92...
But maybe the App is easier?
K2
I have downloaded the ap and will try later today .
It seems simple to use.
I hope it works

Thank you
Dave
 
I have what is essentially the same lathe, but built with simple Vee pulleys and belts and tensioner. It hasn't slipped (Yet!). I regularly stall the motor with low-speed thread cutting above 5/16 ths in or 8MM steel, or 3/8in Brass. So cut larger threads by hand power, not motor.
I am amazed the poly vee belts slip!
When cutting metal normally, I do not allow much drop of revs, as the speed control simply turns up the power against torque, until it reaches the max to maintain the selected speed. - Then speed drops, and it will cook electronics or motor after more than 5 to 10 mins at full power. A 600W motor is good for 0.010" cuts at most speeds and light feeds, but really small for proper metal removal as industrial machines work.
I strongly recommend you ensure the whole lathe is mounted on a STRONG (STIFF) base that is accurately set, as if not correctly set and shimmed level the base will distort the lathe when bolted down. e.g. a 50mm kitchen worktop will twist the lathe bed if not absolutely flat and true....
What are the dimensions of the poly vee belt? - If you bought the lathe NEW you will have a guarantee so can chase South Bend for advice?
K2
What I did with my Minilathe, I welded together a couple of 2" x 3" steel beams, got them as flat as I could, then poured epoxy where the lathe feet would be and placed the lathe in position, not bolting it down yet. When the epoxy had cured I bolted the feet down. I used anchoring epoxy, it seems a lot stronger/firmer than regular. Something like JBWeld would also be a good candidate for this situation.
 

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