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loply
07-30-2012, 07:28 AM
Hi folks,

I have an old Denford Viceroy lathe which I bought for cheap and have been restoring and ultimately plan to convert to CNC.

The headstock contains a layshaft which connects, via a backgear mechanism, to the spindle. The motor is underneath in the stand and connects to this layshaft via a linkbelt.

Unfortunately all of the gears (4 in total) in this backgear mechanism have one or more missing teeth.

I'm replacing the motor with a 3hp 3 phase unit and a VFD, so at the moment I'm planning on ditching the backgears completely and replacing the mechanism with a pair of taperlock timing pulleys connecting the layshaft to the spindle at a 1:1 ratio. The spindle is too close to the top of the headstock to take the main drive pulley, so I can't ditch the layshaft. I would install a mechanism (an arm with a pulley on a bearing) to set the belt tension.

Does anybody see a problem with doing this? I don't think I'll need the backgear any more, the motor has a stepped pulley anyway and I can get it down to 600rpm at full motor speed on the lowest ratio, then I can slow that down further with the VFD, which should provide good power still since the motor is so big.

My other reasoning is that the belt drive will be smoother and quieter than the gears anyway. Only trouble is the belt snapping would require spindle removal in order to replace, but I just thought I'd use a really wide belt to make it strong.

I'm confident about the above plan but being relatively new to this I don't want to bugger it up so thought I'd ask for advice first.

Cheers,
Rich

Abaker
07-30-2012, 09:09 AM
Should work fine. If you want to be sure you will need to find the torque capacity of all of the components. I know this info is available on the web, but my search skills aren't up to finding it.

One other thought - Why not use a poly-V belt(s) to drive the spindle? One or two of those can transmit a lot of torque, but less likely to break in the event of a crash. Could be tensioned in the same way.

loply
07-30-2012, 10:13 AM
Should work fine. If you want to be sure you will need to find the torque capacity of all of the components. I know this info is available on the web, but my search skills aren't up to finding it.

One other thought - Why not use a poly-V belt(s) to drive the spindle? One or two of those can transmit a lot of torque, but less likely to break in the event of a crash. Could be tensioned in the same way.

The spindle and the layshaft are relatively close, and the spindle is close to the top of the headstock casting, meaning the pulleys can be a max of about 60mm diameter (possibly even smaller).

In my experience V belts don't tend to like tight bends due to the deep profile, whereas a timing belt is flatter in profile and thus will take that radius with ease.

Also, to a certain extent, I don't want an unbreakable belt - I would be concerned about a serious stall bending the spindle. Given that the motor will be 3hp (it really only needs a max of 1.5hp) I don't like the idea of a V belt which digs in harder and harder.

Cheers,
Rich

Abaker
07-30-2012, 12:24 PM
I agree, that is way too small for a V belt. I was suggesting a Poly-V belt, which is a different animal. Over here we call them serpentine belts. It's a flat belt with a number of small Vs on the inside. Being flat they are much more flexible than traditional V belts. I don't know how unbreakable they are, but they use friction instead of tooth engagement like a timing belt. If you adjust the tension a bit in the light side then the belt will slip (screech like a banshee too) instead of break.

An internet search on "poly V belt" turned up this site among others.
http://www.goodyearep.com/ProductsDetail.aspx?id=3880

Besides I think spindles are a bit more robust than you do. Some riggers will lift lathes by their spindles.

lakeside53
07-30-2012, 12:28 PM
Remember that with your 3hp motor at full power with 600 rpm, at 60rpm output on the vfd you will only have about 0.3hp output. However, it's not really practical to expect your VFD to perform well at that output frequency. I find 12hz-15hz is pretty much the bottom end for any real torque. At 15hz your bottom speed will be 180rpm. Pretty high...

I also recommend Polyvee belts. Look for J-section, maybe 4-5 ribs. Don't try to use loose belts as a safety fuse! Your vfd correctly programmed will stop any overload in a heart beat!

atty
07-30-2012, 12:48 PM
I agree on the torque factor. I have a 3hp Baldor on my Logan, and while it's not VFD, but DC, I frequently have to switch to the backgear more for torque reasons rather than speed. I have plenty of power at 3hp, but there's just no substitute for a backgear.

lakeside53
07-30-2012, 01:20 PM
This is why you see huge motors on relatively small cnc lathes. A friend of mine has a 20hp motor (factory) on his 16x40, mostly for low speed torque. if you fit such large motors, definitely limit the power in the vfd!

Abaker
07-30-2012, 01:31 PM
Don't try to use loose belts as a safety fuse! Your vfd correctly programmed will stop any overload in a heart beat!

I never thought of that. I do kind of use the belt drive as a safety fuse, but it's on a 1938 Sheldon that was designed to run with flat belts at a max of 600 RPM. I'll take it to 900 if I'm feeling like talking a walk on the wild side.

That's why I like this place, I learn so much new all the time, I don't even mind getting schooled.

Forrest Addy
07-30-2012, 03:35 PM
I just betcha you can find stock gears with the same pitch and close enough tooth counts to replace the existings gears. You will have to modify them perhaps boring out the replacement gears and turning down the existing gears so one be can slipped over the other and be retained by pinning, anerobic bushing locker, or drilling and tapping at the joint to insert a screw. The ratio may not be exact but it will be close.

As others have mentioned, you cannot simply turn down the motor speed and expect decent stock removal at low RPM unless your spindle motor is truely large. For constant HP, RPM is inversely proportional to torque. Thus mechanical advantage in the form of a multi-step mechanical transmission is necessary for machine tool spindle drives. If you employ a VFD and three phase motor you can vary the spindle speed over a wide range and take light cuts simply by dialing the speed. However, if you wish to remove significant stock you have to emply a huge motor whose RPM you can vary or train the multi-speed mechanical drive.

In the home shop where older machine tools are retrofitted with DC/SCR drives or AC/VFD, major RPM ntervals of the original mechanical transmission have to be retained.

I suggest you fix the backgear either along the lines I suggest, obtain replacement parts, or some other remedy like maybe an extra large V belt drive.

loply
07-30-2012, 05:13 PM
.

As others have mentioned, you cannot simply turn down the motor speed and expect decent stock removal at low RPM unless your spindle motor is truely large.


As mentioned though the main pulley between the motor and layshaft allows 4 different ratios which gets me 600rpm at full motor speed in the lowest ratio, so 300rpm at half motor speed, and so forth.

Since I'm converting to CNC anyway and nearly always use carbide tooling I'm happy with this. 300rpm @ 1.5hp (roughly) should be fine.

What I'm more concerned about are potential negative effects of using belts instead of gears, eg longevity, uneven wear on spindle bearings, etc, none of which I know much about.

Cheers for the advice folks.
Rich

The Artful Bodger
07-30-2012, 06:16 PM
I suggest you repair or replace the damaged gears, you will get more satisfaction from your restoration and all options will be open to you.

It is interesting to see what www.lathes.co.uk has to say about your lathe:-


Viceroy machine tools were rarely advertised in other than the educational press and are still little known, even in their country of origin; consequently, the market under-values them and, if you want a neat, strong, very compact lathe with a generous capacity at a bargain price - a Denford Viceroy may be the machine to look for ..

Tony Ennis
07-30-2012, 10:27 PM
I have a VFD on my lathe. Even so I was happy to have back gears when I was turning some cast iron. I could keep the motor RPMs up and gear down the spindle for some torquey goodness.

J Tiers
07-31-2012, 12:16 AM
As mentioned though the main pulley between the motor and layshaft allows 4 different ratios which gets me 600rpm at full motor speed in the lowest ratio, so 300rpm at half motor speed, and so forth.

Since I'm converting to CNC anyway and nearly always use carbide tooling I'm happy with this. 300rpm @ 1.5hp (roughly) should be fine.



Might think differently with some tool steels or other nasty materials..... carbide at 3x HSS speeds might need lower rpm for larger pieces, .... you'd want some serious power, but at 1/4 motor speed, you'd have 1/4 motor power.

The other advantage I think was mentioned, which is that the belt is not as good for raw low speed torque, may slip etc..... gears on the OUTPUT side (to spindle) give a torque MULTIPLYING effect, instead of a power loss.

I have a small horizontal mill which has a 1/4 HP (!) motor. Your first thought is that might be OK for a small slitting saw, but weak otherwise.... But I have back gears and a good belt reduction..... That thing will chew a slab mill through steel without slowing down.... I am more afraid of breaking something than the motor stalling.

The gears can be redone by shrinking on blank hoops and re-cutting, or by putting on a modified stock gear..... the modified stock might have issues with web thickness vs the width of "landing" that it goes onto..... but a blank rim that you cut is makeable to any size/thickness you want.

loply
07-31-2012, 06:49 AM
Thanks for all the replies folks, I appreciate the comments about retaining the back gear.

Unfortunately I'm not confident about my ability to make new gears - they are quite big and relatively complex, and I'd need the lathe to make them on! Even acquiring stock for them may challenge me.

This may seem like an outlandish idea but I have a possible alternative I'm considering.

I've had an idea for a while of a selectable belt drive system whereby I would put two complete sets of pulleys (of different ratios) on the layshaft on bearings so they are free to rotate independent of the layshaft, then I would 'engage' one or another to the layshaft by means of a sliding dog gear which is keyed to the shaft. This is much like how the current system works.

The pulley pair which is not in use would harmlessly rotate at whatever speed was neccessary, but wouldn't be in sync with the layshaft as the dogs aren't engaged in it.

The reason I'm considering this is because:

1) I'm more comfortable that I can modify four cheap, easily acquired pulleys with the dog holes than trying to make four big gears

2) I like the idea of replacing the gears anyway as they are noisy and may introduce a 'vibe' to the surface finish as they look quite coarse

I hope this doesn't seem like a daft idea but would appreciate any comments on it.

Cheers,
Rich

The Artful Bodger
07-31-2012, 05:16 PM
Rich, I am sorry you are not going to try to make/repair/replace the gears as I well recall the satisfaction I got when I managed to make some change gears for my Drummond lathe.

I think your scheme with belts and pulleys may, or may not, be a good idea and I suggest you make sure you have considered all details before starting modification to your lathe.

You might also consider making plastic gears. These are quite easy depending upon the plastic. I have made a few for light duty work. The technique is to fit a gear of the required gear family in your lathe spindle and mount your plastic blank on an axle fixed to your tool post. Then start the lathe and press the blank against the spinning gear, it will be very noisy (rumble) at first but as the teeth begin to form the noise will subside, then you increase the pressure some more and so on until the teeth are formed. A hot air gun playing on the edge of the plastic blank helps the process. Just a thought.

John

Forrest Addy
07-31-2012, 09:55 PM
Let me repeat a portion of my previous post where I pose a solution:


I just betcha you can find stock gears with the same pitch and close enough tooth counts to replace the existings gears. You will have to modify them perhaps boring out the replacement gears and turning down the existing gears so one be can slipped over the other and be retained by pinning, anerobic bushing locker, or drilling and tapping at the joint to insert a screw. The ratio may not be exact but it will be close.

...

You don't have to make new gears. Find suitable stock gears and mod them and the existing back gears. You get new gear teeth on the old hubs. Be sure the tooth counts etc work out to center distances in reach of the back-gear eccentric.

You are proposing a CNC retrofit. If your retrofit is to be successful it must have the range and capacity to justify the time and expense taken to execute it. Your assumptions and numbers have to be analyzed and verified. Proposals and assumptions first analysis and numbers after. Modify the proposals and assumptions and repeat. This is the feedback model of the design cycle. Omit it at your peril.

atty
08-01-2012, 12:10 AM
I fully concur with the gear making. It's not nearly the daunting task as it might seem, and you'll feel good when you crank off the first one. It's sort of a rite of passage that every machinist should go through anyway. You'll be glad you did.

loply
08-01-2012, 07:15 AM
Hi folks,

Thanks for all the advice.

I have been trying to price up replacement gears from stock this morning, but it looks like most suppliers want around 35 for the large gear alone, meaning the whole set would cost me 100-120!

It would be nice to replace the gears for satisfaction but I will have to consider my options as it certainly seems (at this stage in my musings!) to be quicker, cheaper, quieter and smoother if I go to belts, which is a compelling series of arguments.

One of the few complaints about these lathes is the noisy headstock gears, which I don't imagine help the surface finish, let alone if they've been cut by a newbie using scrapbinnium!

I'll take into account the comments above and try to post some photos once I've solved it either way!

Cheers,
Rich

Abaker
08-01-2012, 09:11 AM
Yes, please keep us updated. The multi-belt sliding dog clutch ideas sounds interesting and I hope to shamelessly steal your idea. I hate to stop and change belts, so a drive with 2-4 ranges along with a variable speed motor would be HSM heaven.

Sheldon used a similar idea in what they called the Udrive. They had two pairs of lever shifted pulleys in sequence to get a 4 speed drive. They put the clutch mechanism inside the pulley hubs.