V-belt vibration in Ajax AJ8 horizontal milling machine

[AJAXParticipant @ajax]

Posted by Andrew Johnston on 01/11/2019 11:43:40:

On the assumption that the machine didn't vibrate when new the question is why is it doing so now? Poly V-belts may be better, but they're a red herring while the source of the vibration is unknown. A few ideas are:

A 10mm deflection for 10N (1kgf) force seems pretty slack to me, especially on a long unsupported belt. None of my V-belt driven machine tools have a vibration issue, and none of them have idler pulleys.

The second belt looks a bit odd; is the top pulley in line with the intermediate pulley or should the belt be one pulley further out at the top?

While it's unlikely to be the problem a single phase motor is not as smooth as a 3-phase one. It might be that the pulsing characteristic of the single phase motor is exciting something it shouldn't.

Would we be correct in assuming that the vibration only happens when a cut is in progress? If so what is the feedrate? A good starting point for chip load is 4 thou per tooth. When I started using my horizontal mill I used a much smaller chip load, and the whole machine vibrated. My horizontal mill weighs nearty two tons.

What rpm is the cutter running? Looking at the speed chart the lowest value of 150rpm is on the fast side for a 5" cutter in steel.

Andrew

If the current setup allowed a small increase in belt tension I would do so, but as you are now aware there is no means to do so other than changing the belt. Rightly or wrongly, I feel the belt tension is about right apart from the problem of belt vibration/oscillation at higher speed which I'm starting to believe is due to other problems (e.g. feed rate or unsuitability of the cutter).

I doubt changing to a 3 phase motor will fix this problem but I'm happy to convert as I already have a spare VFD. It might also give me some flexibility over spindle speed that I lack through the very limited pulley options.

Regarding the current setup, the pulleys have been chosen to give the slowest spindle speed and best belt alignment.

The vibration only happens when taking a cut using the full width of a 5 or 6" x 1/2" cutter. I have *many* cutters that were included with this mill but most of them are clearly too big for this machine, including slab mills, etc. However, I have enough smaller cutters to know that I can take decent cuts with no vibration. I should point out the 5 and 6" cutters work well (with no vibration) when I can progress the machining on the y axis taking no more than about 2mm at a time.

I will probably be getting rid of the unneeded tooling in due course. A couple of items have already been sold, so the machine has only cost me £60 plus the motor mount purchased new.

The speed chart is something of a red herring as the motor and motor pulley are not original. 120RPM is about the slowest with the current setup and I now understand that may be somewhat high for a larger cutter on this size machine.

Thanks for all your help,

Brian

[AJAXParticipant @ajax]

Posted by Nigel McBurney 1 on 01/11/2019 19:27:26:

Now there is a picture,the mill small and not up to 5 by 1/2 cutters, where the arbour enters the arbour the diameter appears reduced so to myr eckoning ,the taper is probably 2 MT,too small for a large cutter ,now when milling the golden rule is support the arbour, unless the work or the vice gets fouls the arbour or the column , get the cutter as close to the column is possible and then amount arbour support as close to the cutter as possible. on big mills and heavy work,more than one arbour support is used either side of the cutter, and then there is usually a tie bar from the top arbour support down to the knee,.Just think,that mill is about eqvivalent to a 5 inch lathe say a Boxford, would any one try using a parting tool a half inch wide !! because that is what is being tried on this small mill.with the wide cutter. This is not a belt problem ,apart from requiring a means of tensioning, its just hammering a small machine to death,plus the speed range is far too high,4 inch dia cutters about 3/16 or a 1/4 wide should be its maximum, capacity,in ally or non ferrous and a bit less in steel.

The arbour is 3 MT which is handy as it matches my lathe and ER collet set if I decide to use end mills.

I much appreciate your observation about supporting the arbour and how larger horizontal milling machines are used. I never planned to buy a horizontal mill but this one was too good to pass by and I enjoy a new (old) toy to play with. If I ever find time I may try converting it to a vertical mill.

"would any one try using a parting tool a half inch wide !!" — good point!

Anyone interested in buying a job lot of large milling cutters?

[AJAXParticipant @ajax]

Posted by Alan Jackson on 01/11/2019 16:26:59:

I had an Ajax mill like yours and rebuilt it. The existing pulley had lumps broken from it by the previous owner so I machined the pulley down for polyvee belts. I also repositioned the vertical feed shaft so that it could be fitted with a handwheel.

Edited By Alan Jackson on 01/11/2019 16:29:37

Thanks for posting the photos – it's nice to see other examples of this (and similar) mills to see what others have done. The motor pulley was in poor condition on my machine but that was easily replaced and the others are all in good condition. I think the jury's out on whether it would be worth my while changing to poly-v belts and I should try different cutters / lower spindle speed first.

Out of curiousity, did you have a quick return lever on your machine? Mine was missing the lever and I machined a replacement but haven't really found it necessary. I see you have a t-nut / stop mounted to the front of the table – was this for a power feed add on or simply to limit travel?

[AJAXParticipant @ajax]

An updated video which may answer some of the issues / questions that have been raised in this thread. Thanks to everyone who has taken the time to read my posts and offer their advice – it is hugely appreciated.

[Anonymous]

Posted by AJAX on 01/11/2019 20:07:36:

Anyone interested in buying a job lot of large milling cutters?

Could be; depends upon cost, cutter type, condition, bore size and location. I'd be particularly interested in slab mills.

Andrew

[SillyOldDufferModerator @sillyoldduffer]

Posted by not done it yet on 01/11/2019 12:35:05:

“except to claim the prize for spotting the typo on that belt-settings plate”

And, according to the plate, the top belt should be aligned with the “E” pulley? So someone has moved the top pulley outwards, to achieve the lowest speed possible? Or is the top belt mis-aligned? Plot thickens.

I missed NDIY's comment, sorry NDIY, but has he spotted the problem? Is the top belt fitted correctly to a pair of aligned pulleys? If not, it's running at an angle, which might well explain why the belt vibrates.

Ajax's photos and video seem to confirm the belt is misaligned. First, the info plate strongly suggests belts should be fitted in vertical alignment:

Second, this photo shows the belt is on the inner top pulley and the outer intermediate pulley, ie it runs on a diagonal.

I suspect the original motor was mounted inside the plinth with its 3-way pulley positioned directly under the others. All the pulleys would have been aligned and spaced so the belts could be fitted as per the plate. The position of the replacement motor doesn't allow that perfection, so the new top belt has been fitted to drive the spindle at reasonable speed but not in correct alignment. As the belt is flexible, running sideways may not matter much. I'd expect it to wear faster than an aligned belt and to vibrate, but the arrangement obviously transfers enough power to drive that big 5" cutter!

Dave

[AJAXParticipant @ajax]

Posted by AJAX on 01/11/2019 08:41:00:

The intermediate jackshaft is a fixed stub welded to the milling machine base. It appears to be original and offers no form of adjustment for belt tension. The belts are currently set up for best alignment giving a driven speed of approximately 120 RPM. With a 6" cutter that equates to a cutting speed of 57.5 ms-1 which does not seem excessive.

Correction: I should have said 57.5 metres per minute (not second!) but I think you realised that.

[AJAXParticipant @ajax]

Posted by SillyOldDuffer on 02/11/2019 09:40:47:

Posted by not done it yet on 01/11/2019 12:35:05:

“except to claim the prize for spotting the typo on that belt-settings plate”

And, according to the plate, the top belt should be aligned with the “E” pulley? So someone has moved the top pulley outwards, to achieve the lowest speed possible? Or is the top belt mis-aligned? Plot thickens.

I missed NDIY's comment, sorry NDIY, but has he spotted the problem? Is the top belt fitted correctly to a pair of aligned pulleys? If not, it's running at an angle, which might well explain why the belt vibrates.

Ajax's photos and video seem to confirm the belt is misaligned. First, the info plate strongly suggests belts should be fitted in vertical alignment:

Second, this photo shows the belt is on the inner top pulley and the outer intermediate pulley, ie it runs on a diagonal.

I suspect the original motor was mounted inside the plinth with its 3-way pulley positioned directly under the others. All the pulleys would have been aligned and spaced so the belts could be fitted as per the plate. The position of the replacement motor doesn't allow that perfection, so the new top belt has been fitted to drive the spindle at reasonable speed but not in correct alignment. As the belt is flexible, running sideways may not matter much. I'd expect it to wear faster than an aligned belt and to vibrate, but the arrangement obviously transfers enough power to drive that big 5" cutter!

Dave

Dave, thanks so much for your thoughts on this matter. I posted a video which hopefully shows the top belt is currently in alignment. However, it is ONLY aligned with the current (and slowest) pulley selection which happens to suit my needs. The whole setup appears a bit strange as there can only ever be one pair of correctly aligned pulleys, and there is no means to adjust for top belt tension. However, there is no sideways belt deflection.

You are correct in thinking there used to be a motor mounted inside the plinth. Access was very difficult and the motor old, so I replaced it with a newer motor on a sliding rail mounted outside of the plinth. There appears to be a good market on eBay for vintage motors so that gave me some more money back on what I had originally paid. Judging by the number of old screw holes (since filled up) this machine has seen several motors or attempts to get the belt tension just right. At least that problem is sorted and I can use the plinth for storage / adding extra mass / or possibly a coolant system. The rails and motor foot allow me to move the motor in just about any direction, so if the pulleys need changing / flipping that won't be a problem at all.

[Howard LewisParticipant @howardlewis46836]

Two thoughts

1 As a cheap try, fit new belts.. Belts should run in line from pulley to pulley, not at an angle. (My mill shredded a belt, within 6 months from new, when the motor pulley was misaligned with the intermediate pulley. Having corrected that, the replacement belt has survived, apparently undamaged, for over 15 years. )

2 If you can fit the FGH pulley the other way round, it should be possible to obtain slower speeds, (Or is that the typo already mentioned? )

Howard

[Anonymous]

Posted by AJAX on 02/11/2019 10:54:59:

I should have said 57.5 metres per minute (not second!) but I think you realised that.

Indeed, but it's still pretty fast. It equates to 189fpm, which is close on double the recommended speed for HSS in mild steel. I reckon there are 22 teeth on the cutter. So at 4 thou per tooth times 120rpm that equates to a feedrate of 10.56 inches per minute. If we assume the table feedscrew is 8tpi that's about 0.7 seconds per turn of the handle. That's going some!

From the video the belt looks pretty tight, so I think I'll modify my advice and say too big a cutter and/or running too fast and/or too slow a feedrate.

Andrew

[AJAXParticipant @ajax]

Howard,

The belts are aligned. I have tried swapping the belts with other spares that I have, but I'm not averse to buying a new belt or two just in case. However, the consensus appears to be to use a smaller / less aggressive cutter to cut down on vibration.

The current pulley configuration gives me the best belt alignment and slowest speed. A slower speed is not possible without a new pulley(s) or changing the motor speed, both of which are likely upgrades.

[Howard LewisParticipant @howardlewis46836]

Having looked back at the pictures, the top pulley is the way round that i suggested, so the cast plate seems to be wrong.

My thoughts would be to reassess the orientation of all the pulleys to ensure that the range of lower speeds is available without running belts other than in line..

Also, don't over feed the work into the cutter.

For a side and face cutter like the one shown, on a small machine the feed per tooth is likely to be not much more than 0.005"/ tooth, possibly less. Count the teeth, multiply by the feed / tooth and feed accordingly.

The intermittent cutting action will cause the belt tension to vary, which is likely to cause vibration. Over feeding will make it more apparent.

Howard

[AJAXParticipant @ajax]

OK, probably my last update on this thread and hopefully this is now drawn to a conclusion. I replaced both belts again,but this time with brand new v-belts. Paying attention to earlier advice, I went for slightly increased tension of the top belt by choosing a smaller size. Still no option for fine adjustment of belt tension. I made as trial cut using a small cutter and belt vibration was much diminished. I haven't had a chance to do any more machining but it looks likely to be much better. If I want smoother than this I will have to get round to covering to a vertical head. Many thanks for all your help, Brian.

[oldveloParticipant @oldvelo]

Hi Brian

"Still no option for fine adjustment of belt tension". It is possible by Inserting "Ü" shape shims around the hold down bolts Between the base and the mill.

Eric

[AJAXParticipant @ajax]

I know it has been some while since I last posted to this thread, but I thought it might be useful giving a quick update. Firstly, I should point out the machine has not seen much use over recent months due to other projects, family and work commitments.

In the last week or so I finally got around to changing the single-phase drive for a 0.55 kW 8-pole 3-phase motor. I already had a spare VFD and knew this would give me the option to reduce spindle speed without replacing the existing v-belt pulleys. Selecting an 8-pole motor bumped the price up somewhat (cost me more than the milling machine) but gives a motor speed of 680 RPM at 50 Hz and a spindle speed of around 60 RPM. It gives a very smooth and slick performance.

The motor performs very well from 20 Hz to 60 Hz (I haven't tried outside this range) and has absolutely transformed the ability of this machine. I have taken a few test cuts of mild steel with a 5" cutter and it is now a pleasure to use. The machine now sounds right when cutting and belt vibration is no longer apparent. It's still not a heavy duty machine but absolutely fine for my needs.

The motor should be okay up to 75 Hz (1020 RPM) so that will give me some flexibility when using smaller cutters or when machining aluminium.

I hope that is of interest to someone.

Edited By AJAX on 28/10/2020 00:08:26

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