Small mill periodic vibrations?

Started this thread to get suggestions as to why a second hand bench top horizontal mill is vibrating periodically - when offering up a slitting saw into a bar the cutting action is not continuous from the metal squeaking being obviously periodic (hope this description makes sense!)

I'm waiting on buying circlip pliers to properly disassemble the pulley mechanism in order to work back in stages to isolate the cause of the vibrations.

I can rule out the motor since in runs perfectly in isolation. Is it likely to be the knackered vee belts (there are some rough patches on one of them)? I read somewhere that replacing old vee belts on Myford lathes can cure similar periodic rubbing.

The simplest and first thing to check is if the slitting saw is perfectly centred on the mandrell. Then does the mandrell run true.

That's perfectly normal, no machinist that ever lived got a slitting saw to cut evenly on all teeth. If the cut is causing a squeak then it may well be that some teeth are rubbing rather than cutting. Up the feedrate and see what happens. Use at least 4 thou per tooth and take it from there.

Andrew

Do You mean 0.1mm per tooth?

This gives 8mm per rev on my best 100mm slitting saw.

I am with Andrew on this, it is just the effect of the slitting saw running out. A case of they all do that sir.

There was a machine in one of our bays that was used for sharpening cold saw blades (think about 10-12" diameter when new). It used a cam system to push the blade round one tooth then move the grinding wheel in to sharpen a tooth before repeating the process. The problem with this was that after a number of sharpening cycles the blade was getting more and more off centre. in the end the machine was scrapped and the blades are now sent to an original blade supplied where they are sharpened by a CNC controlled system that maintains the concentricity for the life of the blade. Typical batch size sent for sharpening is 40 blades.

Martin C

Generally you also want to be running a slitting saw at quite low rpm if you want the teeth to last. Think of a 100mm dia saw as though you were turning a 100mm bit of material in the lathe.

Tooth count also plays a part a lot of people will use a fine blade to cut through thick material when less teeth and a bigger gullet works better particularly in material like aluminium

As the others have said I have never found a true slitting saw and get that ching-ching-ching sound as it cuts

Yes I do. Your figures imply 80 teeth on the cutter. I would regard that as a fine pitch cutter, not really suited to anyrhing other than shallow slotting. Here's a picture I took earlier:

The definition isn't clear cut (pun not intended, but couldn't resist it) but the slitting saw bottom right is fine pitch. I only use them for shallow slots, as on screw heads. The problem with deeper cuts is that the gullets get jammed with the swarf; equals a broken cutter. The other slitting saws shown are coarse tooth and will happily cut a deep slot in one pass. If I have a deep slot to cut I'll use a DOC of at least 10mm per pass.

As Jason says slitting saws won't be running fast. So if we take a coarse 100mm slitting saw with 36 teeth in mild steel it'll be running a bit under 100rpm. At 0.1mm per tooth that gives a feedrate around 360mm/min. In practice I run slitting saws slower than calculated, even with flood coolant there's some steam produced. I'd run a 100mm cutter at 60rpm and a proportionate feedrate.

Andrew

Regarding the original posters actual question, he described the problem as 'periodic vibration' although the example he gave was of a 'once per rev' eccentricity of a cutting blade.

My take on the problem was that a regular variation in running speed (or just running smoothness) caused by harmonics in the drive system. The fact that belts, rather than belt, was referred to in conjunction with poor belt condition does create a situation where there are cyclic changes as the uneven belt changes pass in and out of phase.

I too have never found a slitting saw that is truly concentric with its central bore, even when I have turned an arbour in situ and then immediately fitted the blade the cutting action is not equal on all teeth.

Ian P

Yes thanks for directly answering (though the other posts were useful!). I'll take some photos later and add them here. There are two pulleys.