Milling machine operation

Hi Everyone,

I have two questions regarding the milling machine about which I would like some advice:

A I want to mill grooves or keyways along a piece of 40mm round. The workpiece is mounted in a 3 jaw chuck fitted to a rotary table,mounted vertically ,so the work is horizontal,the overhang supported by a tailstock. The cutter is 5mm two flute and the machine is a Chester Lux.

What I want to know is,how deep a cut can I take at each pass,using powerfeed.?

B i am also experimenting with slotting saws,which I find do not always run true [I think someone mentioned this before ]. I find that they tend to cut on some of the teeth but not on all of them. I have clocked the spindle and the saw holder and both are running true,so I presume the error is in the saw

Nothing I can do about that,but how deep a cut should I aim for,using a 3" dia x3/16" thick saw? I have tried a speed of 100 rph,just to see how it went. Should I increase or decrease ? I forgot to mention that the material in each case is mild steel,nothing fancy. I have not yet tried using powerfeed for the saw,just feeding by hand.

Any advice ,of which there is loads out there,much appreciated.

Thanks in advance.

For the 5mm cutter I'd be running a DOC of 1.5 to 2mm.

Whatever you do, on any machine, slitting saws never run true. And in my experience on anything other than a horizontal they have a tendency to go walkies and not cut straight.

A cutter speed of 100rpm is on the top limit for a HSS steel cutter and low carbon steel. I'd back off a bit, say 80rpm. Depth of cut depends on whether the cutter is fine tooth or coarse tooth. In this picture they're coarse tooth except bottom right and possibly bottom middle:

If the cutter is fine tooth only a shallow DOC will do, say around 1mm. For coarse tooth DOC depends on how much power is available; I wouldn't be worried about DOCs in the 10-15mm range.

Andrew

Thank you Andrew, that,s the kind of info I wanted. Looking at your picture,my saws are fine tooth

I have no real need for saws right but I just wanted to know a little more about them.

Thanks again for your reply,

Larry.

Hello Andrew,

I have found the hard way, that slitting saws tend to wander. This being so, are there any applications for them where they can run true? If not why on earth are they made?

This isn't intended to be a smart a**ed enquiry. Just curious and feeling slightly annoyed at my lack of success!

Andrew.

I slot 3/8" deep in 303 Stainless Steel. I use 3 passes of 1/8". Width of cutter is .062. I use a small horizontal mill.

To Andrew Tinsley,

I often wondered that too. In all the saw blades that I used in my woodworking,and I have used many,I never,ever came across one that cut on only a few teeth. I have used top grade blades [cost a fortune ] and blades from Aldi/Lidli and while the latter are not of the same quality,or price,they all run true and all teeth cut.

So,if they can do it for woodwork saws,why not for metalwork ones? It,s enough to put you off bothering with them.

Larry, I would guess that the feed per tooth in wood is hugely more than in metals, so a small eccentricity is noticeable in slitting saws. Plus, in my experience, horizontal mill arbours (where I have used slitting saws) are always bent to some degree.

Bill

Plus at a few thousand revs you won't get that ching, ching, ching sound of an slow eccentric slitting saw.

I think they can wander if teh tram is out on the mill and as they have no set will just get pushed off more and more once they start to wander.

Thanks to all,this is all new information to me,and thankful for it.. I just wondered if I was doing something wrong [not unusual for me]. As was pointed out the difference in speed between wood and metal saws is something which needs to be taken into account. My experience is more with the former.. It,s just handy to know something about these things before having to find out the hard way.

I live and learn.

There's a difference between eccentricity and the tendency to wobble, and to wander. Due to tolerances a slitting saw will always run slightly eccentric, but can still run without side to side wobble. One way of reducing the effect of eccentricity is to up the feedrate. If the saw is 1 thou eccentric and the feedrate is 2 thou per tooth then the load per tooth varies by 100%. Increase the feedrate to 5 thou per tooth and the variation is reduced to 25%.

Taking shallow cuts is no problem. But on the vertical mill (Bridgeport) with deeper cuts the saws always seem to wander slightly. I'm not sure it's to do with tramming. My mill is usually pretty well trammed. I suspect it's more to do with slight flexibility of the spindle and arbor causing the saw itself to flex slightly. Which causes it to wander off and so on.

On my horizontal mill with a larger diameter arbor and outboard support near the cutter I don't have an issue with the saw wandering. I'll quite happily cut 25mm or more deep without a problem. Presumably that is because the setup is much stiffer.

Andrew

Andrew is correct. If one considers the difference between saw blades and slitting saws, things may become more understandable. Woodworking saw blades are always secured between machined dished clamps, thus making the blade far more rigid than otherwise. They also always leave a kerf somewhat wider than the thickness of the blade.

Narrow, fine toothed slitting saws would be much better supported between clamps supporting the tool as close to the outer edge as practicable, but this is not the usual case, particularly as many are driven by the fitted keyway rather than dished clamps like a wood saw blade.

I have experienced saw blades when they are overheated and bend under expansion! The same is likely to occur, to a lesser extent, with slitting saws.

I have made 22mm and 16mm arbors for horizontal use. I am confident that the 22mm one is not quite as well finished as I could have achieved. While using gear cutters, I have found I can reduce the concentricity error by turning the cutter on the arbor. A good job, perhaps, that it does not have a keyway and key!

I don't have any slitting saws and they probably wouldn't last very long if I did. But one thing I've know is that they have cutting edges only on the circumference ie can only cut radially and not axially. In contrast, circular saw blades for wood usually seem to have teeth (often brazed on carbide teeth) that can manage side cutting (axial) as well as radial. You can even cut sideways (at shallow depth), even though they aren't designed to. And of course, the wood is much softer than any metal. So it's not likely the wood could force the blade to deviate from its default position. With a slitting saw, there's no way for a blade to correct if it starts to deviate. I suspect that's part of the answer.

Murray

Do all slitting saws have zero tooth offset? If so, presumably they are stackable to obtain the required width of cut?

I asked RDG the question recently and they did not really have a clue. In the end I purchased just one, of approximately the width I needed, instead of two of lesser thickness, but of a much better total thickness.

I have had good success with a slitting saw. This one has had a lot of use, mainly cutting bronze and brass but also has been used to cut steel. Slow speed, light cuts and lubrication to overcome friction and heating.

Paul.

One readily identifiable cause of wandering on a narrow slitting saw is if the cutting edge of the teeth are either worn or incorrectly ground, such that they are not precisely at a right angle (i.e. exactly parallel to the arbor). If there is thus a "slope" to the cutting edge there is a small resultant force vector that will cause the saw blade to move 'sideways'. I had a 1/64" fine tooth saw that had perhaps hit a hard spot and incurred slight damage this way. This saw subsequently exhibited a sideways bias, even on the horizontal mill. Close examination highlighted the problem and a re-sharpen resolved it.

Similarly on my bandsaw, could not make a straight cut. Even after adjusting the guide rollers and eventually replacing the rollers, could not make it cut straight. Changed the blade and back to normal with no wandering. Close inspection of the blade showed a slight sideways bend in some of the teeth.

That's why you set the fence to the track of the blade on a bandsaw, the guides don't affect teeth that may be bias to one side.

There is a slight hollow grind on a lot of slitting saws so not sure I would want to stack them face to face once a bit of swarf gets in between mor ewill follow and you will soon get a wider kerf. Nothing wrong with gamng cutting using several blade sand spacers though.

To Not Done It Yet, and Muzzer, Good morning,

Yes,I too have seen blades buckle and twist due to overheating. In fact the steel blade supplied with my Skil saw did this from day one. Since I was cutting mostly chipboard and MDF,this was a non;starter,so I bought a tipped blade,which cost almost as much as the saw ! This was back in 1975 and I still have and use the machine !

Tipped blades give no trouble if you keep them sharp,when they get blunt,the timber will dictate to the blade and the cut goes all over the place. The teeth do have a set to each side in order to give clearance to the body of the blade,otherwise a deep cut would be out of the question since the blade would just jam in the cut.

You can cut sideways to a certain extent,useful for making coving if you dont have a spindle moulder,did it many times myself in the early days. Dont think that would work with a slitting saw !

What I found so odd about slitting saws was that they all seem to cut with only a few teeth and to me that seems to indicate that they are bored off-center rather than anything else,although I cannot see why this should be.

I must say a great deal of info has emerged from this post,for which I thank everyone.

PS I am not trying to teach anyone how to suck eggs,just relating my experiences with wood machining. Milling is another ball game,for me anyway !.

The eccenricity has been aptly explained, particularly by JB. One should not compare 4000rpm with less than 100, or feed rates of metres per minute with mm/minute.

I see no particular connection with an overheated saw blade and the machine that was driving it. I doubt you still use that particular saw blade! Woodworking saw blades often have stress relief slots to avoid distortion of the blade as it warms.

With slitting saws there is tolerance of drive arbor, clearance of blade on the arbor and the concentricity of the actual saw. It does not need large tolerances to make observation of these obvious.

The slitting saw may well still be cutting on all, or nearly all, teeth - but rather more on some than others. The obvious ‘it’s only touching on a few teeth’ is usually only actually checked with zeo feed rate, which is not the ordinary operation. The difference in noise, while actually feeding, is likely only that some teeth are cutting more material than others. Clearly, one must set the feed rate for the most prominent teeth, not the least. That may actually halve the rpm - and make the cyclic difference in sound even more pronounced.

Concentricity is unlikely to affect wandering of the cut.

How would a change of feed rate affect the rpm of the saw? might half how fast you turn the handle but speed would be constant.

Assume we are not talking about a heavier cut bogging down the machine but reducing the spindle speed by gear/belt/vari

Also just checked some slitting saws and all wre hollow ground about 4-5 thou thinner in te middle than at the cutting edge so you would distort them if you tried to clamp 2 or more directly together

Edited By JasonB on 29/06/2018 09:23:00

Edited By JasonB on 29/06/2018 09:29:51