Fly cutters,small mills, a few thoughts.

[samuel heywoodParticipant @samuelheywood23031]

I’ve not had great sucess experimenting with flycutting using a X1 mill but recently had a project to do requireing my first serious use of a flycutter.

I may write that project up latter.

A few thoughts, hopefully helpfull for those that know EVEN less than me!

Firstly, whilst i regularly  throw caution to the wind and climb mill using endmills, the little x1 really doesn’t seem to like climb milling with a flycutter, conventional milling seems to be the way to go.

A quick google search will probably throw up something like” a flycutter will render a flat surface if the mill is trammed.”

I don’t know about you, but i’ve never quite been able to get my mill trammed perfectly~ close, but always slightly out in one axis or the other, & in any case, with a small, lightweight mill there will be some deflection under load.

Got me thinking about this a little~it’s good when you actually think things through, rather than blindly accept statements as fact.

Hopefully i’ve thought this through enough~ IF NOT< SOMEONE PLEASE CORRECT ME.

Here’s my conclusions.

If your mill is out of tram in both axis’ it will still render a flat plane, however this plane will not be parallel to the reference plane (ie the mill table / vise)

If you then flip the workpiece over to fly cut the opposite surface, the error will be compounded?(doubled), however if you also rotate the workpiece 180 deg, the error will tend to cancel out, but assuming it started as a cubeoid, it will be more of a rhomboid.

However if you have tram in one axis & use the opposite axis for traverse you should end up with a flat surface parallel to the reference plane.

thus if your mill is in tram back to front, & you use the x axis handwheel to fly cut (which would be the more usual way of working) all should be ok.

 

Now, have i got the right end of the stick or do i nead to re-read “Flatland” by Edwin Abbot?~ which may hold some appeal to those of you wont to contemplate the multidimensional universe.

 

[samuel heywood]

On 3 August 2024 at 20:57 samuel heywood Said:

If your mill is out of tram in both axis’ it will still render a flat plane, however this plane will not be parallel to the reference plane

Wrong, it will leave a curved surface on the X-axis. Think about what would happen if you tilt the head over at 45 degrees and then run an endmill across the workpiece. It will leave a curved groove. Same thing happend with a flycutter and the head slightly out of tram; just that the curve is imperceptible.

Andrew

[samuel heywoodParticipant @samuelheywood23031]

Ok thanks for that Andrew, ~Ok, back to school for me.

Care to elaborate on anything else i’ve said, being wrong  is not a problem, finding the truth of a matter is the goal, & ultimately helpfull to all i think.

[HuubParticipant @huub]

If you flip the part, the error will not change because your part is still lying on the same parallels.

To reduce the tramming error, you can use a smaller fly cutter.

To reduce the bending error you can use a smaller fly cutter and take shallower cuts.

To compensate for tramming errors, you can make parallels from mild steel and mill them flat in the position they are used. That will compensate tramming errors and errors from your vice. It will not compensate for deflection during facing/milling.

 

 

[JasonBModerator @jasonb]

On a small mill (X1?) don’t go too big with the diameter that the cutter swings, maybe 40mm max so you can get the spindle speed up a bit and that also allows for a faster feed rate as it is hard to feed consistently at a low rate by hand.

If you can find which way your machine leans then it can be taken advantage of as the back cut will give a fine finish pass, though as said any lean will give a slightly concave surface though I’m told Myfords are setup to face like this and the users seem happy with that.

Sharp tool and shallow cuts will keep machine deflection to a minimum

[duncan webster 1Participant @duncanwebster1]

I think all lathes are (or should be) set up to face concave, the more you pay the less concave, but concave is preferable to convex.

 

[duncan webster 1]

On duncan webster 1 Said:

I think all lathes are (or should be) set up to face concave, the more you pay the less concave, but concave is preferable to convex.

 

Agreed, Duncan !!

Dr Schlesinger rules

… his recommended ‘Permissible Error’ makes it quite clear.

MichaelG.

.

[samuel heywood]

On 3 August 2024 at 20:57 samuel heywood Said:

…
Firstly, whilst i regularly  throw caution to the wind and climb mill using endmills, the little x1 really doesn’t seem to like climb milling with a flycutter, conventional milling seems to be the way to go.

…

 

Why throw caution to the wind by climb milling?  Unless the mill is powerful and heavy enough to take the strain there’s no advantage in climb milling, which thrashes most hobby machines for no benefit, maybe flexing the machine enough to tilt the cut.  The exception is that a single very light climb cut right at the end often improves the surface finish.

Apologies if it’s me, because stomach ache is destroying any limited ability to think I might have had,  but does a fly-cutter ‘climb-cut’?  My gut feel is that they cut conventionally.

Dave

 

[JasonBModerator @jasonb]

Just depends on what part of the circle swung by the tool you pass the work under

For example if you are swinging a tool at 25mm radius and have the work behind the spindle, say 10mm over the work and you fred the work towards the right it may well pull the work into the cut.e

[Chris MateParticipant @chrismate31303]

My mill seems to be trammed as good as I can get it, so I mounted a lazer switched on, to shine to the roof focussed to  inside the hole of a small washer. I hope this will help if I tilt the head again. One could add markings for degrees chosen & trammed.

[PeteParticipant @pete41194]

I had to learn it the hard way, but every mill is going to deflect towards the opposite direction of the cutting forces at least a tiny amount. Even the most perfectly trammed mill is going to show a slight amount of back cutting from the fly cutters edge as it starts to pass over the area that’s previously been faced by the leading edge of the tool. To allow any part to slide or rotate, there has to be some amount of clearance. It may be very small such as with heavily preloaded spindle bearings, but there’s still a clearance. On top of that, the cutting forces tend to deflect the cutting tool and even the head upwards on light weight mills. Or I’d guess even slightly bend the column on a small mill. How much deflection happens throughout the whole machine is a factor of multiple items. The cutting edges sharpness is even a part of it. So you have multiple effects adding to the problem and how well the mill is trammed is only one of them.

I have a BP clone so it’s fairly adjustable and a lot easier to tram in both directions. It took awhile, but I once spent a few hrs tramming it’s head in using a technique not probably recommended and certainly not in any book. But I managed to get it within about .0001″/ .0025 mm over almost 12″/ 305 mm in X,Y. My fly cutters still showed traces of that back cutting simply due to those inevitable machine and cutting tool deflections. Obviously a properly trammed mill is still what you want, but that’s for multiple other reasons than just fly cutting.

Climb milling with end mills? I suspect the vise on the mill I’m using outweighs an X 1 sized mill. And I only climb mill with mine during very light finishing passes. Not paying attention to those logical recommendations has cost me a fair amount just to then find out they were right all along. But I suppose that might be something everyone has to learn themselves.

 

 

[JasonBModerator @jasonb]

A mill that is in tram should give a very slight back cut but it should not be so deep that it removes the cutting marks from the leading cut. You should end up with an almost holographic effect where the (((( shaped leading cut and the )))) trailing cut cross each other.

 

One I prepared earlier on the X3

[PeteParticipant @pete41194]

A great picture that shows the effect well Jason. Only a guess since I’ve never measured that back cut swarf, but it likely cuts a few .0001″ at most.

There was a post on the PM forums yesterday about using a mill to fly cut automotive cylinder heads. One of the pros there mentioned tilting the head by 1/2 a degree to prevent that back cutting. Personally I don’t agree, for cylinder heads and how they seat or hold against the head gasket, it’s well known you don’t want an extremely smooth or almost reflective surface. One with some slight ridging grips the gasket better once the head is fully torqued down. And fly cutting is only one operation you’d do on a mill, I don’t have any issues checking or re-tramming my mill, but I’m not about to move the head off tram just for fly cutting a part and then move it back.

[JasonBModerator @jasonb]

1/2 a degree sounds quite a lot. You probably only need a thou or two over a foot to tilt the head enough to stop the back cut and any more will increase the amount the surface becomes concave.

A quick bit of cad gives quite a dip. Say the flycutter is swinging 8″ dia which would be about right for a 6″ wide head then at 1/2deg you would get 0.010″ dip in the middle. Would certainly cure the back cutting as the trailing edge would be about half a mil above the work.

Texture is really a product of feed rate and shape of the tool, think of it like threading, if you feed fast then the Vees are more noticable than a fine thread and then reduce it even more you get turning cuts rather than threads.

[PeteParticipant @pete41194]

I didn’t run the numbers like you did, but that half a degree sounded like a lot to me. In fact with the numbers you came up with, I don’t see how that could be right.  Afaik the proper block and head surfacing equipment uses something much like a large bed mill. And a large multi toothed face mill with no tilt to the head at all. Extremely flat and straight is what you want. And just like you said, they use a known and pre set feed rate to obtain that slightly ridged surface finish. I still don’t think any BP type mill is the right tool for the job anyway. In my opinion they don’t have the table support length to not move in an arc as that table, the part and fixture weight on it moves in either direction. Even though I have a BP clone I still wouldn’t try to surface a cylinder head on it. Pay what it costs for the right equipment and an experienced automotive machinist who knows what there doing. I think a BP type mill can be just fine for some head work, but not for that full length head or block resurfacing.

[Neil LickfoldParticipant @neillickfold44316]

I found with flycutters, the greater the mass of the fly cutter, the smoother the cut becomes. Some have made flywheels for their spindles to assist in this more constant load cutting. Vernier mills come to mind. The lighter the rigidity of the machine the finer the radius needs to be on the tool to reduce tool deflection. If the tool has too high a rake, it can sometimes actually pull the cutter into the surface a small amount. The advantages of fly cutting is to use a larger area than the size normal mill cutter can make and with less cutting forces.

When you have a mill like BP clone, I worked at a place and we placed another table ontop of the existing tabled. The knee was brought to the correct height and locked solid. The X axis was locked solid too. The Y axis had the gib adjusted firmly but could be moved. The part was moved on the bolt on table that used linear rails for it’s smoothness. It worked remarkably well. We were able to do work that previously needed a fixed bed mill like a Deckle with it’s axis all hanging off the back of the bed.  With the bigger plates being bored , DTI were set up and then the height was corrected with essentially a large bottle jack. I made the alloy wheel tooling on that set up for what was then the Ford alloy wheel plant in Auckland.  Learnt alot about how much things can flex and move, and the lighter the  cutting forces are, the better the end result will be.

You can balance the fly cutter by having provision for two cutters opposite each other that are the same dimensions, but have one a small amount lower than the other. It gives you an option of just changing the height and you have another edge ready to go. If taking cuts deeper than 0.1 , the second cutter will effectively rough the height to that 0.1 or what ever the difference is you choose.  Two were vertical opposite each other, and the other two were on a shallow 30 deg to cover a wider area. Mild steel worked very well for the holder. The first one I made a bit light and used a chunk of high tensile steel and it had a ring happening sometimes.  2nd one was MS and heavier.

[David George 1Participant @davidgeorge1]

I have a simple fly cutter I made when an apprentice and it has two cutters, one HSS and one tungsten carbide. It is a piece of 1″ siver steel with a 1/2″ shank cross drilled for a Cutter hole at an angle and a grub screw hole to clamp the cutter.  Its my go for skimming cutter especialy when getting a nice finnish.

Make your own similar fly cutter justs keep cutting.

David

 

 

[PeteParticipant @pete41194]

The fly cutter itself is easy enough, I have a commercial version that uses the round RPMT inserts that produces a really good surface finish in most materials. It’s also very cheap to operate since the insert can be slightly rotated as the edge dulls or possibly chips. Neils clever work around to remove that table rocking within it’s own clearances due to gravity obviously worked out well.

This is also one area where those with both a vertical and horizontal spindle machine have a clear advantage since that inevitable table rock really doesn’t matter as long as the whole surface is being fly cut. Fixturing something like an automotive engine head so it’s exactly vertical and true to that horizontal spindle would be more difficult. But I still think that given the fairly reasonable costs and the time involved to set it all up and do one or two automotive heads on something with even a horizontal spindle. It just makes more sense to have a professional with the right equipment and the required experience to do it properly. In a commercial situation where the effort and costs can be justified over many many parts, then what he was doing clearly made it worthwhile.

 

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