Endmill, Slot Drill

[Sub MandrelParticipant @submandrel]

Distracted by the OT comments, I’ve being trying to recall why endmills cut oversize.

 

As I recall, it goes like this:

 

When a four flute cutter makes a slot at different times there are either one or two flutes pulling the cutter sideways. With a two-flute slot drill there is only ever one flute pulling sideways. With a three-flute cutter the number of cutting flutes varies between one and two, but the geometry is balanced so one flute comes in gradually as another leaves.

 

This means the variation in side thrust is least for 3-flute, then 2-flute and worst for 4-flute. Hence with a light setup a 4-flute mill is more likely to wander.

 

I don’t say this is 100% correct, i offer it as musings to be shot down…(gently please)

 

Neil

 

[Sub MandrelParticipant @submandrel]

[John Stevenson 1Participant @johnstevenson1]

4 Flute.

Just take one tooth as an example and imagine looking down on it from the top, cutter revolving clockwise. Slot being cut is upwards.

 

Starting off at the 9 o’clock position.

 

Tooth 1  is cutting one side and tooth 3 is cutting the other so everything is in balance. What we have here is a two flute cutter but we are getting away from the 4 flute [ sorry ]

 

As it revolves tooth 1 is now at 12 o’clock, tooth 3 is at 6 o’clock  but it’s not balanced because tooth 3 is in the slot being cut so the cutter tries to pivot, correction it DOES pivot and makes tooth 2 dig in making a wider slot.

 

Tooth two then moves to 12 o’clock and forces tooth 3 to dig in etc, etc.

 

Two flute cutters don’t do this as they both cut together or when they are unbalance there are no teeth making contact with the sides.

 

Same applies to 3 flute .

 

 

John S.

[TerrydParticipant @terryd72465]

Hi Neil,

 

You are quite correct.  When cutting a slot most of the action is at the front and if you analyse it, for most of the time the cutter is being pulled sideways (especially with smaller diameter cutters) and the job tends to be pulled in the opposite direction (actions and reactions) so the slot tends to wander and also becomes wider.  With a 2 flute cutter there is only one cutting edge acting and for part of the time it is being pulled in one direction but as it revolves around the cut it is being pulled in the other thus tending to even out the forces.

 

You are also correct about the 3 flute cutter, the forces are balanced as the two cutting edges act.

 

I couldn’t remember how it worked until I analysed it again myself last night.  I must have been in the classroom for too long!

 

Terry

[Ian S CParticipant @iansc]

Terry, there was a very good artical in ME a few (maybe quite a few) years ago, I’ll put up the numbers if I find them. Ian S C

[TerrydParticipant @terryd72465]

Thanks Ian,

 

My local library has a complete collection of bound MEs back to the early 40’s so i can easily get access to them.  While doing a thought analysis on this I was also thinking about the arguments for and against rear toolposts for parting especially which was raging here a while ago and have come up with a few ideas which I may share.

 

Terry

Edited By Terryd on 01/12/2010 10:55:35

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

(1)

 

A cut with a slot drill will usually be less accurate and more ragged than one with an endmill .

 

For any given depth of cut and feed rate the load on any one cutter tooth gets higher the lower the number of teeth on the cutter . The higher the load on the active tooth is the more likely the cutter is to deflect . So for a two tooth slot drill the load on the active tooth is (about) twice that of a four tooth endmill .

 

With very small numbers of teeth the deflection is not simple but is cyclical : bend – straighten – bend – straighten as each tooth engages and disengages . The situation with a two tooth slot drill slot drill is particularly bad because of the cross section is not only rather weak it is also stronger on one (cross sectional ) axis than the other which tends to make things worse .

 

(2)

 

The conventional explanations given in the magazines for parting off difficulties on Myfords and the arguments for rear tool posts are so far off the mark as to be almost pure fiction . The true explanation is simple and I will be interested to see if somebody finally gets there after nearly fifty years of argument .

[blowlampParticipant @blowlamp]

The whole parting-off on a Myford thing has always intrigued me too.

My thoughts are that the problem is largely down to chip crowding in the groove as well as a slightly flimsy compound assembly.

So using coolant can help flush the chips, but as the chip is at least as wide as the slot, (but probably wider due to plastic flow) it can still get jammed and cause a cram-up. Turning the tool upside down and mounting at the rear allows gravity to help remove the swarf, but I find that the modern insert style tooling can be used conventionaly in the front position with confidence as the tips are designed to shrink the width of the swarf and thus enable easy ejection.

 

Martin.

[Sub MandrelParticipant @submandrel]

I’ll think again on the parting problem. Thing is I’ve always found that tight slides, well adjusted bearings, right speed and sharp, properly adjusted tools I can part from teh front without problems, in proportion to the capabilities of a modest mini lathe (2″ in mild steel, and 1″ in high-tensile).

 

I think the best advice was that given by George Thomas – have the guts to feed the cutter in heart – most jams are due to lilly livered prodding. Unfortunately mini=lathe can overheat if you make full use of teh torque available for a long cut

I think the thing about the three flute cutter is that the cutting forces are analogous to three phase electricity. The latter always sum to zero, but with the cutter only having a semicircle of cut, the result is not zero force, but still much reduced.

 

Neil

[Anonymous]

Errr, Neil, dare I suggest that that’s not a good analogy? The neutral current in a three phase system is only zero in a balanced system. Of course, if there is no neutral connection the sum of the currents at that point, for a star connection, will be zero, but the currents will not be balanced.

 

Clearly a three flute cutter which is not cutting equally on all flutes will be unbalanced. Now there’s a question; is there a machining operation wherein a three flute slot drill will be cutting equally on all flutes? I can’t offhand think of one. I discount the case of a plunge in Z, as we’re talking about side flutes, and the end flutes are not equal anyway.

 

Regards,

 

Andrew

[Martin WParticipant @martinw]

Hi

 

Just another thought, if the cut is fairly deep and the mill has flutes with a tight twist then there will be potential for cutting forces to be exerted for, possibly, 180 degrees on each flute. The other problem is that the advancing flutes will be cutting with tip speed plus feed speed and on the opposite side the flutes will be cutting at tip speed minus feed speed.

 

Secondly the advancing flutes will be cutting in conventional milling mode while the trailing flutes will effectively be cutting in climb milling mode. The load on the advancing flute, in normal mode, is significantly higher as it is trying to penetrate the surface at an acute angle while the other flutes and portions thereof are cutting at their designed optimum with much less side load .

 

Its a wonder that a mill cuts straight at all . Don’t think I want to ponder it too much but it will give me a scapegoat when I make a pig’s ear of a job.

 

Cheers

 

Martin

Edited By Martin W on 01/12/2010 19:38:05

[John OlsenParticipant @johnolsen79199]

If you can manage to think about it from the rotating point of view of the cutter, it is being wiggled in a rotary manner, rather like the way you wiggle a fencepost when trying to pull it out. The force varies as described in the posts above depending what each flute is trying to do. We can reduce the forces by keeping the tooth load low, which means not feeding too fast. Higher RPM will also reduce the tooth load, but of course is limited by the maximum cutting speed. We have an advantage over the professionals in that we are not generally in a hurry, so can afford to keep the cuts lighter. The profesionals tend to have advantages like more rigid machines and a bigger budget for cutters.

 

regards

John

[Sub MandrelParticipant @submandrel]

Andrew – I was thinking of the voltages in three-phase summing to zero; is that not correct?

 

My experience shows that 3-flute mills slot pretty close to size and 4-flute don’t, at least up to 1/4″ (which I admit is pretty small).

 

Neil

[geoff walker 1Participant @geoffwalker1]

Hi All,

I have a question. The crosshead shown in the photo is made from cast bronze. It is complete except for the slot which needs to be cut for the small end of the crankshaft. The size of the slot is shown in the drawing 5/16" wide and 1/2" long. My question is what type of cutter is best for this job? I will be holding the work, using a vertical slide and vice using the small fixture (or is it a jig?) to set the gudgeon pin hole vertical.

My thoughts are that as this is a "closed" slot a two flute cutter may not be the most appropriate choice. What do you guys think?

cheers geoff

[geoff walker 1Participant @geoffwalker1]

Hi All

doh!!!!!!!!!! connecting rod not crankshaft.

[Russell EberhardtParticipant @russelleberhardt48058]

Two or three flute every time if the slot width is critical.

Russell

[MuzzerParticipant @muzzer]

Hmm. 2 and 3 flute cutters are less rigid due to the smaller remaining "core". And there can only be a fraction of a flute in contact (and cutting) with the work at any one time, especially with the light cuts we tend to see in this context (hobby machinery etc), so the cutting will be very much intermittent which isn't good for surface finish or dimensional accuracy. They are generally recommended for heavy roughing, as the large flutes are good for chip evacuation.

On the other hand, multi flute cutters have a more rigid core and are more likely to have one or more flutes in continuous contact with the work. It's probably not a great surprise to see that these are recommended for finishing. Even better when combined with odd number of flutes and variable pitch flutes that minimise resonance vibrations. If you had a big chequebook you could buy a groovy finishing cutter just for the job with a high flute count.

I suspect there won't be a massive difference between 2, 3 and 4 flute in this instance. It's more likely that the accuracy will be determined by setup, human error, backlash etc.

Murray

[Mick B1Participant @mickb1]

Posted by geoff walker 1 on 06/11/2017 11:12:34:

My thoughts are that as this is a "closed" slot a two flute cutter may not be the most appropriate choice. What do you guys think?

cheers geoff

The only way you could do it with a conventional 4-flute endmill – which don't generally (or maybe ever?) endcut to centre – is to do the 2BA hole through first, then feed the endmill down it prior to opening the slot sideways in both directions.

There'll be an assymetric tendency to form oversize radii at both ends with the 'lobes' in opposite directions, and the whole idea assumes that the noncutting counterbore in the end of the mill is smaller than 2BA tapping size – and you'd have to be a bit cute about getting it right.

Easier to use a vanilla 5/16" slotdrill, or maybe 8mm to get yourself a couple of thou play.

[JasonBModerator @jasonb]

I do the same sort of cut for the inside of IC engine pistons.

Drill to depth with a drill one size smaller than the 5/16. Change to a 2,4 or 4-flute cutter and plunge to depth, move say 10thou to the side and plunge again. Do this until you have one side to finished length then repeat for the other and then finally move the work the full length of the cut to clean the sides.

[geoff walker 1Participant @geoffwalker1]

Hi All,

Thank you all for your responses,

I've done it now, 2 flute, nice and steady 5 thou max cut, clean as whistle.

My original post was perhaps misleading. My concern was not so much accuracy but more the way a 2 flute cutter would react. You could just feel it clipping the gudgeon pin hole but otherwise no problem.

The more I get involved with model engineering the more I enjoy it but there are times when using machinery and you are stepping into the unknown, well it can a little nerve wracking!!!

thanks again geoff

[larry PhelanParticipant @larryphelan54019]

No-one said it was easy but, SOMEONE HAS TO DO IT !!

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