Squaring off the bottom of a blind hole

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Beware of even well intentioned information. Many thousand engraving machines have been produced in England, in gernany and China as clones to make 'flat engraving cutters' Amongst my assorted miscellany, I have a 'Chinese'. It may well be capable of grinding lathe tools to a high degree of precision and can alter fluted tool blanks to twist drills, end drills, mill drills and whatever- in essence that is what it is-- and the original Deckels and Alexanders are or were astronomically priced, the Chines ones are only slightly less breathtaking. Reminds me that I could utilise a lot more 5C collets for mine.
thinks it is possible to buy a se t of plans for an adequate tool and cutter grinder using the scrap box for less than ONE 5C collet

best Wishes

Thanks Norman. Maybe I'll stick to my hand expertise for lathe tools and drills for now. - I don't bother re-sharpening drills below 35p each - Packs of 10 x 0.5mm "jobber drills" for a £1 last about 5 to 7 years.... and the sub- "half-mil" sizes I buy for drilling jets are too femur to touch, so at less than 35p each I buy a pack of 10 from China every few years.
I don't do a lot of milling - haven't bought a new tool in a decade or so - or sharpened one ever! I just like to learn how to select and use them properly, rather than just "having a go". Hence my interest in this thread.
Look after yourself.
following yesterday's 'Thundersnow' and a lot of surprises, I got a update from the Quorn Owners' group. some body has actually demonstrated how to 'round off the corners of a lathe tool- but on na Quorn and then mechanically and with fine precision:) Standing astride the two camps, I must caution that Quorns come in as kits of well over £600 to which must be added additional tool holding of an ER32 collet holder and a box of collets. All this requires additional cost and a lot of time to make the desirable jigs if not already in the builder's possession. Totting it all up requires a deep breath.
Even then I doubt that it will re=sharpen your little Chinese drills. Well the Quorn designer could make 1/10th diameter drills but couldn't re-sharpen them.
Whatever way one thinks, ending up with a Quorn is an expensive time consuming labour of love- or worse.
So for a tenner or so and a cheap Aldi/Lidl grinder or so and scrap steel, and sayb a week's work , people can have remarkable precision. Not only cheap precision but going through a learning pattern and having cheap hss tools to the configuration to suit the builder's way of working.

It's all a lead balloon and perhaps red rags to Andalusian bulls!.
Whilst 'pricing' the tool and cutter costs, I espied a New thingy from Hemmingwaykits n a saddle mounted pillar to take a small versatile dividing head. It suggests the ability to own an amazing bit of kit. I saw the principle decades ago- and next week - deliveries etc helping, I should have the prototype. What is odd is that Hemmingwaykits has the drawings and mould as well.
Meantime, it is all worth future planning.

So best wishes for the future

... With two flutes, as one edge is entering cut, the other is leaving and the sideways forces are balanced, so the cutter stays central to the slot.

My apologies - people keep saying this, but I seem to be too dense to completely understand.

It would seem to me, that with a 2-flute (well, 2 cutting edge) cutter, you get constantly oscillating sideways forces. When the cutting edges are "across" the slot - one entering the material and one leaving - you have net zero sideways force on the cutter. When the cutting edges are "along" the slot - one at the leading edge of the slot cutting, the other in free air in the already cleared slot - you have a significant sideways force applied to the cutter.

With a 4-flute cutter, you constantly have at least one cutting edge engaged in the material, producing a more-constant side thrust.

Since many have said that the 2-flute cutters cut more true to width, I have to believe that this works in practice, but I'm failing to understand how constantly oscillating side forces produce a more-accurate cut than constantly-present forces.

Thanks for any illumination shed into my darkness!

The forces at play are just as you describe. Although it may seem unlikely it is a fact that slot drills do cut a more accurate slot (and very close to the cutter's diameter). If a four flute endmill is used to 'size' a slot to the cutter's diameter, even if roughed out beforehand, it will produce a slightly wider slot than expected.

If a very accurate slot width is required it is best to be done with a slightly undersize cutter, moving each side to establish the width.

Regards - Tug
The core web tends to be more robust on a four flute than a 2flute or 3flute but at the sacrifice of chip clearance. and it is possible to do an angled plunge feed or spiral plunge feed with a non center cutting endmill if the feed angle is slight enough everyone's done it. with really rigid machines like big cnc's there doesn't seem to be much of an advantage if any in using 2 flute cutters over 4 flute as far as cutter cut size accuracy , actually recall anyone going for a two flute for sizing reasons. most often it was use a 2 flute for the rough plunge than shift to 4 flute for finish and size accuracy. the results of people getting more accurate size with a 2 flute may have more to do with their particular machines resonances and stiffness from cutter load. other wise as far as accurate size cut comparisons on equally sharp cutters in comparison with the cutters actual size the stiffer cutter will win and that relates to the web thickness and that usually means shallower flutes . more flutes do mean more cutter pressure from all the engaged cut surfaces so the stiffness of the machine starts to really matter, but there is a benefit from the more constant pressure of multi flute cutters and hence less vibration
If you are having trouble machining bronze, take heart!
I just finished reading "Ford Methods and The Ford Shops" which goes into extreme detail on the procedures Ford used in producing cars.
It was written in 1914/1915, just as Ford was converting to a moving assembly line. (Not just for car assembly, but for ALL the parts they made.)
One of the things they cover is the broaches they used for bronze bushing manufacture. THEY NEEDED SHARPENING EVERY 100 PIECES. The floor hands kept between 16 and 24 reserve cutters at the machine tool, and switched them out as needed.
I happened to cut or broach keyways in shafts and hubs on a daily base. In metals such as Titanium and Zirconium and Stainless and Hastelloy. And use 2 flute or keyway cutters they are made for cutting keyways to very close tolerance they are square cut. One thing everyone forgets is the more flutes a cutter has the weaker the cutter becomes so in turn the more flex you have. Now you can counter that and say why do you get a better finish with a higher flute count if it has more flex. Well that’s a direct result of the amount of cutting edges peeling away the metal. But above all of this it comes down to something so simple. And that is speed vs feed vs doc and picking the right size cutter and the right cutter material and coating.
remember you can polish a turd all day long and at the end of the day you’ll have a turd

Fascinating thread here, a mixture of experiences , theory, humor and personal feelings. I will not address them all except to say that having a smile on one's face is better than a grimace and real experience is better than theory and most important for all is sharing information !. With over 62 years of machine shop experience , let me add a bit of information not discussed.
Yes, two flutes are better than four for slots because they allow chips to freely evacuate, but I personally prefer three flute cutters as they give good chip flow and are stouter than a two flute. ALL endmills....1, 2,3,4,5 flute will deflect when pushed in a straight line. The harder you push (Feedrate and DOC) the more you will deflect.
Making a precision slot with a undersized endmill is all well and good, just make sure the size is sufficiently smaller to account for this deflection.
Another comment about straight flute versus helical flutes. Straight flutes do not exert any axial forces , while positive helix angles cause the cutter to be pulled out of the spindle. This may be fine on a CNC machine with stout tool holders and good bearings, but can mess up a home shop job that uses collets for holding the cutter .
"If" the cutter does not pull out, be assured that it is attempting to pull your work piece out of the vise !

Well I did say I was a dynasaw :)

I'm speaking from a home workshop point of view based on working experience with non CNC machines. With the advent of CNC which after all from a production point of view is a relatively recent but incredible advance in machining techniques and particularly so in some home workshops. I would agree that from that perspective things are mightily different but it's surprising how easy it is for some to forget about pre CNC and how short a time that was ago.

Machine rigidity is another matter altogether - if you've only worked modern machines it may be difficult to visualise how less sturdy machines can cope. Even the all encompassing Bridgeport has it's limits.

I programmed and worked a twenty tool Haas machining centre for my last three years or so (1999-2002) - if a slot had had to be done then it would not have been done with a single sized cutter but a tool path created to mill it to size, radius and all if it were a closed slot. Ramping using a conventionally ground end mill is a perfectly feasible action - very easy to accomplish on CNC and not that difficult on conventional machines either. That's a different process however from direct plunging

Most, though not all of course, home workshop equipment is of a much lower 'grade' than that found in an average machine shop and even conventional kit in them is (has?) dissapearing at a phenomenal rate. The last jobbing shop I worked at (which bought the Haas) had all conventional turret mills and lathes when I left - now there is nothing that isn't CNC.

I mentioned the fact that a Google search produced virtually all solid carbide cutters - having used them I am aware of the rigidity required in the machine and the tooling to use them to advantage but though I will turn to them to get over something hard if required I know the tooling that has done sterling service for many years on HSS would have a hard job being efficient.

All this is long way from my old Linley mill - still sound and accurate but made around 1940-1950 and using tooling that was readily available when I purchased it in the 1970s - I've never given it a thought before but I've just realised that I've been using that for longer than it was old when I bought it :oops:

So, no I don't disagree but I do stand by my views that a slot drill is a slot drill and an end mill is another cutter all together but it's aimed firmly at those using, and sometimes learning to use, pretty basic kit in order to enjoy our hobby.

Regards to all - Tug
It would seem to me, that with a 2-flute (well, 2 cutting edge) cutter, you get constantly oscillating sideways forces. When the cutting edges are "across" the slot - one entering the material and one leaving - you have net zero sideways force on the cutter. When the cutting edges are "along" the slot - one at the leading edge of the slot cutting, the other in free air in the already cleared slot - you have a significant sideways force applied to the cutter.

Thanks for any illumination shed into my darkness!
Will, you are right so far, but you seem to be missing the point that while one edge is cutting at the 'front' and is subject to lateral deflection, with a two lip cutter, there is no edge in a position be cutting into the side.
Will, you are right so far, but you seem to be missing the point that while one edge is cutting at the 'front' and is subject to lateral deflection, with a two lip cutter, there is no edge in a position be cutting into the side.

ahhh - that may indeed be the lightbulb I am missing! Thank You!
Just happened by this thread and thought I would share this with everyone. Before I start, this narrative is going to show that we all may just over think some problems. My 18 year old granddaughter (21 now) was building a Stirling engine. She was in the shop making some parts and came over to me and asked if she could chop off a 3” piece of .500 6061 Al for a displacer piston. I was busy on the Bridgeport and told yes that would work fine. She was working on a Clausing 5904, with a DRO on it. An hour later she brought the displacer piston over to me and asked how she could hold it so she could square off the cutoff pip on the end. With a wall thickness of .010 she was concerned about crushing it if she put it in a 3 Jaw. The displacer dimension is 1.122 long with an OD of .380. It had a .360 ID hole 1.070 deep. The drawing showed a flat bottom hole. I looked at the part and asked - how did you do that? Just for your general information she had been machining since she was 8 years old. In teaching her and her sister I never ever told them something was difficult to machine, when they started out I would give them suggestions and I just let them have at it. Here is what she told me. She faced off the end, turned the OD to .380, center drilled it with 118 degree spot drill I have, and drilled it out to 9/32 (I think) because she said she was going to use my .187 carbide boring bar leaving some amount so she could flattened the bottom. Then she took a .250 endmill and flattened the drilled bottom. Next she took the boring bar and finished the ID. She then moved the boring bar to the center and took a couple of cuts across the bottom until it was flat. She zeroed the DRO, backed the boring bar out, and machined off the entrance until she was at 1.070. I asked why she was so careful about the bottom being flat as it really did not need to be. She told me the drawing showed it flat and I told her to always do what the drawings showed - so she made it flat! I have no idea how she came up with that sequence, and after showing her how to cut off the pip the part was done.

To me the lesson I learned from her was – just make the part and move on. Living 2 hours away she had a limited amount of time at grandpa’s house (2 or 3 times a year) so she made the most of it when she was here. See photo for more parts.


Displacer piston being epoxied to end cap and rod
using a arbor spacer to hold it upright

Other parts


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