Drilling accuracy question.

[Michael GilliganParticipant @michaelgilligan61133]

This doesn’t meet Gerry’s spec but it’s a good example of what can be done in that ‘footprint’

https://airinc.net/wp-content/uploads/2021/09/1530-Profile.pdf

Variations on the theme are produced in vast quantity, and widely available as off-cuts, etc.

MichaelG.

[Graham MeekParticipant @grahammeek88282]

As I opened my old toolbox this morning from the days when I worked for a living. The mass of extension, and long series drills in the top compartment of the box made me realise I had forgotten to point out that a standard jobber drill wont really be long enough for the task in hand. The standard jobber drill being only 110 mm long does not give the best circumstances to maintain a rigid grip on the drill.

Regards

Gray,

[Martin ConnellyParticipant @martinconnelly55370]

If you are buying a drill for this job  would suggest avoiding a coated drill and go for one with a bright finish, less chance of soft alloy sticking to the drill.

Martin C

[not done it yetParticipant @notdoneityet]

I would use a milling cutter – either a slot mill, or multi-cutter mill after drilling a clearance hole for its centre.  Started from both ends with a bush/hollow dowel at one end, so oversized in the centre would be my choice, I think.

That or cut from both ends and finish-reamed.  If the block is not perfectly square, no chance of holes from both ends meeting perfectly.

I would likely just thread from both ends and use cap screws or similar – can’t see the need for an accurately positioned through-bolt.  Kiss princple in operation as often as I can manage it.

[old martParticipant @oldmart]

When commercially made parts have to align perfectly, alignment pins or bushes are commonly used. Your parts would only need two of these on each side and they would not have to be deep. Then drilling the bolt holes with clearance from both sides gets easier.

[HOWARDTParticipant @howardt]

Drill out to say 7 then finish with milling cutter 3 or 4 flute.

[PeteParticipant @pete41194]

I think your expecting far beyond what the average model Engineer might be using for equipment and usual mill would be even halfway capable of Gerry. 6, 8 mm through holes over 100 mm in depth is a 12.5-1 ratio for diameter to length. Simple and shorter hole lengths to diameter ratios can be just fine for many tasks. Industrially speaking, drills would considered as a roughing tool for size, hole roundness, straight and location accuracy when better accuracy for any hole is desired. Yes there’s cheats and work around’s with better or more specialized drills and even solid carbide that might still be better and good enough in industrial situations to save using slower and more expensive methods. Single point boring even with a solid carbide boring bar, maybe verging on impossible without serious chatter and the small diameter to that length ratio. The longer the hole depth, the accuracy of where the exit might be while drilling increases exponentially for it’s true exit location.

For whatever reason, very few hobbyist’s seem to have ever checked how there mill may react with even the appropriate levels of cutting loads we all think very little of using. Actual doing some of that checking for various machine deflections. And you begin to appreciate why accuracy issues in the parts being produced may not be unlikely or might even be inevitable. Since it’s what I’m most experienced with, I have to use them as my examples. For anything else, your going to have to imagine how well what your using compares.

While there not exactly the most rigid mill design around, I’ve got a Taiwanese Bridgeport clone. I’ve checked it’s alignments, have had and could still have any time I wanted it’s spindle squared to the table to a fraction less than .0002″/ .005 mm over roughly a 9″ front to rear in Y, and I’ve managed the same number over about 18″ in X. Time consuming and a bit frustrating to accomplish, but at least easier than most other hobby sized mills with it’s worm adjustments on the heads tilt and nod. And while tough to accomplish, still not impossible. But that’s only a no load and static alignment. That true spindle alignment is going to change the instant any appreciable cutting loads are applied.

The design of my 6″ capacity mill vises uses a one piece casting with there fixed jaws cast integral to the vise body. There classed as a pretty decent industrial level cnc style milling vise using high grade ductile cast iron and weigh around 80 lbs each. And weren’t very much cheaper than the Kurt vises are. The design allows the use of 2, 5/8ths diameter allan head cap screws through the vise bed on each side of the vise to attach them to the mill table. And another short 5/8ths tee bolt on each side between those cap screws, so very rigid using all 3 table tee slots. As far as what I use for initial hole making, I’ve always bought very good and well known industrial brand names. I could easily locate and start all 6 of your through holes with my dro accurately. Starting and exactly where they might exit for location wouldn’t be very unpredictable. Even the usual feed pressure variations while peck drilling add another variable to drill flex and milling head deflection changes. Drilling 100 mm deep through holes only looks like it should be simple and accurate.

What I couldn’t do is actually produce what your wanting to and what you expect for hole location, size, hole straightness and accuracy for the drills exit location. I highly doubt any conventional BP type mill at 1 ton and less with the best conventional HSS type drill could. It might be remotely possible with the parts faces precision ground, and extremely parallel, flat and true, then rigidly fixtured onto something like a Moore or SIP jig borer table while using the very best techniques and multiple step drilling up to finished diameter. Even with that, I still have serious doubts it’s possible without some real luck, and it wouldn’t be something I’d expect to count on doing for all 6 holes.

Larger than the 8 mm hole your trying to end up with since it’s 1/2″ or 12.7 mm, and it’s drilled in different material, but it is an example of what it takes for drill feed pressure just to drill a hole. It’s the only numbers and for a single drill size I’ve seen mentioned, and for material type. But possibly other numbers for other drills sizes and materials might be listed somewhere I haven’t run across yet. According to a test I read about, to drill a 1/2″ / 12.7 mmm diameter hole in mild steel with a brand new high quality factory sharpened drill without any step drilling first. That takes at least 150 ft. lbs of down feed pressure on the drill tip to make that drill cut and continue to drill. No, most of us for our sizes of parts and equipment wouldn’t ever do it without step drilling, but drilling still requires a fair amount of axial thrust to make any drill cut. Given the leverage multiplication on the drill presses and mills I’ve used, and the amount of force required on the spindle feed handle that can be estimated. I have little reason to doubt that number. If anything, it might even be a bit conservative.

My mills head on it’s own and without the 3 ph motor attached to it, I’d estimated at around 75 lbs, the motor adding at least another 35-40. But even drilling an initial .250″ diameter hole in mild steel still shows measurable head flexing on an indicator at the rear of the head of up to .003″ when I’ve checked that. And it doesn’t even take excessive closing force on the mill vise to start to detect fixed jaw deflection even with that one piece vise bed and jaw. Any movement is going to tilt the block at least a bit that your trying to drill. The part could be counter shimmed to compensate for any fixed jaw movement. But it’s just one more item to account for. No it isn’t very much, over 100 mm it is going to affect where the drill exits even if everything else was perfect. And while my vises are very well made and finish ground, there’s no such thing as perfect for any man made object if your metrology equipment has enough accuracy and resolution. I suppose I might get somewhere within workable limits of what your wanting to do. But not to what you seem to be expecting. That’s just the partial realities of this.

In general, parts design is dictated by the current equipment available for a low volume of parts or one offs. If it’s not possible to the accuracy requirements needed, the design gets changed, modified and tolerances loosened until it can be produced, or shipped to an outside source that can with more specialized equipment. To state the obvious, there are machine tool limits where there’s no practical solution to eliminate them.

As far as using an end mill for finishing and sizing? Maybe there made some where, but I don’t know of any 8 mm end mills with at least 120 mm in length since you still need extra just to hold and drive the tool shank even if the cutting flutes were still the standard length. A good tool sharpening shop could make one as a non standard end mill, but that still wouldn’t be cheap.

Without knowing everything about your complete parts and there use. I’d suggest machining the individual parts faces as accurately as you can, use pins to fix your parts location for the completed assembly. Then start small with through drilling of all the parts while pinned as an assembly. Move up in drill sizes until you drill though for what the tap drill size is needed, remove the part that gets tapped, and finish drill the other to your 8 mm. Those pins accurately locate your parts assembly, the bolts your using clamp everything solid and help maintain the alignment as well once tight. But as others have suggested, that’s an unusually tight hole clearance on the bolt shank. If you were using that tight clearance as a way to both help attach and align the parts, that’s really not the way to do it. Bolts are good enough sometimes or a poor part locator when more accurate is needed. Even automotive cylinder heads are first located with solid or hollow dowels. The multiple head bolts hold the head down. Those pins I mentioned, or other methods could be used. You are without question going to get drill wander with holes of that length. Drilling all the parts as an assembly at least ensures the holes in all parts match up even if the entrance and exit locations vary a bit between the top and bottom. More detailed information about what the parts are for and how there meant to be used might get even better suggestions.

[Neil LickfoldParticipant @neillickfold44316]

Few model engineers even know about double margin drills. They drill very straight holes and very close to size. Most home drill presses are just not rigid enough to drill though 100mm in one go and have the hole within 0.1mm of centreline. A lot of home hobby mills will not do that either.

It is why I suggest marking out and drilling 1/2 way from each side. The drilling with a small drill I think is not very sound advise. A smaller drill will wonder more than a larger drill, simply from the length to drill ratio.

The most critical part of drilling any hole, is the 1st 2 diameters of the drill deep. In this case the 1st 16mm. Having a good centre point sharpened drill, for the most part will give you a very good result. There are now Drills sold in Cobalt HSS, that have a 90 deg centre point, like a wood work drill, and also have a small chamfer corner break on the outer edges.

Here in NZ I buy a brand called Kango, with this geometry on them. They work very well for what I call deep hole drilling. They will start from a centre punch or just straight into the smooth flat stock. They will wonder on sawn stock or not flat stock.

You should be able to drill though at the 8mm from each end and have them meet quite nicely in the middle. Just take care and it should go well.

Using things like an accurate angle plate to set the work piece to and clamping it in place will help instead of using a vice for example will help in getting it done. Advantage of an angle plate, is that it can be set up, in such a way, that it will run true to the run of the Mill or Drill quill.

[Robert Atkinson 2Participant @robertatkinson2]

Why don’t people read the whole topic.
In post 817418

Drilling accuracy question.

I suggested using a long series end mill. No comment and now both Not Done it Yet and Howardt make the same suggesion…..

Robert.

 

[JasonBModerator @jasonb]

I would like to know where you are all finding 8mm dia milling cutters with in excess of 100mm flute length to clear the swarf?

long shank ones which are easy to come by would be a bit risky in aluminium as a bit of swarf could get between hole and shank and gall then you have a cutter stuck in the hole.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

Why don’t people read the whole topic.
In post 817418

…. No comment and now both Not Done it Yet and Howardt make the same suggesion…..

Robert.

 

Likely not guilty, rather than  “non readers”.

Be aware a forum bug intermittently causes some posts to temporarily disappear.  I’ve been caught 3 or 4 times. Responding in good faith with a new point, I’ve found later that it was already comprehensively covered.

Not obvious posts are delayed – there’s no way of knowing a post is lost in hyperspace.  One exception: images are occasionally delayed leaving gaps in the text that many members have noticed.  Return to the post later, and the image is present and correct.  I suspect a cache error.

Dave

 

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

Why don’t people read the whole topic.
In post 817418

Drilling accuracy question.

I suggested using a long series end mill. No comment and now both Not Done it Yet and Howardt make the same suggesion…..

Robert.

 

Agreed, Robert.  Tramming errors, over that distance/depth, likely exceed the requirements expected by the OP.  As others just above state, our model engineering machines are simply not good enough for perfection.

[cogdobblerParticipant @cogdobbler]

Why use an 8mm anything to make a clearance hole for a 5/16″ bolt? That’s only 4 thou clearance, which is never going to work well.

Drill it 11/32″ or even 9mm would be the usual clearance.

If the OP (Hello, are you still with us?) Is worried about location or alignment of the two pieces, that is the function of dowel pins, not bolts.

[Nigel Graham 2Participant @nigelgraham2]

Gerry –

If we’ve not sent you round the bend by now,

I think the upshot of this entire discussion is:

Don’t rely on bolt-holes alone. The chance of drilling six accurately-placed holes right through that depth is low no matter how careful you are, even if you have the tools to do so.

Drill and tap screw-holes for the bracket, into its side of the block.

If the position is critical w.r.t. the block locate it with two dowels or perhaps fitted screws (those being bolts or studs with accurate plain shanks that fit reamed bracket holes and counterbores).

An alternative if the overall design permits, is to machine a small lip on either the bracket or block to give a locating edge. Dowels are simpler and normal practice. Sometimes the dowel is tubular with a normal clearance hole for a screw or stud that winds into the tapped region beyond the reamed dowel location.

Presumably the through-holes are so the block will become sandwiched between the bracket and something else, and the criticality lies in their relationship, not to the block as such?

Fasten the “something” from its own side, too.

I would arrange the design so the three components can be assembled using squares, straight-edges, a surface-plate or whatever else appropriate, to align the external parts to each other – not to the block specifically if that is simply a big spacer.

[Neville ChaseParticipant @nevillechase52412]

How about using a pilot point drill?

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, one way of drilling the holes from both sides, and keeping them in line, whether drilling all the way through, or just deep enough for set screws, is maybe by making a template of the holes in a piece of say 3mm thick ground flat stock, with the same dimensions as your block. Mark one side with a T for use on the top, and the other side with a B for use on the bottom. The template holes only need to be a pilot hole size, or for a suitable transfer punch. Of course you will have to line the template up on the same two sides of the block.

Regards Nick.

[Chris GunnParticipant @chrisgunn36534]

The holes do not all have to be the same size surely? drill the outer plates 8.5mm, the inner block 10mm, assemble the lot, line up all the edges, mark up what goes where, pop in a couple of roll pins at each joint to maintain the line up, plus a couple of tapped extraction holes if it all has to come apart. Using a simple jig like Nick suggests will help as well. We often forget that spending time on a simple jig can make a job so much easier to execute.

Chris Gunn

[Nigel Graham 2Participant @nigelgraham2]

If you have to make the bracket as well, then that’s the jig!

[Kiwi BlokeParticipant @kiwibloke62605]

Late to the party, I fear.

Relieve the drilled holes of their location duties. Drill them with a generous clearance however your equipment best allows. Counterbore each hole end (positioning by co-ordinates) with a slot drill, for about two hole-diameters depth. Make hollow dowels. I’m assuming you can also make tight-fitting dowel holes in the mating parts, or use the existing holes with smaller diameter through bolts, but if not, the ‘dowels’ need not poke out from the block’s surface, and will provide the tight bolt clearance you were thinking of, which is only important at entry and exit points, of course. Given a generous diameter, it doesn’t matter if the deep hole drilling is a bit wonky.

[gerry maddenParticipant @gerrymadden53711]

Well, I think finally the dust has settled!  Thank you all for your thoughts, fears and experiences and the time and effort you put into your comments. I have certainly learnt about a bunch of new drill bit types, parabolic, ratio, double margin, to name just a few. Also good to hear about the importance of pecking more frequently as the hole gets deeper, and lots of lubricant.

I didn’t really specify an accuracy target for the bottom hole pattern, other than wanting to get it to be as close to the starting pattern as might be possible. No one actually gave any figures for what might be achievable, but that’s really not surprising. It’s clear that this would depend on so many factors, my machine, bit quality, material, my own force inputs and so many other things as well.

Since I would rather not change the construction of the job at this stage, (it’s a long story!) I think the only way to determine what I’m likely to achieve ‘in my environment’, and then to decide if its acceptable or not, is to do some trials. To this end I have just purchased a ‘medium priced’ YG uncoated long drill from Cutwel. I’ve checked its tip quality under a microscope and it looks good and uniform. As Jason and others have suggested, I will use this long drill only after the centre, stub and jobber. I’ll also report the results to you when I have them.

If after this the bottom face of the block looks more like a Swiss cheese than an engineering component then it’s back to the drawing board.

Gerry

 

 

 

[JasonBModerator @jasonb]

Well having just drilled a little over 2000mm in length I was quite happy with the lack of drill wander.

6mm drills, stub and Jobber Dormer A002 & A022 (Neither new) no spot drill to start. Nothing more than a sharpie line to get the radial position and holes spaced at approx 5.9mm cts  (18 holes on a 34mm PCD) so one just overlapps the next. Came in from both ends of the 112mm long bit of CI bar. No pecking just steady pressure with a feed counted at 70-80mm/min and speed what sounded right. Once the swarf was knocked out the bit in the middle dropped out too.

[renardiere7Participant @renardiere7]

That’s just showing off!  Well done!

[gerry maddenParticipant @gerrymadden53711]

Excellent work, and very encouraging!

[Michael GilliganParticipant @michaelgilligan61133]

Fabulous work, Jason

… ‘though presumably two hundred millimetres

MichaelG.

[JasonBModerator @jasonb]

No 18 holes at 112mm long = 2016mm TOTAL

[Author]

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