Help with making small counterbores

[AJSParticipant @ajs]

I am partway through making the small boring tool from Hemingway and am in need of a counterbore for 4BA cap head screws.

 

My attempt at making one has been unsuccessful so I wondered if anyone could help me with a fool proof method please.  The dia. of the screwhead is approx 0.22 inches.

 

Thanks

 

Alan

[AJSParticipant @ajs]

[Martin Johnson 1Participant @martinjohnson1]

 

Drill the hole for the screw shank, drill down with the next largest drill size, but do not go full depth.  Now take the drill you used for the counterbore and grind the end flat, then grind a clearance angle (about 7degrees) so it will cut.  Take a look at a slot drill to get the idea.

 

You might want to invest in a set of cheap drills from the DIY store to turn into counterbores – it saves re-sharpening all the time.

 

Good luck,

 

Martin

[TerrydParticipant @terryd72465]

Hi Alan,

 

Why not make and use a D bit.  G H Thomas explains how to make these in his ‘Workshop Techniques’ book in the ‘Boring’ Chapter.  These tools are really useful as they will ream as well.  In fact they were used for making gun barrels before accurate machinery evolved.  They are quite easy to make from standard diameter silver steel.  There are easy to use and as the end is more or less at 90 degrees to the centre line (actually about 88 degrees) they are accurate enough to use as counterbores in a previously drilled hole.

 

They will usually give a better finish than jobbing drills, reamers or counterbores. You can make a whole set using different diameters of silver steel and make specials by turning the diameter.  To make, you grind or mill (I do) the last 12 mm or so to a few tenths thicker than half diameter. The end is then ground at a rake of 7 degrees and at an angle of a couple of degrees from the centre line across the non cutting side of the end.  The Bit is then hardened and tempered to dark straw.  Sounds complex but actually is easier to make than describe.  I’ll scan and send you the descriptions from G H Thomas if you don’t have the book, – email me.

 

Also try Here for a discussion.

 

These bits will last for years when properly made and hardened.

 

Terry

Edited By Terryd on 24/05/2010 12:37:51

[Ramon WilsonParticipant @ramonwilson3]

Alan,

 

‘Flat bottoming’ a drill is one way but if it’s the only drill you have its a pain to keep resharpening a new point on but it will get you by. It’s good advice then to have some spare drills as Martin says.

 

 I have several kept for this purpose but only use them to flat bottom the hole that has previously been drilled with the correct (counterbore diam) size drill nearly down to depth. A flat bottomed drill has nothing to guide or centralise it – this way ensures it and it is used only to take out the ‘slope’ at the bottom of the hole. A slot drill has one side ground ‘over centre’ – there is no need to recreate that on a FB  drill, just grind the angles as suggested to the approx centre – there is no material under it to be cut anyway.

 

Another way is to use a FC3 cutter of the nearest size  – enough clarance with a 5.5mm? Either in a holder or in the drill chuck – nothing wrong with the latter as long as you use it as a drill.

 

And of course you can make one from silver steel – turn the diameter you need for long enough to suit the job, Drill for a pin which is slightly smaller than the clearance drill diameter for about 1/2 deep. Turn a pin to fit this hole and which will protrude about 3/16. Set the part vertically in a vice and using a three cornered file cut some teeth – they dont have to be perfectly divided but make sure they point the right way though. Three are ideal – the less teeth you cut the deeper they are and the less quickly they will clog. Harden out  and temper and do the same with the pin then fit the pin with Loctite.

 

Keep the revs well down and use coolant – this will cut a counterbore from solid (with the previously drilled bolt clearance hole in of course) on most materials as long as you retract continually to clear the chips which as said will clog quite quickly. 

 

I’ve taken this pic of some – not very good but it should give you an idea.

 

 

 

hope this helps as well

Regards – Ramon

 

 

Edited By Ramon Wilson on 24/05/2010 13:16:03

[AJSParticipant @ajs]

Thank you Martin Terry and Ramon for your prompt replies.

 

I made a two flute piloted cutter (before asking for help) but the cutting action was not very good and one of the cutting edges broke.

 

I quite lile the D bit solution but the work will need to be accurately centered on the clearance hole before maching.

 

Ramon.  When filing the teeth on your silver steel cutters do you angle the file towards the drilled end, or do you file straight across. I’ thinking that would create the wrong rake angles. i.e. straight across.

 

If all else fails I’ll resharpen a twist drill as suggested by Martin.

 

Thanks all

 

Alan

[Ramon WilsonParticipant @ramonwilson3]

Hi Alan,

 

You seem to be on the right track though two flutes are probably quite a bit weaker than the method adopted. There isn’ta lot of clearance for the swarf but they do work and are strong. And very quick to make

 

With the cutter blank infront of you, pilot hole uppermost,  you need to file your side of the cutter with a vertical cutting edge on your left sloping the right hand side toward where the next cutting edge will be formed and filing upwards toward the cutting face. (As said this is not critical but the more you make them the better you get at it.) Rotate the cutter by eye (I normally do three teeth on small cutters – up to 6mm or so and four or more above depending on dia.) and start the next tooth slightly to the right of the relief continuing to gash the teeth out around the periphery. If you start too close to the previous relief it will shorten the cutting edge however if one tooth is a little out of line or a little shallow it doesn’t matter it will still work well enough to do the job in hand. Don’t forget to debur inside the pilot hole before hardening!

 

I normally use a swiss file # O cut for the first cuts and finishing with a #2 tweaking the cutting edges to give a bit of front rake and reasonable backing off. If you have one, a diamond file is useful too after hardening but not essential – a stone will do. If you are using the cutter on brass or ally I wouldn’t bother to temper – the harder and sharper the edge the better.

 

Come back again if you can’t visualise the above and I’ll take a few more pics  for you – that’s the joy of these digi jobs eh so useful for things like this.

 

Regards – Ramon

[Richard ParsonsParticipant @richardparsons61721]

An easy to make a counter bore is: –>>

1.     Turn a short piece of silver steel down a pin to the required clearing size (about 0.75” – 20mm long).  Round the ends to make it easier to insert>>

2.     Turn a length of silver steel –Drill rod- about 3” – 75mm down to a nice fit on the head of the screw.  Centre and drill the bar with the clearing size of the screw so that it fits the item you made in 1 above.>>

3.     Saw off the item and put in the vice in the vertical mill with hole end on your right.  This is important.>>

4.     Use a small end mill and set it so that the edge of the end mill will just graze the edge of the hole and set a stop or adjust the setting ring to Zero.  Make certain that the work piece is on your (front) side of the end mill.  You can either feed in to about 1/3^rd of the mills diameter.>>

5.     Loosen the vice, turn the bar ¼ turn – do it by eye and cut again.  Repeat the process to cut all 4 faces.>>

6.     With a small fine file (or stone) clean up and back off the faces and clean out the central hole,>>

7.     Harden and temper the new cutter.  Clean up with a fine stone.>>

8.  &nbsp

[CirclipParticipant @circlip]

Duplex gave a description of how to do it in an artical showing a grinding jig starting in the ME of 7th October 1948, but copywrong issues prevents me from sending it to you.

 

  Regards  Ian.

[TerrydParticipant @terryd72465]

Posted by Clockie on 24/05/2010 14:57:30:

Thank you Martin Terry and Ramon for your prompt replies.

 

I quite lile the D bit solution but the work will need to be accurately centered on the clearance hole before maching.

 

Thanks all

 

Alan

 

 

Hi Alan,

 

centring is not a problem with a D Bit.  As Martin suggested in his post, you drill the clearance for the screw, then drill the clearance for the head and then use the D bit to cut away the angled material that the jobbers drill has left.  Hey presto quick and perfectly concentric.You do not try to cut the whole of the counterbore with this tool.  And if you do it all on one setting in the mill or drill without disturbing the workpiece it would be concentric anyway.  I’ve taken a couple of pictures of one of my D bits, It’s done a lot of work and needs sharpening but once used you won’t want to use anything else.  They’re like me, old fashioned but still relevant . This one is 6mm dia.

 

Sorry the pics are out of focus but I don’t have my macro lamps at the moment and had to use a normal lens, but they show the gist of what I mean.  The blue is the tempering colours which still remain, the cutting edge was tempered back to dark straw.

 

 

View from side showing general shape and rake angle of cutting edge.  The material is cut away to just above the centre (5 thou’ or so)

 

 

View from Above showing the relief angle on the non cutting edge (from
centre top half of leading edge)  This gives clearance and prevents
rubbing.

 

Regards

 

Terry

Edited By Terryd on 24/05/2010 17:31:16

Edited By Terryd on 24/05/2010 17:32:00

[Ramon WilsonParticipant @ramonwilson3]

 

Terry,

I’m sure you would agree that there is a big difference between a D bit and a counterbore. The D bit has to have a hole virtually near size drilled first to work at its best. The work has to be held securely in relation to the D bit too. Agreed though, where the D bit comes into it’s own of course is to flat bottom a blind hole.

 

The counter bore on the other hand is self aligning in the previously drilled bolt hole and can be used on the drill press without the work being clamped if desired and deemed safe.

Where I have used D bits this has always been to size a hole rather than use it as a CB but of course it can be used for that too just as long as that work can’t move.

 

The big thing is if there is a whole series of CB’s to do – only one change of tool needed with a CB

 

Sorry, I m teaching Gran I know but it’s something that others should perhaps be aware of.

 

Regards – Ramon

[Sub MandrelParticipant @submandrel]

Clockie

 

I thnk there is a clue where you says one of your cutting edges broke. Sounds like you are making a counterbore like Ramon’s (Ramon-  I wish I could be organised like you!!!) but perhaps aren’t tempering properly? Make sure that after hardening you re-heat the cleaned up tool until it goes what is described as a light straw (I think its a pale bronzy colour myself).

 

And yes, angle the cutting edges at about 5 degrees.

 

Neil

[TerrydParticipant @terryd72465]

Hi Ramon,

 

Of course I agree with you.  The D bit does have it’s limitations.  I wouldn’t produce a spotface and yes you would have to drill a hole close to size, in fact for a counterbore it should be on size, that’s just what I said, it would just be used to flatten the bottom of a pre drilled hole.  But they are so simple to make and work so well I don’t see a problem, I’ve made and used them for years and if good old GHT used them that’s good enough for me.  And they can be made with just a file and a heat source to heat treat them.

 

I wouldn’t personally use a counterbore with small cutting edges, they just clog up too fast and it can be a slow process drilling the hole with one ad I’m too impatient I’d ruin it.  However by using the D Bit most of the material is already removed with the drill bit and a very small amount is left to be removed.   But we all have our own preferred techniques and that is just as it should be, I’m just too lazy to go to the extent of setting up to drill perfectly concentrically (my old 3 jaw isn’t that accurate and I would use a 4 jaw) and then file al those teeth accurately, I’m just not that good. and I’d be afraid that Loctite would lose it’s grip if it got hot (I just can’t bring myself to trust glue) when drilling so I would silver solder the pin etc, just too much trouble for me.

 

Best regards

 

Terry

[Ramon WilsonParticipant @ramonwilson3]

Ah Terry,  I guess we are so alike.

 

I too was (and still am) a devotee of GHT – I used to devour the pages he wrote in the ME at the time and tried to take on board all he said. I only heard recently (Myford day) that he was not always considered by some to be correct but for me he had such a way of putting it over without complicating it I can’t see how anyone could take exception.

 

I had a smile when I just read your post as I was previously in the workshop mulling my previous post over – you’ll see what I mean.

 

I thought I would take a couple of more pics to clarify my last post a little.

 

 

This shows two counterbores quickly made to CB holes in the base of a McOnie engine to accept steel ‘bolt pads’. They are just filed up by hand, no division except by eye and simply Loctited to a piece of silver steel (left soft). They are just for the job in hand though will still have some use in the future ie they are not made to last – just to do the job.

 

The Loctite held as very little heat is generated as the speed is quite slow and the cutting well lubricated. They are not tempered back just left as hardened the smaller one just polished with use. I do agree though that they are more like spot facers than cb’s and yes you do have to clear the chips but they do work.

 

 

This gives some idea of the tooth form – I notice from the traces of bronze remaining  that only two teeth were leading! but it did the job okay.

 

 

This shows the small one (resting on top of the now in situ bolt pad) but this is how it was used to get past the ribbing on the casting. This could have course been done by reverse cb but this did it ‘all in one hanging’ and was much simpler.

 

The drawing just calls for spot facing but this would be an area more built up I feel hence the decision to put in bolt pads. The grey stuff is JB Weld – believe me it will look much more the part once a coat of primer goes on

 

This is not to take anything from your advice on D bits on the contrary – just another approach to do the same thing though on this particular op as you can see straight drilling was not an option.

 

Regards – Ramon

Edited By Ramon Wilson on 24/05/2010 23:03:43

[TerrydParticipant @terryd72465]

Hi Ramon,

 

sorry not to have replied earlier but I’ve been away for a while.  I love your home made counterbores and yes thay are also great for spotfacing as you show.  Surely two teeth are all you need really?

 

When counter boring would you drill to diameter and depth first, and then use the tool just to flatten off the bottom?  Or would you use the tool to drill the whole once the pilot is drilled?

 

I note that Tracy tools sell D bits in hss, either individually or in sets.  Very versatile tools.

 

See Here

 

Terry

[DMBParticipant @dmb]

Hi all,

Whats wrong with using an endmill or slot drill to scrape a small amount of taper down to a flat – bottomed hole? It may be technicaly “wrong” but if it does the job, why worry or amI missing something? Can anyone please explain why not?

[Ramon WilsonParticipant @ramonwilson3]

Hi Terry,

 

I agree, ideally two teeth are all thats required for a CB but that requires a lot more filing. – remember these are not intended as long term tools but quickly made to get a job done without recourse to setting up the blanks and milling the flutes in. If not practical to drill first as in the the example above then they will work providing the chips are kept cleared – not a real problem with a small stiff bristled paint brush. This was how the CB for the bolt pad above was carried out – about 4mm deep.

 

Yes given that the size matched then I would drill first. The real benefit of this type is as previously mentioned that they can be used in a drill press without need to be clamped and aligned as a D bit or indeed as John C has said,  a slot drill or FC3 cutter which incidentally I did suggest on my first reply. Depending on the dia. of the CB an endmill is not always suitable becaues of the way it is ground which will create a slightly reverse taper as opposed to a flat bottom.

 

Haven’t done much of late, but off to see my mate this afternoon who lent me the Racer to copy – I think he has an example of another 2.5 the idea of copying which if he has is rather gaining momentum at the moment!

 

Regards – Ramon

[Ian S CParticipant @iansc]

If you make the cutter for the counter bore from silver steel, file the teeth(say 3 to 6),don’t try to get them evenly spaced, this will stop the cutter from chattering. You only need about 1/2″ of each dia, and a pieceto make the pilot/ shaft, and a pin to hold them together. Doing it that way you can take out the pin and turn the cutter over and counter bore the other side by passing the shaft through the hole and reassembling it. Ian S C

[Author]

Posts