Spot Facing With Slot Drill?

[JasonBModerator @jasonb]

7BA nut is just under 5mm across the Corners so 6mm or 1/4" spotface. On that size I'd more than likely use a short series 3-flute. Seldom use 2-flute unless its for an actual slot or an aluminium specific one where it helps to clear the swarf.

[Dr_GMJNParticipant @dr_gmjn]

I was using a 5mm slot drill. As Jason says that’s the minimum I could get away with. I partly chose to go with the minimum in case I ran into issues…

[HopperParticipant @hopper]

Posted by Dr_GMJN on 16/05/2020 08:03:50:

OK I’ll give Jason’s method a go. Any tips on making the cutting edge – approximate geometry? And also hardening?

Thanks.

One way of doing it is to use a mini hacksaw to cut a cross into the end of the silver steel after OD has been turned to size and the hole drilled up the middle. Make the cut as deep as you want the teeth to be, so maybe about 1.5mm. Then you can use a file to create the angle back from the cutting edge, away from the direction of rotation. Easily done without the centre pilot spigot in place. Also helps if you do this part before parting the piece off from the parent silver steel bar. Makes it easier to hold in the vice while filing. Clearance angle should be maybe about 10 degrees or so. The more the angle, the sharper the cutter but the quicker the edge wears.

To harden silver steel, heat to a dark cherry red colour and plunge into a tin of oil to quench. Then clean with emery and reheat to light straw and quench again. Get the right colour at the cutting edge by heating behind the cutting edge and let the colour run up to to the edge.

That's one way. Im sure there are others.

[Dr_GMJNParticipant @dr_gmjn]

Wow. Sounds like the foundry scene in the Mandalorian.

This is either going to go really well or really badly ha ha.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 15/05/2020 23:13:07:

Posted by Dr_GMJN on 15/05/2020 22:34:08:

[…]

I did lock the axes before machining.

I did a subsequent test on some plain steel, exchanging the drill bits and slot drill, again with the axes locked, and it still gave a non-concentric circle. Perhaps the existing holes are dragging the cutter off centre? I should have tried cutting the spot face first.

.

If I read that correctly … it’s worrying

Assuming that the job is properly secured and the X & Y axes are locked: that would imply that the head is moving

Perhaps your concern about the column is worth investigating further.

MichaelG.

I agree with Michael – cause for concern. It seems something is moving.

I'd reach for a DTI next. With the machine switched off, I'd methodically confirm that everything really is locked down solid.

Start by holding a lever DTI on the quill and have it register a couple of mm on the vice before pushing the quill and then table hard with a thumb. Move DTI as necessary to repeat test in both X and Y directions. For example, with the lever in a table slot, can the locked table be moved in X, and by how much? Repeat test in both X,Y on the vice, and then to the vice's moveable jaw. Test at several different table, head, and quill settings.

When applying the DTI test to a small milling machine minor some movement should be detected : metal bends! But not very much. Worth taking measurements and reporting back I think.

DrGMJN has encountered a number of different problems with his new mill. Could be a combination of Learner Driver syndrome, difficult materials, and a few unlucky tool purchases, but maybe there are problems with the machine. Worth checking I suggest.

My first suspect is maladjusted gibs are allowing the table and/or head and/or quill to move whilst cutting. With incorrectly set gibs the amount of movement is likely to vary depending on table/head position. (Tight in some places, slack in others. ) Loose gibs cause vibration, poor finish, and inaccurate cutting, which seems to match the symptoms.

How bad the ill-effects are depends on the cutting operation, for example drilling doesn't rely much on the gibs because spinning drills stabilise on the spindle axis and encounter few sideways forces. On the other hand tight gibs are vital to most milling operations because the sideways forces are massive and change direction depending on the cut. Gibs problems may explain why the Doc is getting mixed results. If not the gibs, then maybe something else is wrong with the machine. The hunt is on! Avoid assumptions; the goal is to eliminate guesses until the truth emerges.

A word of warning! When testing a machine with a DTI, it's easy to confuse actual faults with poor measurement technique. Fine measurements are extraordinarily sensitive to set-up errors and instrument shortcomings. One can end up lost in a maze of red-herrings, so don't jump to conclusions. My inexpensive DTI is accurate enough but it's slightly sticky at some points; when using it for this sort of work I have to make sure it's working in it's comfort zone! Not really worth me buying a better DTI, but being able to completely trust the instrument does make this sort of measurement task quicker and safer.

Dave

[Dr_GMJNParticipant @dr_gmjn]

Posted by SillyOldDuffer on 16/05/2020 09:56:24:

Posted by Michael Gilligan on 15/05/2020 23:13:07:

Posted by Dr_GMJN on 15/05/2020 22:34:08:

[…]

I did lock the axes before machining.

I did a subsequent test on some plain steel, exchanging the drill bits and slot drill, again with the axes locked, and it still gave a non-concentric circle. Perhaps the existing holes are dragging the cutter off centre? I should have tried cutting the spot face first.

.

If I read that correctly … it’s worrying

Assuming that the job is properly secured and the X & Y axes are locked: that would imply that the head is moving

Perhaps your concern about the column is worth investigating further.

MichaelG.

I agree with Michael – cause for concern. It seems something is moving.

I'd reach for a DTI next. With the machine switched off, I'd methodically confirm that everything really is locked down solid.

Start by holding a lever DTI on the quill and have it register a couple of mm on the vice before pushing the quill and then table hard with a thumb. Move DTI as necessary to repeat test in both X and Y directions. For example, with the lever in a table slot, can the locked table be moved in X, and by how much? Repeat test in both X,Y on the vice, and then to the vice's moveable jaw. Test at several different table, head, and quill settings.

When applying the DTI test to a small milling machine minor some movement should be detected : metal bends! But not very much. Worth taking measurements and reporting back I think.

DrGMJN has encountered a number of different problems with his new mill. Could be a combination of Learner Driver syndrome, difficult materials, and a few unlucky tool purchases, but maybe there are problems with the machine. Worth checking I suggest.

My first suspect is maladjusted gibs are allowing the table and/or head and/or quill to move whilst cutting. With incorrectly set gibs the amount of movement is likely to vary depending on table/head position. (Tight in some places, slack in others. ) Loose gibs cause vibration, poor finish, and inaccurate cutting, which seems to match the symptoms.

How bad the ill-effects are depends on the cutting operation, for example drilling doesn't rely much on the gibs because spinning drills stabilise on the spindle axis and encounter few sideways forces. On the other hand tight gibs are vital to most milling operations because the sideways forces are massive and change direction depending on the cut. Gibs problems may explain why the Doc is getting mixed results. If not the gibs, then maybe something else is wrong with the machine. The hunt is on! Avoid assumptions; the goal is to eliminate guesses until the truth emerges.

A word of warning! When testing a machine with a DTI, it's easy to confuse actual faults with poor measurement technique. Fine measurements are extraordinarily sensitive to set-up errors and instrument shortcomings. One can end up lost in a maze of red-herrings, so don't jump to conclusions. My inexpensive DTI is accurate enough but it's slightly sticky at some points; when using it for this sort of work I have to make sure it's working in it's comfort zone! Not really worth me buying a better DTI, but being able to completely trust the instrument does make this sort of measurement task quicker and safer.

Dave

Dave,

I understand what you're saying. the chances are it's me doing something wrong, or at least not knowing whether I am or it's something, else. It is helpful to bounce a few ideas around though. I can't see it's the SX2P – chances are it'll be an adjustement issue if it's anything to do with the machine. I have done some work on the 10V using the mill that I'm really happy with – in fact most of it so far (even though I've not done that much yet!).

[Dr_GMJNParticipant @dr_gmjn]

So with some trepidation I set about making a spot facing tool this morning. The idea is that the spigot will get pressed into the central hole when it's been heat treated; it's a tight fit. It's also a good fit in the casting clearance holes. I made the O/D 5.5mm, which seems to be the minimum required to clean up the error, and still not go too far into the corner radius. I still need to do a final touch-up on the cutting edges with a fine sanding/polishing stick. Looking at the last picture, I might also turn some more off the plain flank to give more clearance to the casting:

I did some reading, and I think I'll need to hold it cherry red for 5 minutes, dunk in oil or salt water, then straw yellow and dunk in water (does it have to be oil)? Hopefully our gas poker will be ok. Making tools and heat treatment (apart from annealing photo-etched brass) is completely new to me, so any further comments on either are of course very welcome.

I have some garden, diy & family jobs this afternoon, so I will check the thread again this evening before trying the heat treatment.

Cheers.

[Anonymous]

Silver steel is designed for water quench, while gauge plate is designed for oil quench. I don't do enough to justify buying the special oil, so I quench both silver steel and gauge plate in brine. The addition of salt makes the quench slightly less violent. When quenching you need to agitate the work; don't just dunk it in and leave it. And I mean agitate, move it like your life depends on it. To get full hardness the metal needs to cool very quickly, before the atoms realise what is going on and re-arrange themselves. If you don't agitate the hot metal forms a blanket of steam around itself, which acts as an insulator and reduces the cooling rate.

Andrew

[Dr_GMJNParticipant @dr_gmjn]

Posted by Andrew Johnston on 16/05/2020 12:28:26:

Silver steel is designed for water quench, while gauge plate is designed for oil quench. I don't do enough to justify buying the special oil, so I quench both silver steel and gauge plate in brine. The addition of salt makes the quench slightly less violent. When quenching you need to agitate the work; don't just dunk it in and leave it. And I mean agitate, move it like your life depends on it. To get full hardness the metal needs to cool very quickly, before the atoms realise what is going on and re-arrange themselves. If you don't agitate the hot metal forms a blanket of steam around itself, which acts as an insulator and reduces the cooling rate.

Andrew

Thanks Andrew,

So:

1) Heat the cutter end in the gas poker until it's cherry red, and try to keep it at that temperature for a few minutes, then dunk in brine.

2) Re-heat until straw yellow and dunk in brine again.

Cheers.

[JasonBModerator @jasonb]

Looks like you are making a good job of that so far, maybe a bit less of an angle will help with tool life. here is a rough and ready one I knocked up this morning from 1/4" silver steel, just filed and not sawn, I tried 3 teeth as it is small but 4 is actually easier. Photo after hardening and a lick with a diamond slip.

Got the end red and held it there for couple of mins

Quenched in water, make sure you keep it vertical as going in at an angle can make the work bend. As it's a fairly solid lump I did not temper it.

This is the side of a piece of cast iron bar drilled 2.6mm than spot faced, bit chewed up as I did not spend much time on the cutting edges

As I said I would usually go for a 3-flute cutter on small ones like this that are easy to get to without excess tool stickout, and it's a better job

For those that don't like to file it can be done by milling but if you are anything like me the job is already in the milll and drilled when you then want a spotfacer!

[Cabinet EnforcerParticipant @cabinetenforcer]

Posted by Dr_GMJN on 16/05/2020 12:12:09:

I did some reading, and I think I'll need to hold it cherry red for 5 minutes, dunk in oil or salt water, then straw yellow and dunk in water (does it have to be oil)? Hopefully our gas poker will be ok. Making tools and heat treatment (apart from annealing photo-etched brass) is completely new to me, so any further comments on either are of course very welcome.

I have some garden, diy & family jobs this afternoon, so I will check the thread again this evening before trying the heat treatment.

Cheers.

Compare your cutter to Jasons on the previous page, yours has a lot more relief, and will cut quite aggressively, I would tone it down a touch.

Tempering can most easily be done in a domestic electric oven, just clean and thoroughly degrease the part first, this is much easier than judging temper by colour, especially on such a small piece. You do not need to dunk after tempering, it's just convenient.

You only really need the tips of the cutter to actually be hard, so I wouldnt worry excessively about soak times, remember that this process can be attempted on the same part as often as needed, if it all goes wrong just heat it up, cool slowly in air and you have a soft part you can reshape if needed, before having another try.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Johnston on 16/05/2020 12:28:26:

… The addition of salt makes the quench slightly less violent. …

Andrew

Other way round? Water cools faster than Oil and Brine cools faster than water. Brine is good for maximum hardness but the result will be very brittle. Water can also be brittle, in which case the newly quenched tool can be bunged straight into a domestic oven at, say 180°C, and tempered for an hour. Tempering allows the atoms to relax a bit, in effect exchanging a little hardness in in favour of improved toughness. (I rarely bother tempering silver-steel because most of my home-made specials aren't made to last, they just have to cut a few times.)

5 minutes is about right, and StubMandrel's advice on measuring the temperature is useful, but soak time is related to the thickness of metal and DR_GMJN's fine hollow cutting head will need less rather than more time. To get a feel for soak time I'd experiment on a few ordinary silver steel test pieces before trying to harden that nice cutter. Having to make another one just because the heat treatment went wonky would annoy me!

In practice I've found hardening silver-steel to be quite forgiving. But it works best when done 'just so'.

Dave

[not done it yetParticipant @notdoneityet]

You do not need to dunk after tempering, it's just convenient.

Quenching is necessary if the item is being heated with a flame – to prevent the tip temperature rising further. No need if the whole piece is at tempering temperature, as in an oven.

[not done it yetParticipant @notdoneityet]

  Double post, again….

Edited By not done it yet on 16/05/2020 13:36:06

[Dr_GMJNParticipant @dr_gmjn]

Great, thanks all.

OK – to reduce the angle, shall I face it off again in the lathe, maybe to half the tooth depth (giving me four flats all on the same plane), then re-file to a shallower angle? In effect giving me a double angle, but only the flatter one will see the cut?

[Mick B1Participant @mickb1]

+1 for SOD – brine is the more severe quench, but at this sort of scale it's not a big worry.

I can remember heating up a big slab of thick gauge plate (same British Standard as silver steel) that I'd carefully milled as the anvil for a flypress forming tool to cherry red (perhaps more accurately carrot red), tipping it into a bucket of oil in the expectation of a less severe quench than water, then looking in horror at the dreadful crack where the sectional thickness changed!

Cutters in the 5-15 mm range aren't likely to present such issues, though. Here's one I used to cut a location for the breeching-rope ring on a model carronade. I only held it at red-heat for 5 or 10 seconds, quenched in tapwater, emeried the end and almost just waved it in the flame before I saw the cleaned area colour-up. It cut bright mild steel like it was cheese.

[JasonBModerator @jasonb]

Posted by Dr_GMJN on 16/05/2020 13:35:54:

Great, thanks all.

OK – to reduce the angle, shall I face it off again in the lathe, maybe to half the tooth depth (giving me four flats all on the same plane), then re-file to a shallower angle? In effect giving me a double angle, but only the flatter one will see the cut?

You can just file the ends of the ends of the teeth to a shallower angle, if you look at that 3 flute cutter I used you will end up with an end like that.

[Martin ConnellyParticipant @martinconnelly55370]

Jason showed reverse spot facing in one of his posts. I would just like to point out that the mill needs to run in reverse for this otherwise you need a tool ground in the opposite way to standard.

Martin C

[Dr_GMJNParticipant @dr_gmjn]

Posted by Mick B1 on 16/05/2020 13:42:34:

+1 for SOD – brine is the more severe quench, but at this sort of scale it's not a big worry.

I can remember heating up a big slab of thick gauge plate (same British Standard as silver steel) that I'd carefully milled as the anvil for a flypress forming tool to cherry red (perhaps more accurately carrot red), tipping it into a bucket of oil in the expectation of a less severe quench than water, then looking in horror at the dreadful crack where the sectional thickness changed!

Cutters in the 5-15 mm range aren't likely to present such issues, though. Here's one I used to cut a location for the breeching-rope ring on a model carronade. I only held it at red-heat for 5 or 10 seconds, quenched in tapwater, emeried the end and almost just waved it in the flame before I saw the cleaned area colour-up. It cut bright mild steel like it was cheese.

Thanks Mick – beautiful work by the way.

[Dr_GMJNParticipant @dr_gmjn]

Posted by Martin Connelly on 16/05/2020 14:30:18:

Jason showed reverse spot facing in one of his posts. I would just like to point out that the mill needs to run in reverse for this otherwise you need a tool ground in the opposite way to standard.

Martin C

Thanks Martin, I think I'll leave reverse spot facing until maybe tomorrow

[Dr_GMJNParticipant @dr_gmjn]

Posted by JasonB on 16/05/2020 13:50:14:

Posted by Dr_GMJN on 16/05/2020 13:35:54:

Great, thanks all.

OK – to reduce the angle, shall I face it off again in the lathe, maybe to half the tooth depth (giving me four flats all on the same plane), then re-file to a shallower angle? In effect giving me a double angle, but only the flatter one will see the cut?

You can just file the ends of the ends of the teeth to a shallower angle, if you look at that 3 flute cutter I used you will end up with an end like that.

I don't think I can file that accurately – I'll end up with the edges at diffetent heights and angles. I guess if it's spinning fast enough it won't matter, but I think I'll do it in the lathe with a parting tool, then at least I'm starting from a datum that shouldn't change after filing.

Cheers.

[Anonymous]

I didn't explain myself very well, although the inference that I meant not as hard is rather a leap of faith.

What I meant was that brine seems to be less prone to creating a blanket of steam around the work leading to less hardness. So less violent in the sense of less vigorous boiling of the liquid.

Andrew

[Dr_GMJNParticipant @dr_gmjn]

So I reprofiled the teeth, heated , and when it was about cherry red after removing it from the flame, dunked it in brine. The outer parts of the teeth felt pretty sharp to the touch at that stage, but in reality the teeth are tiny and it's very difficult to see and feel the edges further towards the pin:

Then heated to straw, and brine:

I tried it in the casting, and in some scrap steel, and it just polished the surface and mashed the teeth:

I re-filed and re-heated etc, but with the same results.

TBH I think all we're doing here now is introducing a whole new load of unknown variables (tooth geometry and heat treatment) into a problem with several existing unknown variables! Plus I'm a beginner…

I hate giving up on things, but unless anyone can see anything obviously wrong, I think I'd be better just buying a spot facing tool if one's available, and leave tool making to the experts for now. Shame, becasue I thought it was looking pretty good.

As ever, comments & suggestions welcome.

Cheers.

[JasonBModerator @jasonb]

Did you definately start with silver steel?

[Nigel McBurney 1Participant @nigelmcburney1]

Pilot spotfacers or pin cutters were used on capstan lathes in the instrument trade for producing parts for microscopes and telescopes, eg the internals of eyepieces where there were bores with steps to take lenses and locking rings,material brass or nickel silver,being straight carbon steel they were run at a lot lower speed than HSS,with full soluble oil lubrication. and would make a thousand parts without sharpening,allwere made inhouse from Stubbs silver steel ,heating was a paraffin blowlamp,quenching was in a couple of gallons of hot water in a galvanised bucket,work at red heat was plunged vertically. tempering was a medium straw colour,no fancy oven just polish the buisiness end of the work with emery,heat the shank of the tool gently until the heat travels towards the cutter end ,the band of straw colour is followed by a blue band, wait until the straw reaches the tip and quench very quickly in tap water,thats how we did it.now some 60 years ago,it was crude but it worked and worked well.cutters up to one inch were made in this manner,they lasted for a long time,though they could be broken if the turret of the capstan was swung bound a bit too quick and colided with cross slide tooling. Stubbs silver steel was always marked with name at one end of the 13 inch bar,and the rule was, cut the the amount required off the other end so you knew it was silver steel,the cutter edges could be filed with very fine files,the preferred method was to use the Clarkson cutter grinder.We never used them on cast iron or steel. if it is acceptable for a spot face on cast iron to be produced by a hollow ground cutter,to prevent any digging in with cutter the lips of the cutter can be groundback to give zero rake.

[Author]

Posts