|4536 forum posts|
I want to core drill a brass plate thus:
The central pillar is 4mm diameter and the slot 1.6mm wide and 4mm deep.
I made a core drill out out of silver steel as shown but with teeth filed into the cutting edge. (Failed to draw a crown in FreeCAD and cant't do it in Fusion 360 because I've broken Windows10 as well. Not having a good day.)
I heated the silver steel bright red and plunged it into cold water. Testing with a file showed the steel was still a bit soft so I repeated the treatment, holding the steel red-hot longer (over a minute) before cooling in water again. This time the drill resisted the file.
At first drilling into the brass plate went well but after about 2mm the end of the drill disintegrated. Nothing jammed in the hole - the bits fell out.
I suspect the heat treatment made the silver steel very brittle, a problem I've not had before. Any ideas what I'm doing wrong?
|Andy Carruthers||08/03/2019 16:50:28|
249 forum posts
Would tempering help? or plunging into oil instead of water
|1216 forum posts|
I wonder whether after the first, unsatisfactory, hardening you should have first annealed the work before having another go?
I assume you tempered after hardening?
Edited By ega on 08/03/2019 16:53:49
|not done it yet||08/03/2019 17:04:51|
|3166 forum posts|
It is what happens when trying to use a cutting tool in the brittle hardened state. Temper it.
Also did you add any clearance? - the chips need to go somewhere, so I expect they jammed the cutter while trying to 'disappear'!
|Mick B1||08/03/2019 17:20:04|
|1128 forum posts|
I've also found I needed 2 tries to harden a silver steel cutter I was making, and I too had to soak it for a minute or so at red heat for it to 'take'.
Tempering is a careful re-heating till you get a light-straw surface oxide colour - obviously you gotta polish up enough surface to be able to see that..
As NDIY says, the chips have to go somewhere and arguably the teeth you file need to be a good deal deeper than the groove you're cutting to give them an escape route. I'd go for 2 teeth on first try and maybe turn up a couple of blanks to allow for failures.
It'd be difficult to provide side clearances to the teeth, and hopefully you might get away without. I'd guess the biggest risk is the centre column of the brass getting hot and expanding inside the cutter to jam its teeth?
Edited By Mick B1 on 08/03/2019 17:23:46
|Mike Poole||08/03/2019 17:28:47|
2019 forum posts
Plunging from red heat into cold water should make the steel glass hard and brittle, to make a tool that will not shatter then further heat treatment is required. The tempering process to relieve some of the hardness and improve the toughness involves cleaning the steel so it is bright and heating gently until you see the steel surface change colour. Colours will range from a light straw through to a dark blue. Light straw keeps most of the hardness for some improvement in toughness and dark blue will trade the most hardness for the greatest toughness.
|Martin Connelly||08/03/2019 17:54:04|
847 forum posts
Were you clearing out the chips to avoid clogging the gullets?
|Brian Wood||08/03/2019 18:05:57|
|1931 forum posts|
If I might add a comment or two as well.
The teeth need some 'set' on them both inwards and outwards, like the teeth on a wood saw, to give them some clearance from the plate. It doesn't need to be much but it will prevent the plate material grabbing the cutting tool as the depth increases. Do all that shaping before the heat treatment
Some inaccuracy in tooth spacing is also a good design feature as it adds a random nature to the cutting and prevents chatter and ringing getting established.
I would also recommend building a shallow dam round the site with plasticine or something like it to drill the hole submerged in water. Low speed, frequent chip clearing and enough end loading to keep the teeth cutting
Edited By Brian Wood on 08/03/2019 18:06:49
|Andrew Johnston||08/03/2019 18:48:30|
4722 forum posts
I've done something similar in brass, albeit larger diameter, about 1/2", and deeper, 3/8", at the bottom of a 3/4" hole:
First on hardening and tempering; you will not get anything like full hardness simply plunging into water. You need to agitate vigorously so that the cold water quickly replaces the steam that is generated. I use brine rather than water as it gives a less explosive quench, but you still need to agitate. Doing the above should give a hardness better than 65Rc. The tool then needs tempering to remove brittleness. I can't remember what temper I applied to the above tool, but probably a straw colour around 220 degrees Centigrade.
The teeth were hand filed before hardening and tempering. I didn't bother with any tooth set and had no problems with grabbing. I did my cutting in the lathe with the tool in the tailstock - straightforward apart from frequent withdrawal to release the swarf.
842 forum posts
Hi Dave [SOD] I use this type go cutter a lot cutting steel, gauge plate EN45 or EN9 mainly, some diameters similar to yours but not as deep, also some with a much smaller centre pin. For my purpose the cutter is a rougher and I finish machine the part in the lathe.
I use silver steel, harden in oil and temper to a pale straw in DAYLIGHT - not artificial light. I have made 4 tooth cutters but from experience make your cutters with only 2 teeth this gives more room for chip clearance, you need plenty of oil and clear the chips frequently. Probably for brass you can cut dry ? I cut the teeth by hand with hacksaw and file, it can be done by machine if time permits, however in my apprentice days we were taught how to make spot face and reverse spot face, countersink cutters etc by hand.
One more point is bore the centre hole of your cutter to the size you want, don't rely on drills and or reamers -- there is always a small bell mouth. For my smaller centre pins where its not so practical to bore I face off say .100" to negate the probability of a bell mouth.
PS I'll take some photos tomorrow
|Martin Hamilton 1||08/03/2019 19:42:56|
|108 forum posts|
For 40 years when i built & raced tethered hydro's i done a lot of hardening & tempering of silver steel. The ball & socket couplings that we made needed to be hard but tough including a very small pin in the ball side of the couplings, used to heat red hot & plunge into cold water. That made them very hard but to brittle, to temper them down i would clean the outer surface of the bit of scale that had formed with emery cloth to bring the metal back to a shiny colour then heat to straw colour & plunge into oil. Old used motor oil used to work well but also used new motor oil if i had no old stuff about, never ever had any coupling failures or excessive wear.
|Martin Hamilton 1||08/03/2019 19:53:03|
|108 forum posts||
I said that the wrong way round ( senior moment ), part would be heated to red hot & then plunged into oil. Then cleaned up & reheated to straw colour & then plunged into cold water, brain cooked with all the methanol in the fuel.
|Chris Trice||08/03/2019 20:25:05|
1362 forum posts
Most household ovens go up to tempering temperatures so no need to check colours or trying to heat evenly with a flame and potentially undoing all the good work hardening it. Put it in, cook it for a while at top temperature and then remove.
|Martin Hamilton 1||08/03/2019 21:04:17|
|108 forum posts|
When i used to make waterscrew hydro props cut from solid bar i used to harden & temper in the same way as the couplings that i described above except the tempering was done slower in the oven to straw colour over a longer period in the household oven. I done this in the oven so that the very thin blades were an even temp all along the props, using a direct flame got uneven heat in the thin blades towards the tips of the blades. These props were not silver steel as silver steel was not tough enough due to the loads put on them turning @ 30000rpm in water, i used en24T for props to cut them from followed by the hardening & tempering.
Edited By Martin Hamilton 1 on 08/03/2019 21:05:20
|duncan webster||08/03/2019 23:48:53|
2167 forum posts
Get hold of a copy of Tubal Cain's book on hardening, tempering and heat treatment, it explains everything. You can borrow mine if you want, but I want it back as it's very good.
3651 forum posts
I think I would look at simplifying the way the job is made and do away with the need for a special tool.
As we have no idea of the application I'm only guessing, but something like drill a hole through the square block then turn up a stepped pin that is then held in place by press fit/Loctite/silver solder etc. Or drill and bore a stepped hole through the square block and secure a straight 4mm diameter pin in place similarly. You could even drill and tap the hole so the pin is screwed into place and then then loctited.
|4536 forum posts|
Brilliant answers as always chaps! My Mk 1 tool had six teeth, which would have weakened the drill considerably, especially as the heat treatment needs attention too, Two teeth next time. Also, I seriously underestimated the need for very frequent swarf clearance. Believe it or not I had the idea it would go up the hole inside the drill: pretty stupid huh.
I've got lazy about tempering. Usually my silver steel efforts are specials only used a few times on brass or plastic and I've not had a failure before. I think they've been tough enough to do the job only due to luck.
Can't get into my workshop until Monday but I'll report back asap. Meanwhile, my attempts to fix Windows 10 have made the problem much worse.
|jaCK Hobson||09/03/2019 10:08:23|
|164 forum posts|
For the current application I think the tool would be fine if you just added a tempering cycle at 220C as suggested.
A few guidelines for next time:
You need to heat until the steel is non-magnetic at least - that is easy and reliable to check. "'red' heat looks different depending on how bright your surroundings are.
Speed of cooling also depends on size of part - bigger parts may well need brine and agitation. A watch pivot would probably be hard in air cooling.
Toughness depends on grain size in the material. You can manipulate with heat treatment. Heating too high and too long will increase grain size. Repeating treatments just above critical / non magnetic will decrease grain size. Fine grain is much better for toughness. Big grain makes it a bit easier to harden. My experience with silver steel is that it is difficult to get really fine grain (compared with O1) - I speculate that this may be intentional to help machining?
You can get a soft skin on heat treated material due to carbon depletion - an initial file test can fool you - sometimes you will find it is hard under the skin. More likely to happen if part was heated high for a long time (as could happen if you forged something or in initially creating the raw stock).
|Brian Wood||09/03/2019 10:27:40|
|1931 forum posts|
|Andrew Johnston||09/03/2019 10:43:29|
4722 forum posts
To add some more detail on hardening silver steel, although I'm fortunate to have an electric furnace curtesy of Fleabay. I heat to about 780°C and let it soak for tens of minutes depending upon thickness. I then quench in brine with vigorous agitation. Then I let the furnace cool to tempering temperature and let the part sit at tempering temperature for at least an hour. Then quench in brine.
SInce I agree with Lord Kelvin on measurement and understanding I use a set of special files to measure hardness:
Tests show that if you vigorously agitate the part when quenching you can achieve a hardness better than 65Rc. if you don't agitate then the part is more likely to be around 40Rc. In this picture of home made cutting tools the tap bottom right illustrates what happens if you don't harden properly:
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