Material recommendation please

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charkmandler
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Material recommendation please

Post by charkmandler »

I’ve made a batch musical instrument repair collet holders. These were made from stainless discs faced and bored to size and are 22mm thick. The bolt recesses where then put in. The holders need slitting in half and I tried using a slitting saw (hollow ground) but it kept grabbing and jamming. Fine up to medium pitch was tried at very slow speed with different feed rates it was painfully slow. I also did test cuts on EN1a with no problem. Different depth of cut from 1mm up to 4mm were tried, more than 4mm was a disaster waiting to happen. 2 holders were cut like this and took about 25 mins each. In the end I hacksawed the parts and milled them clean - much quicker.

I now have several smaller versions to make where the gap between the holder halves can’t be as wide so hacksawing will not work. The easier option all round would be to make them from two rectangular blocks. Machine the mating faces and create bolt holes, hold the halves together with a 1mm spacer between the halves and bore the hole.

Question, if I were to make these out of 304 stainless, each half will go like a banana when bored - probably after milling the mating faces before boring. Stainless would be the choice they won’t rust but it seems to me that stainless is not an option. What material would be recommended that won’t distort when milled and bored and is preferably free cutting.

Any suggestions of a better way to make these? They can be round or square / rectangular.

There is not a great deal of money in this job so heat treating is not an option.

Thanks for any advise.
Collets and holder.JPG
Russ Hanscom
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Re: Material recommendation please

Post by Russ Hanscom »

Can you post a picture of how you hold them?
charkmandler
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Re: Material recommendation please

Post by charkmandler »

Sorry no as they are done - until the next batch.
Basically the are held vertically in the mill vice with the mid point as near to the vice as possible for rigidity. I then 'slit', attempted, to slit them. I used a pretty weak setup with an EN1a test piece and the saw went through fine. I think it's basically that 303 stainless is very gummy.
Carrdo
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Re: Material recommendation please

Post by Carrdo »

Can use 1144, Stressproof or ETD 150 as alternatives. 1144 is often referred to as Stressproof but they are not quite the same.
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Harold_V
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Re: Material recommendation please

Post by Harold_V »

charkmandler wrote: Wed Apr 21, 2021 3:17 pm I think it's basically that 303 stainless is very gummy.
No, it isn't. I've machined 303 for my entire career in the shop. It is anything but gummy. 410---that's a different story.

You might explore using 416 in the annealed condition. It machines slightly better than 303, and yields a very nice surface if you use sharp tools.

If you are machining dry, you may be experiencing some difficulty. I highly recommend the use of sulfur based cutting oil for cuts. It prevents chip welding and provides for a much nicer finish.

You're dreaming if you think you can find a material that doesn't move when you take cuts. You can expect movement from pretty much all materials, it's just a matter of degree.

You can limit the amount of movement you experience by learning the art of roughing before finishing. If you take cuts on all surfaces, leaving a small amount on each face for finish cuts, the vast majority of movement will take place with the roughing cuts, which relieve stresses inherent to the making of the material. Once you've removed the majority of the material, the parts become far more stable, so the finish cuts don't cause much movement, if any.

Edit: I took note of your suggestion to use 304. It is nearly identical to 303, but lacking either sulfur or selenium, each of which make the material free machining. While it yields a wonderful surface finish, assuming you use sharp tools and run at the appropriate speed, it is more difficult to machine than 303, or any of the free machining grades. I suggest you avoid its use, as well as 316, 321, and 347.

Note, also, that stainless on stainless tends to be a bad combination, as it readily galls. That can give the impression that the material is gummy.

H
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charkmandler
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Re: Material recommendation please

Post by charkmandler »

Thanks Harold, I took your advice from before and made sure I roughed out all surfaces first and the result with the 303 was good apart from the slitting saw. I've always had good results with 303 with good chips but long strings on the lathe with finishing cuts. I will see if I can get some 416 and explore making from two halves. The first batch are done and deliver so I have time to experiment.

How much should I leave for roughing - I can buy the material near size - would a roughing skim all round followed by a finishing cut work or does the roughing cut need to be deeper.
The collets are not from stainless, just mild steel - this is a cost issue/spec from the customer.
Thanks again.
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Harold_V
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Re: Material recommendation please

Post by Harold_V »

The amount of material you leave for finishing should be limited, but enough to ensure that the parts will clean up when finish machined. It's really important that you maintain concentricity and perpendicularity when roughing, so you don't have to make any corrections aside from the minor movement that you'll experience in the roughing cuts. If you can grip the parts nicely without distortion (soft jaws work wonders), you should be able to achieve your goal with only about .030" remaining to be removed in finishing (.015" per side). That should allow for three finish passes, which is my preferred method of finishing when I'm trying to hold a specific size. Fortunately, both 303 and 416 will accept light cuts, very unlike mild steel.

Unfortunately, when you split a part, none of the operations you've performed previously will alter the stresses that get addressed in previous machining. You do have the option of stress relieving by heat, however. That may or may not be worth the time and effort required, especially if you can make the parts from two pieces. That's an excellent way to address the stresses involved.

You should be able to start with material with a minimum amount needed to be removed, which was my preferred method. Stainless used to be sized accordingly, by the way. I used to manufacture precision metering pumps for a blood analyzing machine for Becton-Dickenson. The pumps were made from 1-7/8" diameter 303S, with turns held to ± .0005". American made material was about .003" oversized, belt sanded (centerless belt sander is used), which allowed for a skim cut to round the material and take it to size. I fixtured off the diameter, so size was critical. I had no issues with further movement (my parts were all roughed before finishing, just as I advise), so you should enjoy success with minimal stock removal.

I made mention of the material being American made. One of my suppliers began selling 303 made in Japan, and it was cold finished (not ground). It was also undersized, so I couldn't use the material. Make sure the material you source isn't undersized for your needs.

H
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Bill Shields
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Re: Material recommendation please

Post by Bill Shields »

Cut them with a wire edm
Too many things going on to bother listing them.
charkmandler
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Re: Material recommendation please

Post by charkmandler »

EDM is not an option as the numbers won't make it economical, if lots more orders come in for them then maybe but for now orders will only ever be now and again.

Thanks Harold, you've given me a plan. I will stick with 303 as the finish is good, its been easy to machine apart from stringing on finish cuts, and virtually no distortion after cutting in half.
I will rough out, put in bolt holes, then saw in half. Clean off the halves and bolt together around a mandrel to turn the od back to a circle and face the sides. Then mount in soft jaws to bore the hole to size.
John Hasler
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Re: Material recommendation please

Post by John Hasler »

Make a mandrel that is a sliding fit into the part. Mill shallow slots or flats of a convenient width and depth into the mandrel. Put the part on the mandrel and slit along the slots.

Or you could skip the slots and just cut into the mandrel.
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Harold_V
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Re: Material recommendation please

Post by Harold_V »

charkmandler wrote: Thu Apr 22, 2021 9:25 am I will stick with 303 as the finish is good, its been easy to machine apart from stringing on finish cuts,
That's one of the negative aspects of finish cuts. It's virtually impossible to create a functional chip breaker that would avoid creating those strings. However, if you keep the part wet with sulfur based oil and direct the chips with an acid brush, you can usually avoid any surface scratching. A ten thou pass generates a pretty fine chip. That said, nothing precludes a light spin polish if you find it necessary. 303 responds quite nicely to being polished, as it doesn't tear (assuming sharp tools), so it takes very little to bring up the finish.

Good luck!

H
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johnfreese
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Re: Material recommendation please

Post by johnfreese »

I would split them on a vertical band saw.
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