Hi Ken,
No problem, will update as I get any new info. My question has effectively been answered a few post back. I am looking at milling parts that doesn't have to be worked on by hand after its milled apart from say polishing and anodising the parts. This will definitely cut down the cost of the parts.
Blind hole broaching can be done by rotary or wobble broaching - check it our at www.rotarybroaching.com. Although, I believe the form we're talking about here is way to big for rotary broaching, the limitation is usually forms about 1" dia.Originally Posted by turmite
I had a look at the website and I still don't understand how they can broach a part by simply turning the whole piece. I notice the part to be broached is turning and the tool is turning too. So what they effectively is doing is pushing the tool onto the part. But why the spinning? I don't get it. But its really interesting that they did it.
The reason why it works.
The Rotary Broach Holder holds the broach at a 1 degree angle to the part. So, in the diagram below, only the bottom corner of the broach touches the part at any given time. When it is indexed into the hole, it cuts away the form , bottom corner first. As the part rotates, it drives the free turning spindle inside the holder, and cuts every corner of the form at the same point, where the broach is closest to the part. Click the image to watch our video.
Still confused. I would really have to look at the operation in person to grasp the technology. Interesting but probably beyond what I need to do. Thanks for the info.
Hi, just my tuppence worth, whenever a part is going to be made it is always designed around the machining process.
Designers don't work this way, and I've yet to see a pencil mechanic that fully understands the limitations of machining.
At some stage in the procedure you're going to come up against the square corner/blind bore syndrome and there isn't a machine tool that can overcome it.
Looking at the part in the first post is a typical example of the great expectations, and if you break it down to square versus round planes, then if the two "keys" and recessed slots were not in the design, it is just a simple bit of turning with the square end hole only needing to be broached out square.
But now we have two thin keys and two broad recesses that have the impossible flush bottom and square corners to contend with.
What was the designer thinking of when the design was proposed?
A "magic" tool? You wish. I'll have half a dozen.
To make this part exactly as the designer envisaged it would need a prototype carefully finished in all aspects and then sent out to be investment cast.
There was no mention of that most important factor....VOLUME.
How many parts are you going to make?
If it's just a few for a specific project then dedicated machining operations to get the shape out are needed. No one machine will do it, at least not when I was a boy, and not much has changed even with the magic of CNC.
However if it is to be a product and an ongoing manufactured item, then the volume will definately determine how it will be made.
So how are we going to redesign the part? I'm sure the panel of experts we have here, have some good ideas.
To start off, make the two "keys" that protude into the bore a seperate part that are attached to the shell.
The two recesses could be formed by making inserts that are "glued" to the inside of the shell and so form the two recesses, square corners and blind bottom all in one.
The square hole in the end is simply a broach operation that is done last.
What we have now is a simple turning opn to produce the two parts and that is where the CNC scores. The rest of the bits are just added on.
There must be dozens of ways to do it.
Personally I like the investment casting for a volume job or fully machined in parts if the volume is only a dozen parts.
Ian.
Hi Ian,
In fact my question was answered a few post back but appreciate the comments you have put forward. I am glad you said what you said. When I first started the design I knew nothing of CNC milling. And as far as I was concern at the time, anything is possible with CNC milling. I had the prototype made and the company that fabricated it did an excellent job of getting the parts to my exact specification. I was amazed at what CNC can do.
But while researching on the subject, I started to realise the limitation of CNC and wondered how the fabricator actually did the square section for the recess and the keyway. It turned out to be just simple hand tools and a lot of hard work.
I intend to produce this in large volume, hence the importance of my design change. If I have to manually work on the parts after its been CNCed, I think I will go nuts. I haven't started on redesigning yet but will work on it soon.
I am just glad to know I don't have to use an expensive piece of machinery to produce the parts. I don't think the broaching technique discussed earlier would help me int his instance. For me redesigning it is easier than having to get the parts fabricated in an expensive manner.
Thank you all who have helped me to understand this topic alot better.
Hey, I resemble that remark
You are correct [and this is coming from a designer] that we don't know all the limitations of machining and its operations ALL THE TIME. However, the difference between an average designer and a 'good' designer, is the questions that are asked. A good designer will realize he has limited knowledge of the operations involved as well as the available tooling, machine capabilities and available material. [how many designers out there like to build things from Unobtainium??] I regularly go down to my machinists and show them the 'basic' part profile and the prefered mat'l. I tell them what I'm thinking, and ask them if this is realistic or not. Some times they will add a little something here or there to simplify their job or save a tool change. Sometimes I can't comply because of other issues, but the reality is, it's a team effort, an open dicussion and the machinist realizes that I'm trying to work with them. They also get time to think over the project and it's happened where they come back to me a couple days later and ask if we could do it 'this way' and its a brilliant idea, sometimes it still won't work..
I have the advantage to be able to go down to the machinist's and ask, some people won't as they farm out their machining, even still I'd be contacting them and asking.. [I have in the past before we got our own machine division]. To some this would be viewed as admitting you don't know what your doing, but to the rest, they realize that your making the job a team event and appreciate being included.
We all know that designers can design stuff that no-one has a hope in heck of ever building [well.. other than NASA engineers
This isn't ment as a flame in ANY way, I just thought I'd add my perspective from a 'designers' point of view..
Jerry [machinist in training...]
JerryFlyGuy
The more I know... the more I realize I don't
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
Hi Jerry, I totally agree with your point of view.
What a lot of people don't realise is that a good designer is really an artist, in that he/she can visualise something, even if it is totally impractical and impossible to make.
This is where the 'Nuts and bolts' professors come in. By this I mean the guys who work the handles on the machines and also have an eye for practical detail and can see the end product from a 'making' point of view.
The two types of people are what it takes to finalise a design, and when you have the aloof attitude that some designers adopt when it comes to a pet project, where they think (the higher up the academic ladder you go the more God-like they become) that they know it all, then you end up with the "lead zeppelin" syndrome. Looks good, easy to make, costs little, won't fly.
Designing is an art form, and the balance between the inspired doodle on a scrap of paper and the final assembly is a combination of breaking down the impractical design points to the practical manufacturing aspects.
Without a design the item you want to make will not be made.
How many times have we "designed" something and at the lathe realised the damm thing just won't work.
Take the item at the opening of this thread. For a start, it should not have been put out to tender before the "nuts and bolts professors" had a look at it.
With this action you'd get a lot of hints on how to make it, and from this you would know what you can't do.
Ian.
Hi Alex, just another comment, as this item is so interesting, how about making the body as an alluminium extrusion with the internal keys and the end part with the broached hole as a seperate turned and broached item?
This is tig welded at final assembly.
The only item 'out of the ordinary' would be a simple extrusion die, which is straight forward manufacturing by spark erosion etc.
The end cap would also be a straight forward CNC turned and roto broached item.
Ian.
Hi Ian,
Probably would work but it may be a bit out of my league. I am in the process of getting the design redone to make it more suitable for a mill to work on. Will post the pic when its done. Thanks.
Alex
Ian you said it very well.. I've learned alot in the last 7 yrs working in a steel fab shop, we've added a machine area in the last two yrs and I still catch myself missing little things. I wanted a guy to chuck up some 5" o.d. pipe and he had to tell me he couldn't 'cause the head stock is only good to 4.625" I knew that... but had just forgotten it.. Anyway, now that my own mill is coming on line I hope I can get the hands on experiance to be classified as an Artistical Nut/Bolt Professor
Best
Jerry
JerryFlyGuy
The more I know... the more I realize I don't
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
Is the square corner (i.e. with little or no radii) to allow clearence for a mating piece? If so can it be machined out completely by drilling holes at each corner?
Just a thought.
A.J.L.
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