D bit grinder work head calculations
D bit grinder work head calculations
- This topic has 55 replies, 20 voices, and was last updated 14 May 2011 at 18:52 by
TSH.
TSH.-
Peter Tucker
@petertucker86088John Olsen
@johnolsen79199Lawrie Alush-Jaggs
@lawriealush-jaggs50843
I’m baaack…..
Sorry to have been tardy in my replies to everyone, I’ve had some emergencies with customers Servers.
I am very pleased to see how tenacious people are. It is hard to keep good men down.
Dusty: Thanks for your reply. I am going to be long winded again and again it is not going to be Engineering. This time it is Psychology, just for a change.
Imagine yourself going out into the workshop and looking for a hammer. What is the picture in your mind? The chances are very high that you will see a side view of the particular hammer, not a view of it from the cross pein, the end of the handle or the face of the ball. As some people will attest, as they grow older they find it more difficult to find things with which they are very familiar even when they are in plain view. Why is this? It is frequently because the image they have in their minds does not correlate to the way the tool is positioned. So if they are looking on the workbench for the hammer and it happens to be presenting with the head towards them and the handle either facing away or lying under something else, they do not see it. It is not until they start processing other images of hammers that they see it. This sort of thing is well documented by such luminaries as Dr. Oliver Sachs.
In the first picture – the one that started this whole thing off, I drew one several images I carry in my head of what a D bit looks like.
Think about the production of a D bit. (I am assuming the use of a bench grinder here rather than a T&C grinder.) The first picture that comes to mind is of the blank, a peice of HSS or silver steel. Then applying it to the grinding wheel so that we see a side elevation of the bit as it comes to life. The next is probably of holding and measuring it in profile with a mike. The next is of holding it with the ground face up and grinding a clearance on the end and finally of touching things we shouldn’t – sorry, touching things up with an oil stone. Almost none of the time is spent looking at the tool from the end on.
So the images one carries of a D bit are predominantly side and face views, rather than the view that I am talking about, the thing that defines what the tool is called. That when the tool is viewed from the ground end on, it’s shape resembles the letter D.Please have a look at the following picture. In it you will see two cutters. One an unrelieved D bit and a relieved one.Dusty: “Once you grind clearance on the tool the shape is no longer a ”D” and can therefore no longer be called a ”D” bit, you will have to call it something else”
Why? In the picture you will see eight variants of the letter D. It is accepted that all off these shapes constitute a letter D. All of them are far more distorted than the grinding of a relief on a cutter will yet cause no problem. None are called something else. We do however make branching distinctions like in the tree of Life.
Firstly we have a letter D. Then we have a type face. Then we have an upper case and a lower case letter D, next the style or font – bold, italic, superscript, subscript etc.The same is true of tools. We have a drill bit, then a style of drill bit – straight flute, twist, spade etc. Next we go to purpose – masonary, wood, metal etc.
I the case of a D bit I am quite happy to accept that there is a class called engraving bit but an engraving bit which when viewed end on and has the form a letter D is still a D bit in the same way that a tungsten carbide tipped twist flute masonary hole maker is still a drill bit.Lawrie Alush-Jaggs
@lawriealush-jaggs50843Part the nextI am still stubbornly calling the item I drew a D bit and I think that no amount of proof on my part will change some peoples minds and maybe that is as it should be. After all what we are discussing is really not of consequence. What matters to me is that some people did take the time to kindly respond to my question and I have had some very good suggestions.
It does though go to show why it took sixty years to not come to a descision about a Universal Thread.Clive: You replied with a rather complex answer for which I thank you. The problem is not as difficult as you point out though because to tool does not stay clamped in the one position for the whole of the production time.
Peter: No, Im not out of the game yet, I’m still having fun!
I don’t have any problem with the geometry of the cutter though I thank you for your excellent description of it.
John: I agree with you about there being some leg room as far as the actual amount of clearance the cutter requires. The point you make about the strength of the cutter is well taken. In the past I have been a tad grinder happy and made cutters that did not perform to expectations. It makes you glad for eye protection when a peice of carbide snaps off and flies away propelled by 20K RPM.
I like your example of two scuttles though it does not negate my argument. I agree about dictionaries and lexicographers needing to get out more and broaden their reading and that most languages and English in particular are filled with words that mean different things, the meaning of which can only be gleaned in the context in which they appear. In Thai the word for Near and the word for Far is (approximately) “Gluay” There is a slight difference in the inflection that even many Thai’s have difficulty hearing.You are probably right in saying that what one trade names as a thing another names another.
Some of the respondants to my question have stated that the item I supplied a picture of is an engraving cutter and not a D bit. That is what got this whole sorry mess going in the first place
“Nature abhors a vacumm” stated Aristotle and his view held sway for 1,600 odd years. The Church loved him because one of his other ideas was that physical expermiment was a bit beneath anyone with a brain. All that could be known could be deduced by thought alone. Which is fine for Theology but not much use for electricity or steam. Or in the case of Otto Von Guerik, an air pump.
One of the points I have been making ad nausiem is that the term D bit refers to the shape of the item when viewed end on, not side on and no-one appears to have taken up the challenge.
So: Someone make up a taper profile releived edge D bit and the view it end on and tell me what it looks like.Through all of this I think I have obtained the information I require. So now I will go back to the workshop and finish off the router table I am making, then make the dial scriber and the …..a nad the …….. and eventually the D bit grinder work head for the T&C grinder I haven’t made yet.Thanks everyone.TSH
@tsh73987TSH
@tsh73987Another attempt to post.Hi Lawrie,
What a long thread! I’m not very good at following people’s textual explanations so I offer this on the basis of my (probably incorrect) understanding. Essentially it is along the lines of Andrew’s very first reply. I have included three sketches:
The first shows the basic geometry with the tool offset in the sleeve and offered up to the grinding wheel.
The second shows the effect of now rotating the sleeve complete with the tool-the hatched material is removed but the clearance angle is still zero.
To get a positive clearance we need to move the centre of rotation upwards and then towards the wheel enough so that the tool just touches it. This is equivalent to rotating the sleeve and tool together about the centre of the tool. This is shown in the third sketch.
I’m going to call this angle t, and the resulting clearance c. I’ll call the offset z and the radius of the tool r.
Then an equation which connects these is
z sin c cos t + z sin t cos c = r sin c
So if you have a desired clearance c this equation will give you the required rotation t. Unfortunately I don’t think this can be solved explicitly but a tool like ‘goal seek’ in Excel does it easily. Here’s a table for an offset of z = 1mm and a tool radius of r = 5mm.
Peter Tucker
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Lawrie Alush-Jaggs
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