D bit grinder work head calculations

[Clive HartlandParticipant @clivehartland94829]

During my work as an engraver which was part of my job description, the engraving cutters were placed in a holder that was then placed on the angled tool holder that went against the wheel.

The holder had a set quadrant that allowed the cutter to be rotated against the wheel.

This gave the cutting edge and the relief required by engaging the wheel and moving it forward into the wheel.

By rotating the holder the rest of the metal behind the relief angle was ground away.

Then the angled tool holder was rotated to a greater than 90 deg position and the tip was moved forward into the wheel to give the tip clearance.

This then gave you a cutter that cut cleanly and depending on the width of the tip grind a thin or wide engraving.

This was all preset on the grinding jig and it also had a holder for two or four lip endmills up to about 6mm. The settings gave the correct clearance once it was in the tool holder.

There was a set of single quadrant and two quadrant and four quadrant discs that were clamped to the end of the tool holder to facilitate the type of cutter being ground.

 

Once the cutter clearance has been ground, further rotation just took off the un-needed metal behind the lip.

No offset was required for this at all.

 

Clive

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hello Evry Bardy

I think that what we have established thus far is that everyone’s definition of what constitutes a D bit is based on their experience.
David Clarke has kindly offered his technical opinion which as he states is based on his experience.
Hansrudolf’s experience is that of a D bit which cuts at the front as he mentions both a Cannon Drill and his twisted D bit.
Ramon has added a photo of the tools he has made for his excellent Bentley engine and brings up his recent experience of seeing engraving cutters made.
Terry similarly to Hansrudolf says that an engraving cutter is not a D bit.
I think the reason we are having so much difficulty with this is down to taxonomy. I will give three examples of why this is a taxonomic problem.

1 Consider an end mill. We have absolutely no problem with the discussion of what constitutes an end mill. Basically it is a cutter derived from the twist drill in which the face can have a profile (flat, ball) which cuts material. In that, its end cutting capacity is similar to the twist drill. The second similarity is in having a helix. Unlike a twist drill though, the edges of the helix are ground to permit machining.

2 Drills. The twist drill and the spoon drill. Both cut at the face. The twist drill really cuts where the spoon drill is not really much more than a rotary waste of time.

3 The Slocum Centre Drill. There were up to the introduction of this tool more than ten styles of device all designed to perform the same operation, none of them anywhere nearly as effectively. The Slocum Centre Drill, which is a derivative of the twist drill won out.

In each of these cases we have no dispute as to what name means which tool. The reason I say that this is a taxonomic problem is because the D bit is the only one which is named specifically for its form rather than its function.
None of the functions to which the tools everyone here has described with the exception of the engraving cutter, is encompassed in the name of the tool. Excluding the engraving cutter might seem to nullify my argument except for the fact that there are several forms of engraving cutter of which the D bit is one.
The term End Mill describes the function and not the form. As in the case of Drills, the verb is the same as the noun. The name, drill describes what it does, the function of drilling. We then add an adjective to that to describe the form, Twist, Spoon, Centre etc.
In the case of the Centre Drill, we have dropped the name of the Inventor, Slocum and just gone with the function. Unlike the Twist Drill, the adjective Centre does not describe the form, instead it describes the purpose.
The D bit is named for its form and nowhere does it describe any sort of function. Its name is a noun and unlike a drill or mill does not correspond to a verb. It simply names something for its shape, a piece of stuff which when ground and viewed end on is in the shape of the letter D. Nothing more, nothing less.
The use to which one puts them does not alter the basic fact of the name. If you use one to drill deep holes then one may choose – rather cumbersomely but accurately– to name it a deep hole D bit drill.
The fact that someone chooses instead to cut at the edge rather than the end and use it for engraving or for reaming does not change that. The definition of a D bit is that when ground and viewed end on it has the shape of the letter D.
I was incorrect in an earlier posting in saying that a Gun Drill is a form of D bit. Early Gun Drills like the Cannon Drill referred to by Hansrudolf certainly were D bits but the devices supplied as Gun Drills today, and after looking at my 1940 edition of Colvin and Stanley’s American Machinist’s Handbook are not.
Now can we please get back to my question which was one of geometry, not taxonomy?
MGJ: thanks for your reply. Again, the question is not about the geometry of the cutter per se, but making sure that when I move the cutter by a cam action to one side by .Nmm, the scale I have inscribed corresponds to an angle relative to the ground face of said cutter.
You have given me one clue and that is the have a look in The Other Book – Prof Chaddock’s, not GHT.
I don’t really have the inclination to build a Quorn though.
Ramon: thank you for your as always gentlemanly and courteous reply. Thank you also for the offer to find out more about the machinery you viewed. As I mentioned in an earlier post, I am familiar with them.
Andrew: I think that Donald in mentioning Mr Thomas is referring to Terry as in Terry Thomas.
Steve: In response to your first reply. In using a D bit grinder to grind the clearance angle on the cutting edge, you rotate the barrel I described which is in effect a cam. This gives you an offset in one plane moving the cutter past its centre line. You then rotate the cutter through ninety degrees, lock the collet again and then proceed with the grinding. This give you the second axis.
Chris and Steve: I agree. I am approaching dodderhood, well, a way off yet but I can see the sun moving closer to the hills. In spite of my earlier comments about old men making putt putts that go toot toot, the part about staying senility was genuine. More than that I really enjoy all of this stuff and it is only conscience that occasionally rears its head to suggest that I make something of value to others around the house.
Donald: Thank you for your post. I hope that you can elicit something useful from your engineers at work.
Norman: I’m pleased that you are not offended. Thanks. As to the

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Sorry, ran out of space.

 

Norman: I’m pleased that you are not offended. Thanks. As to the type of grinder……… When I get it made, I shall ask you to name it.

So. Can anyone help me with a geometry question?

The attached picture is something like what my some described to me. He was very excited that he had come up with a solution as was I. However we haven’t had time to discuss it further so I don’t quite understand how it works other than that it looks as though you make tangential measurements in the X axis relative to the centre of the cam and that as the cam rotates, the centre changes in the Y axis and that if you stick your thumb in the pie, you call out “What a good boy am I” Or Bingo or something.

Any takers?

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hi Clive

 

Thank you for your reply.

 

“The holder had a set quadrant that allowed the cutter to be rotated against the wheel.

This gave the cutting edge and the relief required by engaging the wheel and moving it forward into the wheel.

By rotating the holder the rest of the metal behind the relief angle was ground away.

Then the angled tool holder was rotated to a greater than 90 deg position and the tip was moved forward into the wheel to give the tip clearance.”

 

It is a cam action that provides this. A standard D bit drinder of the type you describe is designed to grind D bits in that it does not provide for greater clearance angles than those which are commonly encountered in engraving. I want to be able to set a wider range of angles so that I can also grind other types of cutters easily like router bits.

This is the reason for the original question.

 

Lawrie

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hi Clive

 

Thank you for your reply.

 

“The holder had a set quadrant that allowed the cutter to be rotated against the wheel.

This gave the cutting edge and the relief required by engaging the wheel and moving it forward into the wheel.

By rotating the holder the rest of the metal behind the relief angle was ground away.

Then the angled tool holder was rotated to a greater than 90 deg position and the tip was moved forward into the wheel to give the tip clearance.”

 

It is a cam action that provides this. A standard D bit drinder of the type you describe is designed to grind D bits in that it does not provide for greater clearance angles than those which are commonly encountered in engraving. I want to be able to set a wider range of angles so that I can also grind other types of cutters easily like router bits.

This is the reason for the original question.

 

Lawrie

[mgjParticipant @mgj]

Lawrie – sorry I thought you had a Quorn.

 

To be honest, I reckon your best bet is simply to draw it out with a decent pair of dividers and a protractor at x2 or so. Once you have done that and got sizes into the right parish, I’d put it through Excel and you will get your numbers very easily.

 

Head on you have a fixed radial relationship – its just a matter of getting the cam in the right place. Once you know where your pivot point is you can derive the change in radius at hte points you want to use – work in radians and not degrees and its the back of a fag packet calculation?

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hi MGJ

 

Thanks for heading back onto the topic. I have to agree with you and had thought that comething like that may have to be the way to do it.

As a Mathmatical numbnuts, what is the difference between radians and degrees and why is it easier to use them?

 

Thanks, Lawrie

[Clive HartlandParticipant @clivehartland94829]

Lawrie, I found that I could get much more cutter relief by just increasing the depth of the engagement with the wheel. The cutting edge clearance was then more pronounced. Basuically I would grind the cutter to the materiel and sometimes it was two or three tries to get a good cutting edge on the specific materiel.

One of the reasons was that when cutting alu. sheet and making templates the alu. would build up on the cutter edge.

Remember that the rpm of the cutter when engraving is in the +20000 range.

Also on some soft materiels I would use a quarter shank cutter because of the extra clearance. No chance of the trailing edge getting fouled up with the chips coming off and if it was plastic it would be very soft and sometimes melt.

The engraving cutter only had a cutting edge at the cone point and if I was cutting out a shape and using a parallell cutter I would then sharpen the whole length of the cutter.

The machine was an Alexander No2 and the Cutter Grinder was a Deckel.

The Talor Hobson cutter grinder was much the same and gave similar results.

 

Clive

[Anonymous]

Hi Lawrie,

 

The glib answer is that the difference between radians and degrees is a bit over 57.

 

The degree is an arbitrary unit, and a circle contains 360 of them. Each degree is divided into 60 minutes, and each minute is divided into 60 seconds.

 

The radian is an SI derived measure of angle. It is defined as the angle a segment of a circle of radius r subtends when the arc length is also r. A full circle contains 2pi of them. The transcendental functions (sin, cos, tan etc) are naturally expressed in radians and such things as angular velocity are normally expressed in radians per second.

 

To convert from radians to degrees: multiply by 360/2pi

 

To convert from degrees to radians: multiply by 2pi/360

 

Hope that helps.

 

Regards,

 

Andrew

 

PS: So that means that 1 radian is 360/2pi degrees, which is about 57.3

Edited By Andrew Johnston on 07/05/2011 15:41:05

[NobbyParticipant @nobby]

Hi

This may not help ,but on the chart for the Alexsanders(Decal copy) d bit grinder its says on a 1/4 d bit the fflat should be ten thou plus not on the C/L

Nobby

[Clive HartlandParticipant @clivehartland94829]

Thats interesting, the cutting edge would then be ground with the lip forward of the centerline ensuring a clearance.

When mounted in the cutter holder to grind the cutter and the quadrant fitted to the holder and located against the stop peg, the lip is presented to the grinding wheel not at right angles to the flat on the cutter, the flat iis sloped back from the face of the wheel and this gives more relief to the cutting lip.

 

Clive

[mgjParticipant @mgj]

Lawrie- a radian isn’t just a 2pith of a circle in that sense. Andrew is absolutely right in saying there are 2pi radians in a circle and therfore a rad = whatever it is degrees. I never know and it doesn’t matter. Divide 360 by 2pi)

 

The point is that if you know the distance at the circumference that you are rotating through then

 

distance at circumference/radius = angular rotation in radians. Convert for scale in degrees.

 

Multiply or divide by 2pi and you are moving from rads to deg and vice versa with ease, and its so easy to get the angle in rads because you can relate it directly to distance.

 

(but see note below)

 

So if you need so much relief in mm or decimals thereof, and you know diameter, you barely need to take your socks off to calculate an answer in degrees, because you are not using an arbitrary unit of angular measure any longer, but the relationship of circumference to radius.

 

Its not a thing used much in non engineerlng applications, but its the standard unit for using in calculations involving rotation, angular momentum etc, so a lot of people won’t have met it, but its very easy, and 5 mins with a calculator for practise and you can get distances and angles with ease.

 

Then you can mount your eccentric non eccentrically in the dividing head and mark away in degrees. Obviously the amount of relief applied will vary with diameter for a given angular measure so you’ll need a little chart to relate degrees to amount of relief per unit diameter. But with 2pi at your side thats a 2 second calculation.

 

Thankyou Pythagoras – did he discover pi or first calculate it or somesuch? Think so, but could well be wrong.

 

(there is an error in my statement above because I am actually using a chord and not an arc,  but for small proportions of the circumference it wont matter.)

 

 

Edited By mgj on 07/05/2011 21:32:11

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hi Everyone

Thank you all for your replies.

Clive: Thanks. The workshop in which I did engraving had six Deckel 2D’s a Deckel 2 1/2 D, two Alexanders and a Dahlgren computer driven machine. We didn’t much use the Alexander as although it was a copy of the Deckel it was just not as good. The bearings were not as smooth and the lapping on the spindle was not quite as accurate. That may have been down to age or just that we all seemed to prefer the Deckels. We had two Deckel grinders. I understand what you are saying about different materials but in this case my problem is not about the end to which the cutters will be put, rather an aspect of the machine used to produce them.

MGJ: Thanks for your lucid answers. As you say, some time spent with a calculator should get things right.
I have a further question for you though if you don’t mind.

“Obviously the amount of relief applied will vary with diameter for a given angular measure so you”ll need a little chart to relate degrees to amount of relief per unit diameter.”

I don’t understand why the angle should change with the diameter of the cutter. The selection of angle does not take place with the cutter at 90 deg to the wheel or as shown in the illustrations. Instead the collet is loosened, the cutter rotated through 180 degrees, tighten the collet and then select the angle. So as far as the ground face of the cutter is concerned, the eccentric is moved relative to the verticle rather than the horizontal. Once the setting is done, the collet is rotated back through ninety degrees and you start grinding your clearance angle. I may have the sequence slight out of step, it is twenty five years since I used a D bit grinder but the principle is correct.

I’ve sent a crate of whisky to Pythagoras.

Nobby: Thanks for your response Nobby. I understand what you are saying but as in my reply to Clive, I am not so much interested in a cutter/bit at present as the relationship between the position of the cutter in the grinder and the alignment marks used to determine just how much clearance there is on the cutter.

Lawrie

[Gordon WParticipant @gordonw]

My brain hurts, and I am supposed to be quite good at maths. Surely a D bit is so called because the cross -section is a D shape? If a clearance angle is ground on then it will not be D shaped, so not a D bit? Just for further confusion I always thought of a D bit as being made from round bar, with a long flat ground on from the tip at an angle, so only one section will be a D. Make good reamers tho.

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Steve Garnett on 05/05/2011 17:55:38:
I’ve got the ultimate grinder for you, Norman – it’s a Steworndent!

I would have thought a Squorn would be more appropriate

 

Regards,

Tony

[mgjParticipant @mgj]

Alex its not the angle which changes with dia – you are going to set an angle, but the amount in thou taken off – the curved wedge will increase with dia, for a given angle – look at your drawing head on. My brain is fading, but that may be from the Chaddock/Quorn approach, which I am more familiar with, and I was assuming similar geometry.

 

Only one way to find out – draw it up and check. Sorry, but you’ll do the rithmetic easy enough, so I bow out! Wishing you luck with the compasses!.

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by David Clark 1 on 06/05/2011 17:01:30:

Hi There

What started this thread off was an engraving cutter, not a D bit.

An engraving cutter is backed off on an angle and relieved in its diameter so it cuts.

Edited By David Clark 1 on 06/05/2011 17:03:11

I agree – I believe the OP is talking about engraving cutters. The ones I have bought, commercially ground, in the past have (as you point out) a flat ground at the cutting edge to give the right relief angle, which avoids the problem mentioned earlier in this thread.

 

Pantograph Services have a nice document that shows how this works – see “cutter geometry” on this page:

 

http://www.pantograph.co.uk/engraving-cutters-diamonds.htm

Regards,

Tony

 

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

Am I remembering correctly that Prof. J. Hugei had engraving cutters on the SMEE stand at Sandown last year? I am afraid I had/have no interest in them or how they are sharpened, so did not pay much attention. Perhaps the OP can wait till next December and ask our tame Prof of Engineering.

chriStephens

PS Stuff Quorn, I prefer real meat.

[NJHParticipant @njh]

 

 

Hi Steve (& Tony)

” I’ve got the ultimate grinder for you, Norman – it’s a Steworndent! “

 

Well I rather discounted this when you first posted as it thought it was a name already used elsewhere…………. but when I checked in the bathroom cabinet I find that not to be the case as it’s not quite the brand of toothpaste I use!

 

Lawrie  –   I would be pleased to name your grinder but, by the time you finish it, you may well have several names of your own for it!

 

Apologies to all for my being well off topic. I am following your mathematical arguments with interest but it makes my brain ache and it’s too many years now since I needed to do this. So much of the stuff I sweated and agonised over I’ve never used. I’m sure that if I need info. in the future  there are many here who will be able to steer me.

 

Regards

 

Norman

 

 

Edited By NJH on 08/05/2011 20:24:56

Edited By NJH on 08/05/2011 20:26:06

[TerrydParticipant @terryd72465]

Posted by Lawrie Alush-Jaggs on 07/05/2011 10:26:49:

Hello Evry Bardy

I think that what we have established thus far is that everyone’s definition of what constitutes a D bit is based on their experience.
David Clarke has kindly offered his technical opinion which as he states is based on his experience.
Hansrudolf’s experience is that of a D bit which cuts at the front as he mentions both a Cannon Drill and his twisted D bit.
Ramon has added a photo of the tools he has made for his excellent Bentley engine and brings up his recent experience of seeing engraving cutters made.
Terry similarly to Hansrudolf says that an engraving cutter is not a D bit.
I think the reason we are having so much difficulty with this is down to taxonomy. I will give three examples of why this is a taxonomic problem.

1 Consider an end mill. We have absolutely no problem with the discussion of what constitutes an end mill. Basically it is a cutter derived from the twist drill in which the face can have a profile (flat, ball) which cuts material. In that, its end cutting capacity is similar to the twist drill. The second similarity is in having a helix. Unlike a twist drill though, the edges of the helix are ground to permit machining.

2 Drills. The twist drill and the spoon drill. Both cut at the face. The twist drill really cuts where the spoon drill is not really much more than a rotary waste of time.

3 The Slocum Centre Drill. There were up to the introduction of this tool more than ten styles of device all designed to perform the same operation, none of them anywhere nearly as effectively. The Slocum Centre Drill, which is a derivative of the twist drill won out.

Hi Lawrie,

 

The term D bit is a name given to a specific piece of equipment designed to cut on the front face and produce accurate, flat bottomed holes and make a good substitute for a reamer. There is no confusion, they may be purchased here at Tracy Tools, ready made. Any confusion is due to unfamiliarity with older tools.

 

Terryd

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hi Terry

Thanks for your reply. I think that you have missed the point about experience. I am not out to offend you or anyone else though I may have, especially when speaking about experience being a powerfull determinant in what we see as being right. Our lives are very much governed by it.

“There is no confusion, they may be purchased here at Tracy Tools, ready made”

That Terry’s sell the item that matches you description does not make me incorrect. There are at least ten forms of twist drill on the market all of which may legitimately called twist drill by virtue of the fact that they drill and that there is a twist in their construction. There is a requirement on the part of the purchaser to specify which type he wants or he will just end up with a jobber drill, not a stub, second cut, masonry, tapping drill etc.

“The term D bit is a name given to a specific piece of equipment designed to cut on the front face and produce accurate, flat bottomed holes and make a good substitute for a reamer. “

I am not for a moment saying that the description you have given is incorrect. What I am saying is that the term D bit is very specific to a form, not a function. That a peice of material when ground and viewed end on has the form of a letter D. It is not specific about the use to which it is put as in the case of twist drill etc.
That Tracy tools sell the device you accurately describe only goes to support my argument about experience.

If we talk about an axe, we all know what the item is. We can talk about a pole axe, a double headed axe or any type of axe at all. All are covered under the idea of a peice of hard material which has and will support a robust cutting edge, attached to a shaft and swung to cleave the item at which it is directed. The only type of axe which is specific in regard to nomenclature is a Cleaver. Like the Drill, the name tells you what it does. The term Axe does not, any more than the term D bit tells you that it is “a specific piece of equipment designed to cut on the front face and produce accurate, flat bottomed holes “
Pole Axe tells you that some part of the axe will be a pole, like twist drill will tell you that somewhere in the construction of a twist drill there will be a twist.

There are several artices by Harold Hall in MEW – one in MEW 151 p 11 in which he calls for the making of a D bit with a 10 deg taper. It is designed to ream a hole to take an eleven deg. taper pin to act as a detant.
This does not fit with your definition of a D bit “a specific piece of equipment designed to cut on the front face and produce accurate, flat bottomed holes”? If it is not a D bit then what is it? One can say that because it is made to ream a hole, then it must be a reamer. But does it have the form which most people understand to be a reamer? It does not have multiple cutting edges, it is not a peice of silver steel that has been machined and ground obliquely to provide a single cutting edge. Instead, when viewed end on, it has the form of a letter D.

In MEW 156 p26, Mick Knights has photos of nine D bits, only one of which fits your definition.
I am not saying that Mick Knights or Harold Hall are right or wrong, just that your definition seems a little too narrow.

This has been a lively discussion, thanks for your input.

Lawrie

[John Stevenson 1Participant @johnstevenson1]

Well I was going to resharpen a load of engraving cutters up for the Harrogate show for the KX1 demo machine.

 

Now I don’t know whether I’m doing them right or not.

 

John S.

[Peter TuckerParticipant @petertucker86088]

Hi Lawrie,

I’ve been cogitating on your problem and I think it is more complex than you imagine, let’s start simple. Imagine you have a cylindrical D bit of diameter ‘D’ which you want to use as a sort of single edge end mill so you put it in your eccentric and grind a relief from nothing at the cutting edge to an amount ‘A’ on the other side. The Sine of the relief angle is equal to A divided by half the circumference of the D bit. A 10mm D bit ground to ‘A’ = 1mm would have a relief angle of approximately 3.65°; 7mm dia, same A , relief angle approx. 5.2°; 4mm dia, same A, relief angle approx. 9.2°.

However you are wanting to grind a relief angle on a tapered engraving tool so no one eccentric setting will give the same relief angle all the way down. If you only need to cut on the tip then a relief amount ‘A’ could be calculated to suit the tip diameter, but if you need to have the proper relief angle all the way down the cutting edge then I can only think of two possibilities. 1. The bit must, when it is being ground, move in such a way that it’s axis travels through a conical path, the center of rotation when grinding should be parallel to the cutting edge (I have not attempted to calculate the offset for this axis). 2. Using a cup stone whose inside taper closely matched that of the relief wanted to be ground. Number 1 I think would need a grinder of the complexity of the Quorn with special tool holders, number 2 I think one would be most unlikely to find a stone to suit.

If you want a formula for setting your eccentric offset to give a relief angle at the tip I think I may be able to work that. If on the other hand I am completely wrong I’m sure someone out there will point out the error of my ways.

Good luck (I think you’re going to need it)

Peter.

[DustyParticipant @dusty]

Lawrie

Just to throw a spanner in the works, a flaw in your argument. 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. How about an engraving cutter or single lipped cutter?

[Clive HartlandParticipant @clivehartland94829]

All rotary cutters revolve around the central axis and if you put a cutting edge on that tool, ie, ‘D’ bit, pointed engraving cutter or a drill then that is what it becomes and is named as such. Otherwise no one would know what to ask for!

 

If someone wants to put a ‘Spiral’ clearance on a cutter then the cutting edge has less support and is weaker.

Given that from the calculations, a cutting edge relief is formed, ie, 3 deg. then it will cut.

No benefit will come from trying to change the physical shape.

From what I said before, all cutters are ground to set angles and perform well, its only the ham fisted who grind/sharpen a cutter that does not cut well and do not understand functions of the cutter they are using and the material they are using.

There are enough posts on here asking how to grind lathe tools and endmills that are blunt, wrongly ground or unsuitable in some way.

Nothing like a good diagram to show the interaction of the cutting angles and the clearances for different materials being cut.

 

Clive

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