Cutting Dovetails

I have a dovetail milling cutter.

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The shank is marked “20×8 60 degree HSS-E”, this is quite easy to understand. The base of the cutter measusres 20mm diameter, the vertical height of the cutter is 8mm. It cuts 60 degree dovetails and is made of HSS-E.

The box calls it a 20x12x8x63x60D 6FL D Cutter Flatted Shank, this is a lot more information.

  1. 20x – The base diameter
  2. 12x – The diameter of the shank
  3. 8x – The vertical height of the cutting surface
  4. 63x – The overall length of the tool
  5. 60D – 60 Degree Dovetail
  6. 6FL – Six Flutes
  7. D – Unsure, does this mean flatted shank?
  8. Flatted Shank – Means of holding. I’m using collets, so this isn’t perfect but still OK.

I purchased it from Axminster who were doing special offers – I paid £14.75 for it.

I need to cut a bunch of dovetails and I don’t want to spend ages faffing around. So it is worth spending a bit of time planning..

I’m aiming for a 5mm deep dovetail. The base of the dovetail cutter is 20mm, the flutes angle at 60 degrees, so over 5mm depth the radius will reduce by 5/tan60 = 2.89mm, so the width of the cut at the top would be 20-(2*2.89) = 14.22 mm.

Clearly, I’m not going to plunge a dovetail cutter right into the side of the job. It’s a fairly expensive cutter and easily damaged. I want to use it to remove the minimum amount of material.

The dovetails I’m cutting have a top width of 20mm, so it would be a good idea to open out much of this before using the dovetaill cutter.

Ideally, I’d get a 14.22mm end mill, cut a slot with that, move over by 5.78mm and make another pass, leaving a 20.0x5mm notch ready for the dovetail cutter.

However, I don’t have a 14.22mm cutter, and I’d have to change collets even if I did. I need to use a 12mm collet for the dovetail cutter, so that implies using a 12mm end mill.

So my milling procedure becomes….

  1. Set up 12mm end mill on surface of part.
  2. Back off the part and move down by 5mm.
  3. Start the mill and mill a slot through the part.
  4. Move left by 5.75mm
  5. Mill back through the part.
  6. Lift the head, swap the end mill for the dovetail cutter
  7. Drop the head back down to just touch the dovetail cutter to the milled surface.
  8. Mill back through the part.
  9. Move right by 5.75mm
  10. Mill back through the part.
  11. Take the part off and touch the dovetail corners with a file.

So, there we go, a simple way of milling a series of dovetails without too much measuring and faffing about.

I’d be really interested in any comments about how to do it better!

 

 

Angle Grinder Stand

So, this angle grinder stand arrived today. IMG_20160127_181629 (2)

I’ve long had a bit of a Tennis Elbow problem in my right elbow. It started after a weekend at the UK Pinball Party, and it seems that it isn’t going away any time soon. Usually it’s all fine, just if I do a bit of strenuous sawing it aches a bit and I have to be a bit careful of it.

So I was after a cheap method of cutting stock without any effort. I don’t have room for the traditional metal cutting bandsaw, and as my workshop is my back bedroom, a screaming bandsaw might not go down too well with next door. They have a small baby, it screams, but I’m informed that it is “not the same thing at all”.

I unpacked it, and managed to find the angle grinder that I’d bought for cutting up an oil drum to make a furnace (still got to do that…)

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I’d seen various unboxing and assembly videos on YouTube, but was surprised to find that the base is cast iron. I’d assumed that it would be nasty plastic. The stand cost me the grand total of £18.25 and my expectations were not high.

Assembly is straightforward, it coped well with the angled handle attachments on the Bosch grinder. I used a square the set the rather primitive vise back jaw square and to set the blade vertical. It’s not blob on, but I’ll be tidying the ends up on the mill so no great worries there.

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I’m going to make a few tool holders (using the knurled nuts) so I thought I’d give it a try on the 1 inch square aluminium stock I’ll be using for those. All was not straighforward 😉

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It seems that the grinder can’t quite cut all the way through the stock. It left a tiny whisker attached. This is no big deal, but it would be interesting to look at the reasons why.

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It seems that the guard on the grinder hits the stock before it cuts all the way through. This is a grinder issue – essentially the grinder is only capable of cutting through one inch – pretty much exactly. Ho hum. It’s no big issue to either flip the stock over and cut through the last bit, or just bend the cut portion off, but it does illustrate the limitations of using an angle grinder rather than one of those dandy Evolution chop saws.

Taking the guard off. Yes, I did take the guard off the angle grinder. Yes, it is a really silly idea. The nose of the grinder touched the stock and still left a whisker attached.IMG_20160127_185716 (2)

This was the point where I realised that the wear of the cutting discs was significant. I put the disc that I’d used to make two cuts through the inch square aluminium stock on top of a new disc.

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I reckon it took about 2mm off the disk on each cut. The discs are not expensive – about 60p each, but combined with the depth restriction it is a significant effect.

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You can see each of the four cuts is slightly less deep than before. I was able to snap them apart by just bending them, but I did think about holding the last one in a vise (but then tried a bit harder instead).

So, well worth the few quid, takes up much less space than a gert big chopsaw, but does have restrictions. I’m satisfied with it.

 

 

Knurling update

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A quick update on the knurling.

A little musing and I realised that the wheels on the knurling tool were 3/4″ diameter and had 48 knurl features around their circumference. This means that if they were an inch in diameter, they would leave 64 features. This is a nice round number in imperial-world.

So I needed to ensure my diameters were in 64ths of an inch for the pattern to repeat properly.

I also did a bit of reading up and discovered – much to my amazement – that you can/should feed the knurling tool across your work. Which seemed a bit counter intuitive to me.

So I gave it a go, turned my nice 14mm metric brass rod down to ‘arf hinch, set the centre of the knurling tool on the right height, clamped it reasonably tightly and started at a very slow speed.

Once the knurl had picked up, I was able to tighten the clamp nut a bit more and noted the roughened surface. I kicked in the feed and it very slowly traversed.

After it had done a bit, I gave it a squirt of cutting oil, just to see if there was a difference.

So, looking at the picture above, the knurl is pretty (there are a couple of flaws where I held the nuts in pliers to tap the holes), but I’m quite happy with that.

So, I now have four knurled nuts, two nice, two satisfactory (grumble) and can move on to making a batch of four tool holders.

Knurling

Some time ago, I bought a cheap “Clamp type” Knurling tool from eBay there are many sellers.

I stuck it in a tool holder and bashed it onto a couple of bits of aluminium rod. One bit knurled really nicely, the other didn’t. I’ve not really played with it since.

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I’ve recently been making tool holders for my quick change tool post. I’ll blog about that when I get round to making half a dozen of them and can document the process. You can see the tool holder above, the ones I’m making are tighter and better…

Anyhow, these tool holders need a knurled brass knob to adjust the height of the tool.IMG_20160125_211230 (2)

So let’s make some.

Somewhat naive, I just turned a rod to the approximate diameter required and clamped the knurling tool onto it. This merely produced a roughened indistinct surface. Clearly a bit more subtlety is required.

So I measured the knurling wheels in the tool – 19mm diameter. then I counted the number of knurling features on the wheel. There were 48 features.

So, each element of the knurling pattern occupies pi*19/48 of the circumference of the knurled part.

Using this, I turned my brass rod down to 13.06mm diameter, this is 33 knurling patterns. The results were OK, but not massively impressive. Alignment of the knurling tool is important, the pressure is important, the exact diameter is also important. I’m guessing a bit more experimentation is called for.

Anyhow, I tidied up the edges, added a bit of shape to make them similar to the original one, drilled and tapped them to M5 and while I’m not overly happy with the result, they are satisfactory for the purpose.

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I think a bit more experimentation is called for to get a really crisp pattern, possibly allowing an offset of half the depth of the knurl when calculating the circumference is called for.

Still, I made a thing 🙂

 

Grinding

I’ve recently been attempting to turn some mild steel. I wasn’t getting good results.

I started out trying to use a HSS bit which was included in a set supplied with the lathe. I can’t say it was particularly sharp. I’d used it on a bunch of brass and aluminium and it had performed well.

The results with steel were, frankly, awful. Scored and lumpy (and this was supposed to be a bearing surface).

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So I was inspired to try grinding my own.

I followed the Little Machine Shop guide to grinding tools that I’d found through google, http://littlemachineshop.com/instructions/grindingtoolbits.pdf

It is quite a simple guide to follow.

I don’t have a particularly good grinder. It is a cheapie B&Q special, upgraded with a wider pink wheel for sharpening my woodworking lathe tools and a newly fitted tool rest that I picked up cheaply at Cardiff Axminster’s opening day. Other grinders are far superior.

Anyhow, I set the rest to an eyeballed 10 degrees or so, angled the tool blank at an eyeballed 10 degrees-ish and ground the front face compound angle.

Then I put the bit parallel to the front surface of the wheel, overhanging the left hand side and ground the side relief to the 10 degree angle set on the table. This took some time and I dipped in a glass of water to keep it cool enough to hold comfortably.

The top relief was achieved by flipping the bit over to the right hand side of the wheel and angling it to an eyeballed 10 degrees-ish again.  This grind took a while.

Then I touched the point surface on the wheel – you have to elevate it from the table as all the compound angles mean it’s quite high on the wheel.

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So, all very approximate. I wasn’t expecting great things at all. I was expecting something that would cut, but probably no better than the blunt tool I’d used previously.

However, a quick check on some silver steel gave great results. So I pulled out my rough and horrible mild steel and gave that a spin. It cut like a dream producing lovely tightly curled long strings of springy swarf. The surface finish was vastly improved.

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Either I have been really lucky and hit some magic angles, or maybe it’s just that any sharp tool gives a decent result. I’m sure I’ll find out sooner rather than later.

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So that’s a happy bit of grinding 🙂

 

 

It’s been a while…

It’s been a while since I posted on this blog. You often get the two or three posts blogs where someone has lost interest, and I always find that rather sad.

So I reckon it is time to make a commitment to post regularly. I’ve been fiddling with a lot of stuff. TheLab has undergone a few revisions and is now more techno sparkly than ever.

I did get round to buying a lathe – erm, three actually, and a milling machine.

I shall be posting reviews of the new machines and following progress (when I make some) on creating my first bit of model engineering – an Elmer’s Wobbler tiny engine.

So, hang on til tomorrow when my first Monday update will be posted!

 

Doug.