Helpful Hints – part 3

Pin point accuracy with a Centre Pin

In days gone by, one of the methods of drilling in the exact place required was to cover the area of the work piece with engineers blue, using a vernier height gauge or a square and a scriber, mark a cross hair where you want to start drilling. Firstly though it was general practice to centre punch right in the middle of the cross hair before you start drilling, this is to give your centre drill/drill a chance of somewhere to start drilling, but this method relied very heavily on how good you are at getting the centre punch right in the middle of the cross hair.

As technology moved on, we found ourselves with many aids, like a type of microscope which you put up in the chuck of either your drill press or your mill and which had a cross hair in the lens, by moving the X and Y on your mill or drill table it enabled you to position the cross hair in the ‘scope right over the cross hair on your workpiece and of course the accuracy would be as good as the human eye will allow.

With the introduction of  electronics and in particular digital electronics, it became more of a concept in engineering  replacing more and more of the hand and eye skills, Digital and optical scales gave us the Digital Readout system commonly referred  to as the DRO.

Some of the first DRO that came out used what can only be described as looking like a Valve that had segments inside forming the numbers when lit up, if I remember correctly they were called numercators.

Along with all the wonderful electronic gizmo’s came the edge finders, hole finders, touch probes, laser, optical, etc,etc.

I have them all, but never use them I though at the time I must have, hmmmm.

I use what I call my centre pin (fig 1), nothing more than about an inch and a half of tungsten rod with a very accurate long point on one end. These can be made out of silver steel but would need to be hardened before use.

Mine came from a job lot of spark eroding tools, tungsten and rock hard 40mm long by 4mm diameter, ideal because they fit in a standard 4mm collet, or in the drill chuck.

Prepare the  work piece by blueing up and marking the position of the hole, put the centre pin in the drill chuck or the collet on the mill, secure the work piece in the vice and with the down feed handle bring the centre pin down to within about 1/2mm of the work surface, adjust the X & Y axes until the point on the pin is spot on the cross hair, use an eye glass if you want, you can give a little sudden down pressure, a little dink as I call it and you will see a little spot right on the cross hair.

You will never need another centre punch mark again, if you have to join two pieces of metal and you will always be spot on.

Cross drilling with accuracy

There are many ways, jigs and fixtures that aid cross drilling and in my 40 odd years in engineering I have never seen this method used before, although without doubt there will be engineers out there that have been using this way for years.

Einstein once said that, “If you come up with a brilliant idea there will be at least two other people somewhere in the world that have thought of the same way or method”.

The following is my way of doing the job that guarantees success every time and a way of using my centre pin once again.

As an example let’s say we want to drill a hole 10mm along a piece of 20mm round bar stock  right through the middle. First we face off the bar stock in the lathe, remove from the lathe to the work bench and blue up the face and along one side for about 15mm.

Put the piece of bar stock on a V block with the blued end facing you at a convenient angle so as to allow for marking.  Zero out your height gauge and then rest the foot on top of your bar stock, it doesn’t matter what the reading is  we know the bar is 20mm in diameter and we want to scribe a line exactly  through the centre and along one side for about 15mm.

If we now take half the diameter of our bar, 10mm, from the readings on the height gauge we will be exactly half way through the centre of our bar, we can now scribe a line across the face and along the side for about 15mm, see fig 1. If we now remove from the V block, turn the V block so the V is now vertical and stand our bar on the face so as the line along the side is visible, set your height gauge to 10mm and scribe a line across the vertical line. We now have a scribed line across the face and a cross hair along the side, see fig 2

I will digress for a moment to explain how I made a small tool which aids me in all my cross drilling and has done so for many years, you won’t find this in any book and you can’t buy one but it’s easy to make.

There are at least three ways of making this tool depending on what equipment you have available.

(1). Take a piece of 20mm x 50mm x 150mm bar stock, face of both ends, blue up the long length, i.e., 150mm and with

your height gauge scribe a line the full length through the centre at 10mm, offer up to your milling vice sitting on two parallels, (you must have a vice that the jaws open at least 150mm (6”). Mount a 90 degree horizontal V cutter in the chuck, align with the 10mm mark, lock the appropriate beds and cut to depth to give about 16mm across the V over the whole length of 150mm. If you have band saw or power hacksaw then life becomes easier cut off at 20mm so as you wind up with a bar 20mm x 20mm x 150mm long with a nice V the full length. If you haven’t got a band saw or power hacksaw the you can chain drill the full length, hacksaw off and clean up on the mill.

(2). If you haven’t got a 90 degrees horizontal cutter then this method could suite you. Using the same piece of bar stock as method (1) but this time as well as marking the 10mm line along the full length we can mark the 45 degree angle at one end.  Instead of sitting on parallels, this time we sit on a V block, as the width of the bar is only 50mm there is enough material to give a good clamp when sitting on the V block. We now have our work piece sticking out of the vice at 45 degrees.

Put a 16mm or 20mm end mill in the chuck and position the cutting tip of the endmill right on the 10mm line, gradually cut into the work piece adjusting the X and Z axis as required, you should end up with a V cut all the way along the 150mm length, cut off 20mm square as in method 1. See fig4.

3.) The third method is much easier but a fraction more expensive. On the market today there is a cutter that has V inserts that you place direct in the milling chuck, this allows you to put the piece of bar stock standing upright in the vice with the 10mm line and the 45 degrees visible, line up your V cutter, lock the y axis and start cutting using only the Z and X axis for the full 150mm length.

As in method 1 cut off at 20mm so once again you wind up with a 20mm square bar with a V cut the full length.

You can of course use ordinary V block for larger bar stock

We can now go back to our cross drilling, with our piece of bar stock marked off across the diameter and up one side to the dimensions we want, this is where our centre pin and the above V block come in handy. If we put our piece of bar in the drill/milling vice we can only secure it on the extreme outside point of the diameter which is likely to move when pressure is applied unless we use our long V block.

The way I do it is to sit the V block on a parallel with V on its side, we can now secure our piece of bar in the V the full length giving much better  holding power when drilling or milling. You will also remember that we marked a line across the face of our bar at the centre point and along the side to where we want to drill the hole.

That cross hair along the side of our bar needs to exactly upright, so looking at the bar end with the line exactly vertical,  place a square on the bed of our drill or mill and lining it up by eye using an eye glass if needed.

Place your centre pin in the chuck or collet and adjust the X Y until the point is exactly on the cross hair, lock the X,Y beds, replace the pin with a centre drill and you will be 99.99% spot on in drilling a hole right through the centre of your bar.

I have several of these long V blocks, different sizes for different diameter bar stock and it’s always been my way of cross drilling or milling round bar stock.

Removing work piece from the lathe

Removing work from the chuck when there is still some more cutting to be done is something to be avoided if possible,

but there are occasions when we may have to for a variety of reasons. Below is my way of ensuring I   put it back in exactly the same position.

All machinery has a degree of inaccuracy and of course you pay for what you get, even a very expensive lathe has a small amount of run out in the chuck. What that will mean is, if you put a piece of bar in the chuck and skim the surface for about 50mm that dimension will be perfectly round, if we now release the chuck jaws and turn the piece of bar round in the chuck retighten and spin the lathe up we will see the 50mm length which was perfectly round has now got a slight wobble on it.

That in effect is what happens if you remove you work piece from the lathe and don’t put it back in exactly the same position you will get a wobble, due to the amount of run out of the chuck.  There are ways we can eliminate most of the run out, but more about that later.

My way of ensuring that I put my work piece back in exactly the same position on the rare occasion I have to remove it is to mark it with a marker pen. The way I do this is to mark one jaw only on the face where it clamps the work piece, and on the work piece, I use a permanent marker which allows me to take the work piece out of the chuck work on it and return it in exactly the same position therefore eliminating any wobble.

I have come across second hand lathe that had a permanent line marked on one jaw, so I’m not the only one to think this way.

Making Eccentrics Sheaves for steam engines

Making an eccentric is a relatively easy operation. There are several way of doing it and below are two methods, the first one being my way and the second is the traditional way, how engineers in the past used to do it and probably still do.

If we take a piece of bar stock about 50mm long and face off both ends in the lathe, remove from the lathe and blue up one of the faces. Place the bar on the surface table and measure the diameter with the height gauge, let’s say for example 20mm, so half will be 10mm.

If we now put the bar on a V block (use your saddles to hold the bar firm on the V block) if you haven’t got any just use finger pressure and rest the foot of the height gauge on the bar, we will have a measurement, whatever that measurement is it does not matter we just take 10mm off and that will be exactly half the diameter, (I would suggest that you write these dimensions down, you may need to add and take).
Now scribe a line across the face, turn the bar so as the line is now vertical, because we are working on the surface table we can use a square to set the line exactly vertical – Figure 1.

Remove the square and scribe a line through the face of the bar, we now have a cross hair on the face of the bar – Figure 2.

If we want an offset of say 3mm, just add 3mm to the dimension you got when marking the centre line – Figure 3.
Having finished our marking we can now set our bar up in the milling machine. Stand the bar vertical in the vice with the marked face upper most and between a V block, this is most important, firstly for safety reasons, and secondly this way ensures that the bar is vertical and clamped tight when the vice is closed.

If a V block is not used when drilling a bar vertically, when the vice is tightened the only part of a round bar that is in contact with the jaws of the vice is the extreme outside edge, which is about 1mm. With our bar secured in the vice, put the centre pin in the chuck and position the point exactly over the offset cross hair using the X Y adjustment and the quill down feed lever, lock the XY bed when finished. Replace the centre pin with a centre drill followed by the drill required and ream to the size of your crank shaft.

Before removing the bar from the vice, make sure that the crank shaft fits in the reamed hole, this is your last chance of doing the job with the mill beds locked and everything is on the centre line. Drill deep enough to cover the width of the eccentric strap and the boss for the grub fixing screw.

We can now remove the bar from the mill vice and put back in the lathe. The last time we used the lathe was when we faced off both ends, a three jaw chuck with a slight run out does not matter when facing off but we are now going to turn an accurate piece of bar  to
fit in the eccentric strap and there cannot be any wobble.

For this job and any job that requires accuracy I strongly recommend a four jaw chuck, they are sometimes a little bit tricky to set up but with the four independent jaws and a DTI the bar can be clocked in to give us zero wobble.
If needed, skim the outside diameter such that the eccentric is a nice sliding fit in the eccentric strap and then part off to the required length allowing for the thickness of the eccentric strap plus enough for the grub screw boss.

My eccentric strap is ¼” thick and I wanted another 3/8” for the boss so overall I would part off at 5/8”.
A mandrel needs to be made at this stage, nothing fancy a plain piece of bar stock slightly bigger than the outside diameter of your eccentric, if for example the eccentric is 5/8” diameter then use ¾” bar. The mandrel should be
clocked in the four jaw chuck to ensure there is no wobble, turn down the bar to the exact diameter of the drilled

hole in the eccentric, a (1/16”) under the length (5/8”), drill and tap the mandrel to accept a suitable cap head screw. Make sure that the cap head is smaller than the diameter you want the boss to be.

While the eccentric you just parted off is not fixed or clamped to anything the outside diameter can be blued, lay flat on the surface table and set the height gauge to ¼”, mark a line round the circumference, this is the thickness of the eccentric strap.
Slide the eccentric on the mandrel with the ¼” section on the inside and secure with the cap screw, turn down the rest of the eccentric to the
required diameter of the boss, caution must be exercised when turning the eccentric as this will appear egg shape to start with so gentle cut are the order of the day.

In days gone by a four jaw chuck was used from the start, the bar was faced off and with a clock(DTI)  the  four jaws were adjusted to give, in my case a 3mm offset, drilled and reamed using the tailstock chuck turned and parted off to give an
eccentric all in one operation.

Both methods are safe and acceptable to achieve a very accurate eccentric, the choice is yours.

Setting tool height in the lathe

My usual way is as many other engineers use and that is putting a centre in the tailstock, my Bantam is a Morse 3 and  I keep a brand new centre just for setting my tool height.

Cutting metal at any angle

I often have to mill a flat piece of metal at an angle, and to keep the metal straight I use the following method:

When I first purchased my mill new from Chester UK Ltd,

I carried out the usual checks of checking the Vertical Z axis with the bed, my head can swivel 45 deg left and right so it is important that I started with a dead vertical head setting.

Before putting the vice on the mill I made some minor alterations, my small vice is a secondhand  4” Abwood with ½” thick hard jaws,  here are some ways of going about ensuring a true and accurate vice.

Make sure that the bottom of the vice is clean and the jaw securing screws are done up tight, place the vice on the surface table and run a clock across the top of the jaws, if no deviation then all appears OK, if there is some run off then this is where some minor alteration are needed. If you know a friendly engineer who can put the vice on a surface grinder and skim the top of the jaws then that’s all that is needed, I didn’t  at that time, so I obtained some ½” ground bar stock and made and fitted a pair of jaws slightly taller than the Abwood ones, put the vice on the mill secure and clock it  in, close the vice so the jaws are tight together  and with a carbide end mill skimmed the top of the jaws, they will now be parallel with my Z axis whatever cutter is used.

If I now mark a piece of metal to the angle required and place in the vice and just pinch the jaws shut, put anything like a 6” rule, or a 3mm parallel on top of the jaw bring the line on the metal up to meet the top of the parallel, close the vice tight remove the parallel and you should see the line irrespective how the piece of metal looks. Cut to the line. This method is as accurate as the eye will allow, if you want the angle down to seconds then a digital angle gauge would be used.