AT THE conclusion of my last jottings I proposed that, having talked about positioning holes, we would turn our attention to making them and measuring their size. In fact, I have already commented on some aspects of making them. There is a great deal more that can be said on the matter but I only intend to mention a few aspects of the subject here and shall restrict myself to conventional machining techniques.
The most obvious and commonest method of making a hole is to use a drill. Having said this, the tool that will immediately spring to most people’s mind is the standard jobber’s twist drill.
Occasional problems
These ubiquitous tools meet the vast majority of our model engineering everyday requirements for hole drilling but in their standard form they occasionally give rise to problems. One of the most common of these is the tendency for the drill to ‘grab’ the work when machining materials such as brass and cast gunmetal. The result can be anything from a damaged and oversize hole to a broken drill (and a damaged hand if the job has not been secured properly in a drilling machine).
The problem arises from the excessive rake on the cutting edge. The rake angle is in fact the helix angle of the drill flute, which for a standard jobbers drill is about 30 degrees. Long helix drills and straight flute drills are available for use with brass and similar materials but these are expensive and not many of us are able to equip our workshops with such tools of limited application.
The problem is easily overcome by creating a small flat on the cutting edge with a slip stone, reducing the rake to a more acceptable 0 to 5 degrees as shown in Figure 1. The flat only needs to be very small and the normal drill geometry can easily be restored by re-grinding the drill.
Another problem often experienced when drilling materials such as austenitic stainless steel is for the metal to work harden under the drill tip so that the cutting action will not start. In its standard form the very centre of the drill point has to swage the metal aside, allowing the cutting edge to engage.
It is this swaging action which ‘starts the rot’ in the case of work hardening materials and can be significantly reduced by drilling a small pilot hole first or by thinning the point of the drill as shown in figure 2. Thinning the point in this way is, of course, impractical in the case of very small drills.
Large holes in thin sheet metal
Drilling large holes in thin sheet material can also present problems. Here again, there is a tendency for the drill to ‘grab’ the work as the point breaks through and ‘screw’ its way through the material. This problem can be overcome by clamping the job firmly between two pieces of thicker material - pieces of plywood, for example. It is necessary to drill a small pilot hole, say 1/8in. diameter, in the job and backing material first and locate the three pieces with a dowel before clamping.
Another technique which I have used very successfully to drill the tube holes in boiler tube plates is one suggested by Alec Farmer in his book Model Locomotive Boiler Making. This uses a drill ground as shown in Figure 3. Here again a pilot hole of about 1/8in. diameter is drilled first. In the case of boiler tube plates it is convenient to use the flanging plates as drilling jigs.
My own practice is to make these plates from 5/16 or 3/8in. thick steel and drill 1/8in. diameter holes at the tube locations using coordinate positioning on the milling machine to locate them. After they have been flanged the copper plates locate snugly on the flanging plates and the 1/8in. pilot holes can be drilled through.
The especially ground drill is then used to open up to size. This technique works well for tube holes up to about 3/8 or 1/2in. diameter – the drill shown in Figure 3 is 3/8in. diameter. Above 1/2in. diameter (holes for super heater flues for example) I set the job up on the mill and bore to size using a boring head as shown in Figure 6.
The quality of holes produced using ordinary twist drills varies considerably depending on, among other things, the condition of the drill. A brand new drill will normally produce an accurately sized hole with a reasonable finish and circularity, particularly in the smaller sizes. Once the drill has begun to lose its initial sharpness, however, and particularly if it has been sharpened by ‘off hand’ grinding, it is liable to produce less accurate results.
Guaranteed accuracy and finish
When guaranteed accuracy and a good finish are called for it is usual to finish holes to size using a reamer. There are numerous types of reamer available but we shall consider only the basic format here. The two types of reamer most likely to be found in the model engineer’s workshop are parallel hand or machine reamers and it should be emphasized immediately that both of these types are only intended to remove a few thousandths of an inch of material during their operation.
Hand reamers, as their name implies, are normally driven by hand with the aid of a tap wrench or similar. These reamers are provided with a small lead or taper two or three diameters long to facilitate location and alignment in the prepared hole.
Machine reamers only have a very short lead since they are driven by and their location and orientation controlled by the machine in which they are being used. Either type may have straight or helical flutes.
In the case of the hand reamer the helix should be anti-clockwise if the reamer is to be rotated in a clockwise direction, otherwise the tool will tend to screw itself into the work. There is no reason why a hand reamer should not be used in a machine application but it must be remembered that it must be possible for it to pass right through the work until the lead has cleared the hole.
If a parallel blind hole is to be reamed a machine reamer must be used. The amount of metal to be removed by the reamer should not be more than 0.005 to 0.01in. on diameter. Machine reaming should always be carried out at low speed and a good quality cutting lubricant employed.
Another problem which sometimes arises when drilling a hole in the lathe using a twist drill is that the drill will not start true or will wander slightly, due possibly to some inconsistency in the material. I have found that cast gunmetal often gives rise to this problem.
Effective D-bits
The hole can easily be brought back into line with the aid of a D-bit. These tools are usually associated with producing flat bottomed holes or stepped holes such as those required for ball valve seats and they are, of course ideal for this purpose.
They are also very effective boring tools. The cure for the wandering drilled hole only works if the initial drilling is in the form of a pilot smaller than the finished hole required, but if starting to machine a casting for, say, an axle pump it would be very unwise to try to drill for finished size in one go! To correct the defective hole all that is required is to open up a short length (about one third of the diameter long) to the size of the D-bit to be used with a small boring tool. Use this short length of true running hole to start the D-bit and it will bore the remainder of the hole accurately, true and to size.
D-bits are very easy to make from a length of silver steel which will come ready ground to an accurate diameter. I would recommend any workshop to hold a small stock of silver steel of various sizes. It is invaluable for producing odd cutters for those special jobs for which standard tooling is not available.
Hardening and tempering
The D-form should be produced by milling or filing away exactly half of the section and hardened and tempered to a pale straw colour. Only the business end of the cutter need be hardened by heating to a cherry red and quenching in water. Depending on the size of the tool being hardened the metal should be held at red heat for a short time to allow the structure of the steel to stabilize. The usual rule of thumb is one hour for every inch of thickness. This translates into quite a long time for even small tools but is necessary if optimum hardness is to be achieved.
Care should be taken not to overheat the steel. Quenching should be carried out by plunging the tool vertically into the quench bath and moving gently up and down and in a circular motion. Violent movement within the quenching fluid, whether it be water or oil, will increase the risk of distortion.
The method of tempering usually suggested is to polish the hardened tool and gently heat the end remote from the cutting edge, watching for the colour of the polished surface to change. The changing colour will travel down the tool and when the required shade has been reached at the cutting edge the job is again quenched in water.
The reason for the second quench is to halt the travel of heat from the heated end to the business part of the tool. It contributes nothing to the actual tempering process.
It is all too easy to overdo the heating during tempering, especially in the case of very small tools such as injector reamers. A far better way of tempering is to place the tool in a muffle furnace at the required temperature and leave it to soak for long enough for the steel structure to relax. Since no part of the tool reaches a temperature higher than that required for tempering it is not necessary to quench. Simply remove from the muffle after a suitable soak period and allow to cool.
You may think that this procedure is a luxury that few can enjoy since not many of us have a muffle furnace available. This is not so however, for most of us have a cooker in our kitchen with an oven which is quite capable of doing the job. The tempering temperature for a tool such as a D-bit and indeed most other small cutting tools we might make is 225 to 235 degrees C, corresponding to a pale straw colouring of our polished tool. Most domestic ovens will operate up to 250 degrees C.
All that is required is some diplomatic negotiation with management!
If the oven has a thermometer accuracy is guaranteed. Failing that it is necessary to rely on the ovens control system and set the required temperature. Most are quite accurate. Figure 5 shows an example of a commercial bit and a home made version.
Larger holes
Once the holes we are making exceed about 3/4 to 1in. diameter most of us will be thinking of machining them by boring with single point tools, either in the lathe or milling machine. The set up may be with the job rotating in a chuck or on a face plate or with the tool rotating and the job mounted on the cross slide or vertical slide of the lathe or on the milling machine table.
In the case of a rotating job the tooling will be in the tool post of the lathe. A rotating tool will either be carried in a boring head in the lathe mandrel or milling machine spindle or will be mounted in a boring bar between centres on the lathe. The actual cutting tools used for these applications should be ground with clearances and rake similar to ordinary lathe tools and no comment is required.
The main problem likely to be encountered is lack of rigidity of the tools themselves. Clearly the tools should be of as large a section and as short as is possible within the constraints of the work geometry. Be patient, take small cuts and always pass the tool through the job several times on the final cut to ensure that all the spring has been taken out.
Not all holes are round
So far it has been assumed that we are talking about round holes, but not all holes are round! Before leaving the subject it is worth mentioning a problem I have been asked about in connection with one particular form of hole and the tool associated with it, namely slots and slot drills.
The slot drill is a form of milling cutter which can be plunged directly into the job and then traversed laterally to produce a groove or slot of accurate width. Traditionally slot drills had two teeth but three tooth cutters are also available which will perform as slot drills or end mills. In either case, unlike normal end mills, one tooth extends to the very centre of the cutter enabling it to drill its way into the job before moving laterally to generate a groove or slot.
Typical applications are a blind ended keyway or a steam port for a slide valve engine. In either case the width of the slot is required to be accurately to size and used correctly a slot drill will achieve this. The important thing to remember is that the cutting action will apply a load to the cutter at right angles to the axis of the slot being machined.
If the required depth of the slot is such that it can be achieved in one pass of the cutter there is no problem. If more than one pass is required the temptation is to plunge in for a second cut at the end of the first pass and traverse back to the start point, repeating as necessary until the required depth is reached. This procedure will invariably result in an over width slot because the side load on the cutter will be in the opposite direction on the return pass and any deflection of the cutter, spindle, bearings, or indeed any part of the machine tool will result in the cutter path being deflected in the opposite direction at each pass.
The only way to ensure that this does not occur is to always start the cutting action from the same end of the slot. Ideally the cutter should be withdrawn from the work during the return to the start position.
It had been my intention to move on to discuss measuring techniques, but I see that I have once again rattled on for too long and that subject must wait until next time.
For earlier parts of this series see:
