It is virtually impossible to get the numbers and letters in a straight line without a guide... not easy on a flat surface much less a curved steering head.
To keep things straight I use a piece of 30mm angle iron a bit longer than the VIN. On an unassembled build, I tack weld the angle to the flat surface or steering head. On an assembled chopper, the triple trees and fork legs will get in the way of a steering head job, so I find something I can bolt the angle to.
In this case, a CB750 otherwise ready for inspection, I welded a small tab to the angle and attached it to the front tank mount.
When you use a bolt, make sure the angle is very tightly fixed as it will want to move when you start hitting the punches with a hammer.
If the chopper is already painted, it is best to clear a patch of paint away from where you are punching the numbers. The punch will crack the paint anyway, so better for a neat little area for the numbers.
It is also very difficult to judge the depth of the punched number/letter if the paint is still in place. If your chopper is being inspected by Big Brother, you don't want to give them any excuse to fail you. So make sure the punched numbers/letters are deep into the metal.
This brings up another point to be very aware of; punching a flat number on a curved surface means the more than just a couple of hits with the hammer. After the initial hits you will need to tilt the punch both ways to catch the curve.
Also when setting up your guide, make sure the centre line of the punch is on the centre line of the steering head, otherwise your punch will only make an impression of the bottom or top of the character in the steel...
Now to start...
1. Make sure the punch is orientated correctly; I've punched a number in sideways more than once!
2. Hold the flat side of the punch firmly against the guide and give it a number of hits taking care the punch does not bounce out of the first impression and do a 'double' number.
3. Making sure the punch is located in the partly impressed number, tilt it slightly away from you and give it a couple more hits to ensure the top of the number is impressed in the steel.
4. Second and subsequent numbers... Getting reasonably even spaces between the numbers is a bit tricky, especially with the thin number 1 or i and l. For the rest of the numbers and letters, I squint down the left side of the punch (bit like open sighting on a rifle) and line the edge up with the right edge of the previous letter or number. Following a skinny letter/number you will need to add a bit extra space (1-1.5mm). If you not number punched before, do a practice run in wood or steel or ally.
5. With all the characters punched, remove the guide. On a curved surface, you will need to repunch some of the characters by tilting the punch slightly towards you so the bottoms are deep enough. Be very careful to be sure the punch is located in the already punched character BEFORE you repunch... take your time.
End result should like like this or better...
You can purchase number and letter punches from hardware store or machinery suppliers. Punches come in a variety of heights. 5mm is a good size, though the registering body may also stipulate larger. This job had to be 7-10mm high. Letter are 8mm and numbers are 7mm.
Don't buy cheap punches. Most stuff is made in China and can be soft metal. Worse still, I have bought a couple of sets only to find letters and numbers missing. Open the packets BEFORE you pay and check carefully... obviously this precludes buying letter and number punches over the net.
Just a comment on VIN numbers of older bikes. Harley's DID NOT have Vins on the frame until 1969 (Sportster) and 1970 (Big Twin). Registration bodies can be a real pain in this situation. Genuine HD frames of this period had forged sections on the frame (top engine mount, axle plates of rigids, sidecar mounts) so you can prove it is a genuine HD and with luck avoid hassles if you are good at reasoning with less than intelligent bureaucrats , "But sir if I stamp number in the frame, it loses it's resale value!" Also, in South Australia at least a lot of 70's Honda frames are unnumbered. This is because when the frame was damaged in an accident and had to be replaced, the local Honda dealer, used a new unstamped frame, but often didn't stamp in new numbers. I have two like that and have seen a number of others.
Registering bodies officers can vary in their requirements. This 1976 chopper is a case in point. It has been registered interstate since new and has a 70's Amen frame with Amen numbers. In South Australia, it should have just required a number check and very basic roadworthy. Instead the owner was forced to have it engineered as an ICV which means it has had to comply to current regs with all the bells and whistles . This has cost the owner a couple of thousand extra dollars, an taken up a lot of extra time. What cheeses me off in particular in this case is that despite being made to comply to all current regs, they only required pre 1988 exhaust noise, 100 db rather than 95db. Further, they required longer guards than regulations require and insisted that exhausts had to be higher that frame... when many modern bikes have their collectors well below the frame tubes. Arrogance and ignorance has no place in a regulatory body like this.
Anyway that's my beef over!
Hope this has been helpful.