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Spotting through, Drilling through & which order

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William Ayerst03/08/2021 14:27:44
259 forum posts

Good afternoon all,

This is hopefully my last post which relates to the Stuart10v build I'm just about to finish. One of the last pieces to do is to drill the various fixing holes. The 'Building a Vertical Steam Engine' book blithely says that they can be 'done in later in the process' and then seems to never mention it again.

Though I have a mill I lack a DRO, so I was going to use calipers to mark out the various PCDs before spotting/drilling through. Beyond that, I'm not sure which items I should be drilling through or spotting through, and whether they should be clamped to the target ahead of time.

i.e. the joints between:

  • Top cylinder - cylinder - bottom cylinder - standard - soleplate
  • Steam chest cover - steam chest - cylinder

I'm getting just a little in a twist and chasing my own tail for the order of operations on this! Can anyone let me know how to approach tackling it?

Thor 🇳🇴03/08/2021 15:39:09
1395 forum posts
41 photos

Hi William,

When I need to drill holes on a PCD on a cylinder and its cover I start with the Cylinder Cover. I too use calipers to measure the PCD when marking out. I start with the cover and spot and then drill through, I then use the cover as a jig for drilling/tapping holes in the cylinder. After drilling/tapping the first hole in the cylinder I use a temporary screw to clamp the cover to the cylinder before drilling/tapping the next hole. The covers have slipped onto the studs in the cylinder without problems.


Martin Connelly03/08/2021 15:42:26
1853 forum posts
197 photos

There are a number of options. It all comes down to what tooling and equipment you have access to. For example if you have CAD and a printer you can print off a 1:1 drawing and use that as a template. A rotary table is good for spot drilling and then drilling cylinder caps or templates. Transfer punches are also useful for marking out for drilling from an existing part. Scoring a groove in marking out dye on a piece of metal in the lathe can be used for a PCD prior to marking out the hole positions, You will get more help if you make you question less general and more specific regarding what you are wanting to do.

I would suggest the cylinder caps have to be done before the cylinders if you are matching one to the other, you can't usually do it the other way round for example.

Martin C

Les Jones 103/08/2021 15:50:57
2234 forum posts
153 photos

When drilling the holes in the cylinder cover do not drill them to the final size. (Screw clearance size.) Drill them to the tapping hole size. (Or smaller.).


SillyOldDuffer03/08/2021 17:09:41
7482 forum posts
1657 photos

Depends on what equipment and how cack-handed you are.

A rotary table makes the cylinders easy, and though my DRO doesn't do PCDs automagically, it reduces the chance of mistakes.

I use paper templates but make sure your printer is accurate and circles aren't ovals. Print a big circle and measure it. Another problem with templates is are likely to slip; I glue mine down but beware of the glue softening the paper and allowing it to stretch. Avoid wet glues!

When I did my first mini-lathe only engine - no DRO, rotary table, mill, or other fancy gear - I superglued the end caps precisely in position on the cylinder, blued the end with a felt tip, carefully marked out with a compass and centre-popped the holes. Then I wigglered the pops under my pillar drill and tap-drilled the caps through and into the cylinder. Drilling both parts in one go means mistakes don't matter because everything lines up. Despite double checking I made a small a mistake at one end, and the holes only line up in one of six positions - doesn't matter.

The superglue is released by boiling or heating gently with a blowlamp: a blowlamp is faster. An alternative is to mark out, drill and tap two holes and bolt together instead of gluing. I prefer glue. Old glue can be scraped off and cleaned with Nail Varnish remover.

I've never tried the traditional method. Secure each item on the mill, centre pop the first hole, calculate the PCD coordinates with paper and pencil (or cheat with a published table), zero the mill on the first hole, drill it, and then dial-up the other PCD coordinates one by one, and drill again. Do the same for each object, and they should fit together. Real men do this without marking out, but I'd be happier to mark the job up and eyeball it before drilling because I'm clumsy and mistakes like not allowing for backlash are punished severely!

Another job that's easy after doing it a few times.


not done it yet03/08/2021 17:17:09
6279 forum posts
20 photos

These can be very handy. Once one fixing is in place the others can be systematically marked, spotted and drilled.

Best to mark orientation of parts, particularly if not using a DRO - otherwise larger clearance holes may be required.

Transfer punches do not need to be purchased - the few needed at the time are easily made - but a whole set, suitably hardened is the easy way out.🙂

William Ayerst03/08/2021 18:42:17
259 forum posts

Is it the same suggested process for the steam chest? i.e. the cover first, then the chest, then the cylinder?

All of these holes are 7BA so I guess it would be most straight forward to make my own transfer punch?

Thor 🇳🇴04/08/2021 05:38:33
1395 forum posts
41 photos

Hi William,

I do the Steam Chest as you describe, if you have a piece of suitable steel it isn't difficult to make your own transfer punch.


JasonB04/08/2021 07:54:33
21315 forum posts
2419 photos
1 articles

If you are going to stick with manual marking out then best get the right tools, a pair of dividers is what's needed to mark out not callipers be they firm leg or digital.

I suggest you tackle the Front (top) cover first as that is less critical than the back (lower) one. And go about it like this

1. Blue the face that mates to the cylinder, this means any scribed lines won't be seen.

2. Put cover blued side out in the 3-jaw and using a ctr or spotting drill put a small dimple in the middle. Remove from lathe

3. Set dividers to 9/16" (half PCD) and use them to scribe a circle using the just drilled ctr hole to locate one leg, When setting don't do it from the end of the rule, locate one leg in the 1" engraved line of the rule and then set the other leg to the 1 9/16" position.

4. Take a dot punch and make a small dimple with that somewhere on the scribed 1 1/8" circle, inspect with magnifying glass and adjust if needed.

5. Set dividers to just under 17mm and then with one leg in the punch mark scribe two arcs one either side, now move divider to where the arcs cross the PCD circle and strike another two arcs. Then move again and strike an arc that should cross the PCD circle at the same point as the last arc, if not adjust dividers and repeat until you have 5 equally spaced arcs.

6. Dot Punch the remaining 4 hole positions, check with glass and then go round with centre punch to make a deeper mark.

7. Drill all 5 holes tapping size.

8 Locate cover on correct end of cylinder, lining up the holes so two are parallel to port face and clamp in position.

9. Hold clamped parts in mill or drill and drill one tapping size hole into cylinder using the cover to guide it's position.

10. Mark cover so you know which way to put it back on then unclamp from cylinder and enlarge the hole you just used as a guide to clearance size.

11. Tap hole in cylinder, preferably using some form of tapping guide or guide with the drill/mill.

12. Use one of the supplied screws to fix cover back onto cylinder and then repeat drilling, enlarging and tapping sequence for each of the remaining holes.

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