Machining the pocket in a slide valve

Just messed up the pocket in a 5" Boxhill slide valve - doh! I have a vertical mill with a DRO on the cross axis but find keeping track of the numbers and direction rather challenging, especially when using a small slotdrill which is hidden by the collet chuck - what do you guys do ? Thanks, Phil

I made a similar mistake some years ago on a small traction engine so I made a thin stainless steel plate (about 0.030" separately and fitted it into the recess. The slide valve was reduced to suit. The was secured with two small countersunk screws and the whole thing worked perfectly. Colin

When milling similar pockets I use positive travel stops on x and y axes,may be old fashioned but works, good accuracy can be achieved by setting the stops with Hoffman calibrated rollers plus feeler gauges or slip gauges ,or an internal micrometer on longer travels.

Centre the cutter over the slot location using the DRO centering. Zero X axis.

Plunge cutter to desired depth bearing in mind you are using a small cutter.. Move cutter +/- the required width remembering that the overall cut slot width will be oversize by one cutter daimeter unless you subtract the 1/2 diameter from each travel.

No stops or slips etc

Simples? .

I do it like KWIL with the DRO. Sometimes I will also zero the dials at one side of the cut and note the other reading when the dro is in the right place and just use the dials as I find it easier to stop at the right place on the wheel than sneak up on the DRO readings

I tend to mark the dials on the mill with a pencil to give the passion fore and aft, and use the stops for L and R travel.

Ian S C

Done it ! Used the dials / DRO and worked out the settings for each edge then did a couple of trial runs on thick brass plate. Once happy I went for it and although the job disappeared under a pile of swarf and I was cutting blind the result was bang on

You obviously had "blind faith" in your new ability, and it worked!! Well done.

Just part of the learning curve on a mill.

When doing a job like this where the swarf can't get away I use a vacuum cleaner to continuously remove the chips. If the cutter is buried in swarf it can recut the swarf and give poor surface finish, Don't use this on pyrophoric material, but how many of us emachine magnesium or uranium?

i have been a bit bemused by Phil's thread.

ive had 2 sets of Boxhill valves machined. the castings are so good and near enough dead on size that you could easily file the outer dimensions and true up the exhaust cavity with a file. however i endmilled both my sets. one set was made in 1995 when i made the cylinders and crankaxle. many years later my great friend Don Ashton redesigned the valve gear for me which necessitated larger valves with greater 'lap'. the original valve gear has no suspension offset plus the throw of the eccentrics is excessive. the lifting arm lengths and lifting arm lengths isnt correct either on the martin evans' design. Don ran the original gear on his simulator and the valve events were very poor.

in my case i mill the cavity after having done the outside, then apply a vernier to check the lap + port width. if another 5 thou needs milling then advance the table by 5 thou.

of far great importance re martin evans' Boxhill, Metro, and Stirling, is to fit the valve rod gland body via a flange with bolts/studding into the steam chest. the same studding can be used for proper studded valve glands. this allows the valve buckle plus valve rod fitted to same to be assembled in situ. this tip was provided by fellow 5"g terrier builder Jim Scott and is perhaps the most important design modification ive come across and can be strongly recommended, apart from Don Ashton's redesigned valve gear.

cheers,

julian

Edited By julian atkins on 24/11/2015 23:09:07

Quote: Don't use this on pyrophoric material, but how many of us emachine magnesium or uranium?

Many years ago while working at Stone Manganese on my Uni sandwich course I saw a 1 ton pile of magnesium swarf in a steel catch bin take fire.

Fire brigade came and drank tea as it went ballistic saying there was nothing they could do! never seen anything like it and the mess from the oxide was unbelievable.

Martin