Member postings for Dave Shedman

Update: I disassembled the table and put it on parallels on the surface table - to my surprise the error seen on the mill had disappeared and the table surface was now flat to within +/- 0.02mm.

I also noted that the x-gib was polished to a shine at the very lower edge on the sliding face indicating it had probably never been in proper contact resulting in the distortion to the table surface.

I was unable to obtain a replacement gib from the supplier so I had a go at re-scraping it. I superglued a 2 thou shim to the non-sliding face and embarked on a process of blueing the face of the gib, re-assembly and then sliding the table to-and-fro by hand (I left the leadscrew off to allow feel).

The first round of bluing confirmed that the gib was indeed only in contact with the lower edge of the dovetail, after many passes & a few hours of blueing & careful scraping I eventually ended up with a series of high spots across the whole contact face. To finish, I scraped in some deeper 'scuffs' to help with oil retention and reduce 'stiction'.

Now the table has been re-assembled, the action is very smooth with no rocking and the error previously seen across the table width has now been reduced to about +0.02mm and this occurs at various points across the table rather than being a pronounced hump in the middle.

Thanks for all the advice.

Dave

I will not be looking to carry out any grinding/machining of the table surface now!

I have done a few sketches and am wondering if the gib is not in full contact when viewed in section.

If only the top part edges of the gib were in contact this could have the potential to force apart the dovetail in a direction that may and cause the convex bow.

I also note that the table casting does seem rather thin in section when viewed from underneath.

I will get it on my surface plate and check it against the ways. I will also look at the contact surfaces of the gib with blue.

I am considering bolting down a slab of steel and flycutting this to provide a level playing field to clamp work onto.

I also wonder if fitting a parallel gib and drilling in some tapped holes for several adjustment screws may give a better gib solution than the tapered type?

I spoke to a machine shop engineer who asked me to check the gib adjustment.

I found to my surprise that the largest error at the top of the 'hump' (across the y axis) increased by about 0.02mm when the gib/X axis lock lever was applied.

I then remove the X axis gib and the error immediately decreased from 0.15 down to 0.05. (The gib is a tapered type with a lateral adjusting screw at each end).

What I fail to understand is how the gib seems to be causing the table to bow upward. I assume that if excessive pressure was applied by the gib and/or the lock lever, this would have the opposite effect; in that the force applied would in effect try to 'spread apart' the tables own dovetails and hence force the table surface to distort in a concave manner?

Thankyou for all the advice, a local machine shop looks like the way forward.

Would anyone be able to recommend one in the Nottingham/Derby area?

Many thanks

I doubt if the machine is still under warranty, I have had it for about 3years now (from new) and am only just getting to use it!

That's what I've found. The main question is; can this be suitably corrected with a 'skim' taken off at an auto garage as is done to cylinder heads?

Hi Andrew,

That's actually how I first noticed the hump - I found I could rock a steel rule across the table width. Appreciate your caution regarding measurements; I used the most 'solid' method I could to mount the stylus indicator and used this to measure the table top 'undulations'.

Like I said, when I removed the table for some time I could still detect the hump with the rule.

I have a Seig SX 2.7L and the table is not flat across its width. I put a DTI on the milling head dovetail and traversed the Y front to back (160mm); the max deviation I saw was 0.12mm high, the very front and rear of the table both equalled zero.

It would appear therefore that the table has a 'hump' in it and this is consistent along the length (700mm).

I was thinking of removing the table and getting it skimmed flat at an automotive shop. Is this a good idea or could it relieve stress making the hump even worse?

I have had the table off the machine before and the hump was still evident after several weeks at rest so the hump cannot be attributed to the machine set-up.

Any advice much appreciated as always.

I am in the process of making a between centres boring bar and was wondering what kind of geometry the HSS cutting tool will need for a GM casting (5" gauge loco cyl).

I know that for brass you don't need any relief angle on the top face for plain turning so I was hoping if someone could tell me if this is wise for a boring tool intended for GM, or whether it is wise to have some form of relief on this face?

Thanks

I think my main question here is when a casting has been machined and a bend becomes evident does extra material need to come off the Convex or Concave side to equalise the internal stresses and remove the bend as this is the part I'm unsure about? I would have thought the convex side?

In answer to the above; No, the casting was not subjected to any uneven clamping that could have resulted in a spring-back after clamping removed.

Also a new end mill was used so no issue with rub.

I have milled both sides of a gunmetal casting (loco frame stretcher) and it is now very slightly curved (max deflection approx 1.5mm in middle of 100mm of total length). As the stretcher will carry valve guides I would like it as straight as possible.

Am I correct in thinking that if I return it to the mill vise, hammer it flat on parallels and then take another 'skim' off the convex side, this will relieve the internal stress and remove the bend?

Also, when milling off material for the above situation how much is enough? Obviously I don't want to take too much off end end up with a bend in the opposite direction. Or is it a case of take off 0.1mm remove check/repeat?

Any advice appreciated, Thanks.

I recently acquired the drawings for 'Gert' and I wonder if anyone can explain why there are two sets of drawings for the cylinders, slide valves & eccentrics?

It seems that the 'original' ? definition of the cylinders, valves & eccentrics are on sheet 3; but an additional sheet 9 is provided that has alternative dimensions for all.

I assume that this sheet 9 was created in order to correct errors or give a more satisfactory solution?

Any help much appreciated.

Dave

Thank you for the reply Dave, I will look to get the booklet as I can find no mention of the use of jigs anywhere!

I have been using Martin Evans 'The model steam locomotive' book to guide me through the process so far, but am a little surprised that there is no mention of a sound method for locating the eccentric sheaves accurately.

I thought my question would have had more responses/suggestions as this must be a common problem to overcome on many types of model loco.

Dave

Probably a beginners question here. How does one go about setting the correct positions of the four eccentrics required for a loco with Stephenson’s-link valve gear? I’m referring to the common arrangement where the eccentrics are mounted on the driven axle between the frames.

I can find all sorts of advice on quartering the wheels but seem to be struggling on finding a method for ensuring that the four eccentrics are set as accurately as possible relative to their associated crank-pins.

I assume for reliable and accurate valve events this is just as important as getting the quartering spot-on.

Any advice much appreciated.

Thanks Jason, that is worth consideration - the only drawback I can think of is that the door hinges would have to be made as dummies to allow the door to be rotated when locking/unlocking.

I am building a 5" gauge LMS tank engine with a 'clamped' smoke box door. Due to the small size of the clamps these will not be functional but instead be dummies. I need to come up with some way of securely clamping the door from the inside of the smoke box. This latch/clamp can then be accessed using a tool through the chimney. Does anyone have any suggestions on the best way to tackle this? Thanks Dave.