Avonside ChassisPro - a small problem

[David Thorpe]

I've had a ChassisPro for some time now and it has been a boon - as I reported in an earlier post, the first chassis I built with it ran smoothly from the start without any tweaking, a first for me.

However, there is one problem with it that is driving me mad and i wonder if anyone else has come across it and, if so, whether you've found a solution to it.

In constructing my chassis I follow the instructions and, having loosened the bolts on the sliding block, use the screw adjuster to adjust the pins until these are in such a position that the coupling rods drop onto them. When all is correctly positioned I remove the coupling rods and, using the Allen key, tighten the bolts on the block. This is where the problem arises as it seems that when I tighten the bolts, at the last moment the pins move very slightly so that the coupling rods no longer fall easily into position on them. This movement only seems to occur at the very last turn of the bolt tightening procedure. As a result, I have to start the fine adjustment programme all over again and it can take really quite a long time before I reach a situation where the coupling rods fall easily onto the pins with the bolts fully tightened. If I keep the coupling rods on the pins throughout the procedure, the same thing happens - the rods which were free to lift off until the last turn of the bolt are no longer so free and have to be manipulated off, although a slight slackening of the bolt will restore the previous position but leave the pins a little loose.

I don't know if anyone else has had this problem but if so, have you found a solution?

DT

[LesGros]

DaveyTee wrote:

... I don't know if anyone else has had this problem but if so, have you found a solution? ..

I too have had the same problem. I resolved it by working from the centre pin in each direction in turn so that only two pins were engaged in the coupling rod when tightening the lock screws, ie. reverse the rod so that centre hole is on the end pin.

The pins were tightened in the sliding block, and the lock screws were tightened up to start with, then backed off just enough to allow the slide to move easily, the outer pin was postioned "too Wide", then slowly advanced inwards, meanwhile the locking screws were gently tightened, but not enough to stop the movement; when correctly positioned the screws were nipped. But, it took several tweaks to get it right.

It was found to be worthwhile to mark the rod links front and rear to avoid transferring any small assymetric error in the spacing of the rod holes. A similar method can be used with frame sides to set the jig before fretting out the hornblock slots.

I fear there is no magic bullet, only patience. [ unless some one knows different]

[steamraiser]

I have had a similar problem which I have traced to a slightly bent screw rod passing throught the outer adjustable block to the inner adjustable block. I have managed to put a slight bend in the adjusting scre rod.
The hole in the outer block is not quite big enough to take the distortion of the screw rod passing through, the net result being that it slightly tils the block when the locking screws are loose.
I have a peice of plywood that fits between the frames underneath, which only makes contact with the underside of the blocks. When carrying out the final tightening of a block I place a downward pressure on the jig which pushes the block against the underside of the jig. This has the effect of correcting the position of the block and the pin.
A little patience is all that is needed.

Gordon A
Bristol

[45609]

This is a something I too noticed and commented on it in my review of the jig that appeared in MRJ191. The relevant paragraph is repeated below.

I found that following Avonside’s technique, initially leaving the securing screws fairly loose, setting of the wheelbase with the coupling rods was more of a fuss than it needed to be. As the sliding blocks weren't in contact with the underside of the base the pins were not quite vertical. The instructions do warn of this but I found that nipping the screws up a bit with the allen key supplied and then backing off a ¼ turn worked well. It its true that this technique gives a bit more friction for turning the lead screws but it seemed a satisfactory trade off as I knew that the pins were vertical all of the time.

Gordon explains the reasons for this in more detail above and his plywood technique seems to achieve the same thing as my suggested method. Just use what works best for you.