18-12-2011, 09:03 PM
I wasn't sure whether to put this thread under 'woodwork', or 'restoration' but I've tended to use the 'woodwork' section for woodwork that is unrelated to radio, and because this thread is about an aspect of restoration of a Little Maestro, and relevant to other radios needing new backs, I though perhaps this section was the right one. However, if the mods feel otherwise, do please move it!
At the 2009 NVCF I bought a pre-war Little Maestro with a lovely dial, and a nice cabinet, albeit a bit worse for wear, and with a back that needed replacement. There was enough of the back still intact to enable me to fathom out what it should look like, so I pondered on the various options for making a new back, both in terms of time, and the end result. I narrowed it down to two options, both of them labour intensive:
1) Drill a hole at each end of the 58 slots needed, and using a fretsaw or scrollsaw, saw the sides of each slot. Very tedious, and likely to have variable results however much care was taken. Furthermore, if another identical back had to be made, it would be just as laborious. I discounted this option, having decided that I'd only perform this task under sufference if I was in prison and it guaranteed my early release!
2) Make a router jig. This would involve several hours work, but once made, by using the jig to guide a router, the 58 slots in the back could be neatly cut in no more than half an hour, and the jig could be used if needed again for another back. Furthemore, the exerience gained in developing and constructing the jig would be useful should a similar jig need to be made for another style of back. Hence, I opted to make such a jig.
The question then arose as to what material to make the back from.
The old back was made from 3.5mm thick ((or rather thin!) 'millboard' - a sort of compressed cardboard, which is quite fragile - hence the damage to the old back. You can still get it from firms that supply car restoration materials but it's not cheap, nor do I think it's the best material to use. Ordinary hardboard is too 'fluffy' and 6mm MDF is too thick, so I opted for oil tempered hardboard, which is 3mm thick, and far more robust than normal hardboard. A 3ft x 2ft sheet - enough to make the jig and three or four backs, costs about £3.50 from B&Q. (Even cheaper on old fogey discount day!)
The jig needs to have a means of holding the blank in postion which will become the new back, and a guide which restricts the movement of a plunging router to that which is needed to cut the slot. The slots are 6mm wide, and 28mm long, in three rows of 18 slots. Thus, the router guide needs to move up and down the 'y' axis for 28mm. A guide was therefore made from an offcut of 15mm MDF using a jigsaw to cut a hole to the shape of the router footprint, with the hole the same width as the router, but 28mm longer. This guide would be screwed to the jig in a position which would enable the blank back to be progressivley moved sideways along the x axis beneath the router guide 13mm at a time (the spacing of the slots) after each slot is cut. When the top row of 18 slots has been cut, the guide is moved lower down the base-board and screwed into a position to enable the middle row of slots to be cut, then the process is repeated to cut the bottom row of slots.
For the slots to be positioned accurately between each other, two 'lateral' panels were made from the same hardboard, each to fit either end of the blank back. These panels each have a row of 18 holes at the correct spacing for the slots, and are held in position with a fixed dowel for each end panel. As each slot in a row is cut, these two lateral end panels and the backboard are moved to the left by one hole, until eventuallly all 18 holes in a row have been cut.
None of the above will make much sense without studying the pictures, which I've attached. I've shown how the work progressed, with the first hole in the top row being cut, right until the whole backboard has been cut. It should be evident how the two lateral panels of the jig move progressively to the left, with the next hole in each lateral panel of the the jig positioned over the dowel, so that the vertical slots can be cut with the top to bottom movement of the router in the router guide, while the back panel is prevented from any sideways movement by the two end panels. The depth stop on the router is set so that the router bit just cuts through the board and not into the baseboard. The router is held at the top of the guide, plunged into the back panel, then move down to the bottom of the guide, and released and switched off till the panel has been positioned for the next slot to be cut. Once the jig has been made, it takes only 30 seconds or so to cut a slot, reposition the board and be ready for the next slot. No more than 30 mins to cut the 58 slots.
The backboard needs finishing - two slots along the top which can be quickly cut on the bandsaw, holes for the back to be screwed onto the radio, and a hole for the mains flex. No big deal. Then I'll have to think what paint to use to be a reasonable match to the old back. The old and new backs are shown together for comparison.
I hope these notes and the pics may give some ideas, encouragement and inspiration to others - I should add that I have no intention of setting up a production line - there just aren't enough hours in the day! For such light work as this, a cheap DIY store bottom of the range router for less than £25.00 is all that is needed. A piece of chipboard is fine for the baseboard, and the skills that are needed are very basic DIY skills once the dimensions have been worked out.
I'm not sure how many pics we can attach to one post, so having attached six to this, I'll post some more in a follow up post.
At the 2009 NVCF I bought a pre-war Little Maestro with a lovely dial, and a nice cabinet, albeit a bit worse for wear, and with a back that needed replacement. There was enough of the back still intact to enable me to fathom out what it should look like, so I pondered on the various options for making a new back, both in terms of time, and the end result. I narrowed it down to two options, both of them labour intensive:
1) Drill a hole at each end of the 58 slots needed, and using a fretsaw or scrollsaw, saw the sides of each slot. Very tedious, and likely to have variable results however much care was taken. Furthermore, if another identical back had to be made, it would be just as laborious. I discounted this option, having decided that I'd only perform this task under sufference if I was in prison and it guaranteed my early release!
2) Make a router jig. This would involve several hours work, but once made, by using the jig to guide a router, the 58 slots in the back could be neatly cut in no more than half an hour, and the jig could be used if needed again for another back. Furthemore, the exerience gained in developing and constructing the jig would be useful should a similar jig need to be made for another style of back. Hence, I opted to make such a jig.
The question then arose as to what material to make the back from.
The old back was made from 3.5mm thick ((or rather thin!) 'millboard' - a sort of compressed cardboard, which is quite fragile - hence the damage to the old back. You can still get it from firms that supply car restoration materials but it's not cheap, nor do I think it's the best material to use. Ordinary hardboard is too 'fluffy' and 6mm MDF is too thick, so I opted for oil tempered hardboard, which is 3mm thick, and far more robust than normal hardboard. A 3ft x 2ft sheet - enough to make the jig and three or four backs, costs about £3.50 from B&Q. (Even cheaper on old fogey discount day!)
The jig needs to have a means of holding the blank in postion which will become the new back, and a guide which restricts the movement of a plunging router to that which is needed to cut the slot. The slots are 6mm wide, and 28mm long, in three rows of 18 slots. Thus, the router guide needs to move up and down the 'y' axis for 28mm. A guide was therefore made from an offcut of 15mm MDF using a jigsaw to cut a hole to the shape of the router footprint, with the hole the same width as the router, but 28mm longer. This guide would be screwed to the jig in a position which would enable the blank back to be progressivley moved sideways along the x axis beneath the router guide 13mm at a time (the spacing of the slots) after each slot is cut. When the top row of 18 slots has been cut, the guide is moved lower down the base-board and screwed into a position to enable the middle row of slots to be cut, then the process is repeated to cut the bottom row of slots.
For the slots to be positioned accurately between each other, two 'lateral' panels were made from the same hardboard, each to fit either end of the blank back. These panels each have a row of 18 holes at the correct spacing for the slots, and are held in position with a fixed dowel for each end panel. As each slot in a row is cut, these two lateral end panels and the backboard are moved to the left by one hole, until eventuallly all 18 holes in a row have been cut.
None of the above will make much sense without studying the pictures, which I've attached. I've shown how the work progressed, with the first hole in the top row being cut, right until the whole backboard has been cut. It should be evident how the two lateral panels of the jig move progressively to the left, with the next hole in each lateral panel of the the jig positioned over the dowel, so that the vertical slots can be cut with the top to bottom movement of the router in the router guide, while the back panel is prevented from any sideways movement by the two end panels. The depth stop on the router is set so that the router bit just cuts through the board and not into the baseboard. The router is held at the top of the guide, plunged into the back panel, then move down to the bottom of the guide, and released and switched off till the panel has been positioned for the next slot to be cut. Once the jig has been made, it takes only 30 seconds or so to cut a slot, reposition the board and be ready for the next slot. No more than 30 mins to cut the 58 slots.
The backboard needs finishing - two slots along the top which can be quickly cut on the bandsaw, holes for the back to be screwed onto the radio, and a hole for the mains flex. No big deal. Then I'll have to think what paint to use to be a reasonable match to the old back. The old and new backs are shown together for comparison.
I hope these notes and the pics may give some ideas, encouragement and inspiration to others - I should add that I have no intention of setting up a production line - there just aren't enough hours in the day! For such light work as this, a cheap DIY store bottom of the range router for less than £25.00 is all that is needed. A piece of chipboard is fine for the baseboard, and the skills that are needed are very basic DIY skills once the dimensions have been worked out.
I'm not sure how many pics we can attach to one post, so having attached six to this, I'll post some more in a follow up post.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
