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Gearwheel key


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Anthony Knights15/04/2020 16:32:46
622 forum posts
243 photos

I was using my mitre saw yesterday when the blade stopped revolving. Dismantling the front end,I found that the key on the drive shaft gear had sheared. The best I can measure, the key had the following dimensions:

width-4mm (checked with blank end of 4mm drill)

Length 7.5mm, Depth about 4.5mm. The question is, is this a standard size which I can order? Also managed to shear the head off the bolt which holds the saw blade. This a 20mm long, fine 8mm with a pitch of 1mm and a Left Hand thread. Can these be purchased on-line? I'm not sure I could make one myself as my lathe has an imperial lead screw and I don't have a suitable gear to do metric threads. The saw is a B&Q special which I bought about 15 years ago, so I have more chance of winning the lottery, than obtaining spare parts for it.

Thanks in advance for any advice.


Edited By Anthony Knights on 15/04/2020 16:38:19

old mart15/04/2020 18:18:17
3771 forum posts
233 photos

For the key, you can file up a bit of mild steel to fit. You might be saved by Bill Chugg (see current offer of help) over the lefthand threaded bolt.

not done it yet15/04/2020 18:34:48
6809 forum posts
20 photos

First check the keyway. It may need re-cutting.

Agri menders, for instance, will have large selections of keys - in several boxes, probably.

Martin Connelly15/04/2020 18:35:59
2137 forum posts
222 photos

Try these people for the bolt, they sell a range of left hand thread saw blade bolts.

Power tool centre

Martin C

Ps I would be wary of a home made bolt. What would happen if it snapped during use? 

Edited By Martin Connelly on 15/04/2020 18:46:52

Hopper15/04/2020 22:28:47
6393 forum posts
334 photos

Buy a piece of key steel and cut to length and file the ends round.

Nigel Graham 215/04/2020 22:57:00
2133 forum posts
29 photos

Obviously it would be far better to use the proper part or at least proper thread, but I investigated the pitch error over 20mm if you treat a 1mm pitch as 25tpi (with 60º angle).

1mm = 0.0394 inch, so 20 turns = 0.7874 inch.

25tpi = 0.040 " pitch = 0.800 " over 20 turns.

Cumulative error = 0.013 " so possibly too much.

Nevertheless I'd still prefer a proper replacement, for the reason Martin suggests. The machine may be a badged clone, or its present incarnations may still use the same small parts, so spares might be fairly easy to find. M8 x 1mm is an ISO Metric Fine standard, but left-hand?

Anthony Knights16/04/2020 05:07:53
622 forum posts
243 photos

Thank you for your replies gentlemen. I could quite easily make a new key, but what special properties does Key Steel have?. As for the bolt, I did consider turning down a high tensile 10mm bolt to 8mm and then cutting the thread. Maybe an offcut of the same bolt would do to make the new key?

I have found bolts on the site suggested by Martin Connelly, but they don't give sizes/specifications. I may have to ring them when they are open.

Edited By Anthony Knights on 16/04/2020 05:10:34

Hopper16/04/2020 05:16:00
6393 forum posts
334 photos

Key steel is a slightly higher carbon content than mild steel and comes in square and rectangular sections to match standard sizes. Bit of grade 5 bolt probably ok.

Anthony Knights16/04/2020 09:58:11
622 forum posts
243 photos

A correction to previous post. The saw is "McKeller" brand, purchased from the long defunct Focus DIY chain.

vintage engineer16/04/2020 11:03:21
258 forum posts
1 photos

The key should only be used for location. The drive should be taken by the clamping forces of the bolt.

HOWARDT16/04/2020 12:08:20
908 forum posts
39 photos

Key steel is usually EN9 and is bought in standard finished size bar, ie just cut to length, file the ends, job done.

Roger Baker 216/04/2020 12:12:37
16 forum posts

I agree with VE.

I ran a competition Rally Car engine with NO key in the Cam pulley to get the valve timing correct and forgot about it until the end of season strip down. No problems.


not done it yet16/04/2020 15:31:52
6809 forum posts
20 photos
Posted by vintage engineer on 16/04/2020 11:03:21:

The key should only be used for location. The drive should be taken by the clamping forces of the bolt.

Yeah right! So that is what the keyway is for on the typical horizontal arbor? (just as an example). Clearly not needed on small slitting saws, but ....

Anthony Knights17/04/2020 07:42:38
622 forum posts
243 photos

If you think the key is not necessary, you obviously haven't looked at the picture I posted. The saw blade fits on one end of the shaft with clamping plates and the LH bolt. The shaft goes through a bearing in a housing (not shown), with the gear wheel on the other end, which is in turn driven by the (smaller) gear on the end of the motor shaft.

The original fault was I had lost the drive to the saw blade. The gear was just spinning on the shaft because THE KEY HAD SHEARED. Photo shows the key in 2 pieces.

Howard Lewis17/04/2020 08:45:13
6104 forum posts
14 photos

For cutting metal with a Slitting Saw, a key is not required.

Indeed it can pose a danger.

I was walking past a milling machine when a Slitting Saw shattered. Disobeying instructions, the Apprentice had fitted a key, and when the saw jammed, it shattered, sending metal flying around the shop and up towards the ceiling.

It was lucky that no one was injured by the flying metal!

If the saw is driven by the friction between it and the arbor flanges, should it jam, it can slip without doing any great harm.


Baz17/04/2020 08:53:12
724 forum posts
2 photos

During my apprenticeship I was taught that for cutters larger than 1/4 inch wide you keyed the cutter, for cutters under 1/4 inch wide you keyed the arbor.

Anthony Knights17/04/2020 12:58:49
622 forum posts
243 photos

Made a new key. I'm glad I bought some new files from Arc Euro a few weeks ago.


All re-assembled with new circlip.


The saw is now all put back together. I'm just waiting for a new bolt which is ordered. I just hope it's the correct thread.

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