Tight Connection Of Quick-change Gearbox To Leadscrew

[Xiansheng]

Hi Everyone.

I will be grateful for any opinions/advice re fixing this problem, including whether my proposed solution is a good idea or not.

I have the Hare Forbes equivalent of the Grizzly G4003G lathe. I recently discovered that the quick-change gearbox is very tight (impossible to turn by hand) when connected to the leadscrew, even though the half-nuts are not closed on the leadscrew. Selection of the gear cone linkage to the leadscrew or the drive shaft is made by shifting a gear left or right. I would like to insert photographs of the inside of the gearbox to illustrate this but can't see any means of inserting a photos, so forget that. When the drive shaft is selected, the gearbox can be turned with ease. When the leadscrew is selected, the gearbox is very tight.

By removing the gearbox cover and investigating, I discovered that if the taper pin connecting the leadscrew to the gearbox leadscrew adapter is removed, the stiffness disappears and the gearbox turns easily. By gripping the immediately adjacent gear cone and turning, the difference in tightness can be assessed. If I replace the taper pin the tightness returns.

I surmise that the clever Chinese who assembled the machine drilled the hole for the taper pin in situ and by forcing the drill caused the axes of the adapter and leadscrew to be misaligned when the pin is in place, but allowing them to return their coaxial alignment when the pin is removed.

What do you think of my idea re the cause of this problem?

I don't believe the misalignment is severe - but it is sufficient to cause binding of the adapter in its bearing. My idea is to first test the misalignment by pressing on the adapter without the pin in place and see if it tightens the gearbox. If it does, I think I should place a wooden block firmly between the bed casting and the adapter to hopefully place them in an aligned position and use a taper reamer to re-align the hole. I doubt this will be a perfect solution but maybe it will remove the tightness. The only other solution I can think of is to remove the leadscrew and adapter and supporting them in an aligned position on my milling machine bed, drill another hole and ream it to correctly insert a new pin.

What do you think of this?

 

[Xiansheng]

I had further thoughts after I posted about this problem and now am inclined to think the problem is more likely due to the bore in the adapter being misaligned. This would account for the stiffness of the gearbox varying as the adapter is rotated through 360 degrees. I think I should remove the leadscrew and check this possibility.

 

[joshua43214]

Some pics would really help.
There are two ways to upload images.
You can just drag a pic from your computer right into the area where you type your text. This is the easiest way. Once you dropped the pic into the thread, there are two option buttons under it to select for full size or thumbnail.
The other way is to use a service such as Photobucket. Upload the pic to photobucket, go the to pic you want to use. On the right side is a list of different ways to share the pic, click "direct." Come back here to where you enter text. Along the top of where you enter text is a tool bar. In that tool bar is an icon that looks a bit like a pic of the sun setting over some mountains, the icon is directly to the right of the smiley face. Click the icon of the pic, and a dialogue box will appear, paste the direct link you got from photobucket into the box and click OK.

I looked up the owners manual for the 4003 lathe. just to make sure we are all on the same page, are you talking about part 302 on page 47 of the grizzly manual linked here
http://cdn0.grizzly.com/manuals/g4003_m.pdf
This is the pin that connects the quick change gear box to the lead screw. This pin should be visible on the outside of the lathe with out taking the cover off the gear box. I do not see any taper pins inside the gear box itself.

The other piece of info we need is can you turn the lead screw by hand when the pin is removed? You will have to wiggle the lead screw a bit to keep it clear of the half nuts. It might be the bearing at the far right end is too tight.

If the lead screw and gear box are indeed mis-aligned. I would contact the dealer before making any modifications. That would be a clear problem with workmanship and should be covered under warranty.